A STUDY ON PERFORMANCE OF SOLAR SEAWATER DISTILLATION SYSTEM WITH AN AUXILIARY HEAT SOURCE FROM SOLAR COLLECTOR

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1 L_L A STUDY ON PERFORMANCE OF SOLAR SEAWATER DISTILLATION SYSTEM WITH AN AUXILIARY HEAT SOURCE FROM SOLAR COLLECTOR Pakorn Promkaew,* Sirichai Thepa, Roongrojana Songprakorp Solar Energy in Agriculture Laboratory, Department of Energy Technology, Faculty of Energy, Environment and Materials, King Mongkut s University of Technology Thonburi, Bangkok 10140, Thailand * jame_49me@hotmail.com Abstract: In this research, a pyramid-shaped solar still with Thermosyphon type flat plate solar collector was designed and constructed to examine the distillation rate, heat transfer coefficient and overall efficiency of this passive solar distillation system. The pyramidshaped solar still has a mm 2 square basin and a 50 mm maximum set-level of seawater depth. The base is insulated with 25 mm thick polyurethane foam. The top of solar still is covered by four 3 mm triangle clear glasses formed in pyramid shape with 35 tilted angle. Fin-tubes are installed at the bottom of the still base as a heat exchanger in which hot water from flat plate solar collector flowing through and transfer the heat to seawater. This experiment was carried out between 8.00 a.m. to 5.00 p.m. It was found that water temperature, glass temperature, ambient and inlet water temperatures of solar still affect distillation rate and heat transfer coefficient. The maximum of distillation rate was 7.3 L/m 2 day under solar radiation of 18.5 MJ/m 2 day. Evaporative heat transfer coefficient of water to inner glass cover depends highly on water temperature and solar radiation. Besides, convective heat transfer coefficients for glass cover, solar still and surrounding depend highly on velocity wind. The efficiency of seawater distillation system of 68.34% was observed. Introduction: Water is essential for sustaining human life. Although the three fourths of the world are covered by water, unfortunately, majority is seawater, which cannot be consumed directly. People living on islands surrounded by seawater have direct impact in terms of need to purchase water from other sources that is costly as well as difficulty in transportation. To reduce these problems producing freshwater from seawater can solve this lack of freshwater. Seawater desalination is an emerging alternative for this and such a technology has been already developed. The two important technologies are based on Multi-stage Flash and Reverse Osmosis. 1 In 2003 Fath et al. reported on thermal-economic analysis and the performance comparison of pyramid-shaped and single-slope solar still configurations. The study showed that single-slope solar still has efficiency and economic slightly more than pyramid-shaped solar still, however, pyramid-shaped solar still of which the sides were tilted 50 has the maximum quantity of distilled water. 2 In 2004 Voropoulos et al. demonstrated a double-sloped solar still with auxiliary hot water storage tank to enhance effective distillation. 3 Bodran et al. studied the efficiency of pyramid-shaped and double-slope solar still configurations with auxiliary flat plate solar collector. 4 In 2005 Badran and Al-Tahaineh studied the effect of flat plate solar collector coupling with a solar still in distillation process and found that it has increased the productivity by 36%. 5 Although some works mentioned above have already done on adopting solar collector to work with solar still, most are focused on distillation of fresh water. On the other hand, in this work, we present a design of water distillation system that is suitable for seawater distillation. We have constructed pyramid-shaped solar still with an auxiliary solar collector in water distillation process. The hot water in the solar collector is circulated by Thermosyphon. Fintubes were installed in pyramid-shaped solar still for hot water from flat plate solar collector flowing through and transferring heat to seawater. In our system, hot water storage tank was installed in distillation system.

2 L_L Methodology: The experiment apparatus consists of pyramid-shaped solar still and Thermosyphon type solar hot water system. The schematic diagram of the system is shown in Fig. 1. Figure 1 Experimental setup of seawater distillation system The pyramid-shaped solar still is made of a mm square basin with a 50 mm seawater depth. The basin was insulated with 25 mm thick polyurethane foam. The cover is a pyramid-shaped clear glass of 3 mm thickness. Each side was tilted at 35. Fin-tube heat exchanger is installed at the bottom of the pyramid-shaped solar still for hot water from flat plate solar collector flowing through. The size of the flat plate solar collector is m. The volume of hot water storage tank is 63 L and it is insulated with 25 mm PU foam. Solar radiation is measured by a pyranometer (Kipp and Zonen, Holland, model CM11, Sensitivity = 5.24x10-6 V/Wm -2 ). Wind velocity is measured by a digital anemometer (Testo 435) installed in a horizontal position near the glass cover. Temperatures at various positions within the pyramid-shaped solar still and the flat plate solar collector were measured by thermocouples (Type K, accuracy ± 0.5 C). Temperature and radiation were recorded by Data logger (YOKOGAWA) model DX every 2 seconds. The pyramid-shaped solar still and flat plate solar collector were fixed at an angle 14 facing south. The experiment was performed between 8.00 a.m. to 5.00 p.m. and testing was done on a batch basis. The aim of experiments was to determine the distillation rate, heat transfer coefficient and efficiency of seawater distillation system. Results, Discussion and Conclusion: Fig. 2 shows the resulting data obtained from the experiment. The temperatures at various points and distilled water mass flow rate are plotted against the corresponding time of experiment. Later on those data have been used to determine the distillation rate, heat transfer coefficient and efficiency of seawater distillation system. As a result, the distillation rate increases gradually and reach the maximum value of 1.6 L/m 2 h at 1 p.m. After that distillation rate decreases gradually. Distillation rate depends upon heat transfer from hot water from flat plate solar collector to seawater through fin-tubes. Thus, heat was accumulated inside the pyramid-shaped solar still. This result in water temperature rise, of which a maximum temperature was 82.4 C and different temperature between water temperature and glass temperature, was attributed to water condensation at the inner glass surface. The ambient temperature was in a range of 32.7 to 35.9 C, which affected the rate of heat loss from the solar still. Consequently, it can be concluded that the difference of temperature between solar still and ambient temperatures affect distillation rate as shown in Fig. 2.

3 L_L Figure 2 Water temperature, glass temperature, ambient temperature, inlet water temperature, outlet water temperature and distillation rate vs. time (24/6/12) Figure 3 Heat transfer coefficient vs. time (24/6/12) Fig. 3 presents a comparison of heat transfer coefficient at various times. The maximum of evaporative heat transfer coefficient (h ew ) and convective heat transfer coefficient between water and air inside solar still (h cw ) were W/m 2 h and W/m 2 h, respectively. Both evaporative heat transfer and convective heat transfer coefficients between water and air inside solar still varied with water temperature and solar radiation. On the other hand, the maximum of convective heat transfer coefficient between glass cover and surrounding (h cg ) and convective heat transfer coefficient between solar still and surrounding (h cb ) were 5.29 W/m 2 h and W/m 2 h, respectively. Both convective heat transfer coefficient between glass cover and surrounding and convective heat transfer coefficient between solar still and surrounding were varied by wind velocity.

4 L_L Figure 4 Heat transfer rate and distillation rate at various times (24/6/12) In Fig. 4, it is obvious that the distillation rate is depended on evaporative heat transfer rate and it varies with time. The first stage shows that heat transfer rate and distillation rate were low due to less difference in water, glass and ambient temperatures. However, an increase of solar radiation rises up water, glass and ambient temperatures in different rate resulting in temperature differences. Therefore, heat transfer rate and distillation rate were different. The maximum distillation rate was 1.6 L/m 2 h at 1.00 p.m. as maximum difference between water temperature and glass temperature. Hence, the evaporative heat transfer rate between water and inner glass cover (Q e ) was highest followed with the irradiative heat transfer rate (Q r ). Convective heat transfer rate (Q c ) has less affect because the convective heat transfer coefficient between water and air inside solar still was lowest. On the other hand, at 1.00 p.m. it can be seen that the rate of heat loss by radiation between glass and surrounding (Q ra ) was highest because difference between glass temperature and ambient temperature was highest. However, the rate of heat loss by convection between glasses and surrounding (Q ca ) was low as the bottom and all four sides of the basin were insulated to reduce the heat losses to the surrounding (Q s ). Figure 5 the overall efficiency of solar seawater distillation system and distillation rate at various times (24/6/12)

5 L_L Fig. 5 presents the overall efficiency of solar seawater distillation system and distilled water mass flow rate against time. The distillation rate is depended on solar radiation by which water temperature is increased resulting in more water evaporated and hence increased distillation rate. During p.m. distillation rate decrease is observed meanwhile the overall efficiency of solar seawater distillation system increases gradually. Although solar radiation decreases the amount of accumulated heat can distil water in the still. In conclusion, this study can point out the importance of factors influencing the solar still efficiency as follows: 1. The variation of water temperature (T w ), glass temperature (T g ), ambient temperature (T a ) and inlet water temperature (T wi,still ) affected distillation rate (m*) 2. The maximum of distillation rate of the pyramid-shaped solar still with an auxiliary flat plate solar collector was 7.3 L/m 2 h as solar radiation was 18.5 MJ/m 2 day. 3. Both evaporative heat transfer coefficient and convective heat transfer coefficient between water and air inside solar still were varied with water temperature and solar radiation. 4. Both convective heat transfer coefficient between glass cover and surrounding and convective heat transfer coefficient between solar still and surrounding were varied by wind velocity. 5. The overall efficiency of solar seawater distillation system was 68.34%. References: 1. Khawaji, A.D., Kutubkhanah, I.K. and Wie, J.M. Desalination 2008;221: Fath, H.E.S., El-Samanoudy, M., Fahmy, K. and Hassaba, A. Desalination 2003;159: Voropoulos, K., Mathioulakis, E. and Belessiotis, V. Desalination 2004;164: Badran, A. A., Al-Hallaq, A.A., Eyal Salman, I. A. and Odat, M. Z. Desalination 2004;172: Badran, O.O. and Al-Tahaineh, H.A. Desalination 2005;183: Keywords: pyramid-shaped solar still, thermosyphon, solar seawater distillation system