Comparative Experimental Evaluation of Conventional Solar Still (CSS) and CSS Augmented with Wax Filled Metallic Finned-Cups

Dwindling potable water is a big concern for the whole world in general and developing nations in particular. Solar still is found to be suitable for the production of potable water at low cost, especially in the arid regions. In view to improve the distillate output of the solar still, the augmentation of the sensible and latent heat, a storage material was needed. For the purpose, wax filled metallic finned cups were used. In this paper, an attempt was made to investigate (experimentally and theoretically) the performance of a conventional solar still integrated with wax filled metallic finned-cups (MSS). Outdoor experiments were conducted on conventional solar still (CSS) and CSS augmented with wax filled metallic finned-cups, in the month of January and February 2019, at Raghogarh, Guna ( 24o39’N, 77o19’E, India). Linear regression model proposed by Kumar and Tiwari was used to evaluate the performance of solar stills. An improvement of 15.63 and 16.95% in evaporative and convective heat transfer coefficients (from water to condensing cover) have been observed in MSS, as compared with the CSS respectively. It was found that the overall efficiency of MSS increased by 24.64% in compared with the CSS.


INTRODUCTION
Water is one of the essential commodities needed for the survival of human beings on the earth. Expeditious industrialization and population growth have led to the serious problem of water stress, as our natural water resources are now entered an epoch of paucity. According to the world water council report [1], the global average per person availability of renewable water resource is going to reduce from 6600 to 4800 m 3 during 2000-2025. Although many methods like reverse osmosis [2], film distillation [3] etc. are available for converting brackish water into potable water, but poor and developing nations lack the skills and fund to deploy them. So to solve this problem of water crisis, solar still is recognized long back as a simple low cost device. The first use of solar still has reported way back in 1872 by Wilson [4], for supplying potable water to the nitrate mining society. Later on many engineers and researchers have worked on these basin-type solar stills which are commonly known as conventional solar stills (CSS) [5,6]. A CSS has low distillate output and requires large surface area for mass production. With this background, various researchers have proposed different methodology for enhancing the productivity of passive distiller units [7][8][9][10][11][12][13][14][15][16].
Jamil and Akhtar [17] reported the influence of characteristic length of solar still cavity (aspect ratios from 1.94 to 2.67) on the distillate yield. A detailed theoretical analysis of different climatic parameters on the productivity of CSS have been reported by Afrand and Karimpour [18]. An improvement in distillate yield by augmenting still with earth has been reported by Sodha et al. [19]. Dumka and Mishra [20] have reported a detailed energy and exergy analysis of single basin solar still integrated with the earth surface. Effect on distillate yield by covering nearby ground of sand bed solar still with polythene sheet and coal powder have been reported by Tiwari and Mishra [21]. Using ANN model a hybrid solar distiller unit integrated with an air compressor has been reported by Hidouri et al. [22]. Rabhi et al. [23] have reported the augmentation of fins (in basin area) and external condenser with the solar still. Dumka et al. [24] have experimentally examined the use of permanent ferrite ring magnets for magnetizing the brackish water in conventional solar still. They have reported a substantive increase in the distillate output and efficiency of still augmented with magnets.
Kalidasa et al. [25] have observed that the use of rubber in the basin area of still can enhance the evaporation rate and distillate output. Exergy and economic analysis of CSS integrated with rectangular porous media have been reported by Rashidi et al. [26]. Mishra and Tiwari [27] have reported an enhancement of distillate yield by spreading coal and metal chips within the basin area. Ibrahim et al. [28] have given a comprehensive review on absorption energy storage using (with and without) crystallization of the absorption materials for single and multi effect distiller units. A comprehensive review on an application of nano-fluids in solar stills have been reported by Khanafer and Vafai [29]. Deshmukh and Thombre [30] have reported the use of sand and servo-therm medium oil beneath the basin liner, as sensible heat storage material. Dumka et al. [31]have reported the influence of salt concentration on the internal heat transfer coefficients and efficiency of a CSS.
Gugulothu et al. [32] have reported an experimental study on the performance of a single basin solar still by using potassium dichromate, magnesium sulphate heptahydrate and sodium acetate as an energy storage materials. They observed that magnesium sulphate heptahydrate gives better distillate output as compared to others. Kabeel et al. [33] have delineated a comparative study of experimental and exergy analysis of a passive water desalination system, with and without paraffin wax as phase change material in the basin and parabolic concentrator. They have observed that PCM and parabolic concentrator augmented still produces 55-65% and 35-45% higher yield than the conventional still. Arunkumar et al. [34] examined the effect of carbon impregnated foam (CIF) and bubble-wrap (BW) insulation on distillate output of single slope solar still having basin area of 0.5m 2 , and have validated their CFD results with the experiments. They reported an increment of 21.05%, 63.16% and 15.79% in distillate output by using, BW insulation, BW insulation & CIF and saw dust respectively in CSS as compared to CSS alone. Kabeel et al. [35] reported a comparative theoretical study among different organic and inorganic PCM along with their economic analysis. They have observed that Capric-Palmatic (inorganic) and A48 (organic) have the combined advantage of high yield and low cost, but A48 has an edge over the Capric-Palmatic as it is environment friendly. A comparative study of three stills i.e. CSS, CSS with sensible heat energy storage, and CSS with jute knitted sensible energy storage under different water mass have been reported by Kabeel et al. [35]. They have observed that for 20 kg of water, jute knitted sensible energy storage still yields 18% more than the still with only sensible energy storage material.
In this communication, an attempt was made to determine the effect of wax filled finned-cup (that are supposed to act as sensible and as latent heat storage material) on the internal heat transfer coefficients, distillate yield and efficiency of a conventional solar still. Experimental data obtained by the outdoor experimentations in the month of January and February 2019 were evaluated using, model proposed by Kumar and Tiwari. It waas observed that still containing wax filled metallic-finned cups gives very high nocturnal yield and hence, high distillate output and efficiency.

EXPERIMENTAL SETUP
For experimentation, two identical single slope conventional solar still (CSS) are fabricated with the help of 5 mm thick FRP material keeping 1 m 2 basin area, with lower and higher vertical sides of 0.48 and 0.2 m heights respectively. For better absorption of solar energy, inner surface of stills were painted with the black colour. Solar stills are covered with iron transparent glass of 4 mm thickness, at an inclination of 15.6º with reference to horizontal surface.
As water wets the glass surface, so the condensation of vapours on it will obviously be film condensation. But it has been proved by Bhardwaj et al. [36] that the film condensation allow more solar radiation to pass through them when compared to dropwise condensation, and hence film condensation results better yield in solar still. Recently Arunkumar et al. [34] have shown that the use of bubble wrap insulation can reduce the heat loss from side and bottom considerably. So, both the stills were insulated form sides and bottom with bubble wrap. The schematic arrangement, and actual photograph of CSS are shown in Figure 1 and 2 respectively.  To enhance the productivity of CSS twelve wax filled, finned cylindrical cups ( Figure 3) of 6 cm height and 8 cm inner diameter were placed within the solar still at equal distance (to ensure maximum absorption of incident radiation, minimum wall shadow effect, and equal heat dissipation within the basin area), as shown in the Figure 5.
Each cup contains 44 mm long, and 2 mm diameter fins, painted with black colour to ensure the maximum absorption and radiation from Sun to energy storage material (during charging), and energy storage material to water (during discharging) respectively. Wax is filled till a height of 4 cm in each cup, and the top portion of the cups were sealed with a LLDPE cover (to ensure the isolation of wax from moist air) as shown in Figure 4. The wax used in current study is paraffin wax, whose properties are listed in Table 1.   The photograph of Modified Solar Still (MSS), i.e. the CSS augmented with wax filled finned cups is shown in Figure 6.

Figure 6. Photograph of finned cup with wax covered with LLDPE
Five K-type thermocouples (K 7/32-2C-TEF) were deployed in the still for the measurement of atmospheric, inner glass, outer glass, water, and basin temperatures in CSS, whereas six thermocouples are deployed in MSS. Different temperatures were recorded with the help of DTC324A-2 temperature indicator during the experimentation. LX-107 solar power meter has been used to measure incident solar radiation during the experimentation. For the measurement of distillate output a graduated cylinder has been used respectively. Photo graph of full experimental setup is shown in Figure 7. In the experimentations it is assumed that the data is distributed uniformly so, Type B uncertainties are considered. In this type, the standard uncertainty is evaluated as [38]: where, a is the accuracy of the measuring instrument. Table 2 represents the accuracy, range, and standard uncertainty of the measuring instruments. Each experimental run is of 48 h time duration. Thirty kg of brackish water is maintained in the basin area of solar still throughout the experimentation.
Following observations were made during experiments: • Atmospheric, basin, water, inner glass, outer glass, and finned cup temperatures. • Intensity of incident solar radiation on inclined glass cover.
• Distillate output at an interval of one hour.

MATHEMATICAL BACKGROUND
To understand the physics behind a particular thermodynamic system, one should always write the governing equations which can define the system holistically. For the solar still in current study, following assumptions are considered to develop the energy balance equations: • Leak proof solar still.
• Side, and bottom heat loss from still are neglected. • The water, glass, basin, and finned-cup temperatures have very insignificant variation spatially. • System is in quasi-equilibrium condition between two successive readings during the course of time. • Wax filled metallic finned-cup is considered as one combined system. This is in immediate consequence of third assumption. • Neglecting the shadow effect.

Solar still with finned-cup (MSS)
Based on the above mentioned assumptions, energy balance equations for different sections of MSS can be written as follows:

Glass cover
Based on the Figure 8, the energy equation is as follows: Based on the Figure 9 the energy equation is as follows: where, 0 = →atm bain q (because of second assumption).

Finned-Cup
Based on the Figure 11 the energy equation is as follows: Above equations are for the case when the cup is discharging (i.e. gaining energy from incident solar radiations) of cup the direction of energy interactions from cup just changes their sign, and rest of the equations mentioned above will remain same.

Saline water
Based on the Figure 13 the energy equation is as follows: Figure 13. Energy balance of basin water without finned cup

Basin liner
Based on the Figure 14, the energy equation is as follows: The convective heat transfer rate is proportional to the temperature difference of water and glass, and the constant of proportionality is convective heat transfer coefficient, as written below: The thermo-physical properties of moist air are calculated using property relations reported by Tsilingiris [39]. Numerical values of C and n are required to evaluate h cw .Many theoretical models are there to calculate the numerical magnitude of C and n. For theoretical analysis, authors have used the model proposed by Kumar and Tiwari [40] in this manuscript, which is based on linear regression analysis, as it is not limited to Gr range. It takes in the distillate yield, solar radiation intensity, water, glass and ambient air temperatures as an input, and returns the values of C and n as output.
The values of C and n from this model are as follows: 2 2 ( ) The radiative heat transfer coefficient from water to inner glass surface is evaluated as follows [20]:  The overall efficiency of a solar still is defined as the ratio of thermal energy required to obtain a specific amount of distillate output to that of total solar energy as input, and mathematically it is written as [20]: Figure 15 shows the variation of solar radiation intensity with respect to time during the experimentation.  Figure 16 shows the hourly variation of ambient, water, inner glass, and wax filled finned-cup temperatures for CSS and MSS. At the start of the experiment (i.e. at 9:00 h on 25/02/2019), the condensing cover temperature has been recorded as 43.3 and 65.8% higher than the basin water temperature, for CSS and MSS respectively. Whereas for the next day (26/02/2019), at the same time inner glass temperature shows an upsurge of 50 and 61.2% over water temperature for CSS and MSS respectively. For both the days, after 12:00 h, the water temperature takes the lead and maintains it throughout the experimentation till 8:00 h next day. For CSS, the maximum recorded water temperatures are 63.6ºC (first day) and 60.5ºC (second day) at 15:00 h, which are 9.28 and 14.37% higher than the inner condensing temperature respectively, at ambient temperature of approximately 26ºC. Whereas for MSS, the maximum water temperatures recorded (at 15:00 h) are 61.1ºC (first day) and 58.4ºC (second day), which are 8.14 and 12.31% higher than the inner glass temperature respectively. The reason for water to attain the highest temperature 2 h later (at 15:00 h) than the maximum solar radiation (at 13:00 h) is that: the heat capacity of water (in CSS), and water & wax filled finned cups (in MSS) will not allow water to increase its temperature instantaneously with the solar radiation. It is seen that finned cup temperature (T c ) increases with time due to the increased heat transfer rate by convection from the surrounding water and solar radiations to the finned cup as the solar radiation intensity increases. The wax started to melt after 6 h (at 15:00 h), from the beginning of still exposure to solar radiation, for both the days. Subsequently, T c remains nearly constant until it melts completely; then, it decreases slowly with time after sunset when the discharging process of the energy stored by the wax begins. However, the temperatures of the basin water and glass cover of the still decrease much faster with time due to decrease in the ambient temperature. This causes a significant temperature difference between water and cup till late night for both dates. By looking at the trend it is eminent that from 9:00 h till 18:00 h the finned-cup is charging and then it discharges its energy to the water. Figure 17 shows the variation of ΔT throughout the experiment for both the stills (i.e. MSS and CSS). It is apparent that during morning hours glass receives the radiation first and its temperature increases faster in contrast to the temperature rise of water which results in negative ΔT. The ΔT remains negative till water temperature does not overtake glass temperature. The Maximum positive temperature difference of 12.6 and 12.1ºC has been recorded between water and inner glass at 15:00h for MSS whereas, for CSS it is 9.1 and 8.1ºC at 19:00 h for first and second consecutive days respectively.

OBSERVATIONS, RESULTS AND DISCUSSION
The variation of evaporative heat transfer coefficient with respect to time for MSS and CSS is shown in Figure 17. It has been observed that MSS leads CSS throughout the experimentation except at 13:00 h where CSS leads MSS marginally by 2 and 4.4% for day one (25/02/2019) and two (26/02/2019) respectively. The reason for this rise may be the ΔT (Figure 16), which is just crossing the zero line for MSS at 13:00 h. Augmenting CSS with the finned cups (MSS) has lead to an appreciable improvement in h ew from 14:00 h till 8:00 h next day, for both the days. On first day, at 9:00 h, h ew evaluated for MSS is 59.13% higher than that of CSS. MSS maintains its lead till 12:00 h but, at 13:00 h CSS take over MSS as this is the time when ΔT cross over zero line. Thereafter, h ew for MSS takes its lead over CSS and attains a maximum value of 32.65 W/m 2 K at 15:00 h. From 15:00 h till 10:00 h on second day, MSS maintains its lead, thereafter CSS takes over MSS till 14:00 h. The maximum value of h ew attained by MSS on second day was 29.48 W/(m 2 K) at 15:00 h. After 15:00 h till the end of experimentation, similar trend has been observed. Over all, there has been an improvement of 15.63% in the value of h ew for MSS in comparison to CSS.
The variation of convective and radiative heat transfer coefficients for CSS and MSS are shown in Figure 19 and 20 respectively. There has been a significant improvement in the value of h ew due to presence of wax filled finned cups. For both CSS and MSS a sharp discontinuity in the nature can be seen at points 5, 24, 29 and 48, this is due to the very small value (very much close to zero) of ΔT at these points. As already discussed in equation 17, the overall heat transfer coefficient is the cumulative impact of evaporative, convective, and radiative heat transfer coefficients. So, to understand the behaviour of total heat transfer from water to glass, Figure 21 depicts its variation as a function of time. Energy fraction has been calculated based on equation 19, and their variation as function of time is shown in Figure 22. The maximum contribution is of evaporative heat transfer coefficient, followed by radiative, and least contribution is of convective heat transfer coefficient. The augmentation of finned-cups to CSS have a considerable improvement in the evaporative and radiative energy fractions (as these are the only dominating mode of energy transfer in still). After 1:00 h in the night for both the days, the evaporative and radiative energy fractions change their nature: due to decreasing temperature difference between water and glass temperatures.
For 48 h observation, the distillate yield recorded from MSS is 22.44 % higher than CSS viz. 5.932 l/m 2 . Figure 23 shows the variation of distillate yield from CSS and MSS as a function of time. Table 3 gives the comparative view of cumulative yield recorded during the shine and the off-shine hours for day one and two (from 9:00 h (25/02/2019) till 6:00 h (27/02/2019)). From Table 3 it is clear that the distillate yield recorded during day time (shine hours) is slightly lower for MSS whereas, for night time (off shine hours) it is much higher for MSS in comparison to CSS. For day one the recorded distillate yield for MSS lags by 7.06% during shine hours and leads by 104.92% during off-shine hours in comparison to CSS. Whereas for the second day, the MSS lags by 0.7% during shine hours and leads by 67.65% during off-shine hours in comparison to CSS. The higher value of distillate output in night time is due to the energy discharging from the wax filledfinned cups. Figure 24 shows a bar graph which depicts the overall efficiency of CSS and MSS during 48 h of experimentation. It shows that the augmentation of CSS with the wax filled metallic finned-cups has considerably improved the overall efficiency of MSS by 24.64% in compared to CSS.

CONCLUSION
Two identical conventional solar stills were experimentally and numerically investigated under Guna (24º39'N, 77º19'E, India) weather conditions. Twelve wax filled metallic finned-cups were placed in one still (called MSS) to enhance the internal heat transfer coefficients along with distillate output. Experiments were carried out for continuous 48 h, with different experimental sets spread over the month of January and February of 2019. On the basis of experimental and theoretical analysis of CSS and MSS, following conclusions can been drawn: • Distillate output of MSS is enhanced by 22.44% due to the augmentation of wax filled metallic finned-cups. • During nocturnal hours the distillate yield recorded from MSS is higher by 104.92 and 67.65% in comparison to CSS for day one (25/02/2019) and day two (26/02/2019) respectively (due to the presence of wax filled finned-cups covered with LLDPE) . • The average evaporative heat transfer coefficient from water to inner condensing cover of MSS has been evaluated as 15.63% higher as compared to CSS. • It has been observed that the convective heat transfer coefficient from water to inner condensing cover of MSS improved by 16.95% as compared with the CSS. • The augmentation of wax filled metallic finned-cups with the CSS has improved the internal efficiency by 24.64%