Description
Water For The Future Fulfillment
DESIGN AND PERFORMANCE ANALYSIS OF SINGLE BASIN DOUBLE-SLOPE PASSIVE SOLAR STILL
A PROJECT REPORT Submitted by
ASHICK ALI.S.M BADSHA.M.B IRSATH BISMI.M.A SAHUL HAMEED.N
96810114010 96810114011 96810114016 96810114045
In partial fulfillment for the award of the degree Of
BACHELOR OF ENGINEERING
In MECHANICAL ENGINEERING SARDAR RAJA COLLEGE OF ENGINEERING
ANNA UNIVERSITY: CHENNAI 600 025
APRIL 2014
1
DESIGN AND PERFORMANCE ANALYSIS OF SINGLE BASIN DOUBLE-SLOPE PASSIVE SOLAR STILL
A PROJECT REPORT
Submitted by
ASHICK ALI.S.M BADSHA.M.B IRSATH BISMI.M.A SAHUL HAMEED
96810114010 96810114011 96810114016 96810114045
In partial fulfillment for the award of the degree of
BACHELOR OF ENGINEERING
in
MECHANICAL ENGINEERING SARDAR RAJA COLLEGE OF ENGINEERING
ANNA UNIVERSITY: CHENNAI 600 025
APRIL 2014
2
ANNA UNIVERSITY: CHENNAI 600 025
BONAFIDE CERTIFICATE
Certified that this project report “DESIGN AND PERFORMANCE
ANALYSIS OF SINGLE BASIN DOUBLE SLOPE PASSIVE SOLAR STILL” is
the bonafide work of ASHICK ALI.S.M BADSHA.M.B IRSATH BISMI.M.A SAHUL HAMEED.N Who carried out the project work under my supervision. 96810114010 96810114011 96810114016 96810114045
SIGNATURE Mr. K. CHANDRASEKAR, M.E., HEAD OF THE DEPARTMENT Department of Mechanical Engineering Sardar raja College of Engineering Alangulam
SIGNATURE Mr. P .SETHU RAMALINGAM, M.E., SUPERVISOR Department of Mechanical Engg Sardar raja College of Engineering Alangulam
Submitted for project viva-voce to be held on ………………………
INTERNAL EXAMINER
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EXTERNAL EXAMINER
ACKNOWLEDGEMENT
We are very grateful thanks to our honorable founder chairman Dr.S.A.RAJA and our chairman Er.A.JESUS RAJA for helping us in all respects to carrying out this project. We express our sincere thanks to our principal
Dr.M.JEYAKUMAR M.E., Ph.D for helping us in all respects to carrying out this project. We are also expressing our heartfelt and sincere thanks to Prof.K.CHANDRASEKAR M.E., (Ph.D) and head of the department of mechanical engineering for providing us with necessary facilities to complete our project successfully. It is always great pleasure to do any work with keen interest under eminent guidance. Hence we express our gratitude and heartfelt thanks to Prof. P .SETHU RAMALINGAM, M.E., for his inspiring guidance, and valuable advice and suggestion rendered for the success of this project. We are also thank to Prof. K.CHANDRA SEKAR M.E., (Ph.D) for the co-ordinator of project work for supporting us of doing this project successfully. We extend our sincere thanks to all teaching staff of our department for helping us in all respects in carrying out this project. Finally we extend our thanks to all non-teaching staff of our mechanical department who helped us in making this project a grand success.
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ABSTRACT
Basin type solar still is a simple device which can be used for fresh water production. Single basin solar stills can be used for water desalination. Probably they are considered the best solution for water production in remote, arid to semi-arid, small communities, where fresh water is unavailable. However, the amount of distilled water produced per unit area is somewhat low which makes the single basin solar still unacceptable in some instances. For that simple modifications are done. The main purpose of this project is to check the performance of double slope passive solar still using different absorbing material (Aluminium, Rubber, Asbestos, Mild steel) and enhances the productivity of water
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TABLE OF CONTENTS
CHAPTER
TITLE
PAGE NO
ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES 1 INTRODUCTION 1.1 GLOBAL WATER SUPPLY AND DEMAND 1.2 WATER CRISIS IN INDIA 1.3 APPROACHES TO HANDLE THE PROBLEMS 2 DESALINATION TECHNOLOGIES 2.1 PHASE CHANGE PROCESSES 2.1.1 Multi Stage Flash Desalination(Msf) 2.1.2 Multi Effect Desalination(Mes) 2.1.3 Vapour Compression(Vc) 2.1.4 Freezing 2.1.5 Humidification And Dehumidification(Hd) 2.2 MEMBRANE PROCESS 2.2.1 Reverse Osmosis(Ro) 2.2.2 Electrolysis(Ed) 2.3 SOLAR STILLS 3 LITERATURE REVIEW 3.1 INTRODUCTION
6
iii iv vii ix 1
1 3
6 7 8 8 8 9 10
10 10 10 11 11 13 13
3.2 PASSIVE STILL 3.2.1 Cover Plate 3.2.2 Basin Water Depth 3.2.3 Radiation Absorption 3.2.4 Energy Storing Materials 3.2.5 Increasing The Evoporation Area 3.2.6 Surface Heating Technique 3.2.7 Increase The Radiation Received At The Basin 4 SOLAR ENERGY 4.1 SOLAR DISTILLATION 4.2 SOLAR STILL 4.3 TYPES OF SOLAR STILL 4.3.1 Passive Solar Still 4.3.2 Active Solar Still 4.4 HEAT TRANSFER MECHANISM IN SOLAR STILL 4.5 FACTORS AFFECTING PERFORMANCE OF SOLAR STILL 4.5.1 Free Surface Area
13 14 15 17 20 21 22
24 27 27 28 28 28 29
31
34 34
4.5.2 Water Glass Temperature Difference 34 4.5.3 Absorber Area 4.5.4 Water Depth 5 MATERIAL SELECTION AND DESIGN OF THE SOLAR STILL 5.1 HEAT TRANSFER AND HEAT LOSS 5.2 SOLAR STILL CONSTRUCTION MATERIALS 5.2.1 Transparent Cover
7
35 35
36 36
37 37
5.2.2 Basin Lines 5.2.3Insulation 5.3 DESIGN AND FABRICATION 5.4 WORKING PRINCIPLE 6 7 8 9 10 PHOTOGRAPHS EXPERIMENTAL PROCEDURE RESULT AND DECISION CONCLUSION REFERENCES
37 38 38 42 43 45 59 61 64
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LIST OF TABLES
TABLE NO
TITLE
PAGE NO
2.1 5.1 5.2 7.1 7.2
Desalination Technologies Technical Specification Of The Solar Still Materials And Properties Yield Of Water In Different Absorbing Material Temperature Of Various Thermo Couple Using Asbestos (2cm)
7 41 41 46
47
7.3
Temperature Of Various Thermo Couple Using Asbestos(3cm) 48
7.4
Temperature Of Various Thermo Couple Using Asbestos(4cm) 49
7.5
Temperature Of Various Thermo Couple Using Rubber(2cm) 50
7.6
Temperature Of Various Thermo Couple Using Rubber(3cm) 51
7.7
Temperature Of Various Thermo Couple Using Rubber(4cm) 52
7.8
Temperature Of Various Thermo Couple Using Aluminium(2cm) 53
7.9
Temperature Of Various Thermo Couple Using Aluminium(3cm) 54
7.10
Temperature Of Various Thermo Couple Using Aluminium(4cm) 55
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7.11
Temperature Of Various Thermo Couple Using Mild Steel(2cm) 56
7.12
Temperature Of Various Thermo Couple Using Mild Steel(3cm) 57
7.13
Temperature Of Various Thermo Couple Using Mild Steel(4cm) 58
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LIST OF FIGURES
FIGURE NO
TITLE
PAGE NO
2.1 3.1 3.2 3.3
Schematic Diagram Of A Simple Solar Still Regenerative Solar Still With Double Glass Cover Variation Of Nocturnal On Basin Water Depth Variation Of Yield With Water Depth For Different Initial Water Temperature
12 15 16
17 18 19
3.4 3.5 3.6
Water Production For Various Absorption Materials Variation In Production Rate Of The Solar Still Percentage Gain In Distilled Water Yield For Various Absorbing Materials
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3.7
Relation Between The Local Standard Time And Productivity Due To Asphalt Basin Liner Effect 21 22 22 23
3.8 3.9 3.10 3.11
Solar Still With Sponge In The Basin Single Slope Solar Still With Vertical Jute Cloth Solar Still With Aluminium Black Painted Plate Solar Still With Internal And External Reflectors
25
3.12
Schematic Diagram Of Basin Type Still With Internal And Flat Plate External Bottom Reflectors 26 29
4.1 4.2
Schematic Diagram Of A Passive Type Solar Still Schematic View Of Water Flowing Over The Glass Cover Solar Still Coupled With Flat Plate Collector
30 31 39
4.3 5.1
Heat Transfer In A Basin Type Solar Still Line Diagram Of Double Slope Solar Still
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5.2 6.1
Single Basin Double Slopes Solar Still Design of single basin double slope Passive solar still
40
43 44 44 46
6.2 6.3 7.1 7.2
Basin side view Absorbing material Yield Of Water In Different Absorbing Material Temperature Of Various Thermo Couple Using Asbestos (2cm)
47
7.3
Temperature Of Various Thermo Couple Using Asbestos(3cm) 48
7.4
Temperature Of Various Thermo Couple Using Asbestos(4cm) 49
7.5
Temperature Of Various Thermo Couple Using Rubber(2cm) 50
7.6
Temperature Of Various Thermo Couple Using Rubber(3cm) 51
7.7
Temperature Of Various Thermo Couple Using Rubber(4cm) 52
7.8
Temperature Of Various Thermo Couple Using Aluminium(2cm) 53
7.9
Temperature Of Various Thermo Couple Using Aluminium(3cm) 54
7.10
Temperature Of Various Thermo Couple Using Aluminium(4cm) 55
7.11
Temperature Of Various Thermo Couple Using Mild Steel(2cm) 56
7.12
Temperature Of Various Thermo Couple Using Mild Steel(3cm) 57
7.13
Temperature Of Various Thermo Couple Using
12
Mild Steel(4cm) 8.1 Temperature Of Different Absorbing Materials
58 60
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CHAPTER 1 INTRODUCTION
1.1 GLOBAL WATER SUPPLY AND DEMAND Water is essential for all life forms on earth—plants, animals and human. Water is one of the most abundant resources on earth, covering three fourths of the planet’s surface. About 97% of the earth’s water is salt water in the oceans and the remaining 3% (about 36 million km3) is fresh water contained in the polar region (in the form of ice), ground water, lakes and rivers, which supply most human and animal needs. Less than 1% fresh water is within human reach. Even this small fraction is believed to be adequate to support life and vegetation on earth. Nature itself provides most of the required fresh water, through hydrological cycle. A very large-scale process of solar distillation naturally produces fresh water. The essential features of this process are thus summarized as the production of vapours above the surface of the liquids, the transport of vapours by winds, the cooling of air – vapour mixture, condensation and precipitation. However, rapid industrial growth and a worldwide population explosion have resulted in a huge rise of demand for freshwater, both for household needs and for crops to produce adequate quantities of food. Added to this is the problem of the pollution of rivers and lakes by industrial wastes and the large amounts of sewage discharge. On a global scale, human–made pollution of natural sources of water is becoming one of the greatest causes of freshwater shortage. Uneven distribution of population is another problem. Then provision of freshwater is becoming an increasingly important issue in many areas of the world. According to World Health Organization (WHO), the permissible limit of salinity in water is 500 ppm and for special cases up to 1000 ppm while most of the water available on earth has the salinity up to 10,000 ppm whereas seawater normally has salinity in the range of 35,000–45,000 ppm in the form of total dissolved salts
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Global demand for water has tripled since the 1950s, but the supply of fresh water has been declining .Half a billion people live in water-stressed or water-scarce countries, and by 2025 that number will grow to three billion due to an increase in population. Irrigated agriculture is the dominant user of water, accounting for about 80% of global water use. Population and income growth will increase the demand for irrigation water to meet food production requirements and household and industrial demand. The global population is projected to increase to about 9 billion by 2050. Fulfillment of calorie requirements and dietary trends will translate into even higher water demand if more calories will be supplied from meat. At the same time, the limited easily accessible freshwater resources in rivers, lakes and shallow groundwater aquifers are dwindling due to over-exploitation and water quality degradation. Being the largest user of water, irrigation is the first sector to lose out as water scarcity increases. The challenges of water scarcity are heightened by the increasing costs of developing new water sources, land degradation in irrigated areas, groundwater depletion, water pollution and ecosystem degradation. With current water utilization practices, a fast growing population, and a nutritional transition towards diets that rely more on meat, global water resource limits will be reached sooner. Data on water supply and demand are startling: about 450 million people in 29 countries face severe water shortages, about 20% more water than is now available will be needed to feed the additional three billion people by 2025, as much as twothirds of the world population could be water-stressed by 2025, which supply onethird of the world’s population, are being pumped out faster than nature can replenish them, half of the world’s rivers and lakes are polluted and major rivers, such as the Yellow, Ganges, and Colorado, do not flow to the sea for much of the year because of upstream withdrawals. About 40% of the world’s populations live in regions that directly compete for shared trans boundary water resources. Estimated that under
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their baseline scenario, total global water withdrawals for agricultural, domestic and industrial use will increase by 23% from 1995 to 2025. The availability of sufficient water resources is one of the major crises with overarching implications for many other world problems especially poverty, hunger, ecosystem degradation, desertification, climate change, and even world peace and security. Water scarcity is projected to become a more important determinant of food scarcity than land scarcity, according to the view held by the UN (UNDP, 2007). Hence there is an essential and earnest need to get fresh water from the saline/brackish water present on or inside the earth. This process of getting fresh water from saline/ brackish water can be done easily and economically by desalination. 1.2 WATER CRISIS IN INDIA Although India occupies only 3.29 million km2 geographical area, which forms 2.4% of the world’s land area. India has 16 per cent of the world’s population and four per cent of its fresh water resources. Estimates indicate that surface and ground water availability is around 1,869 billion cubic metres (BCM). Of this, 40 per cent is not available for use due to geological and topographical reasons. Around 4,000 BCM of fresh water is available due to precipitation in the form of rain and snow, most of which returns to the seas via rivers. The fresh water crisis is already evident in many parts of India, varying in scale and intensity at different times of the year. Rainfall in India is dependent on the south-west and north-east monsoons, on shallow cyclonic depressions and disturbances and on local storms. Most of it takes place under the influence of south-west monsoon between June and September except in Tamil Nadu, where it is under the influence of north-east monsoon during October and November. India is gifted with a river system comprising more than 20 major rivers with several tributaries. Many of these rivers are perennial and some of these are seasonal. The rivers like Ganges, Brahmaputra and Indus originate from the
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Himalayas and carry water throughout the year. The snow and ice melt of the Himalayas and the base flow contribute the flows during the lean season. More than 50% of water resources of India are located in various tributaries of these river systems. Average water yield per unit area of the Himalayan Rivers is almost double that of the south peninsular rivers system, indicating the importance of snow and glacier melt contribution from the high mountains. Apart from the water available in the various rivers of the country, the groundwater is also an important source of water for drinking, irrigation, industrial uses, etc. Ninety two per cent groundwater extracted is used in the agricultural sector, five and three per cent respectively for industrial and domestic sector. Eight nine per cent of surface water use is for agricultural sector and two per cent and nine per cent respectively are used by the industrial and domestic sector. As per the international norms, if per-capita water availability is less than 1700 m3 per year then the country is categorized as water stressed and if it is less than 1000 m3 per capita per year then the country is classified as water scarce. In India per capita surface water availability in the years 1991 and 2001 were 2309 and 1902 m3 and these are projected to reduce to 1401 and 1191 m3 by the years 2025 and 2050 respectively. The fresh water crisis is not the result of natural factors, but has been caused by human actions. India’s rapidly rising population and changing lifestyles also increases the need for fresh water. Intense competition among competing us er’s agriculture, industry and domestic sector is driving the ground water table deeper and deeper. Widespread pollution of surface and groundwater is reducing the quality of fresh water resources. Fresh water is increasingly taking centre stage on the economic and political agenda, as more and more disputes between and within states, districts, regions, and even at the community level arises. In India nearly 45 million people are affected by water quality problems caused by pollution, by excess fluoride, arsenic, iron or by the ingress of salt water.
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Millions do not have adequate quantities of safe water, particularly during the summer months. In rural areas, women and girls still have to walk long distances and spend up to four hours every single day to provide the household with water. Scarcity of fresh water problems are facing many arid zones of Gujarat and Rajasthan, luckily these places are getting more amount of solar energy, apart Gujarat and Rajasthan that in western India, which face water shortage and have huge underground saline water sources, certain regions in Haryana state and Maharashtra states also have underground saline water in spite of high rain fall. The village peoples are facing lot of difficulties to get fresh water for their family needs. All families the women and children are responsible for collecting and storage of water. The quality of drinking water also not suitable for human health, it was found by tested the village water samples at Guru Kripa test house at Ajmer district. After analyzing in all the aspects authors concluded that, the village peoples are expecting suitable low cost purification devices for getting pure drinking water. It was estimated that, approximately 13.443 Million Liters per Day (MLD) of wastewater being generated from the 453 cities for consuming 16,814 MLD of water. Around 37.7 million Indians are affected by waterborne diseases annually, 1.5 million children are estimated to die of diarrhea alone and 73 million working days are lost due to waterborne disease each year. The resulting economic burden is estimated at $600 million a year. While ‘traditional diseases’ such as diarrhea continue to take a heavy toll, 66 million Indians are at risk due to excess fluoride and 10 million due to excess arsenic in groundwater. In all, 1,95,813 habitations in the country are affected by poor water quality. It is clear that the large investments have not yielded comparable improvements in health and other socio-economic indicators.
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1.3 APPROACHES TO HANDLE THE PROBLEMS In order to address the problems discussed above, an attempt is made in the present study to suggest a few approaches and methodologies as follows. ? Rain Water Harvesting and subsequent recharge of groundwater can help lower the concentration of minerals in aquifers. ? Adoption of latest technologies in order to reduce the waste water generation and/or to treat them effectively. ? Dual water supply – The success of this system lies in the fact that filtered purified water is used only for drinking purposes while other source of water may be used for purposes other than drinking. ? Improved and innovative planning of water resources – The development of water resources involves the conception, planning, designing, construction, and operation of facilities to control and utilize water with the national objective of improving the quality of life of the people. ? Treatment of sea water and backrish water to convert fresh water by using desalination technologies.
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CHAPTER 2
DESALINATION TECHNOLOGIES Desalination can be achieved using a number of techniques. Industrial desalination technologies either use phase change or involve semi permeable membranes to separate the solvent or some solutes. Therefore, desalination techniques may be classified into the following categories. phase change or thermal processes and membrane or single-phase processes. In Table 2.1, the most important technologies in use are listed. In the membrane processes, electricity is used for either driving high-pressure pumps or ionization of salts contained in the seawater. Table 2.1 Desalination technologies Phase change processes Multi-stage flash (MSF) Multiple (MED) Vapour compression (VC) Freezing Humidificationdehumidification Solar stills effect Membrane processes Reverse osmosis (RO)
distillation Electrodialysis (ED)
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2.1 PHASE CHANGE PROCESSES In the phase change or thermal processes, the distillation of seawater is achieved by utilizing a thermal energy source. The thermal energy may be obtained from a conventional fossil fuel source, nuclear energy, or a non-conventional solar energy source or geothermal energy. These processes involve heating of saline water and collecting the condensed vapour (distillate) to produce pure water. 2.1.1 Multi –Stage Flash Desalination (MSF) The MSF process is composed of a series of elements, called stages. In each stage, condensing steam is used to pre-heat the seawater feed. By fractionating the overall temperature differential between the warm source and seawater into a large number of stages, the system approaches ideal total latent heat recovery. Operation of this system requires pressure gradients in the plant. Current commercial installations are designed with 10–30 stages (2°C temperature drop per stage). 2.1.2 Multi Effect Distillation (MED) The Multi effect distillation process is composed of a number of elements, which are called effects. The steam from one effect is used as heating fluid in another effect, which, while condensing, causes evaporation of a part of the salty solution. The produced steam goes through the following effect, where, while condensing, it makes some of the other solution evaporate, and so on. For this procedure to be possible, the heated effect must be kept at a pressure lower than that of the effect from which the heating steam originates. The solutions condensed by all effects are used to pre-heat the feed. In this process, vapour is produced by flashing and by boiling, but the majority of the distillate is produced by boiling. Unlike an MSF plant, the MED process usually operates as a once-through system without a large mass of brine recirculating around the plant. This design reduces both pumping requirements and scaling tendencies.
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2.1.3 Vapour Compression (VC) Vapour compression desalination refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapour. Since compression of the vapour increases both the pressure and temperature of the vapour, it is possible to use the latent heat rejected during condensation to generate additional vapour. The effect of compressing water vapour can be done by two methods. The first method utilizes an ejector system motivated by steam at manometric pressure from an external source in order to recycle vapour from the desalination process. The form is designated Ejecto or Thermo Compression. Using the second method, water vapour is compressed by means of a mechanical device, electrically driven in most cases. This form is designated mechanical vapour compression (MVC). The MVC process comprises two different versions: Vapour Compression (VC) and Vacuum Vapour Compression (VVC). VC designates those systems in which the evaporation effect takes place at manometric pressure, and VVC the systems in which evaporation takes place at sub-atmospheric pressures (under vacuum). The compression is mechanically powered by something such as a compression turbine. As vapour is generated, it is passed over to a heat exchanging condenser which returns the vapour to water. The resulting fresh water is moved to storage while the heat removed during condensation is transmitted to the remaining feedstock. The VVC process is the more efficient distillation process available in the market today in terms of energy consumption and water recovery ratio. As the system is electrically driven, it is considered a "clean" process, it is highly reliable and simple to operate and maintain.
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2.1.4 Freezing Freezing desalination has been proposed as a method for desalination for several decades, only demonstration projects have been built to date. The concept is appealing in theory because the minimum thermodynamic energy required for freezing is less than for evaporation since the latent heat of fusion of water is 6.01 kJ/mole while the latent heat of vaporization at 100°C is 40.66 kJ/mole. 2.1.5 Humidification-Dehumidification (HD) The HD process is based on the fact that air can be mixed with important quantities of vapor. The amount of vapor able to be carried by air increases with the temperature; in fact, 1 kg of dry air can carry 0.5 kg of vapor and about 670 kcal when its temperature increases from 30°C to 80°C. When airflow is in contact with salt water, air extracts a certain quantity of vapor at the expense of sensitive heat of salt water, provoking cooling. On the other hand, the distilled water is recovered by maintaining humid air at contact with the cooling surface, causing the condensation of a part of vapor mixed with air. Generally the condensation occurs in another exchanger in which salt water is preheated by latent heat recovery. 2.2 MEMBRANE PROCESSES In the membrane processes, electricity is used for either driving high-pressure pumps or ionization of salts contained in the seawater. In this method membranes with very fine holes in the order of microns are employed. When the water is passing through this membrane, it gets purified and desalinated. 2.2.1 Reverse osmosis (RO) Reverse osmosis (RO) is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute
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is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. 2.2.2 Electro dialysis (ED) Electro dialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configureuration called an electro dialysis cell. The cell consists of a feed (diluate) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electro dialysis processes, multiple electro dialysis cells are arranged into a configureuration called an electro dialysis stack, with alternating anion and cation exchange membranes forming the multiple electro dialysis cells. Electro dialysis processes are different compared to distillation techniques and other membrane based processes (such as reverse osmosis) in that dissolved species are moved away from the feed stream rather than the reverse. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electro dialysis offers the practical advantage of much higher feed recovery in many applications. 2.3 SOLAR STILLS A simple way of distilling water is by evaporation and condensation method. In solar still, the water is evaporated using solar energy. In solar still, impure water is taken in a well insulated air tight basin covered with transparent plastic/glass cover. When the cover is exposed sun, radiation energy is transmitted through transparent cover, falls on the basin, absorbed by basin absorber plate, converted into heat and transferred to water. Water gets heated up, transfer heat to air inside the still and the air become unsaturated. The water evaporates and makes the air inside still saturated. This air subjected circulatory motion due to the temperature difference between water surface and cover lower surface. When high temperature air touches the cover,
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become cool and the water present in the air condenses at the lower surface of the cover. This condensed water slides down and collected using a drain. A schematic diagram of simple single basin double slope solar still is shown in Figure 2.1
Figure 2.1 Schematic diagram of a simple solar still
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CHAPTER 3 LITERATURE REVIEW
3.1 INTRODUCTION Basin type solar still is a simple device which can be used for fresh water production. The main drawback of a traditional solar still is the low amount of distilled water production per unit area which makes the single-basin solar still as uneconomical. (O.O. Badran)There is a great scope to improve the efficiency of such type of solar stills. Depending on the methods used to enhance the stills are classified as passive and active type solar still. In active type of stills, the evaporation and condensation of the water is activated externally. Additional space for evaporation, condensation and radiation collectors may also available. In passive type still, (K. Kalidasa Murugavel, K.Srithar) simple modifications are done or some materials are used in basin along with saline water to improve the performance. 3.2 PASSIVE STILL Depending on the energy available for evaporating the water in the still, the stills are classified into passive and active. In passive stills, the water in the still basin receives heat only by the radiation transmitted through the transparent cover. Hence the performances of this type of stills are improved by optimizing the cover and basin conditions. The cover plate is optimized to receive and transmit maximum radiation into the still. Also, the cover should be capable of condensing maximum mass of water vapour by exhausting heat to the atmosphere. The basin has to receive the maximum radiation transmitted into the still, convert into heat, transfer heat to the water and provide maximum surface area for evaporation. It should also retain excess heat and utilize it when the radiation level falls.
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3.2.1 Cover plate The transparent cover receives and transmits radiation into the still. Also, the condensation takes place at its lower surface. Also, it should suppress thermal radiation to the atmosphere. Glass is the best material for cover. since it has higher transmittance and less reflectivity. Also glass is opaque for thermal radiation. For higher latitude places, single slope still is preferable and for northern chemosphere the still faces south. The inclination of the cover is optimized to collect the accumulated condensate through drain before it drops down to basin. The condensate mass accumulation depends on solar intensity and condensation rate. The conducted indoor simulation experiment and found that the production rate is higher for 30o cover inclination. The glass has to withstand its self weight and thermal stresses. For higher surface area, high thickness glass will be used. But, radiation transmittance and heat transfer through the glass decrease with thickness. Glass cover plate with 3 mm thickness gives 16.5% more production than the cover with 6mm glass thickness. The transmittance loss at cover is less due to film wise condensation at the lower surface. The other transparent materials are not having these characteristics. Glass temperature affects the condensation rate at its lower surface. Lower glass surface temperature increases the circulation of air inside the still which enhances convective and evaporative heat transfer between basin water and glass. Also cooler glass lower surface increases condensation. The glass cover temperature is reduced by continuous flow or intermittent flow of raw cooling water on the cover. The cooling water gains latent heat from condensing water and regenerates it in the basin. Second effect of evaporation and
condensation takes place between the covers as shown in Figure 3.1 result
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shows increase in production by 20%. Results also show that, the use of the film-cooling increases the still efficiency up to 20%. Wind velocity is also having its effect on temperature of the glass. At higher wind velocity, due to higher convection heat transfer from the glass to atmosphere the productivity of the still is increasing. Theoretically analyzed the effect of water flowing over the glass cover in a single basin still. The result shows the productivity is increased with flowing water over the glass cover. Also, the yield decrease when the water flow rate increases.
Figure 3.1 Regenerative solar still with double glass cover 3.2.2 Basin water depth Water depth is the one of the main parameters affecting the solar still. Some investigations show that the total production of the still varies with water depth. Since the bulk motion of the air inside the still is proportional to temperature difference between the water and glass which is responsible for carrying water vapour from basin to glass surface. The volumetric heat capacity of the basin depends on the depth of water in the basin. For given radiation, the temperature of the basin is high when the volumetric heat capacity of the basin is less. But, experiments with deep basin reveal that, the productivity of the still also decreases with an increase in depth. For given radiation condition, the water temperature is less for high depth stills, but it store higher amount of energy due to its higher amount of volumetric heat capacity. This increases the
28
nocturnal production in Figure 3.2 and delays the morning production. The basin temperature and production rate are not affected by intermittent cloud passing. But, the shallow basin still is having immediate effect on change radiation intensity.
Figure 3.2 Variation of nocturnal production on basin water depth
The water depth was decreased from 0.1 to 0.005 m resulted in an increase of 19.6% in the still output, while a decrease in the depth from 0.29 to 0.1 m resulted only in a 6.3% increase in the still yield. The conducted an experiment in plastic solar still. This result also indicates that increase in water depth decrease the productivity. The productivity increased by 44.28% when the water depth was decreased from 0.18 m to 0.04 m. Results show that, the daily yield was decreased about 44% when changing the water depth from 0.01 to 0.20 m with the initial water temperature was 35ºC. Also, the yield was increased about 25% when changing the water depth from 0.01 to 0.20 m with the initial water temperature was 50ºC Figure 3.3.
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Figure 3.3 Variation of yield with water depth for different initial water temperatures 3.2.3 Radiation absorption Around 11% of radiation received by the still basin is reflected back without using it. This loss can be minimized, if the absorption coefficient of the still basin and water is increased. (A.A. El-Sebaii, A.A.Al-Ghamdi )Various methods are used to increase the absorption capacity of the basin. A simple method of increasing the absorption of the basin water is to add dye with the water. When the dye is added with water, the solar radiation is absorbed by the upper layer and the temperature of upper layer water increases, which in turn increases the evaporation rate. Different dye with different materials and concentration are having different effect on productivity at different depth water. Black napthylamine dyes at 172.5 ppm give higher increase in production rate by 29% Compared with red carmoisine and dark green. The effect of dye on productivity is more on deep basin still than shallow basin still. Different types of absorbing materials are used in the basin along with water to increase the absorption of the still basin. Rubber mate and charcoal are some materials used in the basin. An average yield of 2.5-4 l/m2.d was obtained when carbon powder (40-50/µm size) used as a basin material. Also removing
30
the basin's insulation reduces the productivity by 13-17% [36]. Basin lined with coal was used to enhance the absorption capacity of the basin. Figure 3.4 compares the effect of different energy absorbing materials placed along with basin water. Results show that, the black dye was the best absorbing material to increase the still productivity.
Figure 3.4 Water productions for various absorption materials The effects of coal and charcoal on solar-still performance. Both materials improve the still thermal performance and the high efficiency obtained when using charcoal in the basin. The effect of dissolved salts such as: copper sulphate, potassium permanganate, potassium dichromate, cobalt chloride. The effect of using various absorbing materials on the productivity of a single-basin solar still. The materials used to enhance the absorptivity of water for solar radiation include dissolved salts, violet dye, and charcoal. The salts were potassium permanganate and potassium dichromate. They found that the addition of potassium permanganate resulted in 26% improvements in efficiency. The best result was obtained by using violet dye with an increase of about 29%. The effect of using spreader materials in the basin. In this a single basin double slope passive type solar still was tested with minimum mass of water (approximately 2 mm depth) in the basin with different basin spreader materials
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like cotton cloth, jute cloth and sponge sheet, and porous materials like washed natural rock and quartzite rock as spread materials. From the above materials, black cotton cloth in the basin gives more productivity then others. Figure 3.5 shows the variation in the productivity. The effects of using various different black-paint absorber materials on the thermal performance of a solar desalination unit. Also the effect of using different absorbing materials in a solar still like black ink, black rubber mate and black dye. They found that the black dye was the best absorbing material. The effect of using uncoated metallic wiry sponge, coated metallic wiry sponge and black volcanic rocks. Figure 3.6 shows the variation in percentage of gain in distilled water yield.
Figure 3.5 Variation in production rate of the solar still
Figure 3.6 Percentage gain in distilled water yield for various absorbing materials
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The effect of using finned and corrugated absorbers in solar stills. The results indicated the productivity of finned and corrugated solar stills was higher than conventional still. It increases the amount of distilled water produced by about 40% and 21% respectively. 3.2.4 Energy storing materials Some black materials can store more amount of heat energy and increase the heat capacity of the basin in addition to increasing the basin absorption. (A. Safwat Nafey, M. Abdel kader, A. Abdel Motalip & A.A. Mabrouk) Glass, rubber and gravel are some material having these properties. Experimental results show that, the black rubber with 10 mm size increases the productivity of the deep basin still by 20% and black gravel with 20 –30 mm size increases the productivity of a shallow basin still by 19%. A solar still was tested with a special phase changing material as energy storing. Specially formulated mixture consisting of an emulsion of paraffin wax, paraffin oil and water with aluminum turnings to promote heat conduction has been used effectively to store the heat during day time, and then give off its heat at night time, thus increasing the productivity appreciably. The effect of using asphalt in the basin as an energy storing material. The use of asphalt in the basin resulted in a significant improvement in still production for an increase of 29% it’s shown in Figure 3.7. When the sprinkler was combined with asphalt the production rate increased up to 51%. Also the productivity during night contributed to around 16% of total day around productivity. The effect of using energy storing materials in the basin. The materials were quartzite rock, red brick pieces, cement concrete pieces, washed stones and iron scraps. They found that the still with 3/4 in. sized quartzite rock was the effective basin material.
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Figure 3.7 Relation between the local standard time and productivity due to asphalt basin liner effect 3.2.5 Increasing the evaporation area When the exposed area of basin water is high, then the air mass subjected to natural convection inside the still will take more amounts of water particles. The water wets the surface of the materials available in the basin and exposed to larger area and ready for diffusion. The rubber, gravel and charcoal used in the basin to improve the absorption, heat capacity and also the evaporation area hence the production. The performance of a solar still with different size sponge cubes placed in the basin was studied experimentally as shown in Figure 3.8. The increase in distillate production of the still ranged from 18% to 27% was observed compared to an identical still without sponge cubes under the same conditions. The small openings in the sponge cubes also reduce the surface tension between the water molecules, thus making it easier for the water molecules to evaporate. Experimental study of solar still with floating-wick showed that, the productivity of this type of still was higher than the common tilted-wick type and the conventional basin type solar stills. In this blackened jute wick floated with a polystyrene sheet. To improve the efficiency of a solar still by introducing a medium in the basin to provide large evaporation surface and utilize the latent heat of condensation in Figure 3.9. They found that the
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cumulative still yield in the regenerative still with jute cloth increases approximately by 20% and efficiency increases by 8%.
Figure 3.8 Solar still with sponge in the basin
Figure 3.9 Single slope solar still with vertical jute cloth 3.2.6 Surface heating technique The evaporation rate is increasing with the basin water temperature. To increase the temperature of entire mass of water, higher energy is required. Heating the top surface of water alone require less amount of heat and also this results in higher temperature for top surface water. Adding black dye in the basin water to improve the absorption results in surface heating of water and
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increase the production since the maximum portion of radiation is received by the top layers of water. In another method of surface heating technique, a plate separates top surface water with remaining water of the deep basin still as shown in Figure 3.10. The radiation is received by the separating plate and a part is used to heat the top layer of water and increases its temperature to enhance the productivity. Remaining part of the heat is stored in the bottom water and released later during low solar intensity periods. The material used and the thickness of the layer are the parameters affecting the productivity. Black painted aluminum with 2 cm of water layer increases the efficiency of the system by 28%. It was found that the floating perforated black aluminum plate in the solar still increases solar still productivity by 15% for 3 cm water layer and increases by 40% for 6 cm water layer. The effect of thermal conductivity of the suspended absorber on the daily productivity of the still was investigated experimentally using aluminum, copper, stainless steel, glass, and mica plates as suspended absorbers. The results obtained are compared with those obtained for the conventional still. It shows the daily productivity of the still with mica plate was found to be 42% higher than a conventional still.
Figure 3.10 Solar still with aluminium black painted plate
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3.2.7 Increase the radiation received at the basin Even though the glass cover inclination is optimized to receive the sun rays, a part of sun rays are received by the back plate and side plates of the basin. This effect reduces the amount of radiation available to the basin water for heating. (A.Tamimi) To reflect the sun rays fall on the side plates on to the basin reflecting mirror is used as shown in Figure 3.11. The effect of using reflecting mirrors on the vertical walls. Results show that, the production rate was increased by 86.2% for winter seasons and 22% for summer seasons when compared with conventional still. Another result shows that, the productivity of a single slope still with reflecting mirrors was 20% more than the double slope still. The increase in the daily amounts of distillate by adding the internal and external reflectors to the single-slope basin type still for the entire year would be averaged as 48%. The effect of inclining the external reflector on distillate productivity of a basin type still. The productivity of the still is increased by inclining the external reflector slightly backwards in summer and forwards in other seasons, and the inclination angle of the external reflector would be less than 25º throughout the year at 30ºN latitude. The benefit of both the internal and inclined external reflectors would be considerably less in summer than in winter. The increase in the average daily amount of distillate throughout the year of a still with inclined external reflector with optimum inclination in addition to an internal reflector, compared to a conventional basin type still was predicted as 29%, 43% or 67% when the glass cover inclination was 10º, 30º or 50º and the length of external reflector was half the still’s length . The system efficiency was increased by 20 to 26. Theoretically analyzed solar still with internal and external reflector in winter. The theoretical results show distillate of the inclined reflector was about 16% greater than the vertical external mirror and 2.3 times higher yield than still without mirrors. The experimental result shows daily productivity of the still increased by 70% to 100% with internal and
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external reflector in winter. Using of internal and external reflector inclined at 20º and the cover angle at 20º in winter session increases the productivity around 2.45 times than a simple still with no reflectors. A single basin type solar still with flat plate external bottom reflector in Figure 3.12
Figure 3.11 Solar still with internal and external reflectors The optimum inclination of the external reflector maximizes the daily amount of distillate of the still. Solar radiation distribution in a solar still with internal and external reflectors. They conclude as (i) the efficiency of radiation exchange between any two surfaces was significantly affected by their view factor, (ii) the use of global irradiance observed on a horizontal surface in the heat balance equations of a basin-type solar still would lead to inaccurate estimation of the distillate output, and (iii) the accuracy of modelling the performance of a basin-type solar still with internal and external reflecting surfaces can be improved by incorporating view factors and the diffuse component in the reflected solar radiation.
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Figure 3.12 Schematic diagram of basin type still with internal and flat plate external bottom reflectors
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CHAPTER 4 SOLAR ENERGY
Energy from the sun is said to be the solar energy. This is the earth’s primary energy flow and equally too, the most abundant on the earth crust. There are two major ways in which the solar energy can be used or harnessed. It can be used either as a thermal energy by heating a fluid or by converting it into electricity using photovoltaic arrays (PV). The former would be looked in detail in the design of the solar still. Solar energy is a relatively diffuse source of energy. It is also available almost everywhere, unlike geothermal, wave, wind or even conventional fuels. Depending on the energy demand of the application, it may require large areas. Yet, most solar energy conversion systems are modular and can be installed almost everywhere which relieves the space availability problem The solar energy can be deployed and use for simple desalination systems especially, the solar still for production of potable water in the tropics and arid regions of the world where there is abundance of this natural resource. These regions are well endowed with this resource and it should be fully exploited. 4.1 SOLAR DISTILLATION There is an important need for clean, pure drinking water in many developing countries. Often water sources are brackish (i.e. contain dissolved salts) and/or contain harmful bacteria and therefore cannot be used for drinking. In addition, there are many coastal locations where seawater is abundant but potable water is not available. Water has been distilled using solar energy. Distillation is a process of boiling the water and re-condensing the steam into clean container pure water is also useful for batteries and in hospitals or schools. Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. In this process, water is evaporated, thus separating water
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vapour from dissolved matter, which is condensed as pure water. Solar distillation uses, in common with all distillation processes, the evaporation and condensation modes, but unlike other processes energy consumption is not a recurrent cost but is incorporated in the capital cost of the solar collector. The solar still therefore, is of a simple design, construction and maintenance with ease of operation. It is best suitable for regions of the world with high solar intensities. The mechanism of operation is based on the transmitting, absorption and reflective properties of glass and other transparent materials. 4.2 SOLAR STILL Solar still is a device which converts unwanted water into drinkable water with the help of solar energy. Most important part of the solar still is absorber surface which absorbs solar energy. The stills apply the principles of evaporation and condensation that is seen within the precipitation cycle. The mechanism of operation is based on the transmitting, absorption and reflective properties of glass and other transparent materials. 4.3 TYPES OF SOLAR STILL The solar still can be classified into two types. They are ? Passive type solar still ? Active type solar still
4.3.1 Passive Solar Still In passive type still, simple modifications are done or some materials are used in basin along with saline water to improve the performance. Figure 4.1 shows the schematic diagram of a passive type solar still.
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Figure 4.1 Schematic diagram of a passive type solar still The water in the still basin receives heat only by the radiation transmitted through the transparent cover. Hence the performances of this type of stills are improved by optimizing the cover and basin conditions. In a passive solar still, the solar radiation is received directly by the basin water and is the only source of energy for raising the water temperature and consequently, the evaporation leading to a lower productivity.
4.3.2 Active solar still The temperature difference between water in the basin and condensing glass cover has a direct effect in the performance of the still. To achieve better evaporation rate and productivity, the temperature of water in the basin could be increased. In active still, some external sources are could be used to increase the temperature of water in the basin. The external sources connected with the simple basin still are flat plate collector, concentratic collector, hybrid PV/T system, heat exchanger, solar pond, multiple basins and additional condenser. Figure 4.2 shows the schematic view of water flowing over the glass cover solar still coupled with flat plate collector.
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Figure 4.2 Schematic view of water flowing over the glass cover solar still coupled with flat plate collector. The enhancement of daily yield in a double basin solar still with the effect of water flow over the glass cover and flow of hot water in the lower basin by a flat plate collector. The numerical results shows, the yield of the system was 50% higher than the ordinary double basin still. The double effect solar still with and without water flow and over the glass cover. The study shows that, an active solar still with water flow over the glass cover gives the maximum yield. A double-effect solar still does not enhance the distillate output significantly due to practical difficulties of attaining extremely low and uniform flow rates over the glass cover (10 ml/min). Double effect distillation under active mode of operation. The results show, when the water flow rate was small the efficiency of the still was increased about 30. The effect on daily yield of double effect distillation with water flow. The results show that, active double effect solar still was more effective in production than the passive still.
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4.4 HEAT TRANFER MECHANISM IN SOLAR STILL
Figure 4.3 Heat transfer in a Basin type Solar Still
The mechanisms of heat transfer within a solar still are basically dependent on the climatic effects and the amount of solar radiation that enters the basin. More importantly and frankly too, the performance of the still depends on how much of the solar irradiance that reaches the water in the basin of the solar still. When the sun’s radiation reaches the Earth it is both scattered and absorbed by the atmosphere. The radiation that then travels through the Earth’s atmosphere is known as “sky” radiation, this is the radiation incident on the Earth’s surface after the initial waves from the sun have been absorbed and scattered by the atmosphere. The “sky” radiation that travels to the Earth’s surface can then be used as a valuable energy source for desalination. The direct and diffuse radiation enters the still through the glass cover after partially being reflected and absorbed by the glass itself. Once in the evaporating chamber the radiation is further transmitted, reflected and absorbed by the water until it reaches the blackened basin where most of it is fully absorbed. The basin then begins to heat up and in turn through convective
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processes heats the water causing it to evaporate still. Due to the fact that the glass cover remains at a temperature lower than the dew point temperature (the temperature at which water saturates) the vapour begins to condense on the inside of the glass surface through the mechanism of drop-wise condensation. This is where the vapour condenses in discrete droplets and grows by means of some form of accumulation until it becomes large enough to move under gravity down the glass and can be collected in a pipe at the lower end of the still. This method of condensation has a heat transfer rate of 10 times that of film condensation which allows the heat to be dissipated at a faster rate this allows the excess heat absorbed by the glass to be dissipated and is lost to the atmosphere. Often the formation of drop-wise condensation can reduce the amount of radiation entering the still, and can contribute to a reduction in distillate production in the latter part of the day. The heat received by the film of condensed water, by radiation from the brine surface, by convection from air-vapour, and by conduction of vapour is conducted through the water film and glass to the external surface of the cover. The small amount of solar energy absorbed in the cover is also conducted outward. The heat which the cover (glass) has received is then transferred from the outer surface to the atmosphere by convection and radiation. The heat transfer processes in the solar still are all dependent on the difference in temperature between the brine surface and the glass. The higher the difference, the greater is the energy transfer rate by each mechanism. Furthermore, the higher the brine temperature, the greater the proportion of energy usefully transferred by evaporation. The solar energy transmitted by the glass is partly absorbed in the brine, with the majority of it being absorbed on the basin base. Heat is conducted from the basin base surface into the brine, thereby increasing its temperature and vapour pressure; partial vaporisation then occurs. The warm vapour saturated
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air is carried by the convection currents to the transparent glass cover, which is generally cooler than the brine. A thin film of condensate flows down the transparent glass cover to the collecting trough, from which it passes to storage. The incoming solar radiation, usually composed of direct radiation from the sun and diffuse radiation from the clouds and sky, is partly reflected by the outer and inner cover surfaces, very slightly absorbed in the cover, slightly reflected by the brine and the base of the basin; the balance is absorbed by the brine and the bottom of the basin. Another small portion of energy is lost by conduction through the bottom into the ground or through insulation under the base from the energy absorbed by the basin bottom. The brine is warmed by the convection currents in the shallow basin to the air-water interface, where transfer of mass and energy takes place. Since the vapour pressure of the surface water is greater than the partial pressure in the air space, evaporation into the overlying air film occurs. This transfer of water is accompanied by sensible heat transfer from the warm brine into the air-vapour mixture in contact with it. Both processes produced a temperature rise and density decreased in the airvapoumixture, causing it to rise toward the transparent glass cover. Supplementary to the convective heat transfer from brine surface, is a transfer of heat to the cover by radiation. The glass cover is cooler than the brine partly due to the breeze from the outer side and partly due to the condensate on the inner underside, so the radiant transfer process is essentially between two water surfaces, net radiation being from the brine in the direction of the glass cover. Since the glass cover is cooler than the air-vapour mixture coming in contact with it, the difference in vapour pressure causes diffusion of water vapour through the air film to the water layer on the underside of the cover. Condensation occurs due to the latent heat being released from the water film.
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4.5 FACTORS AFFECTING PERFORMANCE OF STILL The various factors affecting the productivity of solar still are ? Solar intensity ? Wind velocity ? Ambient temperature ? Water-glass temperature difference ? Absorber plate area ? Temperature of inlet water ? Glass angle ? Depth of water The solar intensity, wind velocity, ambient temperature cannot be controlled as they are metrological parameters. Whereas the remaining parameters, free surface area of water, absorber plate area, temperature of inlet water, glass angle and depth of water can be varied to enhance the productivity of the solar stills. By considering the various factors affecting the productivity of the solar still, various modifications are being made to enhance the productivity of the solar
4.5.1 Free surface area The evaporation rate of the water in the solar still is directly proportional to the exposure area of the water. Thus the productivity of the solar still increases with the free surface area of the water in the basin.
4.5.2 Water-glass temperature difference The yield of a solar still mainly depends on the difference between water and glass cover temperatures. The temperature difference between water and glass are acting as a driving force of the distillation process. Regenerative
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solar still solar still with double glasses and triple-basin solar still were used to increase the temperature difference between glass and the water.
4.5.3 Absorber area Productivity of the solar still increases with increase in absorber area.
4.5.4 Water depth It has been reported that the yield is maximum for the least water depth. While maintaining minimum depth in the solar still, dry spot may occur. So, it is very difficult to maintain minimum depth in the solar still. Wick type solar stills a plastic water purifier and stepped solar still were developed.
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CHAPTER 5 MATERIAL SELECTION AND DESIGN OF THE SOLAR STILL
5.1 HEAT TRANSFER AND HEAT LOSS The main objective of the present work is to develop a solar still with improved distillate output. This can be achieved by developing and applying a well-structured design methodology. Still design factors can broadly be classified into optical, heat transfer and heat loss characteristics. Optical characteristics are absorption, reflection and transmission of solar radiation when incident on the still. Moreover, solar radiation is the most influential environmental factor in solar energy systems. Once the radiation is absorbed by the still, it is converted to heat which is transferred from the absorber to other components of the still and the environment. Heat generated by the basin liner of a solar still is transferred to the saline water in the basin by convection. Then, the hot saline water releases heat to the transparent cover and walls of the still through convection, evaporation and radiation. This elevates the temperature of the transparent cover, thereby reducing the temperature gradient between the cover and the water, and the rate of distillation. Heat is lost to the environment through the top, bottom and sides of the system. Heat loss through the top is desirable because it helps to keep the transparent cover temperature low, thereby increasing the rate of condensation and distillate production. Top heat loss occurs through convection and radiation. Convective heat loss from the top is influenced by the speed of wind over the transparent cover while radiative heat loss from the top to the sky depends on the temperature and emittance of the transparent cover, and temperature of the sky.
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On the other hand, heat loss through the bottom and side walls reduces useful thermal energy for the distillation process, and the productivity of the still. The problem of heat loss from the bottom of a solar still is worse because of the higher temperature gradient between the basin liner and the ambient air temperature outside the still. This leads to a reduction in the distillate production. To overcome this problem insulation of the bottom part of a solar still. 5.2 SOLAR STILL CONSTRUCTION MATERIALS 5.2.1 Transparent cover The most influential property of a transparent material needed in solar technology is transmittance. Clear glass has transmission values of 88- 92% at normal incidence. A glass cover is fitted on the top part of the evaporator unit to allow solar radiation to reach saline water in basin placed under the cover. One or more transparent covers can be used with an air gap between them to reduce heat losses from the top of the evaporator. Multiple glazing reduces top heat loss significantly which leads to high temperatures of the glazing and a decrease in the rate of evaporation-condensation. Thus, single glazing is commonly used for solar distillation systems. Therefore clear window glass is selected as a transparent cover material. 5.2.2 Basin liner Solar radiation that passes through the transparent cover is absorbed by saline water and the basin liner of a solar still. So, the basin liner acts as an absorber of solar radiation and it is important for the liner to have a relatively high absorptance for solar radiation. In practical applications, basin liners can be made of plastic or metal-sheet. Some plastics are relatively cheap while others are expensive. Common metal sheets applied in solar collection are copper, aluminium and steel. The important property of a metal for application
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in solar engineering is thermal conductivity. Copper and aluminium have relatively high thermal conductivities (k=200 Wm-1K-1 for aluminium and k=390 Wm-1K-1 for copper) while the thermal conductivity of steel is relatively low (k=48 Wm-1K-1). Nevertheless, copper and aluminium are more expensive (more than two times the cost of galvanized steel). With these considerations, black coated steel plate is selected for increase its solar absorption. 5.2.3 Insulation Heat loss from the bottom and sides of a solar still is undesirable because it reduces distillate yield. Consequently, it is necessary to minimize this loss by insulating the relevant surfaces. This enables most of the absorbed solar radiation to contribute to the evaporation of saline water and thereby augment the distillate yield. The most important property of an insulator is the coefficient of heat conduction (k). Materials with low values of k are suitable for use as insulators due to their relatively high resistance to flow of heat. Glass wool and thermo cool contain low conductivity and low cost. Therefore glass wool and thermo cool is chosen as insulating material. 5.3 DESIGN AND FABRICATION This project took into cognizance the fact that the structure to be used should possess a number of features intended to guarantee an efficient and effective evaluation of the results. The design is a basin-type solar still (horizontal water-filled basin), covered by a sloping surface transparent to solar radiation, on which water is condensed and collected. Salt water was supplied to the basin with a depth of maximum 40mm. Figure 5.1 shows the Line diagram of double slope solar still.
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All dimensions are in mm
Figure 5.1 Line diagram of double slope solar still
The bottom of the still has a black surface to absorb solar energy. A transparent glass cover is placed on top of the basin such that its surface slopes down into a small trough at its lower edge. The trough is connected to a flexible hose for collection of the distillate. The basin was constructed with stainless steel (2mm thickness) and painted black to absorb the radiant heat. It was then secured an insulated casing of expanded thermo cool (30mm thickness) in bottom and side (20mm thickness). A semi-circular PVC pipe was attached at the lower end of the box to collect the distillate and directed it out to be collected. Figure 5.2 shows the Single basin double slope solar still.
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Figure 5.2 Single basin double slopes solar still
A glass panel (4mm thickness) was then placed on top of the still at an angle of 30 to the horizontal to ensure that the amount of condensate dripped back into the basin. The bottom and sides of the basin are insulated to reduce the heat losses to the surrounding. Once all of the modifications were made the solar still was set-up on the roof top of the building for testing. There are many adaptations and variations that could be made to this design. As a result the other modifications were not considered in great detail.
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Table 5.1 Technical specifications of the solar still
Width Length Thickness Base Area Glass Area Glass Angle
0.50m 0.70m 0.015m 0.30m2 0.20m2 30o
Table 5.2 Materials and properties
MATERIALS
DENSITY (kg/m3) 2707 1200 2110 7833
Thermal conductivity (w/m.k) 204.2 162.8 697.8 53.6
Specific heat (kj/kg.k) 0.896 1.382 2.093 0.465
Aluminium Rubber Asbestos Mild steel
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5.4 WORKING PRINCIPLE ? Salty or dirty water in an airtight container is heated by the sun, causing it to evaporate. ? The water then condenses on the clear container covering, which is slated to allow the fresh water to drain into a collection unit. ? The pure water evaporates and the impurities do not, distilling the water and making it safe to drink.
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CHAPTER 6 PHOTOGRAPH
Figure 6.1 Design of single basin double slope passive solar still
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Figure 6.2 Basin side view
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Figure 6.3 Absorbing material
CHAPTER 7 EXPERIMENTAL PROCEDURE
A single basin double slope solar still has been fabricated with mild steel plate. The overall size of the basin inner (absorbing medium) is 0.65mx0.45mx0.05m and that of outer basin is 0.70mx0.5mx1.5m. The top is covered with two glass of thickness 4mm inclined at 30° on both side. The basin is covered with thermo coal from all the sides to prevent the heat loss. The experiment was carried out 9.00 Am to 6.00Pm in a day. In this study, four absorbing materials were tested under different water level conditions. These were Asbestos, Rubber, Aluminium and Mild steel with black paint coated. Experiments were carried out at 2cm to 4cm water level in the basin. Data were taken during the February 2014. The thermocouples are used to measure temperature of water of glass cover, temperature of absorbing material and ambient temperature is measured every hour. The measuring jar is used to collect the distilled water. The yield of different absorbing material is tabulated shown in table 7.1
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Table7.1 Yield of water in different absorbing material
Sl No
Water level (cm)
Yield of water (ml)
Absorber material
700 630 1 2 780 620 600 550 2 3 690 530
Asbestos Rubber Aluminium Mild steel Asbestos Rubber Aluminium Mild Steel
530 3 4 430 570 420
Asbestos Rubber Aluminium Mild Steel
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800 700 yield of water/0.3m2 600 500 400 300 200 100 0 Asbestos Rubber Aluminium Mild steel Absorbing Medium 2 cm 3 cm 4 cm
Figure 7.1 Yield of water in different absorbing material
The ASBESTOS is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL : 14/02/2014 : 2 cm : ASBESTOS
WATER COLLECTED PER DAY: 700 ml Table 7.2 Temperature of thermo couple using Asbestos (2cm) Water TIME Temp. (T1) oC 10.00 am 11.00 am 35 47 Glass1 Temp (T2)oC 38 45 Asbestos Temp. (T3) oC 36 43 Glass2 Temp (T4) oC 41 47
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12.00 pm
50
49 52 56 48 40 35 30
46 44 54 40 46 34 33
48 44 39 40 48 41 31
1.00 pm 52 2.00 pm 3.00 pm 53 55
4.00 pm 48 5.00 pm 6.00 pm 39 31
60 50 Temp. in oC 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Ta Tg2
Figure 7.2 Temperature of thermo couple using Asbestos (2cm)
DATE WATER DEPTH ABSORBING MATERIAL WATER COLLECTED PER DAY
: 15/02/2014 : 3 cm : ASBESTOS : 600 ml
Table 7.3 Temperature of thermo couple using Asbestos (3cm) Water TIME Temp. Glass1 Temp Asbestos Temp.
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Glass2 Temp
(T1) oC 10.00 am 11.00 am 12.00 pm 33 48 53
(T2)oC 41 45 55 53 54 49 42 35 31
(T3) oC 34 36 40 48 52 43 37 31 28
(T4) oC 45 49 52 51 48 45 40 39 37
1.00 pm 50 2.00 pm 3.00 pm 49 48
4.00 pm 47 5.00 pm 6.00 pm 46 45
60 50
Temp. in oc
40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM
Time in hr
Tw Tw2
Tw3
Tw4
Figure 7.3 Temperature of thermo couple using Asbestos (3cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 16/02/2014 : 4 cm : ASBESTOS
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WATER COLLECTED PER DAY
: 530 ml
Table 7.4 Temperature of thermo couple using Asbestos (4cm) Water TIME
o
Glass1 (T2)oC 40 42 43 45 53 47 39 34 31
Asbestos Temp. (T3) oC 35 39 41 46 50 46 42 36 33
Glass2 Temp (T4) oC 31 33 34 35 37 38 35 33 31
Temp. (T1) Temp C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
32 37 38 40 43 47 42 36 33
60 50
Temp. in oc
40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM
Time in hr
Tw Tw2 Tw3 Tw4
Figure 7.4 Temperature of thermo couple using Asbestos (4cm)
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The RUBBER is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL : 17/02/2014 : 2 cm : RUBBER
WATER COLLECTED PER DAY : 630 ml Table 7.5 Temperature of thermo couple using Rubber (2cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 39 42 44 46 47 50 41 36 30
Rubber Temp. (T3) oC 37 40 41 43 48 46 42 37 31
Glass2 Temp. (T4) oC 33 35 36 38 40 42 37 35 30
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
35 39 41 44 45 48 40 37 31
60 50 Temp. in oc 40 30 20 10 0 10:00 AM 11:00 AM 12:00 PM Tw Tg1
Tr
Tg2
64
01:00 02:00 03:00 04:00 05:00 PM PM PM PM PM Time in hr 06:00 PM
Figure 7.5 Temperature of thermo couple using Rubber (2cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 21/02/2014 : 3 cm : RUBBER
WATER COLLECTED PER DAY : 550 ml Table 7.6 Temperture Of thermo couple using Rubber (3cm) Water Time
o
Glass1 (T2) oC 40 43 45 46 47 49 39 36 30
Rubber Temp. (T3) oC 34 38 40 42 43 45 50 38 31
Glass2 Temp. (T4) oC 32 34 36 37 41 42 37 33 29
Temp.(T1) Temp. C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
33 37 41 45 48 50 41 37 32
65
60 50 40 Temp. in oc 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Tr Tg2
Figure 7.6 Temperture Of thermo couple using Rubber (3cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 23/02/2014 : 4 cm : RUBBER
WATER COLLECTED PER DAY : 430 ml Table 7.7 Temperature of thermo couple using Rubber (4cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 39 41 42 44
Rubber Temp. (T3) oC 33 35 37 40
66
Glass2 Temp. (T4) oC 30 32 33 36
C
10.00 am 11.00 am 12.00 pm 1.00 pm
32 36 38 41
2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
43 45 42 39 37
45 46 40 35 32
43 45 41 35 33
38 39 35 33 31
50 45 40 Temp in oc 35 30 25 20 15 10 5 0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 03:00 PM PM Time in hr 04:00 PM 05:00 PM 06:00 PM Tw Tg1 Tr Tg2
Figure 7.7 Temperature of thermo couple using Rubber (4cm)
The ALUMINIUM is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL : 28/02/2014 : 2 cm : ALUMINIUM
WATER COLLECTED PER DAY : 780 ml Table 7.8 Temperature of thermo couple using Aluminium (2cm)
67
Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 51 54 56 58 59 53 44 39 32
Aluminium Temp. (T3) oC 44 46 50 53 51 50 45 38 34
Glass2 Temp. (T4) oC 38 47 56 55 53 46 43 38 34
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
43 50 57 59 60 58 51 45 38
70 60 50 temp. in oc 40 30 20 10 0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 03:00 PM PM Time in hr 04:00 PM 05:00 PM 06:00 PM Tw Tg1 Tal Tg2
Figure 7.8 Temperature of thermo couple using Aluminium (2cm)
DATE WATER DEPTH
: 01/03/2014 : 3 cm
68
ABSORBING MATERIAL
: ALUMINIUM
WATER COLLECTED PER DAY : 690 ml Table 7.9 Temperature of thermo couple using Aluminium (3cm) Water Time Temp.(T1)
o
Glass1 Temp. Aluminium (T2) oC Temp. (T3) oC 41 52 54 55 57 54 50 47 33 36 48 53 54 51 50 47 43 35
Glass2 Temp. (T4) oC 32 47 51 53 55 51 45 39 32
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
34 47 55 57 58 56 51 45 38
70 60 Temp. in oC 50 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Tal Tg2
Figure 7.9 Temperature of thermo couple using Aluminium (3cm)
69
DATE WATER DEPTH ABSORBING MATERIAL
: 02/03/2014 : 4 cm : ALUMINIUM
WATER COLLECTED PER DAY : 570 ml Table 7.10 Temperature of thermo couple using Aluminium (4cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 36 45 46 47 48 46 40 38 30
Aluminium Temp. (T3) oC 29 38 40 43 50 52 44 34 32
Glass2 Temp. (T4) oC 30 35 39 44 45 40 38 34 30
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
31 36 42 45 47 49 44 38 36
70
60 50 Temp.in oC 40 30 20 Tw Tg1 Tal Tg2
10
0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 PM Time in hr 03:00 PM 04:00 PM 05:00 PM 06:00 PM
Figure 7.10 Temperature of thermo couple using Aluminium (4cm)
The MILD STEEL is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL WATER COLLECTED PER DAY : 07/03/2014 : 2 cm : MILD STEEL : 620 ml
Table 7.11 Temperature of thermo couple using Mild steel (2cm) Water Time Glass1 (T2) oC 10.00 am 11.00 am 12.00 pm 1.00 pm 38 47 50 51 42 50 55 56
71
Mild steel Temp. (T3) oC 40 48 52 53
Glass2 Temp. (T4) oC 36 39 55 52
Temp.(T1) oC Temp.
2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
52 53 50 43 37
57 51 47 39 33
54 55 53 44 36
50 45 42 36 32
60 50 Temp. in oC 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Tm Tg2
Figure 7.11 Temperature of thermo couple using Mild steel (2cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 08/03/2014 : 3 cm : MILD STEEL
WATER COLLECTED PER DAY : 530 ml Table 7.12 Temperature of thermo couple using Mild steel (3cm) Water Glass1 Temp. Mild steel Glass2
72
Time
Temp.(T1) oC (T2) oC
Temp. (T3) oC
Temp. (T4) oC 32 35 45 49 50 52 46 37 33
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
34 41 47 50 52 53 49 42 36
37 42 46 52 56 51 48 37 32
35 42 50 52 54 55 51 42 37
60 50 Temp. in oC 40 30 20 Tw Tg1 Tm Tg2
10
0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 PM Time in hr 03:00 PM 04:00 PM 05:00 PM 06:00 PM
Figure 7.12 Temperature of thermo couple using Mild steel (3cm)
DATE
: 09/03/2014
73
WATER DEPTH ABSORBING MATERIAL
: 4 cm : MILD STEEL
WATER COLLECTED PER DAY : 420 ml Table 7.13 Temperature of thermo couple using Mild steel (4cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 37 42 50 52 54 51 47 43 36
Mild steel Temp. (T3) oC 35 39 43 45 48 47 46 41 35
Glass2 Temp. (T4) oC 32 36 49 50 51 48 44 39 33
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
33 38 45 48 50 46 43 41 37
60 50 Temp. in oC
40
30 20 Tw Tg1 Tm Tg2
10
0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 PM Time in hr 03:00 PM 04:00 PM 05:00 PM 06:00 PM
74
Figure 7.13 Temperature of thermo couple using Mild steel (4cm)
CHAPTER 8 RESULT AND DISCUSSION
Highest temperature is obtained by the temperature of water which is evaporated with the help of solar radiation. Hence, we can say that the higher temperature of water. So higher evaporation and condensation and higher yield. Figure 8.1 shows that, yield is very low up to noon and for all the absorber plate and goes on increasing after noon due to the solar radiation falling on the absorber plate. And as we know that, when sun is east. Solar evaporation takes place at east side and west side acts as a condenser and during west side of sun, reverse will be occur. Figure 8.1 shows that the yield for mild steel absorber plate is low compared to the aluminium sheet Asbestos as well as rubber sheet. Hence, it has been observed that yield of Alunminium is higher compared with other three absorbing materials (Asbestos, Rubber, MS). The absorber temperature at different timing as shown in Figure 8.1. The absorbing temperature of the “Aluminium” is higher than the Asbestos and others. Because we know that the Aluminium has higher thermal conductivity compared with others. So due to higher thermal conductivity higher temperature will obtained.
75
Temp. of absorbing material
60 50 Temp. in oC 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time
Asbestos
Rubber Aluminium mild steel
Figure 8.1 Temperature of different absorbing material
76
CHAPTER 9 CONCLUSION
? The orientation of the glass cover depends on the latitude of the place. For northern latitude south facing and southern latitude north facing stills are used. ? The inclination of the cover is optimized for rate of condensation of water on the bottom surface of the cover and to collect it without the mass accumulated drops fall back into the basin. Hence it depends on the intensity of solar radiation, rate evaporation and condensation, material used for cover and its wetting property. ? Glass is the most preferred material for cover, since it has higher solar transmittance and long service life. The surface wets with condensed water and allow film condensation at the bottom surface which results in less loss in transmittance. Other plastic materials do not possess the above required qualities. ? Lowering the cover temperature helps in increasing the productivity. The glass cover temperature is reduced by a film of cooling water continuously flowing over the glass or intermittent flow of cooling water on the cover. ? The dependence of yield on water depth is a strong function of initial temperature of the water in the basin. The productivity of the still decreases with an increase in depth of water during daytime and the reverse is the case of overnight production. Higher depth of water in the
77
basin could be used only in the places where higher solar radiation is available. Otherwise it is best to use shallow basin still. ? To improve the radiation absorption in the black painted basin is most suitable with deep basin still. ? Other basin materials like rubber, gravel, copper sulphate, potassium permanganate, potassium dichromate, cobalt chloride, coal, charcoal, uncoated metallic wiry sponge, coated metallic wiry sponge and black volcanic rocks are having the properties of absorbing and storing of solar radiation in different proportions along with increasing the exposed area for evaporation of water. ? Volcanic rock and asphalt is the best basin materials to improve the productivity by absorption and storage of heat. ? Using finned plate in the basin increase the productivity by increasing the surface area of absorber and rate of heat transfer between saline water and absorber. ? Rubber is the best basin material to improve absorption, storage and evaporation effects. ? Mica sheet as suspended absorber is better material for surface heating. ? The distillate output increases with increase of the initial water temperature in the basin. ? To reflect the solar rays falling on the side walls of the still onto the basin reflecting mirrors are fixed on the side walls. This increases the insulation effect also. ? The maximum productivity is achieved, when energy storing materials were used in the stepped solar still coupled with mini solar pond.
78
? In higher latitude places using additional condenser in the single slope still increases the productivity. ? Using the double-effect solar still instead of single-effect still is leads to increased productivity and cost saving. ? Increasing the number of basins beyond three does not yield significant improvements in the still productivity.
79
CHAPTER 10 REFERENCES
1. K. Kalidasa Murugavel, K.Srithar (2010). performance study on basin type double slope solar still with different with materials and minimum mass of water, Department of Mechanical Engineering, Thiagarajar college of Engg., Madurai, Tamilnadu 625015. 2. A.Tamimi (1986), Performance of a solar still with Reflectors and Black dye, Solar & wind Technology vol 4, No 4, Chemical Engineering Dept, Jordan University & Tech, Irbid, Jordan. 3. A Safwat Nafey, M. Abdel kader, A. Abdel Motalip & A.A. Mabrouk 1999. Parameters affecting solar still productivity, Energy Conversion & Management 41. Faculty of petroleum and mining Engineering, Engineering Science Dept, Suez Canal University, Port Said, Egypt . 4. O.O. Badran (2006). Experimental study of the enhancement parameters on a single slope solar still productivity, Elsevier Publications. Mechanical Engg., Dept., Al-Balqa Applied University, Jordan
80
doc_415004949.pdf
Water For The Future Fulfillment
DESIGN AND PERFORMANCE ANALYSIS OF SINGLE BASIN DOUBLE-SLOPE PASSIVE SOLAR STILL
A PROJECT REPORT Submitted by
ASHICK ALI.S.M BADSHA.M.B IRSATH BISMI.M.A SAHUL HAMEED.N
96810114010 96810114011 96810114016 96810114045
In partial fulfillment for the award of the degree Of
BACHELOR OF ENGINEERING
In MECHANICAL ENGINEERING SARDAR RAJA COLLEGE OF ENGINEERING
ANNA UNIVERSITY: CHENNAI 600 025
APRIL 2014
1
DESIGN AND PERFORMANCE ANALYSIS OF SINGLE BASIN DOUBLE-SLOPE PASSIVE SOLAR STILL
A PROJECT REPORT
Submitted by
ASHICK ALI.S.M BADSHA.M.B IRSATH BISMI.M.A SAHUL HAMEED
96810114010 96810114011 96810114016 96810114045
In partial fulfillment for the award of the degree of
BACHELOR OF ENGINEERING
in
MECHANICAL ENGINEERING SARDAR RAJA COLLEGE OF ENGINEERING
ANNA UNIVERSITY: CHENNAI 600 025
APRIL 2014
2
ANNA UNIVERSITY: CHENNAI 600 025
BONAFIDE CERTIFICATE
Certified that this project report “DESIGN AND PERFORMANCE
ANALYSIS OF SINGLE BASIN DOUBLE SLOPE PASSIVE SOLAR STILL” is
the bonafide work of ASHICK ALI.S.M BADSHA.M.B IRSATH BISMI.M.A SAHUL HAMEED.N Who carried out the project work under my supervision. 96810114010 96810114011 96810114016 96810114045
SIGNATURE Mr. K. CHANDRASEKAR, M.E., HEAD OF THE DEPARTMENT Department of Mechanical Engineering Sardar raja College of Engineering Alangulam
SIGNATURE Mr. P .SETHU RAMALINGAM, M.E., SUPERVISOR Department of Mechanical Engg Sardar raja College of Engineering Alangulam
Submitted for project viva-voce to be held on ………………………
INTERNAL EXAMINER
3
EXTERNAL EXAMINER
ACKNOWLEDGEMENT
We are very grateful thanks to our honorable founder chairman Dr.S.A.RAJA and our chairman Er.A.JESUS RAJA for helping us in all respects to carrying out this project. We express our sincere thanks to our principal
Dr.M.JEYAKUMAR M.E., Ph.D for helping us in all respects to carrying out this project. We are also expressing our heartfelt and sincere thanks to Prof.K.CHANDRASEKAR M.E., (Ph.D) and head of the department of mechanical engineering for providing us with necessary facilities to complete our project successfully. It is always great pleasure to do any work with keen interest under eminent guidance. Hence we express our gratitude and heartfelt thanks to Prof. P .SETHU RAMALINGAM, M.E., for his inspiring guidance, and valuable advice and suggestion rendered for the success of this project. We are also thank to Prof. K.CHANDRA SEKAR M.E., (Ph.D) for the co-ordinator of project work for supporting us of doing this project successfully. We extend our sincere thanks to all teaching staff of our department for helping us in all respects in carrying out this project. Finally we extend our thanks to all non-teaching staff of our mechanical department who helped us in making this project a grand success.
4
ABSTRACT
Basin type solar still is a simple device which can be used for fresh water production. Single basin solar stills can be used for water desalination. Probably they are considered the best solution for water production in remote, arid to semi-arid, small communities, where fresh water is unavailable. However, the amount of distilled water produced per unit area is somewhat low which makes the single basin solar still unacceptable in some instances. For that simple modifications are done. The main purpose of this project is to check the performance of double slope passive solar still using different absorbing material (Aluminium, Rubber, Asbestos, Mild steel) and enhances the productivity of water
5
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE NO
ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES 1 INTRODUCTION 1.1 GLOBAL WATER SUPPLY AND DEMAND 1.2 WATER CRISIS IN INDIA 1.3 APPROACHES TO HANDLE THE PROBLEMS 2 DESALINATION TECHNOLOGIES 2.1 PHASE CHANGE PROCESSES 2.1.1 Multi Stage Flash Desalination(Msf) 2.1.2 Multi Effect Desalination(Mes) 2.1.3 Vapour Compression(Vc) 2.1.4 Freezing 2.1.5 Humidification And Dehumidification(Hd) 2.2 MEMBRANE PROCESS 2.2.1 Reverse Osmosis(Ro) 2.2.2 Electrolysis(Ed) 2.3 SOLAR STILLS 3 LITERATURE REVIEW 3.1 INTRODUCTION
6
iii iv vii ix 1
1 3
6 7 8 8 8 9 10
10 10 10 11 11 13 13
3.2 PASSIVE STILL 3.2.1 Cover Plate 3.2.2 Basin Water Depth 3.2.3 Radiation Absorption 3.2.4 Energy Storing Materials 3.2.5 Increasing The Evoporation Area 3.2.6 Surface Heating Technique 3.2.7 Increase The Radiation Received At The Basin 4 SOLAR ENERGY 4.1 SOLAR DISTILLATION 4.2 SOLAR STILL 4.3 TYPES OF SOLAR STILL 4.3.1 Passive Solar Still 4.3.2 Active Solar Still 4.4 HEAT TRANSFER MECHANISM IN SOLAR STILL 4.5 FACTORS AFFECTING PERFORMANCE OF SOLAR STILL 4.5.1 Free Surface Area
13 14 15 17 20 21 22
24 27 27 28 28 28 29
31
34 34
4.5.2 Water Glass Temperature Difference 34 4.5.3 Absorber Area 4.5.4 Water Depth 5 MATERIAL SELECTION AND DESIGN OF THE SOLAR STILL 5.1 HEAT TRANSFER AND HEAT LOSS 5.2 SOLAR STILL CONSTRUCTION MATERIALS 5.2.1 Transparent Cover
7
35 35
36 36
37 37
5.2.2 Basin Lines 5.2.3Insulation 5.3 DESIGN AND FABRICATION 5.4 WORKING PRINCIPLE 6 7 8 9 10 PHOTOGRAPHS EXPERIMENTAL PROCEDURE RESULT AND DECISION CONCLUSION REFERENCES
37 38 38 42 43 45 59 61 64
8
LIST OF TABLES
TABLE NO
TITLE
PAGE NO
2.1 5.1 5.2 7.1 7.2
Desalination Technologies Technical Specification Of The Solar Still Materials And Properties Yield Of Water In Different Absorbing Material Temperature Of Various Thermo Couple Using Asbestos (2cm)
7 41 41 46
47
7.3
Temperature Of Various Thermo Couple Using Asbestos(3cm) 48
7.4
Temperature Of Various Thermo Couple Using Asbestos(4cm) 49
7.5
Temperature Of Various Thermo Couple Using Rubber(2cm) 50
7.6
Temperature Of Various Thermo Couple Using Rubber(3cm) 51
7.7
Temperature Of Various Thermo Couple Using Rubber(4cm) 52
7.8
Temperature Of Various Thermo Couple Using Aluminium(2cm) 53
7.9
Temperature Of Various Thermo Couple Using Aluminium(3cm) 54
7.10
Temperature Of Various Thermo Couple Using Aluminium(4cm) 55
9
7.11
Temperature Of Various Thermo Couple Using Mild Steel(2cm) 56
7.12
Temperature Of Various Thermo Couple Using Mild Steel(3cm) 57
7.13
Temperature Of Various Thermo Couple Using Mild Steel(4cm) 58
10
LIST OF FIGURES
FIGURE NO
TITLE
PAGE NO
2.1 3.1 3.2 3.3
Schematic Diagram Of A Simple Solar Still Regenerative Solar Still With Double Glass Cover Variation Of Nocturnal On Basin Water Depth Variation Of Yield With Water Depth For Different Initial Water Temperature
12 15 16
17 18 19
3.4 3.5 3.6
Water Production For Various Absorption Materials Variation In Production Rate Of The Solar Still Percentage Gain In Distilled Water Yield For Various Absorbing Materials
19
3.7
Relation Between The Local Standard Time And Productivity Due To Asphalt Basin Liner Effect 21 22 22 23
3.8 3.9 3.10 3.11
Solar Still With Sponge In The Basin Single Slope Solar Still With Vertical Jute Cloth Solar Still With Aluminium Black Painted Plate Solar Still With Internal And External Reflectors
25
3.12
Schematic Diagram Of Basin Type Still With Internal And Flat Plate External Bottom Reflectors 26 29
4.1 4.2
Schematic Diagram Of A Passive Type Solar Still Schematic View Of Water Flowing Over The Glass Cover Solar Still Coupled With Flat Plate Collector
30 31 39
4.3 5.1
Heat Transfer In A Basin Type Solar Still Line Diagram Of Double Slope Solar Still
11
5.2 6.1
Single Basin Double Slopes Solar Still Design of single basin double slope Passive solar still
40
43 44 44 46
6.2 6.3 7.1 7.2
Basin side view Absorbing material Yield Of Water In Different Absorbing Material Temperature Of Various Thermo Couple Using Asbestos (2cm)
47
7.3
Temperature Of Various Thermo Couple Using Asbestos(3cm) 48
7.4
Temperature Of Various Thermo Couple Using Asbestos(4cm) 49
7.5
Temperature Of Various Thermo Couple Using Rubber(2cm) 50
7.6
Temperature Of Various Thermo Couple Using Rubber(3cm) 51
7.7
Temperature Of Various Thermo Couple Using Rubber(4cm) 52
7.8
Temperature Of Various Thermo Couple Using Aluminium(2cm) 53
7.9
Temperature Of Various Thermo Couple Using Aluminium(3cm) 54
7.10
Temperature Of Various Thermo Couple Using Aluminium(4cm) 55
7.11
Temperature Of Various Thermo Couple Using Mild Steel(2cm) 56
7.12
Temperature Of Various Thermo Couple Using Mild Steel(3cm) 57
7.13
Temperature Of Various Thermo Couple Using
12
Mild Steel(4cm) 8.1 Temperature Of Different Absorbing Materials
58 60
13
CHAPTER 1 INTRODUCTION
1.1 GLOBAL WATER SUPPLY AND DEMAND Water is essential for all life forms on earth—plants, animals and human. Water is one of the most abundant resources on earth, covering three fourths of the planet’s surface. About 97% of the earth’s water is salt water in the oceans and the remaining 3% (about 36 million km3) is fresh water contained in the polar region (in the form of ice), ground water, lakes and rivers, which supply most human and animal needs. Less than 1% fresh water is within human reach. Even this small fraction is believed to be adequate to support life and vegetation on earth. Nature itself provides most of the required fresh water, through hydrological cycle. A very large-scale process of solar distillation naturally produces fresh water. The essential features of this process are thus summarized as the production of vapours above the surface of the liquids, the transport of vapours by winds, the cooling of air – vapour mixture, condensation and precipitation. However, rapid industrial growth and a worldwide population explosion have resulted in a huge rise of demand for freshwater, both for household needs and for crops to produce adequate quantities of food. Added to this is the problem of the pollution of rivers and lakes by industrial wastes and the large amounts of sewage discharge. On a global scale, human–made pollution of natural sources of water is becoming one of the greatest causes of freshwater shortage. Uneven distribution of population is another problem. Then provision of freshwater is becoming an increasingly important issue in many areas of the world. According to World Health Organization (WHO), the permissible limit of salinity in water is 500 ppm and for special cases up to 1000 ppm while most of the water available on earth has the salinity up to 10,000 ppm whereas seawater normally has salinity in the range of 35,000–45,000 ppm in the form of total dissolved salts
14
Global demand for water has tripled since the 1950s, but the supply of fresh water has been declining .Half a billion people live in water-stressed or water-scarce countries, and by 2025 that number will grow to three billion due to an increase in population. Irrigated agriculture is the dominant user of water, accounting for about 80% of global water use. Population and income growth will increase the demand for irrigation water to meet food production requirements and household and industrial demand. The global population is projected to increase to about 9 billion by 2050. Fulfillment of calorie requirements and dietary trends will translate into even higher water demand if more calories will be supplied from meat. At the same time, the limited easily accessible freshwater resources in rivers, lakes and shallow groundwater aquifers are dwindling due to over-exploitation and water quality degradation. Being the largest user of water, irrigation is the first sector to lose out as water scarcity increases. The challenges of water scarcity are heightened by the increasing costs of developing new water sources, land degradation in irrigated areas, groundwater depletion, water pollution and ecosystem degradation. With current water utilization practices, a fast growing population, and a nutritional transition towards diets that rely more on meat, global water resource limits will be reached sooner. Data on water supply and demand are startling: about 450 million people in 29 countries face severe water shortages, about 20% more water than is now available will be needed to feed the additional three billion people by 2025, as much as twothirds of the world population could be water-stressed by 2025, which supply onethird of the world’s population, are being pumped out faster than nature can replenish them, half of the world’s rivers and lakes are polluted and major rivers, such as the Yellow, Ganges, and Colorado, do not flow to the sea for much of the year because of upstream withdrawals. About 40% of the world’s populations live in regions that directly compete for shared trans boundary water resources. Estimated that under
15
their baseline scenario, total global water withdrawals for agricultural, domestic and industrial use will increase by 23% from 1995 to 2025. The availability of sufficient water resources is one of the major crises with overarching implications for many other world problems especially poverty, hunger, ecosystem degradation, desertification, climate change, and even world peace and security. Water scarcity is projected to become a more important determinant of food scarcity than land scarcity, according to the view held by the UN (UNDP, 2007). Hence there is an essential and earnest need to get fresh water from the saline/brackish water present on or inside the earth. This process of getting fresh water from saline/ brackish water can be done easily and economically by desalination. 1.2 WATER CRISIS IN INDIA Although India occupies only 3.29 million km2 geographical area, which forms 2.4% of the world’s land area. India has 16 per cent of the world’s population and four per cent of its fresh water resources. Estimates indicate that surface and ground water availability is around 1,869 billion cubic metres (BCM). Of this, 40 per cent is not available for use due to geological and topographical reasons. Around 4,000 BCM of fresh water is available due to precipitation in the form of rain and snow, most of which returns to the seas via rivers. The fresh water crisis is already evident in many parts of India, varying in scale and intensity at different times of the year. Rainfall in India is dependent on the south-west and north-east monsoons, on shallow cyclonic depressions and disturbances and on local storms. Most of it takes place under the influence of south-west monsoon between June and September except in Tamil Nadu, where it is under the influence of north-east monsoon during October and November. India is gifted with a river system comprising more than 20 major rivers with several tributaries. Many of these rivers are perennial and some of these are seasonal. The rivers like Ganges, Brahmaputra and Indus originate from the
16
Himalayas and carry water throughout the year. The snow and ice melt of the Himalayas and the base flow contribute the flows during the lean season. More than 50% of water resources of India are located in various tributaries of these river systems. Average water yield per unit area of the Himalayan Rivers is almost double that of the south peninsular rivers system, indicating the importance of snow and glacier melt contribution from the high mountains. Apart from the water available in the various rivers of the country, the groundwater is also an important source of water for drinking, irrigation, industrial uses, etc. Ninety two per cent groundwater extracted is used in the agricultural sector, five and three per cent respectively for industrial and domestic sector. Eight nine per cent of surface water use is for agricultural sector and two per cent and nine per cent respectively are used by the industrial and domestic sector. As per the international norms, if per-capita water availability is less than 1700 m3 per year then the country is categorized as water stressed and if it is less than 1000 m3 per capita per year then the country is classified as water scarce. In India per capita surface water availability in the years 1991 and 2001 were 2309 and 1902 m3 and these are projected to reduce to 1401 and 1191 m3 by the years 2025 and 2050 respectively. The fresh water crisis is not the result of natural factors, but has been caused by human actions. India’s rapidly rising population and changing lifestyles also increases the need for fresh water. Intense competition among competing us er’s agriculture, industry and domestic sector is driving the ground water table deeper and deeper. Widespread pollution of surface and groundwater is reducing the quality of fresh water resources. Fresh water is increasingly taking centre stage on the economic and political agenda, as more and more disputes between and within states, districts, regions, and even at the community level arises. In India nearly 45 million people are affected by water quality problems caused by pollution, by excess fluoride, arsenic, iron or by the ingress of salt water.
17
Millions do not have adequate quantities of safe water, particularly during the summer months. In rural areas, women and girls still have to walk long distances and spend up to four hours every single day to provide the household with water. Scarcity of fresh water problems are facing many arid zones of Gujarat and Rajasthan, luckily these places are getting more amount of solar energy, apart Gujarat and Rajasthan that in western India, which face water shortage and have huge underground saline water sources, certain regions in Haryana state and Maharashtra states also have underground saline water in spite of high rain fall. The village peoples are facing lot of difficulties to get fresh water for their family needs. All families the women and children are responsible for collecting and storage of water. The quality of drinking water also not suitable for human health, it was found by tested the village water samples at Guru Kripa test house at Ajmer district. After analyzing in all the aspects authors concluded that, the village peoples are expecting suitable low cost purification devices for getting pure drinking water. It was estimated that, approximately 13.443 Million Liters per Day (MLD) of wastewater being generated from the 453 cities for consuming 16,814 MLD of water. Around 37.7 million Indians are affected by waterborne diseases annually, 1.5 million children are estimated to die of diarrhea alone and 73 million working days are lost due to waterborne disease each year. The resulting economic burden is estimated at $600 million a year. While ‘traditional diseases’ such as diarrhea continue to take a heavy toll, 66 million Indians are at risk due to excess fluoride and 10 million due to excess arsenic in groundwater. In all, 1,95,813 habitations in the country are affected by poor water quality. It is clear that the large investments have not yielded comparable improvements in health and other socio-economic indicators.
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1.3 APPROACHES TO HANDLE THE PROBLEMS In order to address the problems discussed above, an attempt is made in the present study to suggest a few approaches and methodologies as follows. ? Rain Water Harvesting and subsequent recharge of groundwater can help lower the concentration of minerals in aquifers. ? Adoption of latest technologies in order to reduce the waste water generation and/or to treat them effectively. ? Dual water supply – The success of this system lies in the fact that filtered purified water is used only for drinking purposes while other source of water may be used for purposes other than drinking. ? Improved and innovative planning of water resources – The development of water resources involves the conception, planning, designing, construction, and operation of facilities to control and utilize water with the national objective of improving the quality of life of the people. ? Treatment of sea water and backrish water to convert fresh water by using desalination technologies.
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CHAPTER 2
DESALINATION TECHNOLOGIES Desalination can be achieved using a number of techniques. Industrial desalination technologies either use phase change or involve semi permeable membranes to separate the solvent or some solutes. Therefore, desalination techniques may be classified into the following categories. phase change or thermal processes and membrane or single-phase processes. In Table 2.1, the most important technologies in use are listed. In the membrane processes, electricity is used for either driving high-pressure pumps or ionization of salts contained in the seawater. Table 2.1 Desalination technologies Phase change processes Multi-stage flash (MSF) Multiple (MED) Vapour compression (VC) Freezing Humidificationdehumidification Solar stills effect Membrane processes Reverse osmosis (RO)
distillation Electrodialysis (ED)
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2.1 PHASE CHANGE PROCESSES In the phase change or thermal processes, the distillation of seawater is achieved by utilizing a thermal energy source. The thermal energy may be obtained from a conventional fossil fuel source, nuclear energy, or a non-conventional solar energy source or geothermal energy. These processes involve heating of saline water and collecting the condensed vapour (distillate) to produce pure water. 2.1.1 Multi –Stage Flash Desalination (MSF) The MSF process is composed of a series of elements, called stages. In each stage, condensing steam is used to pre-heat the seawater feed. By fractionating the overall temperature differential between the warm source and seawater into a large number of stages, the system approaches ideal total latent heat recovery. Operation of this system requires pressure gradients in the plant. Current commercial installations are designed with 10–30 stages (2°C temperature drop per stage). 2.1.2 Multi Effect Distillation (MED) The Multi effect distillation process is composed of a number of elements, which are called effects. The steam from one effect is used as heating fluid in another effect, which, while condensing, causes evaporation of a part of the salty solution. The produced steam goes through the following effect, where, while condensing, it makes some of the other solution evaporate, and so on. For this procedure to be possible, the heated effect must be kept at a pressure lower than that of the effect from which the heating steam originates. The solutions condensed by all effects are used to pre-heat the feed. In this process, vapour is produced by flashing and by boiling, but the majority of the distillate is produced by boiling. Unlike an MSF plant, the MED process usually operates as a once-through system without a large mass of brine recirculating around the plant. This design reduces both pumping requirements and scaling tendencies.
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2.1.3 Vapour Compression (VC) Vapour compression desalination refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapour. Since compression of the vapour increases both the pressure and temperature of the vapour, it is possible to use the latent heat rejected during condensation to generate additional vapour. The effect of compressing water vapour can be done by two methods. The first method utilizes an ejector system motivated by steam at manometric pressure from an external source in order to recycle vapour from the desalination process. The form is designated Ejecto or Thermo Compression. Using the second method, water vapour is compressed by means of a mechanical device, electrically driven in most cases. This form is designated mechanical vapour compression (MVC). The MVC process comprises two different versions: Vapour Compression (VC) and Vacuum Vapour Compression (VVC). VC designates those systems in which the evaporation effect takes place at manometric pressure, and VVC the systems in which evaporation takes place at sub-atmospheric pressures (under vacuum). The compression is mechanically powered by something such as a compression turbine. As vapour is generated, it is passed over to a heat exchanging condenser which returns the vapour to water. The resulting fresh water is moved to storage while the heat removed during condensation is transmitted to the remaining feedstock. The VVC process is the more efficient distillation process available in the market today in terms of energy consumption and water recovery ratio. As the system is electrically driven, it is considered a "clean" process, it is highly reliable and simple to operate and maintain.
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2.1.4 Freezing Freezing desalination has been proposed as a method for desalination for several decades, only demonstration projects have been built to date. The concept is appealing in theory because the minimum thermodynamic energy required for freezing is less than for evaporation since the latent heat of fusion of water is 6.01 kJ/mole while the latent heat of vaporization at 100°C is 40.66 kJ/mole. 2.1.5 Humidification-Dehumidification (HD) The HD process is based on the fact that air can be mixed with important quantities of vapor. The amount of vapor able to be carried by air increases with the temperature; in fact, 1 kg of dry air can carry 0.5 kg of vapor and about 670 kcal when its temperature increases from 30°C to 80°C. When airflow is in contact with salt water, air extracts a certain quantity of vapor at the expense of sensitive heat of salt water, provoking cooling. On the other hand, the distilled water is recovered by maintaining humid air at contact with the cooling surface, causing the condensation of a part of vapor mixed with air. Generally the condensation occurs in another exchanger in which salt water is preheated by latent heat recovery. 2.2 MEMBRANE PROCESSES In the membrane processes, electricity is used for either driving high-pressure pumps or ionization of salts contained in the seawater. In this method membranes with very fine holes in the order of microns are employed. When the water is passing through this membrane, it gets purified and desalinated. 2.2.1 Reverse osmosis (RO) Reverse osmosis (RO) is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute
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is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. 2.2.2 Electro dialysis (ED) Electro dialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configureuration called an electro dialysis cell. The cell consists of a feed (diluate) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electro dialysis processes, multiple electro dialysis cells are arranged into a configureuration called an electro dialysis stack, with alternating anion and cation exchange membranes forming the multiple electro dialysis cells. Electro dialysis processes are different compared to distillation techniques and other membrane based processes (such as reverse osmosis) in that dissolved species are moved away from the feed stream rather than the reverse. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electro dialysis offers the practical advantage of much higher feed recovery in many applications. 2.3 SOLAR STILLS A simple way of distilling water is by evaporation and condensation method. In solar still, the water is evaporated using solar energy. In solar still, impure water is taken in a well insulated air tight basin covered with transparent plastic/glass cover. When the cover is exposed sun, radiation energy is transmitted through transparent cover, falls on the basin, absorbed by basin absorber plate, converted into heat and transferred to water. Water gets heated up, transfer heat to air inside the still and the air become unsaturated. The water evaporates and makes the air inside still saturated. This air subjected circulatory motion due to the temperature difference between water surface and cover lower surface. When high temperature air touches the cover,
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become cool and the water present in the air condenses at the lower surface of the cover. This condensed water slides down and collected using a drain. A schematic diagram of simple single basin double slope solar still is shown in Figure 2.1
Figure 2.1 Schematic diagram of a simple solar still
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CHAPTER 3 LITERATURE REVIEW
3.1 INTRODUCTION Basin type solar still is a simple device which can be used for fresh water production. The main drawback of a traditional solar still is the low amount of distilled water production per unit area which makes the single-basin solar still as uneconomical. (O.O. Badran)There is a great scope to improve the efficiency of such type of solar stills. Depending on the methods used to enhance the stills are classified as passive and active type solar still. In active type of stills, the evaporation and condensation of the water is activated externally. Additional space for evaporation, condensation and radiation collectors may also available. In passive type still, (K. Kalidasa Murugavel, K.Srithar) simple modifications are done or some materials are used in basin along with saline water to improve the performance. 3.2 PASSIVE STILL Depending on the energy available for evaporating the water in the still, the stills are classified into passive and active. In passive stills, the water in the still basin receives heat only by the radiation transmitted through the transparent cover. Hence the performances of this type of stills are improved by optimizing the cover and basin conditions. The cover plate is optimized to receive and transmit maximum radiation into the still. Also, the cover should be capable of condensing maximum mass of water vapour by exhausting heat to the atmosphere. The basin has to receive the maximum radiation transmitted into the still, convert into heat, transfer heat to the water and provide maximum surface area for evaporation. It should also retain excess heat and utilize it when the radiation level falls.
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3.2.1 Cover plate The transparent cover receives and transmits radiation into the still. Also, the condensation takes place at its lower surface. Also, it should suppress thermal radiation to the atmosphere. Glass is the best material for cover. since it has higher transmittance and less reflectivity. Also glass is opaque for thermal radiation. For higher latitude places, single slope still is preferable and for northern chemosphere the still faces south. The inclination of the cover is optimized to collect the accumulated condensate through drain before it drops down to basin. The condensate mass accumulation depends on solar intensity and condensation rate. The conducted indoor simulation experiment and found that the production rate is higher for 30o cover inclination. The glass has to withstand its self weight and thermal stresses. For higher surface area, high thickness glass will be used. But, radiation transmittance and heat transfer through the glass decrease with thickness. Glass cover plate with 3 mm thickness gives 16.5% more production than the cover with 6mm glass thickness. The transmittance loss at cover is less due to film wise condensation at the lower surface. The other transparent materials are not having these characteristics. Glass temperature affects the condensation rate at its lower surface. Lower glass surface temperature increases the circulation of air inside the still which enhances convective and evaporative heat transfer between basin water and glass. Also cooler glass lower surface increases condensation. The glass cover temperature is reduced by continuous flow or intermittent flow of raw cooling water on the cover. The cooling water gains latent heat from condensing water and regenerates it in the basin. Second effect of evaporation and
condensation takes place between the covers as shown in Figure 3.1 result
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shows increase in production by 20%. Results also show that, the use of the film-cooling increases the still efficiency up to 20%. Wind velocity is also having its effect on temperature of the glass. At higher wind velocity, due to higher convection heat transfer from the glass to atmosphere the productivity of the still is increasing. Theoretically analyzed the effect of water flowing over the glass cover in a single basin still. The result shows the productivity is increased with flowing water over the glass cover. Also, the yield decrease when the water flow rate increases.
Figure 3.1 Regenerative solar still with double glass cover 3.2.2 Basin water depth Water depth is the one of the main parameters affecting the solar still. Some investigations show that the total production of the still varies with water depth. Since the bulk motion of the air inside the still is proportional to temperature difference between the water and glass which is responsible for carrying water vapour from basin to glass surface. The volumetric heat capacity of the basin depends on the depth of water in the basin. For given radiation, the temperature of the basin is high when the volumetric heat capacity of the basin is less. But, experiments with deep basin reveal that, the productivity of the still also decreases with an increase in depth. For given radiation condition, the water temperature is less for high depth stills, but it store higher amount of energy due to its higher amount of volumetric heat capacity. This increases the
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nocturnal production in Figure 3.2 and delays the morning production. The basin temperature and production rate are not affected by intermittent cloud passing. But, the shallow basin still is having immediate effect on change radiation intensity.
Figure 3.2 Variation of nocturnal production on basin water depth
The water depth was decreased from 0.1 to 0.005 m resulted in an increase of 19.6% in the still output, while a decrease in the depth from 0.29 to 0.1 m resulted only in a 6.3% increase in the still yield. The conducted an experiment in plastic solar still. This result also indicates that increase in water depth decrease the productivity. The productivity increased by 44.28% when the water depth was decreased from 0.18 m to 0.04 m. Results show that, the daily yield was decreased about 44% when changing the water depth from 0.01 to 0.20 m with the initial water temperature was 35ºC. Also, the yield was increased about 25% when changing the water depth from 0.01 to 0.20 m with the initial water temperature was 50ºC Figure 3.3.
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Figure 3.3 Variation of yield with water depth for different initial water temperatures 3.2.3 Radiation absorption Around 11% of radiation received by the still basin is reflected back without using it. This loss can be minimized, if the absorption coefficient of the still basin and water is increased. (A.A. El-Sebaii, A.A.Al-Ghamdi )Various methods are used to increase the absorption capacity of the basin. A simple method of increasing the absorption of the basin water is to add dye with the water. When the dye is added with water, the solar radiation is absorbed by the upper layer and the temperature of upper layer water increases, which in turn increases the evaporation rate. Different dye with different materials and concentration are having different effect on productivity at different depth water. Black napthylamine dyes at 172.5 ppm give higher increase in production rate by 29% Compared with red carmoisine and dark green. The effect of dye on productivity is more on deep basin still than shallow basin still. Different types of absorbing materials are used in the basin along with water to increase the absorption of the still basin. Rubber mate and charcoal are some materials used in the basin. An average yield of 2.5-4 l/m2.d was obtained when carbon powder (40-50/µm size) used as a basin material. Also removing
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the basin's insulation reduces the productivity by 13-17% [36]. Basin lined with coal was used to enhance the absorption capacity of the basin. Figure 3.4 compares the effect of different energy absorbing materials placed along with basin water. Results show that, the black dye was the best absorbing material to increase the still productivity.
Figure 3.4 Water productions for various absorption materials The effects of coal and charcoal on solar-still performance. Both materials improve the still thermal performance and the high efficiency obtained when using charcoal in the basin. The effect of dissolved salts such as: copper sulphate, potassium permanganate, potassium dichromate, cobalt chloride. The effect of using various absorbing materials on the productivity of a single-basin solar still. The materials used to enhance the absorptivity of water for solar radiation include dissolved salts, violet dye, and charcoal. The salts were potassium permanganate and potassium dichromate. They found that the addition of potassium permanganate resulted in 26% improvements in efficiency. The best result was obtained by using violet dye with an increase of about 29%. The effect of using spreader materials in the basin. In this a single basin double slope passive type solar still was tested with minimum mass of water (approximately 2 mm depth) in the basin with different basin spreader materials
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like cotton cloth, jute cloth and sponge sheet, and porous materials like washed natural rock and quartzite rock as spread materials. From the above materials, black cotton cloth in the basin gives more productivity then others. Figure 3.5 shows the variation in the productivity. The effects of using various different black-paint absorber materials on the thermal performance of a solar desalination unit. Also the effect of using different absorbing materials in a solar still like black ink, black rubber mate and black dye. They found that the black dye was the best absorbing material. The effect of using uncoated metallic wiry sponge, coated metallic wiry sponge and black volcanic rocks. Figure 3.6 shows the variation in percentage of gain in distilled water yield.
Figure 3.5 Variation in production rate of the solar still
Figure 3.6 Percentage gain in distilled water yield for various absorbing materials
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The effect of using finned and corrugated absorbers in solar stills. The results indicated the productivity of finned and corrugated solar stills was higher than conventional still. It increases the amount of distilled water produced by about 40% and 21% respectively. 3.2.4 Energy storing materials Some black materials can store more amount of heat energy and increase the heat capacity of the basin in addition to increasing the basin absorption. (A. Safwat Nafey, M. Abdel kader, A. Abdel Motalip & A.A. Mabrouk) Glass, rubber and gravel are some material having these properties. Experimental results show that, the black rubber with 10 mm size increases the productivity of the deep basin still by 20% and black gravel with 20 –30 mm size increases the productivity of a shallow basin still by 19%. A solar still was tested with a special phase changing material as energy storing. Specially formulated mixture consisting of an emulsion of paraffin wax, paraffin oil and water with aluminum turnings to promote heat conduction has been used effectively to store the heat during day time, and then give off its heat at night time, thus increasing the productivity appreciably. The effect of using asphalt in the basin as an energy storing material. The use of asphalt in the basin resulted in a significant improvement in still production for an increase of 29% it’s shown in Figure 3.7. When the sprinkler was combined with asphalt the production rate increased up to 51%. Also the productivity during night contributed to around 16% of total day around productivity. The effect of using energy storing materials in the basin. The materials were quartzite rock, red brick pieces, cement concrete pieces, washed stones and iron scraps. They found that the still with 3/4 in. sized quartzite rock was the effective basin material.
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Figure 3.7 Relation between the local standard time and productivity due to asphalt basin liner effect 3.2.5 Increasing the evaporation area When the exposed area of basin water is high, then the air mass subjected to natural convection inside the still will take more amounts of water particles. The water wets the surface of the materials available in the basin and exposed to larger area and ready for diffusion. The rubber, gravel and charcoal used in the basin to improve the absorption, heat capacity and also the evaporation area hence the production. The performance of a solar still with different size sponge cubes placed in the basin was studied experimentally as shown in Figure 3.8. The increase in distillate production of the still ranged from 18% to 27% was observed compared to an identical still without sponge cubes under the same conditions. The small openings in the sponge cubes also reduce the surface tension between the water molecules, thus making it easier for the water molecules to evaporate. Experimental study of solar still with floating-wick showed that, the productivity of this type of still was higher than the common tilted-wick type and the conventional basin type solar stills. In this blackened jute wick floated with a polystyrene sheet. To improve the efficiency of a solar still by introducing a medium in the basin to provide large evaporation surface and utilize the latent heat of condensation in Figure 3.9. They found that the
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cumulative still yield in the regenerative still with jute cloth increases approximately by 20% and efficiency increases by 8%.
Figure 3.8 Solar still with sponge in the basin
Figure 3.9 Single slope solar still with vertical jute cloth 3.2.6 Surface heating technique The evaporation rate is increasing with the basin water temperature. To increase the temperature of entire mass of water, higher energy is required. Heating the top surface of water alone require less amount of heat and also this results in higher temperature for top surface water. Adding black dye in the basin water to improve the absorption results in surface heating of water and
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increase the production since the maximum portion of radiation is received by the top layers of water. In another method of surface heating technique, a plate separates top surface water with remaining water of the deep basin still as shown in Figure 3.10. The radiation is received by the separating plate and a part is used to heat the top layer of water and increases its temperature to enhance the productivity. Remaining part of the heat is stored in the bottom water and released later during low solar intensity periods. The material used and the thickness of the layer are the parameters affecting the productivity. Black painted aluminum with 2 cm of water layer increases the efficiency of the system by 28%. It was found that the floating perforated black aluminum plate in the solar still increases solar still productivity by 15% for 3 cm water layer and increases by 40% for 6 cm water layer. The effect of thermal conductivity of the suspended absorber on the daily productivity of the still was investigated experimentally using aluminum, copper, stainless steel, glass, and mica plates as suspended absorbers. The results obtained are compared with those obtained for the conventional still. It shows the daily productivity of the still with mica plate was found to be 42% higher than a conventional still.
Figure 3.10 Solar still with aluminium black painted plate
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3.2.7 Increase the radiation received at the basin Even though the glass cover inclination is optimized to receive the sun rays, a part of sun rays are received by the back plate and side plates of the basin. This effect reduces the amount of radiation available to the basin water for heating. (A.Tamimi) To reflect the sun rays fall on the side plates on to the basin reflecting mirror is used as shown in Figure 3.11. The effect of using reflecting mirrors on the vertical walls. Results show that, the production rate was increased by 86.2% for winter seasons and 22% for summer seasons when compared with conventional still. Another result shows that, the productivity of a single slope still with reflecting mirrors was 20% more than the double slope still. The increase in the daily amounts of distillate by adding the internal and external reflectors to the single-slope basin type still for the entire year would be averaged as 48%. The effect of inclining the external reflector on distillate productivity of a basin type still. The productivity of the still is increased by inclining the external reflector slightly backwards in summer and forwards in other seasons, and the inclination angle of the external reflector would be less than 25º throughout the year at 30ºN latitude. The benefit of both the internal and inclined external reflectors would be considerably less in summer than in winter. The increase in the average daily amount of distillate throughout the year of a still with inclined external reflector with optimum inclination in addition to an internal reflector, compared to a conventional basin type still was predicted as 29%, 43% or 67% when the glass cover inclination was 10º, 30º or 50º and the length of external reflector was half the still’s length . The system efficiency was increased by 20 to 26. Theoretically analyzed solar still with internal and external reflector in winter. The theoretical results show distillate of the inclined reflector was about 16% greater than the vertical external mirror and 2.3 times higher yield than still without mirrors. The experimental result shows daily productivity of the still increased by 70% to 100% with internal and
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external reflector in winter. Using of internal and external reflector inclined at 20º and the cover angle at 20º in winter session increases the productivity around 2.45 times than a simple still with no reflectors. A single basin type solar still with flat plate external bottom reflector in Figure 3.12
Figure 3.11 Solar still with internal and external reflectors The optimum inclination of the external reflector maximizes the daily amount of distillate of the still. Solar radiation distribution in a solar still with internal and external reflectors. They conclude as (i) the efficiency of radiation exchange between any two surfaces was significantly affected by their view factor, (ii) the use of global irradiance observed on a horizontal surface in the heat balance equations of a basin-type solar still would lead to inaccurate estimation of the distillate output, and (iii) the accuracy of modelling the performance of a basin-type solar still with internal and external reflecting surfaces can be improved by incorporating view factors and the diffuse component in the reflected solar radiation.
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Figure 3.12 Schematic diagram of basin type still with internal and flat plate external bottom reflectors
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CHAPTER 4 SOLAR ENERGY
Energy from the sun is said to be the solar energy. This is the earth’s primary energy flow and equally too, the most abundant on the earth crust. There are two major ways in which the solar energy can be used or harnessed. It can be used either as a thermal energy by heating a fluid or by converting it into electricity using photovoltaic arrays (PV). The former would be looked in detail in the design of the solar still. Solar energy is a relatively diffuse source of energy. It is also available almost everywhere, unlike geothermal, wave, wind or even conventional fuels. Depending on the energy demand of the application, it may require large areas. Yet, most solar energy conversion systems are modular and can be installed almost everywhere which relieves the space availability problem The solar energy can be deployed and use for simple desalination systems especially, the solar still for production of potable water in the tropics and arid regions of the world where there is abundance of this natural resource. These regions are well endowed with this resource and it should be fully exploited. 4.1 SOLAR DISTILLATION There is an important need for clean, pure drinking water in many developing countries. Often water sources are brackish (i.e. contain dissolved salts) and/or contain harmful bacteria and therefore cannot be used for drinking. In addition, there are many coastal locations where seawater is abundant but potable water is not available. Water has been distilled using solar energy. Distillation is a process of boiling the water and re-condensing the steam into clean container pure water is also useful for batteries and in hospitals or schools. Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. In this process, water is evaporated, thus separating water
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vapour from dissolved matter, which is condensed as pure water. Solar distillation uses, in common with all distillation processes, the evaporation and condensation modes, but unlike other processes energy consumption is not a recurrent cost but is incorporated in the capital cost of the solar collector. The solar still therefore, is of a simple design, construction and maintenance with ease of operation. It is best suitable for regions of the world with high solar intensities. The mechanism of operation is based on the transmitting, absorption and reflective properties of glass and other transparent materials. 4.2 SOLAR STILL Solar still is a device which converts unwanted water into drinkable water with the help of solar energy. Most important part of the solar still is absorber surface which absorbs solar energy. The stills apply the principles of evaporation and condensation that is seen within the precipitation cycle. The mechanism of operation is based on the transmitting, absorption and reflective properties of glass and other transparent materials. 4.3 TYPES OF SOLAR STILL The solar still can be classified into two types. They are ? Passive type solar still ? Active type solar still
4.3.1 Passive Solar Still In passive type still, simple modifications are done or some materials are used in basin along with saline water to improve the performance. Figure 4.1 shows the schematic diagram of a passive type solar still.
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Figure 4.1 Schematic diagram of a passive type solar still The water in the still basin receives heat only by the radiation transmitted through the transparent cover. Hence the performances of this type of stills are improved by optimizing the cover and basin conditions. In a passive solar still, the solar radiation is received directly by the basin water and is the only source of energy for raising the water temperature and consequently, the evaporation leading to a lower productivity.
4.3.2 Active solar still The temperature difference between water in the basin and condensing glass cover has a direct effect in the performance of the still. To achieve better evaporation rate and productivity, the temperature of water in the basin could be increased. In active still, some external sources are could be used to increase the temperature of water in the basin. The external sources connected with the simple basin still are flat plate collector, concentratic collector, hybrid PV/T system, heat exchanger, solar pond, multiple basins and additional condenser. Figure 4.2 shows the schematic view of water flowing over the glass cover solar still coupled with flat plate collector.
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Figure 4.2 Schematic view of water flowing over the glass cover solar still coupled with flat plate collector. The enhancement of daily yield in a double basin solar still with the effect of water flow over the glass cover and flow of hot water in the lower basin by a flat plate collector. The numerical results shows, the yield of the system was 50% higher than the ordinary double basin still. The double effect solar still with and without water flow and over the glass cover. The study shows that, an active solar still with water flow over the glass cover gives the maximum yield. A double-effect solar still does not enhance the distillate output significantly due to practical difficulties of attaining extremely low and uniform flow rates over the glass cover (10 ml/min). Double effect distillation under active mode of operation. The results show, when the water flow rate was small the efficiency of the still was increased about 30. The effect on daily yield of double effect distillation with water flow. The results show that, active double effect solar still was more effective in production than the passive still.
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4.4 HEAT TRANFER MECHANISM IN SOLAR STILL
Figure 4.3 Heat transfer in a Basin type Solar Still
The mechanisms of heat transfer within a solar still are basically dependent on the climatic effects and the amount of solar radiation that enters the basin. More importantly and frankly too, the performance of the still depends on how much of the solar irradiance that reaches the water in the basin of the solar still. When the sun’s radiation reaches the Earth it is both scattered and absorbed by the atmosphere. The radiation that then travels through the Earth’s atmosphere is known as “sky” radiation, this is the radiation incident on the Earth’s surface after the initial waves from the sun have been absorbed and scattered by the atmosphere. The “sky” radiation that travels to the Earth’s surface can then be used as a valuable energy source for desalination. The direct and diffuse radiation enters the still through the glass cover after partially being reflected and absorbed by the glass itself. Once in the evaporating chamber the radiation is further transmitted, reflected and absorbed by the water until it reaches the blackened basin where most of it is fully absorbed. The basin then begins to heat up and in turn through convective
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processes heats the water causing it to evaporate still. Due to the fact that the glass cover remains at a temperature lower than the dew point temperature (the temperature at which water saturates) the vapour begins to condense on the inside of the glass surface through the mechanism of drop-wise condensation. This is where the vapour condenses in discrete droplets and grows by means of some form of accumulation until it becomes large enough to move under gravity down the glass and can be collected in a pipe at the lower end of the still. This method of condensation has a heat transfer rate of 10 times that of film condensation which allows the heat to be dissipated at a faster rate this allows the excess heat absorbed by the glass to be dissipated and is lost to the atmosphere. Often the formation of drop-wise condensation can reduce the amount of radiation entering the still, and can contribute to a reduction in distillate production in the latter part of the day. The heat received by the film of condensed water, by radiation from the brine surface, by convection from air-vapour, and by conduction of vapour is conducted through the water film and glass to the external surface of the cover. The small amount of solar energy absorbed in the cover is also conducted outward. The heat which the cover (glass) has received is then transferred from the outer surface to the atmosphere by convection and radiation. The heat transfer processes in the solar still are all dependent on the difference in temperature between the brine surface and the glass. The higher the difference, the greater is the energy transfer rate by each mechanism. Furthermore, the higher the brine temperature, the greater the proportion of energy usefully transferred by evaporation. The solar energy transmitted by the glass is partly absorbed in the brine, with the majority of it being absorbed on the basin base. Heat is conducted from the basin base surface into the brine, thereby increasing its temperature and vapour pressure; partial vaporisation then occurs. The warm vapour saturated
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air is carried by the convection currents to the transparent glass cover, which is generally cooler than the brine. A thin film of condensate flows down the transparent glass cover to the collecting trough, from which it passes to storage. The incoming solar radiation, usually composed of direct radiation from the sun and diffuse radiation from the clouds and sky, is partly reflected by the outer and inner cover surfaces, very slightly absorbed in the cover, slightly reflected by the brine and the base of the basin; the balance is absorbed by the brine and the bottom of the basin. Another small portion of energy is lost by conduction through the bottom into the ground or through insulation under the base from the energy absorbed by the basin bottom. The brine is warmed by the convection currents in the shallow basin to the air-water interface, where transfer of mass and energy takes place. Since the vapour pressure of the surface water is greater than the partial pressure in the air space, evaporation into the overlying air film occurs. This transfer of water is accompanied by sensible heat transfer from the warm brine into the air-vapour mixture in contact with it. Both processes produced a temperature rise and density decreased in the airvapoumixture, causing it to rise toward the transparent glass cover. Supplementary to the convective heat transfer from brine surface, is a transfer of heat to the cover by radiation. The glass cover is cooler than the brine partly due to the breeze from the outer side and partly due to the condensate on the inner underside, so the radiant transfer process is essentially between two water surfaces, net radiation being from the brine in the direction of the glass cover. Since the glass cover is cooler than the air-vapour mixture coming in contact with it, the difference in vapour pressure causes diffusion of water vapour through the air film to the water layer on the underside of the cover. Condensation occurs due to the latent heat being released from the water film.
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4.5 FACTORS AFFECTING PERFORMANCE OF STILL The various factors affecting the productivity of solar still are ? Solar intensity ? Wind velocity ? Ambient temperature ? Water-glass temperature difference ? Absorber plate area ? Temperature of inlet water ? Glass angle ? Depth of water The solar intensity, wind velocity, ambient temperature cannot be controlled as they are metrological parameters. Whereas the remaining parameters, free surface area of water, absorber plate area, temperature of inlet water, glass angle and depth of water can be varied to enhance the productivity of the solar stills. By considering the various factors affecting the productivity of the solar still, various modifications are being made to enhance the productivity of the solar
4.5.1 Free surface area The evaporation rate of the water in the solar still is directly proportional to the exposure area of the water. Thus the productivity of the solar still increases with the free surface area of the water in the basin.
4.5.2 Water-glass temperature difference The yield of a solar still mainly depends on the difference between water and glass cover temperatures. The temperature difference between water and glass are acting as a driving force of the distillation process. Regenerative
47
solar still solar still with double glasses and triple-basin solar still were used to increase the temperature difference between glass and the water.
4.5.3 Absorber area Productivity of the solar still increases with increase in absorber area.
4.5.4 Water depth It has been reported that the yield is maximum for the least water depth. While maintaining minimum depth in the solar still, dry spot may occur. So, it is very difficult to maintain minimum depth in the solar still. Wick type solar stills a plastic water purifier and stepped solar still were developed.
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CHAPTER 5 MATERIAL SELECTION AND DESIGN OF THE SOLAR STILL
5.1 HEAT TRANSFER AND HEAT LOSS The main objective of the present work is to develop a solar still with improved distillate output. This can be achieved by developing and applying a well-structured design methodology. Still design factors can broadly be classified into optical, heat transfer and heat loss characteristics. Optical characteristics are absorption, reflection and transmission of solar radiation when incident on the still. Moreover, solar radiation is the most influential environmental factor in solar energy systems. Once the radiation is absorbed by the still, it is converted to heat which is transferred from the absorber to other components of the still and the environment. Heat generated by the basin liner of a solar still is transferred to the saline water in the basin by convection. Then, the hot saline water releases heat to the transparent cover and walls of the still through convection, evaporation and radiation. This elevates the temperature of the transparent cover, thereby reducing the temperature gradient between the cover and the water, and the rate of distillation. Heat is lost to the environment through the top, bottom and sides of the system. Heat loss through the top is desirable because it helps to keep the transparent cover temperature low, thereby increasing the rate of condensation and distillate production. Top heat loss occurs through convection and radiation. Convective heat loss from the top is influenced by the speed of wind over the transparent cover while radiative heat loss from the top to the sky depends on the temperature and emittance of the transparent cover, and temperature of the sky.
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On the other hand, heat loss through the bottom and side walls reduces useful thermal energy for the distillation process, and the productivity of the still. The problem of heat loss from the bottom of a solar still is worse because of the higher temperature gradient between the basin liner and the ambient air temperature outside the still. This leads to a reduction in the distillate production. To overcome this problem insulation of the bottom part of a solar still. 5.2 SOLAR STILL CONSTRUCTION MATERIALS 5.2.1 Transparent cover The most influential property of a transparent material needed in solar technology is transmittance. Clear glass has transmission values of 88- 92% at normal incidence. A glass cover is fitted on the top part of the evaporator unit to allow solar radiation to reach saline water in basin placed under the cover. One or more transparent covers can be used with an air gap between them to reduce heat losses from the top of the evaporator. Multiple glazing reduces top heat loss significantly which leads to high temperatures of the glazing and a decrease in the rate of evaporation-condensation. Thus, single glazing is commonly used for solar distillation systems. Therefore clear window glass is selected as a transparent cover material. 5.2.2 Basin liner Solar radiation that passes through the transparent cover is absorbed by saline water and the basin liner of a solar still. So, the basin liner acts as an absorber of solar radiation and it is important for the liner to have a relatively high absorptance for solar radiation. In practical applications, basin liners can be made of plastic or metal-sheet. Some plastics are relatively cheap while others are expensive. Common metal sheets applied in solar collection are copper, aluminium and steel. The important property of a metal for application
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in solar engineering is thermal conductivity. Copper and aluminium have relatively high thermal conductivities (k=200 Wm-1K-1 for aluminium and k=390 Wm-1K-1 for copper) while the thermal conductivity of steel is relatively low (k=48 Wm-1K-1). Nevertheless, copper and aluminium are more expensive (more than two times the cost of galvanized steel). With these considerations, black coated steel plate is selected for increase its solar absorption. 5.2.3 Insulation Heat loss from the bottom and sides of a solar still is undesirable because it reduces distillate yield. Consequently, it is necessary to minimize this loss by insulating the relevant surfaces. This enables most of the absorbed solar radiation to contribute to the evaporation of saline water and thereby augment the distillate yield. The most important property of an insulator is the coefficient of heat conduction (k). Materials with low values of k are suitable for use as insulators due to their relatively high resistance to flow of heat. Glass wool and thermo cool contain low conductivity and low cost. Therefore glass wool and thermo cool is chosen as insulating material. 5.3 DESIGN AND FABRICATION This project took into cognizance the fact that the structure to be used should possess a number of features intended to guarantee an efficient and effective evaluation of the results. The design is a basin-type solar still (horizontal water-filled basin), covered by a sloping surface transparent to solar radiation, on which water is condensed and collected. Salt water was supplied to the basin with a depth of maximum 40mm. Figure 5.1 shows the Line diagram of double slope solar still.
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All dimensions are in mm
Figure 5.1 Line diagram of double slope solar still
The bottom of the still has a black surface to absorb solar energy. A transparent glass cover is placed on top of the basin such that its surface slopes down into a small trough at its lower edge. The trough is connected to a flexible hose for collection of the distillate. The basin was constructed with stainless steel (2mm thickness) and painted black to absorb the radiant heat. It was then secured an insulated casing of expanded thermo cool (30mm thickness) in bottom and side (20mm thickness). A semi-circular PVC pipe was attached at the lower end of the box to collect the distillate and directed it out to be collected. Figure 5.2 shows the Single basin double slope solar still.
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Figure 5.2 Single basin double slopes solar still
A glass panel (4mm thickness) was then placed on top of the still at an angle of 30 to the horizontal to ensure that the amount of condensate dripped back into the basin. The bottom and sides of the basin are insulated to reduce the heat losses to the surrounding. Once all of the modifications were made the solar still was set-up on the roof top of the building for testing. There are many adaptations and variations that could be made to this design. As a result the other modifications were not considered in great detail.
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Table 5.1 Technical specifications of the solar still
Width Length Thickness Base Area Glass Area Glass Angle
0.50m 0.70m 0.015m 0.30m2 0.20m2 30o
Table 5.2 Materials and properties
MATERIALS
DENSITY (kg/m3) 2707 1200 2110 7833
Thermal conductivity (w/m.k) 204.2 162.8 697.8 53.6
Specific heat (kj/kg.k) 0.896 1.382 2.093 0.465
Aluminium Rubber Asbestos Mild steel
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5.4 WORKING PRINCIPLE ? Salty or dirty water in an airtight container is heated by the sun, causing it to evaporate. ? The water then condenses on the clear container covering, which is slated to allow the fresh water to drain into a collection unit. ? The pure water evaporates and the impurities do not, distilling the water and making it safe to drink.
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CHAPTER 6 PHOTOGRAPH
Figure 6.1 Design of single basin double slope passive solar still
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Figure 6.2 Basin side view
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Figure 6.3 Absorbing material
CHAPTER 7 EXPERIMENTAL PROCEDURE
A single basin double slope solar still has been fabricated with mild steel plate. The overall size of the basin inner (absorbing medium) is 0.65mx0.45mx0.05m and that of outer basin is 0.70mx0.5mx1.5m. The top is covered with two glass of thickness 4mm inclined at 30° on both side. The basin is covered with thermo coal from all the sides to prevent the heat loss. The experiment was carried out 9.00 Am to 6.00Pm in a day. In this study, four absorbing materials were tested under different water level conditions. These were Asbestos, Rubber, Aluminium and Mild steel with black paint coated. Experiments were carried out at 2cm to 4cm water level in the basin. Data were taken during the February 2014. The thermocouples are used to measure temperature of water of glass cover, temperature of absorbing material and ambient temperature is measured every hour. The measuring jar is used to collect the distilled water. The yield of different absorbing material is tabulated shown in table 7.1
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Table7.1 Yield of water in different absorbing material
Sl No
Water level (cm)
Yield of water (ml)
Absorber material
700 630 1 2 780 620 600 550 2 3 690 530
Asbestos Rubber Aluminium Mild steel Asbestos Rubber Aluminium Mild Steel
530 3 4 430 570 420
Asbestos Rubber Aluminium Mild Steel
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800 700 yield of water/0.3m2 600 500 400 300 200 100 0 Asbestos Rubber Aluminium Mild steel Absorbing Medium 2 cm 3 cm 4 cm
Figure 7.1 Yield of water in different absorbing material
The ASBESTOS is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL : 14/02/2014 : 2 cm : ASBESTOS
WATER COLLECTED PER DAY: 700 ml Table 7.2 Temperature of thermo couple using Asbestos (2cm) Water TIME Temp. (T1) oC 10.00 am 11.00 am 35 47 Glass1 Temp (T2)oC 38 45 Asbestos Temp. (T3) oC 36 43 Glass2 Temp (T4) oC 41 47
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12.00 pm
50
49 52 56 48 40 35 30
46 44 54 40 46 34 33
48 44 39 40 48 41 31
1.00 pm 52 2.00 pm 3.00 pm 53 55
4.00 pm 48 5.00 pm 6.00 pm 39 31
60 50 Temp. in oC 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Ta Tg2
Figure 7.2 Temperature of thermo couple using Asbestos (2cm)
DATE WATER DEPTH ABSORBING MATERIAL WATER COLLECTED PER DAY
: 15/02/2014 : 3 cm : ASBESTOS : 600 ml
Table 7.3 Temperature of thermo couple using Asbestos (3cm) Water TIME Temp. Glass1 Temp Asbestos Temp.
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Glass2 Temp
(T1) oC 10.00 am 11.00 am 12.00 pm 33 48 53
(T2)oC 41 45 55 53 54 49 42 35 31
(T3) oC 34 36 40 48 52 43 37 31 28
(T4) oC 45 49 52 51 48 45 40 39 37
1.00 pm 50 2.00 pm 3.00 pm 49 48
4.00 pm 47 5.00 pm 6.00 pm 46 45
60 50
Temp. in oc
40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM
Time in hr
Tw Tw2
Tw3
Tw4
Figure 7.3 Temperature of thermo couple using Asbestos (3cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 16/02/2014 : 4 cm : ASBESTOS
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WATER COLLECTED PER DAY
: 530 ml
Table 7.4 Temperature of thermo couple using Asbestos (4cm) Water TIME
o
Glass1 (T2)oC 40 42 43 45 53 47 39 34 31
Asbestos Temp. (T3) oC 35 39 41 46 50 46 42 36 33
Glass2 Temp (T4) oC 31 33 34 35 37 38 35 33 31
Temp. (T1) Temp C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
32 37 38 40 43 47 42 36 33
60 50
Temp. in oc
40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM
Time in hr
Tw Tw2 Tw3 Tw4
Figure 7.4 Temperature of thermo couple using Asbestos (4cm)
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The RUBBER is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL : 17/02/2014 : 2 cm : RUBBER
WATER COLLECTED PER DAY : 630 ml Table 7.5 Temperature of thermo couple using Rubber (2cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 39 42 44 46 47 50 41 36 30
Rubber Temp. (T3) oC 37 40 41 43 48 46 42 37 31
Glass2 Temp. (T4) oC 33 35 36 38 40 42 37 35 30
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
35 39 41 44 45 48 40 37 31
60 50 Temp. in oc 40 30 20 10 0 10:00 AM 11:00 AM 12:00 PM Tw Tg1
Tr
Tg2
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01:00 02:00 03:00 04:00 05:00 PM PM PM PM PM Time in hr 06:00 PM
Figure 7.5 Temperature of thermo couple using Rubber (2cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 21/02/2014 : 3 cm : RUBBER
WATER COLLECTED PER DAY : 550 ml Table 7.6 Temperture Of thermo couple using Rubber (3cm) Water Time
o
Glass1 (T2) oC 40 43 45 46 47 49 39 36 30
Rubber Temp. (T3) oC 34 38 40 42 43 45 50 38 31
Glass2 Temp. (T4) oC 32 34 36 37 41 42 37 33 29
Temp.(T1) Temp. C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
33 37 41 45 48 50 41 37 32
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60 50 40 Temp. in oc 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Tr Tg2
Figure 7.6 Temperture Of thermo couple using Rubber (3cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 23/02/2014 : 4 cm : RUBBER
WATER COLLECTED PER DAY : 430 ml Table 7.7 Temperature of thermo couple using Rubber (4cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 39 41 42 44
Rubber Temp. (T3) oC 33 35 37 40
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Glass2 Temp. (T4) oC 30 32 33 36
C
10.00 am 11.00 am 12.00 pm 1.00 pm
32 36 38 41
2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
43 45 42 39 37
45 46 40 35 32
43 45 41 35 33
38 39 35 33 31
50 45 40 Temp in oc 35 30 25 20 15 10 5 0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 03:00 PM PM Time in hr 04:00 PM 05:00 PM 06:00 PM Tw Tg1 Tr Tg2
Figure 7.7 Temperature of thermo couple using Rubber (4cm)
The ALUMINIUM is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL : 28/02/2014 : 2 cm : ALUMINIUM
WATER COLLECTED PER DAY : 780 ml Table 7.8 Temperature of thermo couple using Aluminium (2cm)
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Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 51 54 56 58 59 53 44 39 32
Aluminium Temp. (T3) oC 44 46 50 53 51 50 45 38 34
Glass2 Temp. (T4) oC 38 47 56 55 53 46 43 38 34
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
43 50 57 59 60 58 51 45 38
70 60 50 temp. in oc 40 30 20 10 0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 03:00 PM PM Time in hr 04:00 PM 05:00 PM 06:00 PM Tw Tg1 Tal Tg2
Figure 7.8 Temperature of thermo couple using Aluminium (2cm)
DATE WATER DEPTH
: 01/03/2014 : 3 cm
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ABSORBING MATERIAL
: ALUMINIUM
WATER COLLECTED PER DAY : 690 ml Table 7.9 Temperature of thermo couple using Aluminium (3cm) Water Time Temp.(T1)
o
Glass1 Temp. Aluminium (T2) oC Temp. (T3) oC 41 52 54 55 57 54 50 47 33 36 48 53 54 51 50 47 43 35
Glass2 Temp. (T4) oC 32 47 51 53 55 51 45 39 32
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
34 47 55 57 58 56 51 45 38
70 60 Temp. in oC 50 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Tal Tg2
Figure 7.9 Temperature of thermo couple using Aluminium (3cm)
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DATE WATER DEPTH ABSORBING MATERIAL
: 02/03/2014 : 4 cm : ALUMINIUM
WATER COLLECTED PER DAY : 570 ml Table 7.10 Temperature of thermo couple using Aluminium (4cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 36 45 46 47 48 46 40 38 30
Aluminium Temp. (T3) oC 29 38 40 43 50 52 44 34 32
Glass2 Temp. (T4) oC 30 35 39 44 45 40 38 34 30
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
31 36 42 45 47 49 44 38 36
70
60 50 Temp.in oC 40 30 20 Tw Tg1 Tal Tg2
10
0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 PM Time in hr 03:00 PM 04:00 PM 05:00 PM 06:00 PM
Figure 7.10 Temperature of thermo couple using Aluminium (4cm)
The MILD STEEL is used as a absorbing medium DATE WATER DEPTH ABSORBING MATERIAL WATER COLLECTED PER DAY : 07/03/2014 : 2 cm : MILD STEEL : 620 ml
Table 7.11 Temperature of thermo couple using Mild steel (2cm) Water Time Glass1 (T2) oC 10.00 am 11.00 am 12.00 pm 1.00 pm 38 47 50 51 42 50 55 56
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Mild steel Temp. (T3) oC 40 48 52 53
Glass2 Temp. (T4) oC 36 39 55 52
Temp.(T1) oC Temp.
2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
52 53 50 43 37
57 51 47 39 33
54 55 53 44 36
50 45 42 36 32
60 50 Temp. in oC 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time in hr Tw Tg1 Tm Tg2
Figure 7.11 Temperature of thermo couple using Mild steel (2cm)
DATE WATER DEPTH ABSORBING MATERIAL
: 08/03/2014 : 3 cm : MILD STEEL
WATER COLLECTED PER DAY : 530 ml Table 7.12 Temperature of thermo couple using Mild steel (3cm) Water Glass1 Temp. Mild steel Glass2
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Time
Temp.(T1) oC (T2) oC
Temp. (T3) oC
Temp. (T4) oC 32 35 45 49 50 52 46 37 33
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
34 41 47 50 52 53 49 42 36
37 42 46 52 56 51 48 37 32
35 42 50 52 54 55 51 42 37
60 50 Temp. in oC 40 30 20 Tw Tg1 Tm Tg2
10
0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 PM Time in hr 03:00 PM 04:00 PM 05:00 PM 06:00 PM
Figure 7.12 Temperature of thermo couple using Mild steel (3cm)
DATE
: 09/03/2014
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WATER DEPTH ABSORBING MATERIAL
: 4 cm : MILD STEEL
WATER COLLECTED PER DAY : 420 ml Table 7.13 Temperature of thermo couple using Mild steel (4cm) Water Time Temp.(T1)
o
Glass1 Temp. (T2) oC 37 42 50 52 54 51 47 43 36
Mild steel Temp. (T3) oC 35 39 43 45 48 47 46 41 35
Glass2 Temp. (T4) oC 32 36 49 50 51 48 44 39 33
C
10.00 am 11.00 am 12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00 pm 6.00 pm
33 38 45 48 50 46 43 41 37
60 50 Temp. in oC
40
30 20 Tw Tg1 Tm Tg2
10
0 10:00 AM 11:00 AM 12:00 PM 01:00 PM 02:00 PM Time in hr 03:00 PM 04:00 PM 05:00 PM 06:00 PM
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Figure 7.13 Temperature of thermo couple using Mild steel (4cm)
CHAPTER 8 RESULT AND DISCUSSION
Highest temperature is obtained by the temperature of water which is evaporated with the help of solar radiation. Hence, we can say that the higher temperature of water. So higher evaporation and condensation and higher yield. Figure 8.1 shows that, yield is very low up to noon and for all the absorber plate and goes on increasing after noon due to the solar radiation falling on the absorber plate. And as we know that, when sun is east. Solar evaporation takes place at east side and west side acts as a condenser and during west side of sun, reverse will be occur. Figure 8.1 shows that the yield for mild steel absorber plate is low compared to the aluminium sheet Asbestos as well as rubber sheet. Hence, it has been observed that yield of Alunminium is higher compared with other three absorbing materials (Asbestos, Rubber, MS). The absorber temperature at different timing as shown in Figure 8.1. The absorbing temperature of the “Aluminium” is higher than the Asbestos and others. Because we know that the Aluminium has higher thermal conductivity compared with others. So due to higher thermal conductivity higher temperature will obtained.
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Temp. of absorbing material
60 50 Temp. in oC 40 30 20 10 0 10:00 11:00 12:00 01:00 02:00 03:00 04:00 05:00 06:00 AM AM PM PM PM PM PM PM PM Time
Asbestos
Rubber Aluminium mild steel
Figure 8.1 Temperature of different absorbing material
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CHAPTER 9 CONCLUSION
? The orientation of the glass cover depends on the latitude of the place. For northern latitude south facing and southern latitude north facing stills are used. ? The inclination of the cover is optimized for rate of condensation of water on the bottom surface of the cover and to collect it without the mass accumulated drops fall back into the basin. Hence it depends on the intensity of solar radiation, rate evaporation and condensation, material used for cover and its wetting property. ? Glass is the most preferred material for cover, since it has higher solar transmittance and long service life. The surface wets with condensed water and allow film condensation at the bottom surface which results in less loss in transmittance. Other plastic materials do not possess the above required qualities. ? Lowering the cover temperature helps in increasing the productivity. The glass cover temperature is reduced by a film of cooling water continuously flowing over the glass or intermittent flow of cooling water on the cover. ? The dependence of yield on water depth is a strong function of initial temperature of the water in the basin. The productivity of the still decreases with an increase in depth of water during daytime and the reverse is the case of overnight production. Higher depth of water in the
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basin could be used only in the places where higher solar radiation is available. Otherwise it is best to use shallow basin still. ? To improve the radiation absorption in the black painted basin is most suitable with deep basin still. ? Other basin materials like rubber, gravel, copper sulphate, potassium permanganate, potassium dichromate, cobalt chloride, coal, charcoal, uncoated metallic wiry sponge, coated metallic wiry sponge and black volcanic rocks are having the properties of absorbing and storing of solar radiation in different proportions along with increasing the exposed area for evaporation of water. ? Volcanic rock and asphalt is the best basin materials to improve the productivity by absorption and storage of heat. ? Using finned plate in the basin increase the productivity by increasing the surface area of absorber and rate of heat transfer between saline water and absorber. ? Rubber is the best basin material to improve absorption, storage and evaporation effects. ? Mica sheet as suspended absorber is better material for surface heating. ? The distillate output increases with increase of the initial water temperature in the basin. ? To reflect the solar rays falling on the side walls of the still onto the basin reflecting mirrors are fixed on the side walls. This increases the insulation effect also. ? The maximum productivity is achieved, when energy storing materials were used in the stepped solar still coupled with mini solar pond.
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? In higher latitude places using additional condenser in the single slope still increases the productivity. ? Using the double-effect solar still instead of single-effect still is leads to increased productivity and cost saving. ? Increasing the number of basins beyond three does not yield significant improvements in the still productivity.
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CHAPTER 10 REFERENCES
1. K. Kalidasa Murugavel, K.Srithar (2010). performance study on basin type double slope solar still with different with materials and minimum mass of water, Department of Mechanical Engineering, Thiagarajar college of Engg., Madurai, Tamilnadu 625015. 2. A.Tamimi (1986), Performance of a solar still with Reflectors and Black dye, Solar & wind Technology vol 4, No 4, Chemical Engineering Dept, Jordan University & Tech, Irbid, Jordan. 3. A Safwat Nafey, M. Abdel kader, A. Abdel Motalip & A.A. Mabrouk 1999. Parameters affecting solar still productivity, Energy Conversion & Management 41. Faculty of petroleum and mining Engineering, Engineering Science Dept, Suez Canal University, Port Said, Egypt . 4. O.O. Badran (2006). Experimental study of the enhancement parameters on a single slope solar still productivity, Elsevier Publications. Mechanical Engg., Dept., Al-Balqa Applied University, Jordan
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