A study on effects of water capacity on the performance of a simple solar still.Introduction
Solar distillation represents a most attractive and simple technique among other distillation processes, and it is especially suited to small-scale units at locations where solar energy is considerable. Solar energy is abundant, never lasting, available on site with free of cost and pollution free energy. Because of the simplicity of apparatus design, requirement of fresh water, and free thermal energy, work in the field of solar distillation is in the most advanced stage of development. The most common type of solar stills are the basin type. Many researchers have investigated the effects of climatic, operational and design parameters on the performance of a basin type solar still to improve the productivity. M.A.S. Malik et. al  have reviewed the work on solar distillation that includes various designs of solar stills, like single basin still, multiple effect still, inclined solar stills, solar still greenhouse and effect of meteorological and still parameters, etc. In their studies, it is concluded that there is a variation of 30% in daily yield for variation in depth from 12.7mm to 305 mm. Effect of heat capacity of basin water on the productivity of solar stills was investigated by many authors [2-4] and they have been concluded that the productivity decreases with an increase of water depth. Further, in a previous work, it has been found that the overnight productivity of single basin solar still increases with an increase of the basin water depth and it represents a great part of the daily productivity of solar stills [5, 6]. However, a detailed study on the effect of heat capacity of basin water on the solar still performance is still considerable interest due to some difficulties associated with experimental measurements of the still productivity overnight. The main objectives of this present study are, to study the effect of saline water capacity in the basin on the internal, external heat transfers and performance of the single slope single basin solar still. It is also includes analyze cumulative energy balance of the solar still for different water levels in the basin.
Single basin solar still were fabricated and tested under field condition at the testing field of the Mechanical Engineering department, Adhiyamaan College of Engineering, Hosur, Tamilnadu, India. The basin liner is made of galvanized iron sheet of 0.5m x 1 m with maximum height of 288mm, and 1.4 mm thickness. The basin surfaces are painted with black paint to absorb the maximum amount of solar radiation incident on them. The condenser surface of the still is made of glass with 4mm thickness and angle of inclination is 10[degrees] with horizontal. There are certain specifications needed for the used glass cover in the still, and they are (a) Minimum amount of absorbed heat, (b) Minimum amount of reflection for solar radiation energy, (c) Maximum transmittance for solar radiation energy, and (d) high thermal resistance for heat loss from the basin to the ambient. Glass cover has been framed with wood and sealed with silicon rubber, which plays an important role to promote efficient operation as it can accommodate the expansion and contraction between dissimilar materials. A collecting trough made by G.I. sheet is used in the still to collect the distillate condensing on the inner surfaces of the glass covers and to pass the condensate to a collecting flask. Steel rule is fixed along with inside wall for measuring water levels. The bottom and sides are insulated with thermocole and wood materials. The geometrical constructions of the solar still are shown in Fig.1, and pictorial view of the solar still is given in Fig.2. The technical specifications and design parameters of the system are given in Table 1.
[FIGURE 1 OMITTED]
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The solar still is oriented with their long axis in the E-W direction and glass surface is facing north side. The experiments on the still were carried out during March to May 2008 under the same climatic conditions. During experiments, the solar radiation intensity, ambient temperature, water temperature, basin liner temperature, inner wall temperatures, outer wall temperatures, bottom side temperature and wind velocity were recorded every 60 minutes. The hourly productivity of fresh water is collected through a graduated flask. The daily productivity is obtained as a summation of day and night productivity. The night productivity is the total collection from the end of test to start of test in the next day. The measuring devices used in the system are as follows:
1. A Pyranometer is used to measure the solar radiation. This device measures the instantaneous intensity of radiation in (kW/[m.sup.2]).
2. Twelve thermocouples (type-k) coupled to digital thermometer with a range from 0 to 99.9[degrees]C with [+ or -] 1[degrees]C accuracy are used to measure the temperatures of the various components of the still system.
3. A digital anemometer is used to measure wind speed.
4. A 30mm steel rule is fixed inside wall used to measure water depth.
Results and Discussions
The results and discussions for the behavior and performance of the solar desalination system presented here in the form of graphs and tables. Experiments have been conducted from 9:00 o'clock to 18:00 o'clock by considering a wide range of parameters such as temperatures of basin water, glass cover, inside walls (back and side walls) and bottom wall, hourly yield and solar intensity. The experiments were conducted for number of days, so that the analysis and comparison could be fairly done under the same climatic conditions and to get concurrent results. In Fig.3, the hourly variations of different component temperatures of the solar still are compared for 10mm water depth on 16.04.08. It can be seen that, the water, glass and basin temperatures increases gradually and reaches the maximum value in the afternoon because the absorbed solar radiation exceed the losses from the solar still to the ambient. In the morning hours (9:00 o'clock-10:00 o'clock), the glass temperature is higher than the water and vapor temperatures; this may due to the radiation absorbed by the glass and also the less heat capacity of the glass, so the temperature is higher than the water during this period.
[FIGURE 3 OMITTED]
Fig.4-Fig.6 show that the effect of the basin water capacity on the water, vapor and basin liner temperatures. Between morning hours (9:00 o'clock) and mid afternoon (15:00 o'clock), the temperature of water, basin liner and vapor are high for lower water levels (10mm, 20mm); but, it is reverse in the evening hours between 15:00 o'clock and 18:00 o'clock. This may due to the fact, that lower capacity water takes less time to raise its temperature (morning hours). Similarly, it releases the stored energy faster than the higher capacity water (Evening hours). In higher water levels, the maximum temperature of the basin water, vapor and water is attained in the late afternoon hour between 15:00 o'clock and 18:00 o'clock. This may due to; it stores energy (more amount) in the late afternoon due to high heat capacity. This is clearly indicated in Fig.4-Fig.6.
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The productivity rate varies with the time, which grows from morning to the late afternoon and decreases in the evening hours. Fig.7 shows the hourly variations of productivity of solar still. It is clearly indicates that the 10-mm and 20-mm water level give more yielding from 9:00 o'clock to 13:00 o'clock, and then the productivity decreases in the evening. In higher water levels (40, 50 and 60mm); the yielding starts slowly (i.e. lesser productivity) between 9:00 o'clock-13:00 o'clock and it approaches the higher value after 13:00 o'clock-18:00 o'clock up to the end of the evening. This may due to the temperature difference between glass and water is high in evening hours. It can be seen from Fig.7, between 18:00 o'clock and 9:00 o'clock (next day), the productivity is more in higher water levels due to the more heat energy stored in the basin water. The experimental results shows, the distilled water production during nighttime (18:00 o'clock and 9:00 o'clock (next day)) for various water levels like 10, 20, 30, 40, 50 and 60mm, its values is 0.210, 0.225, 0.300, 0.350 and 0.350 kg, respectively. The result also indicates the maximum productivity of 1.717 kg (3.434 kg / [m.sup.2]-day) for 24 hours period (9:00 o'clock-09:00 o'clock) is obtained from 20mm water level. The solar still efficiency is considered as the most important parameter to be evaluated, to ensure the best still design. From Fig.8, it is understood that 10mm, 20mm water level gives higher efficiency than the others between 9:00 o'clock to 13:00 o'clock, and then it is decreases till the evening. For higher water levels, the efficiency slowly increasing from morning ant it will reach maximum in the late evening. Between 17:00 o'clock and 18:00 o'clock, 50mm and 60mm water levels are given more than 100% efficiency; due to the output of distillate water energy (Qd) is much higher than the solar radiation (I) input during this period. The maximum efficiency of 41.29% is obtained from 20mm water level and lowest efficiency of 35.33 from 60mm water level.
[FIGURE 7 OMITTED]
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Fig.9 shows the maximum temperature of water, vapor, basin liner and inner wall obtained from various water levels. The inner wall temperatures for all the water levels are high and it is nearly constant. The maximum temperature of inner wall surface was 83.9[degrees]C obtained in 20mm depth. The inner wall surfaces are not fully makes direct contact with the water. This may be reason for constant temperature at inner surface for all water levels where as the water, vapor and basin liner temperatures are much related with water capacity, from Fig.9, it can seen that, if water level (capacity) is increased the temperature of vapor, basin water and basin liner are decreases. The maximum and minimum water temperature is recorded in 20mm and 60mm water levels are 75.5[degrees]C and 63.9[degrees]C respectively. The results also indicates the maximum (78.8[degrees]C) and minimum (65.9[degrees]C) vapor temperature is noted in 20mm and 50mm depth respectively. It can also be noted that the basin temperature gets closer to the water temperature, because of the continuous contact between them and leads to heat equilibrium. From Fig.10, it is understood, the maximum hourly yield (0.320 kg/hr) is obtained from 10mm water depth between 12:00 o'clock-13:00 o'clock and the lowest hourly maximum yielding (0.204 kg/hr) is from 60mm depth at 16:00 o' clock-17:00 o'clock. It also indicates the maximum hourly yield of solar still is based on the capacity of basin water, which is started from mid afternoon for lower levels and vice versa.
[FIGURE 9 OMITTED]
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Fig.11 shows the hourly variation of various external heat transfers involving in the solar still for 10mm depth with 5kg of water. Between 9:00 o'clock and 13:00 o'clock, the radiation and convection heat losses from glass to ambient are more, and then it decreases in the afternoon. Due to high solar intensity and more condensation the temperature of the glass is increases during this period, this will lead the temperature gradient between glass and ambient; this may be the reason for increasing the heat transfer from glass to ambient. In the morning hours, the outer wall surface temperatures are more, compared with the inner wall surfaces. Due to this temperature difference, the heat enters from outer to inner surfaces through the wall materials in the morning hours. This heat transfer introduces some negative values in the Fig.11 between 9:00 o'clock and 11:00 o'clock. From results we obtained, the heat transfer from water contact surface to ambient for entire day is only 2.3W for 10mm water depth; it is very much lesser than the other losses. The maximum amount of energy utilization is 1083.56W, for converting the saline water into fresh water for 10mm water depth. The remaining external heat transfer for all water levels is presented in the Tables 2.
[FIGURE 11 OMITTED]
The external heat losses are mainly based on temperature of the components of the still. Fig.12 shows the hourly variation of total external losses for various water levels. For 10mm and 20mm levels, due to its low capacity of basin water, the water temperature is increases faster than the other levels between 9:00 o'clock-14:00 o'clock. This will lead to increase the external heat losses during this period, then decreases in the afternoon. Where as in higher water levels the losses are more in the late afternoon than the morning, this is because in the morning maximum energy taken by the water to increase the sensible heat for phase change. In the afternoon, it will release the heat for condensation due to this reason, in the late afternoon basin water and other components temperatures of the still are high for higher water levels. Fig.11 also indicates the maximum external losses occur from glass to ambient, in the mode of radiation and convection heat transfer. From Table 3, easily understood, the radiation and convection heat transfer from glass to ambient is more during 9:00 o'clock-14:00 o'clock in lower water depth, due to high temperature of glass, vapor and water. From Fig.7, and Table 2, it is understood, that the external losses are also influencing the distillate output.
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Fig.13 indicates the percentage of energy distribution of various energy transfers for various water levels. Apart from all energy transfer, unaccountable losses are also exit in the energy balance of the system due to vapor leakage from gaskets, joints, energy stored in water etc. All the energy transfers from the still are very close value for all water depth, except unaccountable losses. Unaccountable losses are more in higher water levels; this may be the reason for its lower productivity. Fig.7 indicates the reasonable productivity starts for higher water levels around 14:00 o'clock. From this we can understood, that the high capacity water will take more time to obtain the required phase change condition during well sunshine hours between 9:00 o'clock and 14:00 o'clock. So, now only less amount of time is left for efficient operations in a full experimental day (i.e.14:00 o' clock-18:00 o'clock), may be in this remaining time only the unaccountable losses will be high for higher water levels.
[FIGURE 13 OMITTED]
Internal heat transfer is another important factor which influencing the productivity of the solar still. Fig.14 shows the hourly variations of internal heat transfer for different water levels. From Figs.7 and 14, we can easily understand that, when the internal heat transfer is less, the productivity will also less. Similarly, if internal heat transfer is high, the productivity will also high. In morning hours (9:00 o'clock-11.00 o'clock) the value of internal heat transfer will be negative for higher water levels; this may be the reason for decreasing the productivity during that period. The amount of internal heat transfer for all the water levels are reported in the Table 3.
[FIGURE 14 OMITTED]
In this present study, several conclusions can be obtained as follows. If the water capacity of the basin is increases, then the operating temperature of vapor, basin water and basin liner are decreases. In higher water levels, the maximum temperature of the basin water, vapor and water is attained in the late afternoon hours between 15:00 o'clock and 18:00 o'clock where as in lower levels that will attain in the mid noon.
The maximum and minimum water temperature is obtained from 20mm and 60mm water levels are 75.5[degrees]C and 63.9[degrees]C respectively. Similarly the maximum (78.8[degrees]C) and minimum (65.9[degrees]C) vapor temperatures are obtained from 20mm and 50mm depths respectively. The overall highest temperature (83.9[degrees]C) of the solar still is obtained at the inner wall surfaces and is almost constant for all water levels and the next highest temperature is the vapor temperature (78.8[degrees]C). The lowest temperature of the still component is found at the bottom (32.2[degrees]C). The basin liner temperature is almost closer to the water temperature, because of the continuous contact between them and it leads to heat equilibrium.
The lower water levels gives more yield from 9:00 o'clock to 13:00 o'clock, where as in higher water levels the productivity is more between 18:00 o'clock and 9:00 o'clock (next day). If the water level decreases from 60mm to 10mm then the productivity of the still is increased by 12%.
The maximum efficiency of the still occurs in the lower water levels is at the afternoon and for higher water levels is at the evening. The maximum efficiency of 41.29% is obtained from 20mm water level and lowest efficiency of 35.33 from 60mm water level. The higher water levels will give greater than 100% efficiency of still during the period of 17:00 o'clock-18.00 o'clock; it is the distillate water energy ([Q.sub.d]) output is much higher than the solar radiation (I) input during this period.
The maximum hourly yield of 0.320 kg/hr is obtained from lower water depth (10mm) between 12:00 o'clock-13:00 o'clock and the lowest maximum hourly yield of 0.204 kg/hr is from higher depth (60mm) at 16:00 o'clock -17:00 o'clock. The maximum hourly yield of solar still is depends on the capacity of basin water. The maximum amount of external energy losses (approximately 42%) from the still is by the combined effect of convection and radiation from glass to ambient, and the lowest losses in the still is conductive heat loss from basin liner to bottom side (1%).
In lower water levels the maximum energy (both external and internal) losses will occurs in the mid noon and higher water levels, it is in the evening. Unaccountable losses (vapor leakage from gaskets, joints, and energy stored by water itself) are more in higher water levels.
Both internal and external heat transfers are very much influencing the productivity of still. If the internal and external heat losses are high, the productivity of the solar still is also high.
Nomenclature [m.sub.w] Mass of hourly yield, kg [Q.sub.bot] Conductive heat transfer from basin liner to ambient through bottom side, W [Q.sub.bw] Conduction heat transfer from inner to outer side through back wall, W [Q.sub.cg] Convection heat transfer from glass to ambient, W [Q.sub.cw] Convection heat transfer from water to glass, W [Q.sub.d] Rate of heat energy of distilled output, W [Q.sub.ew] Evaporative heat transfer from water to glass, W [Q.sub.rg] Radiation heat transfer from glass to ambient, W [Q.sub.rw] Radiation heat transfer from water to glass, W [Q.sub.sw] Conduction heat transfer from inner to outer side through side wall, W [Q.sub.sww], Conductive heat transfer from the inside still to outside through water contact vertical sides, W [T.sub.a] Ambient temperature, [degrees]C [T.sub.b] Basin liner temperature, [degrees]C [T.sub.bwi] Inside back wall temperature, [degrees]C [T.sub.bwo] Outside back wall temperature, [degrees]C [T.sub.g] Glass temperature, [degrees]C [T.sub.swi] Inside side wall temperature, [degrees]C [T.sub.swo] Outside side sidewall temperature, [degrees]C [T.sub.v] Vapour temperature, [degrees]C [T.sub.w] Water temperature, [degrees]C
 M.A.S. Malik, G.N. Tiwari, A. Kumar and M.S. Sodha, Solar Distillation, Pergamon, Oxford, 1982.
 H. P. Harg. and H S. Mann, 1976, Effect of climatic, operational and design parameters on the year round performance of single-sloped and double-sloped solar still under Indian arid zone conditions. Solar Energy, Volume 18, Issue 2, Pages 159-163.
 S.A. Lawrence, S.P. Gupta, G.N. Tiwari, 1990, Effect of heat capacity on the performance of solar still with water flow over the glass cover, Energy Conversion and Management, Volume 30, Issue 3, Pages 245-250.
 Y. P. Yadav, Y. N. Prasad, 1991, Parametric investigations on a basin type solar still, Energy Conversion and Management, Volume 31, Issue 1,Pages 7-16.
 C. E. Okeke, S. U. Egarievwe, A. O. E. Animalu, 1990, Effects of coal and charcoal on solar-still performance, Energy, Volume 15, Issue 11, Pages 1071-073.
 Abdel-Monem A. El-Bassuoni, 1986, Enhanced solar desalination unit: Modified cascade still, Solar & Wind Technology, Volume 3, Issue 3, Pages 189-194.
T.V. Arjunan (1), H.S. Aybar * (2) and N. Nedunchezhian (3)
(1) Automobile Engineering Department, PSG College of Technology, Coimbatore Tamilnadu, India E-mail: email@example.com
(2) Mechanical Engineering Department, Eastern Mediterranean University, G. Magosa, Mersin, 10, Turkey
(3) Automobile Engg., Institute of Road and Tpt. Technology, Erode, T.N., India
* Corresponding Author E-mail: firstname.lastname@example.org
Table 1: Technical specification and design parameters of the solar still Specification Dimension Basin area ([A.sub.b]) 0.5 [m.sup.2] Absorptivity of Basin liner ([[alpha].sub.b]) 0.96 Glass area ([A.sub.g]) 0.508[m.sup.2] Glass thickness 4mm Number of glass 1 Slope of glass 10[degrees] Emissivity of glass ([[epsilon].sub.g]) 0.88 Absorptivity of glass ([[alpha].sub.g]) 0.12 Thermocouples 12 no's Absorptivity of water ([[alpha].sub.w]) 0.45 Emissivity of water ([[epsilon].sub.wg]) 0.96 Mass of saline water 5,10,15,20,25,30 kg Thickness of thermocole ([L.sub.th]) 25.4mm Thermal conductivity of thermocole ([K.sub.th]) 0.015 W/mK Thickness of wood ([L.sub.wood]) 12.5mm Thermal conductivity of wood ([K.sub.wood]) 0.055 W/mK