CHAPTER 5 PERFORMANCE ENHANCEMENT STUDY FOR SOLAR STILL

Transcription

1 82 CHAPTER 5 PERFORMANCE ENHANCEMENT STUDY FOR SOLAR STILL In this chapter, the performance enhancement studies are evaluated based on the experimental data collected by considering the effect of the following parameters on the output, temperatures of various components, difference between water and glass temperatures etc., in detail. 1. Effect of water depth 2. Effect of sponge liner thickness 3. Effect of sponge liner colour 4. Effect of energy storage materials 5. Combined effect of energy storage material and sponge liner The salient points which are necessary for the analysis are discussed in the following sections. 5.1 EFFECT OF WATER DEPTH IN THE BASIN The main objective of this study is to find the suitable water depth of the typical solar still in the conventional mode operation for conducting the remaining experimental studies with modifications like sponge liner, energy storage materials, coloured sponge liner etc. This experiment is performed during April 2008 May 2008 under the same climatic conditions by varying the water depth in the basin, like 10, 20, 30, 40, 50 and 60 mm. The hourly average solar intensity and ambient temperature for different water depths are

2 83 given in Table 5.1. It can be noted from the Table 5.1 that the productivity is high when ambient temperature is high. This may be due to reduction in convection heat loss from glass surface to ambient by decreasing the temperature difference between glass surface and ambient. Table 5.1 Hourly average solar intensity and ambient temperature for different water depths Water depth (mm) Solar intensity, W/m 2 h Ambient Temperature Mass of distilled output Figure 5.1 Hourly variation of water temperature for different water depths

3 84 The temperatures of water, glass and vapour observed hourly for different water depths have been shown in Figures respectively. The hourly difference in water and glass temperature, i.e. ΔT is also shown in Figure 5.4, for all water depths concerned. It is explicit that the temperatures of water, glass and vapour decreases with increase in water depths due to the storage effect of basin water as expected. Further, the maxima of these temperatures are shifted to later hours for higher water depths. Figure 5.2 Hourly variation of basin liner temperature for different water depths Figure 5.4 shows the variations of ΔT throughout the day for different water depths in the still. It is clear that, during the morning hours glass encounters the radiation first and its temperature rises very fast in comparison to the rise in water temperature and as a result, ΔT becomes negative. This remains negative till water temperature does not supersede glass temperature. The higher the depth, the more the time taken by water to exceed the glass temperature and thus more will be the rise in glass temperature and causing the negative value of ΔT as shown in Figure 5.4. Once ΔT becomes positive, it remains positive till the end of experiment. It

4 85 can be seen very clearly that for lower water depths of 10 mm and 20 mm, ΔT becomes positive at noon time, whereas the same condition for higher water depths of 40 mm, 50 mm and 60 mm it attains in the evening hours i.e. higher depth takes three hours to four hours later than the lower depth to attain the positive ΔT. Figure 5.3 Hourly variation of vapor temperature for different water depths

5 86 Figure 5.4 Hourly variation of ΔT for different water depths Figure 5.5 Hourly variation of distilled output for different water depths The effect of water depth on the hourly yield of solar still is given in Figure 5.5. It is noted that with the 10 mm depth of water, the maximum yield per hour is about 0.33 kg, but it is only for a very short duration of less than an hour between 12:00 o clock 13:00 o clock. For 20 mm depth, the hourly yield is around 0.25 kg for the period of three hours from 13:00 o clock to 15:00 o clock. From observation, it is noticed that the maximum solar intensity is received at 12:00 o clock, but the maximum yield of 0.33 kg for 10 mm depth of basin water is achieved at 13:00 o clock. The time delay is due to heat capacity of the basin water and heat losses in the still. The more the water depth the more energy is stored in the water in the form of sensible heat and rise in temperature of water surface is less which causes delay in the maximum hourly yield for later hours. Also it is noticed that, due to release of stored heat, the yield between 18:00 o'clock and 9:00 o'clock (next day), is more in higher water depths. The yield during the night time for various

6 87 water depths 10, 20, 30, 40, 50 and 60 mm, obtained are 0.120, 0.210, 0.295, 0.300, and kg respectively. By comparing Figure 5.4 and Figure 5.5, it is clearly understood that the output of solar distillation system is highly dependent on temperature difference between water and glass. Figure 5.6 Variation of daily yield for different water depths Figure 5.6 shows the variation of daily yield with respect to all water depths. It is found that for a certain lower water depth (10 mm) of basin water, the yield per day is low and increases with depth and again starts decreasing beyond certain depth of basin water. It is observed from Figure 5.4 that even though the still experienced a higher temperature difference for 10 mm water depth, it led to more heat loss from the still and was not utilized for evaporation, which is evident in Table 5.2. This may be the reason for lower output in 10 mm water depth. It is observed that for a particular depth of water (20 mm), yield per day is maximum. The overall cumulative energy balance of the solar stills is very important for analyzing the energy flow between the solar distillation system and the surrounding. The heat transfer

7 88 equations used for calculating the cumulative heat balances in the still are given in Appendix 2. Table 5.2 shows the typical cumulative energy balance for different water depth. It is found that the maximum losses occur on the glass surface by the combination of convection and radiation heat transfer for all water depths. The conductive heat losses from side and bottom wall surfaces are considerably low. Apart from all energy transfer, unaccountable losses are noticed in the energy balance of the system, which may be due to the vapor leakage through gaskets, joints and sensible heat stored by the still components like basin liner, water, glass etc. The unaccounted losses are found quiet high in the higher water depths at 40, 50 and 60 mm, due to the higher heat storage capacity. The cumulative heat balances for the remaining experimental studies like effect of sponge liner, effect of coloured sponge liner etc, are found to be closely match with the 20 mm water depth study. Hence the cumulative heat balance for other experimental studies are not discussed in the corresponding discussion part; however the values of external heat transfers and cumulative heat balances for all the cases are given in Appendix 3 and Appendix 4 respectively. From Figure 5.4 to Figure 5.6, it is understood that the 20 mm basin water depth for the typical solar still is taken as a reference for further studies.

8 Table 5.2 A typical cumulative heat balance of still for different water depth S.NO Description Amount of heat transfer, W 10 mm 20 mm 30 mm 40 mm 50 mm 60 mm 1 Radiation heat loss from glass to atmosphere Q rg Convection heat loss from glass to atmosphere Q cg Conductive heat loss from inner side to outer side through back wall Conductive heat loss from inner side to atmosphere through side walls. Conductive heat loss from inner side to outer side through bottom. Conductive heat loss from inner side to atmosphere through inside wall (water contact surface) Amount of heat utilized for converting saline water in to fresh water. Un accountable heat loss ( due to vapour leakage, heat loss through joints, etc) Q bw Q sw Q bot Q sww Q d Q u

9 EFFECT OF SPONGE LINER THICKNESSES The main objectives of this experimental study are to increase the output of the solar still by increasing the temperature difference between water and glass using sponge liners at the inner wall surfaces and find the optimum thickness of the sponge liner. The effect of sponge liner on the inner wall surfaces made large difference in the temperature of the components. In Figure 5.7, it is seen that, the water temperatures of 3 mm and 5 mm thick sponge liner still are slightly lower than the conventional still, whereas water temperature of 7 mm, 10 mm and 12 mm thick sponge liner still is higher than for the case of the conventional still. This may be due to the following reasons: (i) higher thick sponge liners (7 mm, 10 mm, 12 mm) extract more amount of water by capillary force from the basin. Hence water depth as well as water capacity reduces in the basin, resulting in increase of basin water temperature and (ii) the lower sponge liners extract considerably low water, and will lead to lower the water temperature. 70 Water Temperature, C No sponge 3mm 5mm 7mm 10mm 12mm Time Figure 5.7 Hourly variations of water temperature for different sponge liner thicknesses

10 91 Figure 5.8 Hourly variations of vapor temperature for different sponge liner thicknesses Figure 5.8 shows the hourly variations of vapor temperature for different sponge liner thicknesses. It is seen that, the vapor temperature of sponge liner stills are lower than the conventional still. This may be since the water present in the sponge liners extract heat from the vapor region due to the temperature difference between them, and hence vapor temperature becomes low. Figure 5.9 shows the hourly variation of glass temperature. It is clearly seen that, the glass temperature of 3 mm and 5 mm sponge liner stills are lower than the conventional still, which may be due to the decrease of vapor temperature inside the cavity of the still. As mentioned earlier, the temperature of water is high for the 10 mm, 12 mm thick sponge liner stills and will lead to increase in the radiation effect from water to glass and this may be the reason for higher glass temperature for higher thickness sponge liners.

11 92 Figure 5.9 Hourly variations of glass temperature for different sponge liner thicknesses Figure 5.10 shows the effect of sponge liner on the inner wall surfaces for different thickness of sponge liners. It clearly shows that, the inner wall surface temperature of sponge liner stills are much lower than the conventional still. It may be because the water available in the sponge liner extracts heat from inner wall surfaces; hence its temperature gets lowered. Also, it is noticed that the inner wall surface temperature decreases with the increase in sponge liner thickness. The results also show that the sponge liner stills reduces the conduction heat losses from inner wall surface to outer wall surfaces by 50% and it surface temperature by 24% (Table 5.3).

12 93 Figure 5.10 Hourly variations of inner wall surface temperature for different sponge liner thicknesses Table 5.3 Conductive heat transfer from inner wall surface to outer wall surface for different sponge liner thicknesses S.NO Sponge liner Conductive heat transfer from inner wall thickness surface to outer wall surfaces, W 1 No sponge mm mm mm mm mm The effect of the thickness of sponge liner on the ΔT is given in Figure It is clearly shown that in the still with 3 mm, 5 mm and 7 mm sponge liner ΔT values are higher than the conventional still throughout the day. It may be due to the glass temperature of 3 mm, 5 mm and 7 mm sponge liner stills are lower than conventional still glass temperature (Figure 5.9), and the water temperature of 3 mm, 5mm, and 7mm sponge liner stills are having closer values with conventional still water temperature (Figure 5.7). It is also noticed from Figure 5.11, that the 10 mm and 12 mm thick sponge liners reduce the ΔT values, which may be due to the higher water temperature as said earlier.

13 94 Figure 5.11 Hourly variation of temperature difference between water and glass for different sponge liner thicknesses The variation of ΔT values in sponge liner stills are indicated clearly from Figures 5.12 and 5.13 for 5 mm thick sponge liner. Figure 5.12 Comparison of water temperature for 5 mm sponge liner still and conventional still

14 95 Figure 5.13 Comparison of glass temperature for 5 mm sponge liner still and conventional still In Figure 5.12, it is seen that, the difference between water temperature of sponge liner and conventional still is comparatively low. The water temperature is almost same for both the stills, but in Figure 5.13, the glass temperature of sponge liner still is lower than the conventional one. It is understood that, the sponge liner stills increase the ΔT values by reducing the glass temperature. Figure 5.14 shows the hourly variation of output yield for various thicknesses of sponge liners. It is seen that the output of the sponge liner stills is higher than conventional still since morning. It may be because the water available in the sponge extracts heat from inner wall surfaces and vapor surfaces, and gets evaporated, this results in getting additional yield. The yield of the solar still with 10 mm, and 12 mm sponge liner is lesser than that of 3 mm, 5 mm and 7 mm sponge liner stills. It may be due to the following reason, (i) due to higher water and glass temperatures (Figure 5.7 and Figure 5.9), the convection and radiation losses from water to glass and glass to ambient are more, which makes the output as low (Table 5.4) and (ii) the heat

15 96 energy available at the inner wall surfaces is not sufficient for complete evaporation of water present in the higher thick sponge liner. Figure 5.14 Hourly variation of distilled yield for different sponge liner thicknesses Table 5.4 Convection and radiation heat losses from glass to ambient for different sponge liner thicknesses S.NO Sponge liner thicknesses Radiation heat transfer from glass to ambient (Q rg ) Convection heat transfer from glass to ambient (Q cg ) 1 No sponge mm mm mm mm mm

16 97 Figure 5.15 Variation of daily yield with different sponge liner thicknesses Figure 5.15 shows the variations of daily yield of sponge liner with various thicknesses. It is observed that the 5 mm thick sponge still gives more yield than others and is equal to kg, which is 35.2% higher than the conventional still. The lowest output is obtained from the sponge liner still with 12 mm sponge liner and is equal to 1.21 kg/day. The solar still efficiency is considered as the most important parameter to evaluate the system and to ensure the best still design. Figure 5.16 shows clearly that the efficiency of the sponge liner stills is higher than that of conventional still. This may be due to the sponge liner stills yield more by the combined evaporation of water from basin liner and inner wall surfaces. The overall efficiency of the sponge liner still with 5mm thick is much higher than the other, which is equal to 45.61%, which is 9% higher than the conventional still. Therefore from Figures 5.15 and 5.16, it is understood that the yield and efficiency of 5 mm sponge liner is high. Hence, the remaining parametric studies like effect of colour of sponge liner and combined effect of sponge liner and energy storage materials have been conducted with 5 mm thickness of sponge and 20 mm basin water depth.

17 98 Figure 5.16 Variations of overall efficiency for different sponge liner thicknesses 5.3 EFFECT OF COLOURED SPONGE LINER In conventional solar still a fraction of solar radiation directly falls on the inner wall surfaces especially on the back wall surface. In the present study, the sponge liners are placed on the inner wall surfaces, and it may grasp certain amount of the radiation, which falls on its surface. In this experimental study an attempt is made to find the effect of coloured sponge liner at the inner wall surfaces on the output yield by using the basic colours like blue, black, green, white and red. From the experimental results discussed in previous section the 5 mm thickness sponge liner is found to provide higher yield. Therefore in this experimental study the sponge liner thickness is considered as 5 mm. From the data gathered it is observed that temperature of the components like vapour and inner wall surfaces for different coloured sponge liner have small variation compared to the previous sponge liner thicknesses

18 99 analysis. Discussion is mainly focused on the temperature difference between water and glass (ΔT), yield and efficiency. In Figure 5.17, it is clearly seen that, the ΔT values of all the coloured sponge liners are higher than the conventional still. It is a force behind the output of any type of solar distillation system. It is also observed that the ΔT value of black coloured sponge liner still is higher than the other coloured sponge liner stills. It may be due to higher radiation absorbtivity of black coloured sponge liner on which sun light falls directly. Figure 5.17 Hourly variations of temperature difference between water and glass for coloured sponge liners Figure 5.18 shows the hourly variations of output yield for coloured sponge liner stills. It is clearly seen that, from the morning hour onwards the sponge liner still yields are higher than the conventional still. The daily output yield for different coloured sponge liners is shown in Figure It is seen that the maximum output yield of kg/day is obtained from black colour sponge liner still, which is 43.4% higher than the conventional still. Among the various coloured sponge liners, blue coloured liner has the lowest yield

19 100 i.e. is equal to 1.45 kg/day, which is 21 % higher than the conventional still. It is also observed that, yield is less even when the temperature of components are higher in the green and red colour sponge liner stills, which may be due to more convection and radiation heat losses from glass to ambient. Figure 5.18 Hourly variations of output yield for different coloured sponge liners Figure 5.19 Variations of daily output for different coloured sponge liners

20 101 The effect of coloured sponge liner on efficiency is given in Figure It is seen that, the overall efficiency of the black coloured sponge liner still is 51.07% which is 14% higher than the conventional still. The lowest efficiency of 45.2% is observed in blue coloured sponge liner still, which is 8% higher than the conventional still. Figure 5.20 Variations of overall efficiency for different coloured sponge liners Therefore from Figures 5.19 and 5.20, it is understood that the black colour sponge liner is most suitable for typical simple solar still to enhance the output. The further experimental study of sponge liner still with the energy storage materials (section 5.5) has been conducted in black coloured sponge liner. 5.4 EFFECT OF ENERGY STORAGE MATERIALS The effects of different energy storage materials resulted in larger differences in the temperature of the still components. Figures show the hourly variations of basin water, basin liner, glass and vapour temperature for the conventional still and still with different energy storage

21 102 materials. It is observed that in energy storage material stills considerable amount of heat is stored by the storage materials during the morning hours and the heat is released into the water in the late afternoon hours when radiation is low. This may be the reason for the components of the energy storage material stills to attain low temperature during the morning hours and high temperature during the evening hours. However, in conventional still, due to the absence of storage materials, the components attain high temperature from morning to noon and it cools down during the late evening hours. Figure 5.21 Hourly variations of water temperatures for different energy storage materials

22 103 Figure 5.22 Hourly variations of basin liner temperatures for different energy storage materials Figure 5.23 Hourly variations of glass temperatures for different energy storage materials

23 104 Figure 5.24 Hourly variations of vapour temperatures for different energy storage materials Figure 5.25 shows that, the output is increasing until it reaches the maximum in the afternoon, then decreases in the late afternoon. Due to the absence of energy storage materials, the basin and water temperatures are high in the conventional still between 9:00 o clock 12:00 o clock; this may be the reason for higher output in conventional still during this period. From 13:00 o clock onwards the output of energy storage material stills is higher than the conventional still, due to release of extracted heat into water by the storage materials. The black granite still gives higher yield than other energy storage material stills and is equal to kg/day, which is 9.69 % higher than that of the conventional still. This is evident in Figure It is also observed that, the night time production (17:00 o clock 9:00 o clock) is high in the paraffin wax still due to its high specific heat capacity which is equal to 2140 J/kgK. Figure 5.27 shows the night time production of conventional, sponge liner and energy storage materials stills, it is observed

24 105 that the night time production is more when energy storage materials used in the solar still. Figure 5.25 Hourly variations of distilled output for different energy storage materials Figure 5.26 Variations of daily output for different energy storage materials

25 106 Figure 5.27 Night time yield for different parameters Figure 5.28 Variation of temperature difference between water and glass for different energy storage materials Figure 5.28 shows the hourly variations in temperature between water and glass for the different energy storage materials. It clearly indicates

26 107 that, in the black granite still the temperature difference between water and the glass (ΔT) is higher than other solar stills during 13:00 o clock 17:00 0 clock. This may be the reason for higher output during this period, whereas in the conventional still this difference is high in the morning hours (9:00 o clock- 12:00 o clock). From Figure 5.29, it is seen that, the overall efficiency of the black granite solar still is higher than the other stills, which is 8.5 % higher than the conventional still. By comparing Figures 5.26 and 5.28, it is understood that the black granite gravels are the most suitable energy storage materials for a simple solar still. Figure 5.29 Variation of overall efficiency for different energy storage materials 5.5 COMBINED EFFECT OF SPONGE LINER AND ENERGY STORAGE MATERIALS Based on higher yield, from the previous experimental studies, the following parameters are used for this study, (i) Water depth is 20 mm (ii) Thickness of sponge liner is 5 mm

27 108 (iii) Black coloured sponge liner (iv) The black granite gravel The combination of energy storage materials and the black coloured sponge liner stills are compared with other parameters in Figure It can be seen that, the black coloured sponge liner on the inner wall surfaces produces more yield than other modifications during morning hours and it decreases in the evening hours. Whereas the yield of the combined still is moderate during sunshine hours due to black sponge liner and it is observed to be high during the night time due to the storage effect of the black granite gravels. The highest output (1.71 kg/day) is observed from black sponge liner with black granite still (combination still), which is 50.6% higher than the conventional still (Table 5.5). Figure 5.30 Hourly output variations of different parameters

28 109 Table 5.5 Comparative performance study of simple solar still with different parameters S. No Parameters Distilled output (kg) Highest output (kg) Effect of water depth (mm) Effect of sponge liner thickness (mm) 1 No sponge (Conventional) Effect of sponge liner colour 1 White Red Green Black Blue 1.45 Effect of energy storage materials 1 Pebbles (20 mm water depth) 1.54 (5 mm thick sponge liner) (Black coloured sponge liner) Blue metal stones Black granite gravels Paraffin wax (Black granite gravels) Combined effect of sponge liner and energy storage materials 1 Black granite gravels and black sponge liner Average improvement from the conventional still 17.0% 35.2% 43.4% 10.3% %

29 CONCLUSIONS as follows: From the experimental studies, the several conclusions are obtained The basin liner temperature is almost closer to the water temperature, because of the continuous contact between them and it leads to thermal equilibrium. The highest temperature of the solar still was recorded on the inner wall in the conventional still and the second highest temperature was on the vapor side. The lowest temperature of the still component was found at the bottom. The maximum amount of energy losses (approximately 40%) from the still is by the combined effect of convection and radiation from glass to ambient and the minimum amount of energy loss is from the lowest side in the still due to conductive heat loss from basin liner to bottom side (1%). The increase in either ambient temperature or the solar intensity can lead to the increase of output. Water depth influences the output, when the water depth increases, the output of the still is decreased. It is also pointed out that the operating temperature of vapour, basin water and basin liner are decreased when the water depth is increased. The highest daily output of solar still, has been found for lower water depth (20 mm). The daily yield at water depth 20 mm has been found to be about 17 % more than the daily yield at water depth of 60 mm.

30 111 Sponge liner at the inner wall surfaces makes improvements in the solar still in many ways, It works towards increasing the temperature difference between water and glass by reducing the temperature of glass. It reduces the conduction heat losses from inner wall surfaces to outer wall surfaces by 50% and reduces its surface temperature by 24%. The thickness of sponge liners influences the performance of the still. The 5 mm thickness sponge liner has given 35.2% higher yield than the conventional still. It has the advantage of using a low cost cheap material of sponge liner to enhance the still yield and its efficiency. The sponge liner colours also influences the operating conditions of the still considerably Solar still with black coloured sponge liner gives higher yield (1.63 kg/day) than the others, which is 43.4 % higher than the conventional still and the lowest output is obtained from blue colour sponged still; it is equal to 1.45 kg/day, which is 21 % higher than the conventional still The green and red coloured sponged stills are giving lesser yield than the black coloured sponge still and even its components temperature are higher, due to lower ΔT.

31 112 The energy storage materials have the ability to increase the output of the solar still considerably, The energy storage materials in the still store considerable amount of heat during noon hours and release the stored heat to the basin water in the late afternoon hours when radiation is low. The energy storage materials influence the temperature of the solar still components considerably. Output of the conventional still is higher in the morning whereas the output of the energy storage material still is higher in the evening hours. Black granite gravels are more efficient than pebbles and blue metal stone when used as energy storage materials. Combination of black coloured sponge liner and black granite gravels improve the distilled output considerably. The night time productivity is observed more in energy storage material stills. The energy storage materials which are used for this investigations system are economically suitable for solar still to improve the output and efficiency. The heat transfered from water surface to the glass in simple solar still is by free convection heat transfer mode. It highly depends on convective heat transfer coefficients and their correlation C and n. The next Chapter 6 will deal with the methodology formulated for the evaluation of convective heat transfer correlations C and n using regression analysis. The flow chart of a computer model for the evaluation of coefficient using experimental data is also been discussed.