*Author for correspondence E-mail: algarni@kfupm.edu.sa
Effect of glass slope angle and water depth on productivity of double slope solar still
Ahmed Z Al-Garni*, Ayman H Kassem, Farooq Saeed and Faizan Ahmed
Aerospace Engineering Department, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia Received 29 December 2010; revised 16 August 2011; accepted 29 August 2011
This study presents design, fabrication and testing of double slope solar still and also optimization of glass tilt angle (25, 30, 35 and 40°) and water depth (1, 2 and 3 cm) in Saudi Arabian climatic conditions. Optimum tilt angle was found to be 35° for both summer and winter seasons, and also productivity increases with reduction in water depth. A number of existing energy and mass balance equations were used to validate experimental results. Numerical and experimental results were in good agreement with an RMS error of 5-10%.
Keywords: Renewable energy, Solar distillation, Solar still
Introduction
Solar stills used for water distillation are widely practiced in remote and desert areas having sweet water scarcity problems. Critical parameters related to productivity of solar still are glass tilt angle (GTA) and water depth (WD). Singh et al1 carried out a numerical analysis on active and passive single slope solar still (S4) considering effect of solar intensity, wind velocity, WD, and GTA on productivity. Kumar et al2 observed that annual performance of an active S4 is optimum at GTA of 15° in Indian conditions. Kamal3 analyzed double slope solar still (DS3) for Doha (Qatar) climatic conditions and found a GTA of 10° for summer and 15° for winter to obtain high quantity of distilled water. Enein et al4 reported that GTA of a S4 should be as low as possible in summer and 50° in winter for Egyptian conditions. Nafey et al5 found a similar trend in the results as obtained by Enein et al4. A thermal analysis6 was done to optimize glass cover inclination of S4 for maximum yield in Indian conditions. Al-Hinai et al7 predicted performance of DS3 in Omani climatic conditions that productivity increases with decrease in GTA in summer and vice versa in winter.
Dev et al8 observed that optimum inclination angle for best performance of a solar still is 45° for both seasons in Indian conditions. This is in strong contrast to the results
obtained by earlier studies4-7. Akash et al9 found that 35° GTA of DS3 gives maximum yield in May in Jordan.
Elkader10 obtained similar results with 35° GTA giving maximum yield. Several studies2,3,5,7-9 on effect of WD indicated that productivity decreases in a linear relation with increase in WD. Efforts have also been made to find optimum value of WD3,7. Although a lot of studies have been done on GTA optimization in different countries, but results are contradictory1,3,4,8. The present study is partial implementation of two patents11,12, which have been submitted on solar distillation.
This study optimizes GTA and WD for maximum output in Saudi Arabian climatic conditions.
Experimental Section Mathematical Model
In a DS3 with various HT modes, solar radiation incident on still is partly absorbed and partly reflected by glass cover while most of it is transmitted through glass cover into the still. Transmitted radiation from glass cover is absorbed in large amounts by basin and water while a little radiation is again reflected. Heat gained by basin water is transferred to inner glass cover by convection, radiation, and evaporation of water. Some amount of heat gained by glass cover in this process is lost to the atmosphere by convection and radiation. Equation for conservation of mass is written as1 3 . Energy balance equation for glass cover is given as14
Energy balance equation for basin and water contained in it is given as14
…(3)
…(4) Eqs. (1) and (3) are solved simultaneously for Tw and Tg. Assumptions made while solving energy balance equations are as follows: 1) Temperature of glass is uniform over the glass cover; 2) Temperature of water is uniform over the water and basin material; 3) Bottom and sides of basin are well insulated, thus and are negligible; and 4) Reflection of heat from water surface and energy storage material used in the basin is negligible, thus is negligible.
Heat reflected from glass to air is also negligible.
Distilled water production rate is
calculated as13 . Solar
radiation on a tilted surface is a combination of three components and is calculated as1 5
For northern hemisphere, geometric factor
Rbeam is calculated using1 5
. Declination angle is found as15
Heat is transferred from water to glass surface by convection of air trapped inside still, evaporation of water, and radiation of heat from water surface. HT
is estimated as13 ,
and . Convective and evaporative HT coefficients for water to glass surface are calculated respectively,
temperature and glass surface are calculated
as16 and
. Specific heat of air trapped inside solar still is written in terms of average temperature of basin water and glass as1 7
and . Latent heat of evaporation
of water is found using1 8
. Some amount of heat is absorbed by glass due to incident solar radiation falling on glass surface. This can be calculated as1 9 . Solar radiation incident on still is absorbed in huge amounts by blackened base and water.
Heat absorbed by water is estimated using1 9 . Heat lost from glass cover to atmosphere is found
by13 and
Convective HT coefficient from glass cover to atmosphere is given by20 . Heat added to the system by the supply of feed water is written as1 3 . Heat loss from the system due to distillate leaving the still is estimated by13 . Heat due to blow down in
base tank is given as13 .
Experimental Set up
Four units of DS3 (GTA: 25, 30, 35 and 40°) were designed and fabricated (Fig. 1). Various components of still were collected locally from the workshops.
Galvanized iron (3 mm thick) was used for manufacturing base tank (1 m x 1 m x 0.06 m). Two holes were provided in base tank of each unit so that distilled water can be collected in a measurable bottle kept beneath the still. A
layer of black paint was applied on inner and outer sides of base tank so as to improve absorptivity of tank. Silicon rubber was used to seal glass covers and base tank since silicon has good bonding between glass and many other materials. L shaped aluminum strips were fixed on all the edges of glass and base tank to provide solidity to solar still. A distillate channel is provided in base tank so that condensed water flows through this channel and is collected in a measurable bottle through a flexible pipe.
All four water distillation units were tested to examine the effect of various operating parameters under same weather conditions. Still was placed in South-North orientation. Experiments were carried out from sunrise to sunset while making hourly recordings for distillate water and temperatures of glass and water in the basin.
Ambient temperature, wind speed and direction were also noted every hour. Experiments were conducted in an open ground in KFUPM campus, Dhahran (26°16 N, 050°10 E), a city in the eastern province of Saudi Arabia, during summer (June) and winter seasons (December).
Effect of GTA and WD on the still performance was studied and compared.
Results and Discussion
Effect of Glass Tilt Angle (GTA)
In summer (June) with 1 cm WD, productivity of still increased from 25° to 35° GTA (Fig. 2a) and then decreased. Hence at optimum GTA (35°), maximum productivity is 4.64 l/m2. This is in good agreement to a very recent study8. Productivity increased by 2.6% from 25° to 30° GTA and an increase by 7.9% was observed when angle is changed from 30° to 35°. Further, there was a reduction in productivity by 3.4% when GTA was increased to 40°. In winter (December) with 1 cm WD, optimum GTA was again found to be 35° with a maximum productivity of 2.1 l/m2 (Fig. 2b). This result is also in
agreement with reported study8. Productivity increased by 3.2% from 25° to 30° GTA whereas an increase of 9.3% was observed from 30° to 35° variation in angle. A reduction of 2.3% in productivity was found when angle was further increased to 40°.
Effect of Water Depth (WD)
For summer (June), as WD was increased from 1 cm to 3 cm, productivity decreased (Fig. 3a), as also reported2,3,5,7-9. When WD is increased, heat capacity of water is increased, thereby decreasing in productivity for high WD. For all WDs considered, 35° GTA gave best results. Productivity decreased by 6.2% and 6.7%
when WD was increased from 1-2 cm, and 2-3 cm, respectively. For winter (December) also, as WD was increased from 1 cm to 3 cm, productivity decreased (Fig. 3b). A maximum output of (2.1 l/m2) was obtained at 1 cm depth for 35° GTA. Output reduced by 13.8%
and 16% for 2 cm and 3 cm WDs respectively. In hourly variation of still output for a typical day in summer (June),
Fig. 1—Fabricated double slope solar stills with cover slope angles of 25, 30, 35 and 40° in order
Fig. 2—Accumulated productivity of solar still for different GTA in: a) summer; and b) winter
b)
Cover tilt angle, o
Still out put, l/m2
a)
Cover tilt angle, o
Still out put, l/m2
' '
'
productivity increases from morning to noon, and trend is reversed thereafter (Fig. 4a); peak value is obtained between 12 PM to 1 PM. Hourly variation of still output for a typical day in winter (December) indicates a peak value between 1 PM and 2 PM (Fig. 4b). Since the day is shorter in winter, increase in productivity is very slow in morning hours as much of the solar radiation is consumed in warming up the solar still. With passage of time, productivity increases due to increase in ambient temperature and solar radiation.
Numerical Results
Geographical parameters of experimental site were as follows: latitude, 26°15´N; longitude, 50°09´E; elevation (above mean sea level), 84 ft/26 m; ambient temp. for summer, 40°C (average temp. for a typical day in June);
wind velocity for summer, 5 m/s (average velocity for a
typical day in June); ambient temp. for winter, 21°C (average temp. for a typical day in December); and wind velocity for winter, 2 m/s (average velocity for a typical day in December). A computer program was written in MATLAB software and solved using ode23 function.
Simulation was carried out for all the cases with varying GTAs and WDs. A sample result of productivity for optimum GLA and WD is shown for summer (Fig. 5) and winter (Fig. 6) seasons. A comparison between experimental and numerical temperature profiles for basin water and glass temperatures is also shown for summer (Fig. 7) and winter (Fig. 8) seasons. It is found that simulation results are in good agreement with experimental results. In numerical calculations, mass and area of glass surface are constants varying with the change in GTA. Mass of water in basin is also a constant varying with the change in WD. Hour angle ω varies at
Fig. 3—Effect of water depth on productivity of solar still with various slope angles in: a) summer; and b) winter
Fig. 4—Hourly variation of solar still productivity for a typical day in: a) summer; and b) winter a)
Still out put, l/m
Water depth, cm
b)
Still out put, l/m
Water depth, cm
a)
Still out put, ml/m2
Time, h (AM-PM)
b)
Still out put, ml/m2
Time, h (AM-PM)
a) b)
a) b)
a) b)
Fig. 5—Experimental and numerical comparison of daily productivity in summer for slope angle 35° and water depth of: a) 1 cm; and b) 3cm
Fig. 6—Experimental and numerical comparison of daily productivity in winter for slope angle 35° and water depth of: a) 1 cm; and b) 3 cm
Fig. 7—Experimental and numerical comparison of water and glass temperatures in summer for slope angle 35° and water depth of: a) 1 cm;
and b) 3 cm
Time, h (AM-PM) Time, h (AM-PM)
Time, h (AM-PM) Time, h (AM-PM)
Time, h (AM-PM) Time, h (AM-PM)
Still out put, l/m2 Still out put, l/m2
Still out put, l/m2 Still out put, l/m2
Temp.,oC Temp.oC
15° per h from morning to evening. Other design parameters were found as follows: Ab, 1 m2; Cb, 486 J/
kgK; Cg, 840 J/kgK; Cw, 4178 J/kgK; mb, 30 kg;
; ; ; ;
; ; ; ;
and . Variation in GTA had little effect on temperature profiles of glass and basin water, could be because of very low difference in WDs. The differences between experimental and numerical results obtained for productivity, glass and water temperatures could be because of neglecting reflected radiation from glass to atmosphere.
For lower WD (1 cm), maximum water temperature obtained experimentally in summer is found to be 61.8°C (Fig. 7a), while in winter it is 49.4°C (Fig. 8a). When compared with numerical results, there was an RMS error of 6.4% (Fig. 7a) and 6% (Fig. 8a) for winter. For higher WD (3 cm), maximum water temperatures obtained are 60.8°C for summer (Fig. 7b) and 44.2 °C (Fig. 8b) for winter with corresponding RMS error of 3.8% and 7%. Hence, from summer to winter, there is a reduction in water temperature by 20% for lower WD (1 cm) and 27% for higher WD (3 cm). Also, from summer to winter, there was a reduction in productivity by 55% with lower WD (1 cm) and 62% for higher WD (3 cm). High decrease in productivity for winter season can be attributed to cool winds blowing for most part of the season for this geographical location. Moreover, ambient temperature and solar radiation in winter is substantially less than summer. Thus productivity in summer is slightly more than double that of winter.
Performance of a solar still depends on ambient, operating and design conditions. With change in geographical location, solar radiation, ambient temperature, humidity, wind speed, sunshine hours also changes. Productivity of desalinated water of solar still will be less for the locations with more humidity (near sea) and the productivity will be more for arid regions because of abundant solar radiation.
Error Analysis
Minimum error occurred in any instrument is equal to the ratio between its least count and minimum value of the output measured. Accuracies and error% of various measuring instruments used in the experiments are as follows: thermocouple, ±10C, 0.25; Kipp–Zonen solarimeter, ±1 Wm-2, 0.25; anemometer, ±0.1 ms-1; and collection tank, ±10 ml, 10.
Cost Analysis
Payback period of experimental setup depends on overall cost of fabrication, cost of land, maintenance cost, operating cost and cost of feed water. Cost of feed water is negligible. Investment cost for still is $350 and cost of land is $50. Maintenance cost is $ 50/y. Productivity of solar still is 4.64 l/m2/day. Cost of water produced is the cost of water / l × productivity = 0.3125 × 4.64 = $ 1.45.
Cost of mineral per liter is $ 0.0071. Thus cost of minerals for 4.64 l is $ 0.033. Net earnings is the cost of water produced – maintenance cost “ cost of minerals = 1.45 – 0.136 “ 0.033 = $1.28. Payback period = investment/net earning = 400/1.28 = 312.5 days.
Fig. 8—Experimental and numerical comparison of water and glass temperatures in winter for slope angle 35° and water depth of: a) 1 cm;
and b) 3 cm
a) b)
Time, h (AM-PM) Time, h (AM-PM)
Temp., Temp.,
Conclusions
The best GTA for high performance of DS3 operating in eastern Saudi Arabian climatic conditions is 35° for both summer and winter seasons. The best WD for highest productivity in summer and winter is 1 cm. WD below 1 cm are not recommended as a lot of brine accumulation in base tank is expected based on experimental observations. Highest experimental productivity obtained on a typical day for summer and winter is 4.64 l/m2 and 2.10 l/m2, respectively. Numerical model is found to predict the experimental results with a tolerable RMS error of 5-10%. Hence, the model can be used as a reference to simulate the results for different climatic conditions and design parameters.
Acknowledgements
Authors thank KFUPM for providing facilities and support to carry out this research. Authors also acknowledge the Deanship of Scientific Research (DSR)-KFUPM for funding this project.
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