Effect of glass slope angle and water depth on productivity of double slope solar still

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1 884 Journal of Scientific & Industrial Research J SCI IND RES VOL 70 OCTOBER 2011 Vol. 70, October 2011, pp Effect of glass slope angle water depth on productivity of double slope solar still Ahmed Z Al-Garni*, Ayman H Kassem, Farooq Saeed 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 testing of double slope solar still also optimization of glass tilt angle (25, 30, ) water depth (1, 2 3 cm) in Saudi Arabian climatic conditions. Optimum tilt angle was found to be 35 for both summer winter seasons, also productivity increases with reduction in water depth. A number of existing energy mass balance equations were used to validate experimental results. Numerical 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 desert areas having sweet water scarcity problems. Critical parameters related to productivity of solar still are glass tilt angle (GTA) water depth (WD). Singh et al 1 carried out a numerical analysis on active passive single slope solar still (S4) considering effect of solar intensity, wind velocity, WD, GTA on productivity. Kumar et al 2 observed that annual performance of an active S4 is optimum at GTA of 15 in Indian conditions. Kamal 3 analyzed double slope solar still (DS3) for Doha (Qatar) climatic conditions found a GTA of 10 for summer 15 for winter to obtain high quantity of distilled water. Enein et al 4 reported that GTA of a S4 should be as low as possible in summer 50 in winter for Egyptian conditions. Nafey et al 5 found a similar trend in the results as obtained by Enein et al 4. A thermal analysis 6 was done to optimize glass cover inclination of S4 for maximum yield in Indian conditions. Al-Hinai et al 7 predicted performance of DS3 in Omani climatic conditions that productivity increases with decrease in GTA in summer vice versa in winter. Dev et al 8 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 *Author for correspondence algarni@kfupm.edu.sa obtained by earlier studies 4-7. Akash et al 9 found that 35 GTA of DS3 gives maximum yield in May in Jordan. Elkader 10 obtained similar results with 35 GTA giving maximum yield. Several studies 2,3,5,7-9 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 WD 3,7. Although a lot of studies have been done on GTA optimization in different countries, but results are contradictory 1,3,4,8. The present study is partial implementation of two patents 11,12, which have been submitted on solar distillation. This study optimizes GTA 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 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 water while a little radiation is again reflected. Heat gained by basin water is transferred to inner glass cover by convection, radiation, evaporation of water. Some amount of heat gained by glass cover in this process is lost to the atmosphere by convection radiation. Equation for conservation of mass is written as 13. Energy balance equation for glass cover is given as 14

2 AL-GARNI et al: EFFECT OF GLASS SLOPE ANGLE AND WATER DEPTH ON PRODUCTIVITY OF DOUBLE SLOPE SOLAR STILL 885 (1) (2) Energy balance equation for basin water contained in it is given as 14 (3) (4) Eqs. (1) (3) are solved simultaneously for T w T g. 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 basin material; 3) Bottom sides of basin are well insulated, thus are negligible; 4) Reflection of heat from water surface 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 as 13. Solar radiation on a tilted surface is a combination of three components is calculated as 15 For northern hemisphere, geometric factor R beam is calculated using 15 angle is found as 15. Declination Heat is transferred from water to glass surface by convection of air trapped inside still, evaporation of water, radiation of heat from water surface. HT is estimated as 13,. Convective evaporative HT coefficients for water to glass surface are calculated respectively, as 13,14 Partial vapor pressure at a given basin water temperature glass surface are calculated as 16.. Specific heat of air trapped inside solar still is written in terms of average temperature of basin water glass as 17. Latent heat of evaporation of water is found using 18. Some amount of heat is absorbed by glass due to incident solar radiation falling on glass surface. This can be calculated as 19. Solar radiation incident on still is absorbed in huge amounts by blackened base water. Heat absorbed by water is estimated using 19. Heat lost from glass cover to atmosphere is found by 13 Convective HT coefficient from glass cover to atmosphere is given by 20. Heat added to the system by the supply of feed water is written as 13. Heat loss from the system due to distillate leaving the still is estimated by 13. Heat due to blow down in base tank is given as 13. Experimental Set up Four units of DS3 (GTA: 25, 30, ) were designed 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

3 886 J SCI IND RES VOL 70 OCTOBER 2011 Fig. 1 Fabricated double slope solar stills with cover slope angles of 25, 30, in order layer of black paint was applied on inner outer sides of base tank so as to improve absorptivity of tank. Silicon rubber was used to seal glass covers base tank since silicon has good bonding between glass many other materials. L shaped aluminum strips were fixed on all the edges of glass base tank to provide solidity to solar still. A distillate channel is provided in base tank so that condensed water flows through this channel 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 temperatures of glass water in the basin. Ambient temperature, wind speed direction were also noted every hour. Experiments were conducted in an open ground in KFUPM campus, Dhahran (26 16' N, ' E), a city in the eastern province of Saudi Arabia, during summer (June) winter seasons (December). Effect of GTA WD on the still performance was studied compared. Results Discussion Effect of Glass Tilt Angle (GTA) In summer (June) with 1 cm WD, productivity of still increased from 25 to 35 GTA (Fig. 2 then decreased. Hence at optimum GTA (35 ), maximum productivity is 4.64 l/m 2. This is in good agreement to a very recent study 8. Productivity increased by 2.6% from 25 to 30 GTA 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/m 2 (Fig. 2. This result is also in Cover tilt angle, o Cover tilt angle, o Fig. 2 Accumulated productivity of solar still for different GTA in: summer; winter agreement with reported study 8. 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. 3, as also reported 2,3,5,7-9. 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% 6.7% when WD was increased from 1-2 cm, 2-3 cm, respectively. For winter (December) also, as WD was increased from 1 cm to 3 cm, productivity decreased (Fig. 3. A maximum output of (2.1 l/m 2 ) was obtained at 1 cm depth for 35 GTA. Output reduced by 13.8% 16% for 2 cm 3 cm WDs respectively. In hourly variation of still output for a typical day in summer (June), '

4 AL-GARNI et al: EFFECT OF GLASS SLOPE ANGLE AND WATER DEPTH ON PRODUCTIVITY OF DOUBLE SLOPE SOLAR STILL 887 Water depth, cm Water depth, cm Fig. 3 Effect of water depth on productivity of solar still with various slope angles in: summer; winter Still out put, ml/m 2 Still out put, ml/m 2 Fig. 4 Hourly variation of solar still productivity for a typical day in: summer; winter productivity increases from morning to noon, trend is reversed thereafter (Fig. 4; 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 2 PM (Fig. 4. 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 solar radiation. Numerical Results Geographical parameters of experimental site were as follows: latitude, N; longitude, 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); wind velocity for winter, 2 m/s (average velocity for a typical day in December). A computer program was written in MATLAB software solved using ode23 function. Simulation was carried out for all the cases with varying GTAs WDs. A sample result of productivity for optimum GLA WD is shown for summer (Fig. 5) winter (Fig. 6) seasons. A comparison between experimental numerical temperature profiles for basin water glass temperatures is also shown for summer (Fig. 7) winter (Fig. 8) seasons. It is found that simulation results are in good agreement with experimental results. In numerical calculations, mass 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

5 888 J SCI IND RES VOL 70 OCTOBER 2011 Fig. 5 Experimental numerical comparison of daily productivity in summer for slope angle 35 water depth of: 1 cm; 3cm Fig. 6 Experimental numerical comparison of daily productivity in winter for slope angle 35 water depth of: 1 cm; 3 cm Temp., o C Temp.o C Fig. 7 Experimental numerical comparison of water glass temperatures in summer for slope angle 35 water depth of: 1 cm; 3 cm

6 AL-GARNI et al: EFFECT OF GLASS SLOPE ANGLE AND WATER DEPTH ON PRODUCTIVITY OF DOUBLE SLOPE SOLAR STILL 889 Temp., o C Temp., o C Fig. 8 Experimental numerical comparison of water glass temperatures in winter for slope angle 35 water depth of: 1 cm; 3 cm 15 per h from morning to evening. Other design parameters were found as follows: A b, 1 m 2 ; C b, 486 J/ kgk; C g, 840 J/kgK; C w, 4178 J/kgK; m b, 30 kg; ; ; ; ; ; ; ; ;. Variation in GTA had little effect on temperature profiles of glass basin water, could be because of very low difference in WDs. The differences between experimental numerical results obtained for productivity, glass 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. 7, while in winter it is 49.4 C (Fig. 8. When compared with numerical results, there was an RMS error of 6.4% (Fig. 7 6% (Fig. 8 for winter. For higher WD (3 cm), maximum water temperatures obtained are 60.8 C for summer (Fig C (Fig. 8 for winter with corresponding RMS error of 3.8% 7%. Hence, from summer to winter, there is a reduction in water temperature by 20% for lower WD (1 cm) 27% for higher WD (3 cm). Also, from summer to winter, there was a reduction in productivity by 55% with lower WD (1 cm) 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 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 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 se 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 minimum value of the output measured. Accuracies error% of various measuring instruments used in the experiments are as follows: thermocouple, ±1 0 C, 0.25; Kipp Zonen solarimeter, ±1 Wm -2, 0.25; anemometer, ±0.1 ms -1 ; collection tank, ±10 ml, 10. Cost Analysis Payback period of experimental setup depends on overall cost of fabrication, cost of l, maintenance cost, operating cost cost of feed water. Cost of feed water is negligible. Investment cost for still is $350 cost of l is $50. Maintenance cost is $ 50/y. Productivity of solar still is 4.64 l/m 2 /day. Cost of water produced is the cost of water / l productivity = = $ Cost of mineral per liter is $ Thus cost of minerals for 4.64 l is $ Net earnings is the cost of water produced maintenance cost cost of minerals = = $1.28. Payback period = investment/net earning = 400/1.28 = days.

7 890 J SCI IND RES VOL 70 OCTOBER 2011 Conclusions The best GTA for high performance of DS3 operating in eastern Saudi Arabian climatic conditions is 35 for both summer winter seasons. The best WD for highest productivity in summer 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 winter is 4.64 l/m l/m 2, 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 design parameters. Acknowledgements Authors thank KFUPM for providing facilities support to carry out this research. Authors also acknowledge the Deanship of Scientific Research (DSR)-KFUPM for funding this project. References 1 Singh H & Tiwari G, Monthly performance of passive active solar stills for different Indian climatic conditions. Desalination, 168 (2004) Kumar S, Tiwari G & Singh H, Annual performance of an active solar distillation system, Desalination, 127 (2000) Kamal W, A theoretical experimental study of the basin-type solar still under the arabian gulf climatic conditions, Solar Wind Technol, 5 (1988) Aboul-Enein S, El-Sebaii A & El-Bialy E, Investigation of a single-basin solar still with deep basins, Renewab Energy, 14 (1998) Nafey A, Abdelkader M, Abdelmotalip A & Mabrouk A, Parameters affecting solar still productivity, Energy Convers Mgmt, 41 (2000) Tiwari G, Thomas J & Khan E, Optimisation of glass cover inclination for maximum yield in a solar still, Heat Recovery Syst & CHP, 14 (1994) Al-Hinai H, Al-Nassri M & Jubran B, Effect of climatic, design operational parameters on the yield of a simple solar still, Energy Convers Mgmt, 43 (2002) Dev R & Tiwari G, Characteristic equation of a passive solar still, Desalination, 245 (2009) Akash B, Mohsen M, Osta O & Elayan Y, Experimental evaluation of a single-basin solar still using different absorbing materials, Renewab Energy, 14 (1998) Elkader A, An investigation of the parameters involved in simple solar still with inclined yute, Renewab Energy, 14 (1998) Al-Garni A, Kassem A & Saeed F, Double action solar distiller, US Pat Appl No A1, 3 April Al-Garni A, Saeed F & Kassem A, Wind-solar desalination farm park system, US Pat No. 7,771,568 B2, 10 April Malik M, Tiwari G, Kumar A & Sodha M, Solar Distillation (Pergamon Press, Oxford) Dunkle R. V, Solar water distillation: the roof type still a multiple effect diffusion still, international developments in HT, in ASME Proc of Int HT, part V (University of Colorado, New York) Duffie J & Beckman W, Solar Engineering of Thermal Processes (John Wiley & Sons, Inc., New York) Toure S & Meukam P, A numerical model experimental investigation for a solar still in climatic conditions in Abidjan (Côte d Ivoire), Renewab Energy, 11 (1997) Zurigat Y & Abu-Arabi M, Modelling performance analysis of a regenerative solar desalination unit, Appl Therm Engg, 24 (2004) Fath H & Hosny H, Thermal performance of a single-sloped basin still with an inherent built-in additional condenser, Desalination, 142 (2002) Murugavel K K, Sivakumar S, Riaz Ahamed J, Chockalingam K & Srithar K, Single basin double slope solar still with minimum basin depth energy storing materials, Appl Energy, 87 (2010) Watmuff J H, Charters W W S & Proctor D, Solar wind induced external coefficients solar collectors, Revue Int Helio tech, 2 (1977) 56.