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An experimental study on modified simple solar stills
Energy Conversion & Management 40 (1999) 1835±1847
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An experimental study on modi®ed simple solar stills A.-J.N. Khalifa*, A.S. Al-Jubouri, M.K. Abed Solar Energy Research Center, Jadiriya, PO Box 13026, Baghdad, Iraq Received 9 July 1998; accepted 22 January 1999
Abstract An experimental study was conducted on new designs of basin type solar stills. Several single and double slope stills were constructed. Tests were conducted to show the eect of some modi®cations on the performance, such as productivity and eciency. The modi®cations examined included preheating of feed water by means of a solar heater and utilizing external and internal condensers for vapor condensation as well as for feed water preheating. The results showed improvements in the output and eciency of the solar stills due to the employment of the above modi®cations. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Solar stills; Performance; Modi®cations
1. Introduction Maximization of the production rate per unit basin area has been a prime objective in the development of simple solar distillers. Some of the most promising methods to achieve this goal are the use of extra condensation facilities, such as internal and external condensers and extra condensation surfaces. Many attempts have been made to improve the performance of simple solar stills. Collins and Thompson [1] made tests on forced and natural convection solar stills. A fan was used to bring the formed air-vapor mixture to an external heat exchanger coupled with the forced convection still, where the vapor was condensed by circulating cooling water in an open circuit. An air-vapor mixture ¯ow rate of 0.00269 m3/s was used. Malik et al. [2] reported a design of a composite system of a basin type solar still and a ¯at plate collector used for feed water
* Corresponding author. 0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 0 4 9 - 7
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Nomenclature Ac As E I Ih LH mÇ Q qa qu Ta Ti To w Y
eective area of solar collector, m2 eective basin area of still, m2 eciency, % solar radiation incident on inclined surface, W/m2 solar radiation incident on horizontal surface, W/m2 latent heat of evaporation, J/kg mass ¯ow rate of water, kg/min volume ¯ow rate of air-vapor mixture, m3/s solar power incident on solar heater, W solar power utilized by solar heater, W ambient temperature, 8C inlet water temperature to solar heater, 8C outlet water temperature from solar heater, 8C power consumed by fan, W still productivity, kg/hrm2
preheating. Many other attempts have been made to improve the performance of basin type solar stills by other methods, such as using dyes with the basin water [3], using extra re¯ectors to increase solar intensity [4] and cooling the glazing of the still [5]. This study describes several new designs of solar stills aimed at increasing the productivity and eciency. Simple stills were combined with a solar heater and internal and external condensers. 2. Description of the new designs Several solar stills were constructed, and these are grouped as shown below. The main features of each design are given in Table 1.
Fig. 1. Cross-sectional view of the still of group (A).
Still
shape
Modi®cation
Type of mounting
Base area (m2)
Type of feeding
A1 A2 A3 B1 B2 B3 C
Double-sloped = = = Single-sloped = =
± Combined Combined ± ± Combined Combined
Ground still = = Mounted still = = =
5.5 = = 0.91 = = 0.55
Batch feeding Continuous feeding = Batch feeding = Continuous feeding Continuous and batch feeding
a
with solar heater with external condenser with internal condenser (I) with internal condenser (II)
Tray material for stills A1, A2 and A3 is Asbestos coated with a layer of butyl-rubber, and galvanized steel painted black for stills B1, B2, B3 and C.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
Table 1 The main features of the investigated stillsa
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Stills of group A consist of three identical stills of the type shown in Fig. 1. Still A1 is a simple still; still A2 is a combination of the simple still A1 and a concentrated parabolic collector solar heater using copper heat pipes for feed water preheating. The details of this heater are shown in Fig. 2 and Appendix A. Still A3 is a combination of the simple still A1 and a 1-m long external condenser made of two concentric 5-mm thick glass pipes, of 55 and 75 mm inside diameters. Stills of group B consist of three stills, as shown in Fig. 3. Stills B1 and B2 are simple double slope and single slope stills, respectively, while B3 is a combination of the simple single slope still B2 and an eight pass internal condenser. The condenser was made of 10 mm diameter copper pipes each 1.2 m long. Still C is a single slope still ®tted with a double pass internal condenser made of 10 mm diameter copper pipes each 1 m long, as shown in Fig. 4. 3. Test procedure The eect of feed water preheating on the performance of a simple still is studied from the combination of a simple still with a solar heater (still A2). Dierent water ¯ow rates of preheated water were examined. The combination of a simple still and an external condenser (still A3) was designed to utilize the following operating principles: . Substituting forced convection for natural convection to increase the evaporation of water. This goal was achieved by circulating the air-vapor mixture by means of a fan from the simple still to the external water cooled condenser to gain eciency from a lower condensing temperature. Dierent ¯ow rates of circulating air-vapor mixtures were tested. . Recovery of energy by exchanging with feed water in an external exchanger while extracting condensate from the saturated vapor. Dierent ¯ow rates of feed water were tested. . Return of air from the heat exchanger to provide a saturated mixture to the still. The combination of a simple still and internal condensers (stills B3 and C) were designed to utilize the following operating principles:
Fig. 2. Cross-sectional view of the solar heater used for water preheating.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
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Fig. 3. Cross-sectional view of the stills of group B.
. Recovery of part of the latent heat of condensation for feed water preheating. . Creating a relative vacuum inside the still caused by the lower temperature of condensation. Still C was tested with and without the internal condenser (tests C1 and C2, respectively). The ¯ow rate of feed water was controlled to be approximately equal to that evaporated in order to maintain a steady water level of 10 mm in the basin.
Fig. 4. Cross-sectional view of still C.
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The ¯ow rate of water and air-vapor mixture were kept constant during any single test; however, dierent ¯ow rates were examined for dierent tests. Several measurements were conducted which included the productivity of each still, the incident solar radiation, the ambient temperature, the inlet and outlet water temperatures to and from the solar heater, the mass ¯ow rate of the cooling water (by means of a rotameter) and the average velocity of airvapor mixture (by means of a thermo-anemometer). The glass cover of all stills was sloped at 258 from the horizontal. All stills have an insulated base, and all joints were sealed by silicon rubber sealant. The tests were conducted on clear days.
4. Results and discussion The method used to calculate the eciency of each still is shown in Appendix B. To obtain the ideal ¯ow rate for the solar heater, tests were made for dierent ¯ow rates, as shown in Fig. 5. The maximum eciency was obtained at a ¯ow rate of about 0.3 kg/min. At this ¯ow rate, the collector inlet and outlet temperatures, ambient temperature and solar radiation were measured and plotted in Fig. 6. Fig. 7 shows the variation of the daily eciency of still A3 with mass ¯ow rate for dierent ¯ow rates of the air-vapor mixture. A volume ¯ow rate of 0.00572 m3/s gave the maximum
Fig. 5. Variation of daily eciency of the solar heater with mass ¯ow rate.
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Fig. 6. Variation with time of solar radiation, inlet and outlet temperature from the solar heater and ambient temperature.
eciency. The maximum eciency for all cases was found to be at a water ¯ow rate of about 0.3 kg/min. A comparison between the eciencies of the stills of group A is shown in Fig. 8. The eciency of still A3, using the ideal ¯ow rate given above, was found to give the best performance. The time variation of the accumulated productivity for the stills of group B is shown in Fig. 9. A feed water ¯ow rate of 0.021 kg/min was found to give the highest productivity. Fig. 10 shows the time variation of the solar radiation and the hourly eciency for the dierent stills of group B. It is clear that still B3, using the ideal ¯ow rate given above, gives the best performance. The daily eciencies of the stills of group B are given in Appendix C. The time variation of the hourly eciency for the two tests conducted on still C are shown in Fig. 11. It can be seen that the performance of the still with the condenser is better for most of the time. The daily eciencies for the tests conducted on this still are shown in Appendix D.
5. Conclusions 1. Still eciency can be improved by using extra condensation facilities and by preheating the feed water. The following improvements in the daily eciency of the stills were noticed:
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Fig. 7. Variation of daily eciency of still A3 with water mass ¯ow rate for dierent volume ¯ow rates of air-vapor mixture.
In comparison to the simple still A1, eciency was increased by about 14% when an external condenser was used (still A3) and by about 4% when the feed water was preheated by a solar heater (still A2). From the tests on the stills of group B, it was found that the eciency of the still with the internal condenser B3 is higher by about 33.8% than that of the reference still B2 and higher by about 57.6% than that of the simple double sloped still, B1. The eciency of still C was increased by about 8.7%, due to the use of the internal condenser. 2. The increase in eciency of the stills may be attributed to the increase in the convection currents inside the still. Such increase was achieved by a fan in the case of still A3 and by introducing a local relative vacuum caused by faster condensation on the water cooled internal condensers in the case of stills B3 and C. Preheating of feed water (in the case of still A2) caused earlier evaporation which means higher productivity at a relatively lower solar intensity. 3. The use of the modi®cations examined may be recommended when the improvement in the system performance osets the extra cost of the additional modi®cations of the simple still.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
Fig. 8. Variation of daily eciency with mass ¯ow rate for the stills of group A.
Fig. 9. Variation of accumulated productivity for still B3 with time for dierent mass ¯ow rates.
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Fig. 10. Variation of solar radiation and hourly eciency with time for the stills of group B.
Appendix A The main details of the solar heater used with still A2; refer also to Fig. 2 Part
Material
Type
Details
Re¯ector Absorber
Stainless steel Copper pipes, diameter=22 mm
Parabola Finned heat pipes
Glazing Insulation
Glass Styrofoam
Window type
Width=0.3 m, Length =1.5 m. Evaporater length=1 m, Condenser length=0.5 m, Working ¯uid:distilled water 3 mm thick 50 mm thick
Appendix B The methods used for calculating the eciencies of the dierent stills. Daily and hourly eciency of the solar heater:
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Fig. 11. Variation of hourly eciency with time for dierent tests conducted on still C.
E
qu qa
B1
Daily and hourly eciencies of the solar stills: Ðfor still A1, stills of group B and still C E
LH Y Ih
B2
Ðfor still A2 E
LH As Y As Ac Ih
Ðfor still A3
B3
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E
LH As Y Ih As w
B4
Appendix C Daily eciencies of the stills of group B Still B1
Still B2
Still B3
Ç kg/min m,
36.53 36.89 37.83 37.49 37.39
39.25 43.45 44.05 40.43 41.77
56.70 58.14 54.55 46.43 53.96
0.0175 0.0210 0.0500 0.0600 Average
Appendix D Summary of results carried out on still C Daily insolation (W/m2) Test C1 6720 6060 6146 6666 Test C2 6298 6520
Productivity (kg/m2day)
Daily eciency (%)
5.655 4.820 5.210 5.330
55.91 53.18 56.55 52.68
5.665 5.955
59.35 59.31
References [1] Collins R, Thompson T. Proceedings of the UN Conference on New Sources of Energy, Rome, 1961. p. 205± 217. [2] Malik M, Tiwari G, Kumar A, Sodha M. Solar distillation. Oxford: Pergamon Press, 1982.
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[3] Rajvanshi A. Solar Energy 1981;27:51±65. [4] Mousa M, Abdul Fatah F, Sakar I. In: Proceedings of the International Symposium on Solar Energy, Egypt, 1978. [5] Satcunanathan S, Hanson H. Solar Energy 1973;14:353.
www.elsevier.com/locate/enconman
An experimental study on modi®ed simple solar stills A.-J.N. Khalifa*, A.S. Al-Jubouri, M.K. Abed Solar Energy Research Center, Jadiriya, PO Box 13026, Baghdad, Iraq Received 9 July 1998; accepted 22 January 1999
Abstract An experimental study was conducted on new designs of basin type solar stills. Several single and double slope stills were constructed. Tests were conducted to show the eect of some modi®cations on the performance, such as productivity and eciency. The modi®cations examined included preheating of feed water by means of a solar heater and utilizing external and internal condensers for vapor condensation as well as for feed water preheating. The results showed improvements in the output and eciency of the solar stills due to the employment of the above modi®cations. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Solar stills; Performance; Modi®cations
1. Introduction Maximization of the production rate per unit basin area has been a prime objective in the development of simple solar distillers. Some of the most promising methods to achieve this goal are the use of extra condensation facilities, such as internal and external condensers and extra condensation surfaces. Many attempts have been made to improve the performance of simple solar stills. Collins and Thompson [1] made tests on forced and natural convection solar stills. A fan was used to bring the formed air-vapor mixture to an external heat exchanger coupled with the forced convection still, where the vapor was condensed by circulating cooling water in an open circuit. An air-vapor mixture ¯ow rate of 0.00269 m3/s was used. Malik et al. [2] reported a design of a composite system of a basin type solar still and a ¯at plate collector used for feed water
* Corresponding author. 0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 0 4 9 - 7
1836
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
Nomenclature Ac As E I Ih LH mÇ Q qa qu Ta Ti To w Y
eective area of solar collector, m2 eective basin area of still, m2 eciency, % solar radiation incident on inclined surface, W/m2 solar radiation incident on horizontal surface, W/m2 latent heat of evaporation, J/kg mass ¯ow rate of water, kg/min volume ¯ow rate of air-vapor mixture, m3/s solar power incident on solar heater, W solar power utilized by solar heater, W ambient temperature, 8C inlet water temperature to solar heater, 8C outlet water temperature from solar heater, 8C power consumed by fan, W still productivity, kg/hrm2
preheating. Many other attempts have been made to improve the performance of basin type solar stills by other methods, such as using dyes with the basin water [3], using extra re¯ectors to increase solar intensity [4] and cooling the glazing of the still [5]. This study describes several new designs of solar stills aimed at increasing the productivity and eciency. Simple stills were combined with a solar heater and internal and external condensers. 2. Description of the new designs Several solar stills were constructed, and these are grouped as shown below. The main features of each design are given in Table 1.
Fig. 1. Cross-sectional view of the still of group (A).
Still
shape
Modi®cation
Type of mounting
Base area (m2)
Type of feeding
A1 A2 A3 B1 B2 B3 C
Double-sloped = = = Single-sloped = =
± Combined Combined ± ± Combined Combined
Ground still = = Mounted still = = =
5.5 = = 0.91 = = 0.55
Batch feeding Continuous feeding = Batch feeding = Continuous feeding Continuous and batch feeding
a
with solar heater with external condenser with internal condenser (I) with internal condenser (II)
Tray material for stills A1, A2 and A3 is Asbestos coated with a layer of butyl-rubber, and galvanized steel painted black for stills B1, B2, B3 and C.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
Table 1 The main features of the investigated stillsa
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Stills of group A consist of three identical stills of the type shown in Fig. 1. Still A1 is a simple still; still A2 is a combination of the simple still A1 and a concentrated parabolic collector solar heater using copper heat pipes for feed water preheating. The details of this heater are shown in Fig. 2 and Appendix A. Still A3 is a combination of the simple still A1 and a 1-m long external condenser made of two concentric 5-mm thick glass pipes, of 55 and 75 mm inside diameters. Stills of group B consist of three stills, as shown in Fig. 3. Stills B1 and B2 are simple double slope and single slope stills, respectively, while B3 is a combination of the simple single slope still B2 and an eight pass internal condenser. The condenser was made of 10 mm diameter copper pipes each 1.2 m long. Still C is a single slope still ®tted with a double pass internal condenser made of 10 mm diameter copper pipes each 1 m long, as shown in Fig. 4. 3. Test procedure The eect of feed water preheating on the performance of a simple still is studied from the combination of a simple still with a solar heater (still A2). Dierent water ¯ow rates of preheated water were examined. The combination of a simple still and an external condenser (still A3) was designed to utilize the following operating principles: . Substituting forced convection for natural convection to increase the evaporation of water. This goal was achieved by circulating the air-vapor mixture by means of a fan from the simple still to the external water cooled condenser to gain eciency from a lower condensing temperature. Dierent ¯ow rates of circulating air-vapor mixtures were tested. . Recovery of energy by exchanging with feed water in an external exchanger while extracting condensate from the saturated vapor. Dierent ¯ow rates of feed water were tested. . Return of air from the heat exchanger to provide a saturated mixture to the still. The combination of a simple still and internal condensers (stills B3 and C) were designed to utilize the following operating principles:
Fig. 2. Cross-sectional view of the solar heater used for water preheating.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
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Fig. 3. Cross-sectional view of the stills of group B.
. Recovery of part of the latent heat of condensation for feed water preheating. . Creating a relative vacuum inside the still caused by the lower temperature of condensation. Still C was tested with and without the internal condenser (tests C1 and C2, respectively). The ¯ow rate of feed water was controlled to be approximately equal to that evaporated in order to maintain a steady water level of 10 mm in the basin.
Fig. 4. Cross-sectional view of still C.
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The ¯ow rate of water and air-vapor mixture were kept constant during any single test; however, dierent ¯ow rates were examined for dierent tests. Several measurements were conducted which included the productivity of each still, the incident solar radiation, the ambient temperature, the inlet and outlet water temperatures to and from the solar heater, the mass ¯ow rate of the cooling water (by means of a rotameter) and the average velocity of airvapor mixture (by means of a thermo-anemometer). The glass cover of all stills was sloped at 258 from the horizontal. All stills have an insulated base, and all joints were sealed by silicon rubber sealant. The tests were conducted on clear days.
4. Results and discussion The method used to calculate the eciency of each still is shown in Appendix B. To obtain the ideal ¯ow rate for the solar heater, tests were made for dierent ¯ow rates, as shown in Fig. 5. The maximum eciency was obtained at a ¯ow rate of about 0.3 kg/min. At this ¯ow rate, the collector inlet and outlet temperatures, ambient temperature and solar radiation were measured and plotted in Fig. 6. Fig. 7 shows the variation of the daily eciency of still A3 with mass ¯ow rate for dierent ¯ow rates of the air-vapor mixture. A volume ¯ow rate of 0.00572 m3/s gave the maximum
Fig. 5. Variation of daily eciency of the solar heater with mass ¯ow rate.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
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Fig. 6. Variation with time of solar radiation, inlet and outlet temperature from the solar heater and ambient temperature.
eciency. The maximum eciency for all cases was found to be at a water ¯ow rate of about 0.3 kg/min. A comparison between the eciencies of the stills of group A is shown in Fig. 8. The eciency of still A3, using the ideal ¯ow rate given above, was found to give the best performance. The time variation of the accumulated productivity for the stills of group B is shown in Fig. 9. A feed water ¯ow rate of 0.021 kg/min was found to give the highest productivity. Fig. 10 shows the time variation of the solar radiation and the hourly eciency for the dierent stills of group B. It is clear that still B3, using the ideal ¯ow rate given above, gives the best performance. The daily eciencies of the stills of group B are given in Appendix C. The time variation of the hourly eciency for the two tests conducted on still C are shown in Fig. 11. It can be seen that the performance of the still with the condenser is better for most of the time. The daily eciencies for the tests conducted on this still are shown in Appendix D.
5. Conclusions 1. Still eciency can be improved by using extra condensation facilities and by preheating the feed water. The following improvements in the daily eciency of the stills were noticed:
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Fig. 7. Variation of daily eciency of still A3 with water mass ¯ow rate for dierent volume ¯ow rates of air-vapor mixture.
In comparison to the simple still A1, eciency was increased by about 14% when an external condenser was used (still A3) and by about 4% when the feed water was preheated by a solar heater (still A2). From the tests on the stills of group B, it was found that the eciency of the still with the internal condenser B3 is higher by about 33.8% than that of the reference still B2 and higher by about 57.6% than that of the simple double sloped still, B1. The eciency of still C was increased by about 8.7%, due to the use of the internal condenser. 2. The increase in eciency of the stills may be attributed to the increase in the convection currents inside the still. Such increase was achieved by a fan in the case of still A3 and by introducing a local relative vacuum caused by faster condensation on the water cooled internal condensers in the case of stills B3 and C. Preheating of feed water (in the case of still A2) caused earlier evaporation which means higher productivity at a relatively lower solar intensity. 3. The use of the modi®cations examined may be recommended when the improvement in the system performance osets the extra cost of the additional modi®cations of the simple still.
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
Fig. 8. Variation of daily eciency with mass ¯ow rate for the stills of group A.
Fig. 9. Variation of accumulated productivity for still B3 with time for dierent mass ¯ow rates.
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Fig. 10. Variation of solar radiation and hourly eciency with time for the stills of group B.
Appendix A The main details of the solar heater used with still A2; refer also to Fig. 2 Part
Material
Type
Details
Re¯ector Absorber
Stainless steel Copper pipes, diameter=22 mm
Parabola Finned heat pipes
Glazing Insulation
Glass Styrofoam
Window type
Width=0.3 m, Length =1.5 m. Evaporater length=1 m, Condenser length=0.5 m, Working ¯uid:distilled water 3 mm thick 50 mm thick
Appendix B The methods used for calculating the eciencies of the dierent stills. Daily and hourly eciency of the solar heater:
A.-J.N. Khalifa et al. / Energy Conversion & Management 40 (1999) 1835±1847
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Fig. 11. Variation of hourly eciency with time for dierent tests conducted on still C.
E
qu qa
B1
Daily and hourly eciencies of the solar stills: Ðfor still A1, stills of group B and still C E
LH Y Ih
B2
Ðfor still A2 E
LH As Y As Ac Ih
Ðfor still A3
B3
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E
LH As Y Ih As w
B4
Appendix C Daily eciencies of the stills of group B Still B1
Still B2
Still B3
Ç kg/min m,
36.53 36.89 37.83 37.49 37.39
39.25 43.45 44.05 40.43 41.77
56.70 58.14 54.55 46.43 53.96
0.0175 0.0210 0.0500 0.0600 Average
Appendix D Summary of results carried out on still C Daily insolation (W/m2) Test C1 6720 6060 6146 6666 Test C2 6298 6520
Productivity (kg/m2day)
Daily eciency (%)
5.655 4.820 5.210 5.330
55.91 53.18 56.55 52.68
5.665 5.955
59.35 59.31
References [1] Collins R, Thompson T. Proceedings of the UN Conference on New Sources of Energy, Rome, 1961. p. 205± 217. [2] Malik M, Tiwari G, Kumar A, Sodha M. Solar distillation. Oxford: Pergamon Press, 1982.
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[3] Rajvanshi A. Solar Energy 1981;27:51±65. [4] Mousa M, Abdul Fatah F, Sakar I. In: Proceedings of the International Symposium on Solar Energy, Egypt, 1978. [5] Satcunanathan S, Hanson H. Solar Energy 1973;14:353.