The need for fresh water is increasing by the hour. The demand for freshwater is quickly rising as a result of the increasing population and rapid urbanization. Besides serving as a source of drinking water, fresh water is essential in a wide range of industrial uses, including batteries, pharmaceutical manufacturing, and research facilities. Even though India has a population that accounts for 16 % of the world's total population, the nation possesses just 4 % of the world's freshwater resources. India is experiencing water scarcity as a result of shifting weather patterns and recurrent droughts (Sathyamurthy et al. (2017); Vaithilingam et al. (2021); Arani et al. (2021)). For ages, fetching water has been considered a woman's work in India, particularly in rural regions. Wells, ponds, and tanks are drying up as groundwater supplies are depleted owing to overuse and excessive usage. This has exacerbated the water situation, placing an even larger strain on women in terms of water availability. Water desalination with solar energy is the ideal option for developing water desalination systems in developing and developed nations because of the water shortage problem and the availability of solar energy is abundant, clean energy and available free of cost. Solar distillation has been utilized for thousands of years and is still in use today. A promising procedure and an alternate method of supplying potable water to tiny settlements on islands and in isolated places, solar distillation seems to be gaining traction (Chamkha et al. (2020); Madhu et al. (2017, 2019)).
Arunkumar et al. (2020) studied the influence of adding a sensible heat energy storage medium inside a traditional solar still to augment freshwater production. Different materials such as pebbles, clay balls, CuO nano-coated absorber, and PVA sponges were used in their study. Results showed that fresh water produced from the solar still using pebbles, clay balls, CuO nano-coated absorber, and PVA sponges were 2.8, 2.62, 2.9, and 1.9 L/m2 respectively with their corresponding cost per liter for water produced as 0.0073, 0.008, 0.007, and 0.012 $. The influence of higher thermal conductivity of clay balls with porosity improved the rate of water produced from the solar still. Also, it was observed that at a higher solar radiation period, the yield of all the solar still with sensible heat energy storage exhibited similar characteristics on potable water produced.
Kabeel et al. (2019) studied the effect of using composite heat storage medium on energy, exergy, and economic analysis of traditional solar still. Black gravels were added to the paraffin wax for improved thermophysical properties. The cumulative yield from the SS with PCM and composite PCM were found as 2.44 and 3.72 L/m2 with an average energy efficiency of 48.22 and 66.87% respectively. Also, studies reported that the average exergy efficiency using composite PCM was higher while compared to SS with PCM. It was also reported that the cost per liter for composite PCM was varied from 0.0014 to 0.00163 $ which is lesser than the solar still with PCM alone.
Kabeel et al. (2017) used a solar parabolic concentrator along with PCM to the bottom of the solar still in order to augment the potable water produced. Depth of water maintained inside the solar still was the only parameter that was analyzed in their study to assess the performance such as cumulative yield, energy, and exergy efficiency. The depth of water maintained in the basin was varied from 1, 2, 3, 4, 5, and 6 cm. Their study revealed that the potable water produced from the solar still under proposed modification reduced as the depth of water inside the still was increased from 1-6 cm. The accumulated potable water produced from the solar still with dish type of concentrator and PCM were 4.2, 5.1, 5.5, 6.2, 6.8, and 7.2 L/m2 for a water depth of 1, 2, 3, 4, 5, and 6 cm during summer condition. Similarly, the exergy efficiency was higher at the lowest water depth of 1 cm and on increasing the water depth the exergy efficiency reduced for both summer and winter conditions.
Pumice stones were used as an energy storage medium in a conventional type of solar still for augmenting the potable water produced which was experimentally investigated by Bilal et al. (2019). The mass of the storage medium was varied between 5 and 10 kg inside the basin. Results revealed that the use of energy storage inside the basin reduced the daytime productivity to about 10.38 and 17.02% for the mass of storage medium as 5 and 10 kg respectively on comparing the daytime yield of conventional solar still without any storage medium. This phenomenon was completely due to the change in internal heat energy storage by the material inside the basin. While comparing the distillate output during the overnight, there is a significant improvement of about 1.58 and 12.67% using 5 and 10 kg of pumice stones inside the absorber.
The effect of convex type of absorber with wicks (jute and cotton) spread in the absorber of TSS was experimentally studied by Essa et al. (2021). Along with the proposed modification, composite (graphene and TiO2) were added to the basin water as a working fluid. The convex type of absorber increases the surface contact of water with the solar radiation for maximum evaporation which leads to increased thermal efficiency and potable water produced. The height of the convex absorber was varied between 5 and 20 cm and optimized to 15 cm as the contact angle of the solar radiation was maximum. The daily potable water produced from TSS using jute wick with convex absorber, jute wick with convex absorber and nanocomposite, cotton wick with convex absorber, and cotton wick with convex absorber and nanocomposite were found as 92, 114, 88, and 110% respectively. The increase in potable water produced from the TSS using wick material is due to the higher capillary effect while compared to that of cotton wick.
Dehmukh and Thombre (2017) used sand and servotherm oil as energy storage material and optimized the depth of SE material for improved thermal performance. In both cases, the depth of SE material was varied from 0.5 to 1.5 cm while the depth of water is constantly maintained at 0.6 cm. It was reported that in both cases, with increasing depth of SE material, the daylight productivity decreases, and the overnight productivity increases. The heat stored in the daytime is utilized by the water during night time which improved the rate of evaporation. Also, the optimized depth of SM oil and sand was limited to 0.5 cm as there was no significant improvement in increasing the depth of SE material beneath the basin. The daily yield from solar still using SM oil and sand at a water depth of 0.5 cm were found as 2525 L/m2 and 2502 L/m2 respectively.
The use of mushrooms and carbon black-based nanoparticles on mushrooms for improving the potable water produced from TSS was experimentally analyzed by Sharshir et al. (2021). Three different quantities of carbon black namely 25, 50, and 75 g/m2 was coated on mushrooms and a comparison was made with mushrooms and without mushrooms on the basin. The mushrooms on the plate increase the capillary effect for effective evaporation from the water to get evaporated. The daily productivity of TSS with mushroom and mushroom with 25, 50, and 75 g/m2 carbon black nanoparticles were found as 4.37, 5, 5.36, 5.46 kg/m2 respectively, and which is higher than TSS without mushroom (3.41 kg/m2). On adding 25 g/m2 of carbon black nanoparticle with 50 g/m2, there is a megre improvement in the potable water produced from the mushroom coating in the absorber of TSS. The average thermal efficiency from TSS with mushroom and mushroom with 25, 50, and 75 g/m2 carbon black nanoparticles and TSS without mushroom were found as 54.3, 54.74, 49.61, 44.9, and 35.23 % respectively. The improvement in thermal performance was due to the effective increase in the thermal conductivity, pileus surface roughness, absorption of solar radiation for an effective increase in temperature further leading to an enhanced rate of evaporation. The thermal performance improvement of TSS using circular and hollow fins filled with phase change material was experimentally investigated by Abdelgaied et al. (2021). Results revealed that the use of square fins improved the daily potable water by 33% (5.52 L/m2) compared to TSS with a flat absorber (4.15 L/m2). On using hollow circular fins the heat exposure area was further improved which leads to an improvement in yield to about 47.12% than TSS with a flat absorber. Adding paraffin wax into the circular and square fins attached to the absorber further improves the potable water produced by 90.2% than the conventional case. The daily energy efficiency of TSS, TSS with hollow square fins, hollow circular fins, and circular fins with paraffin wax were found as 36.9, 49.1, 54.4, and 70.2% respectively. Similarly, from the economic point of view, the cost per liter of water for TSS with paraffin wax in the hollow circular fins was reduced to about 0.009 $ whereas, in the case of TSS, TSS with square and circular fins were 0.015, 0.012, and 0.011 $ respectively.
Kabeel et al. (2020) used parabolic concentrators on TSS to augment the performance of TSS by optimizing the thickness of water in the absorber. It was observed that the varied thickness of water in the semicircular trough the fresh water produced was improved by 87.9, 90.8, 81.9, and 68.1% for the thickness of water as 1, 2, 3, and 4 cm respectively while compared to rectangular absorber in TSS without parabolic concentrator. The improvement in the yield of fresh water from the proposed modification was that the entire heat from the solar radiation is focused on the lower circumference of the absorber which leads to increased temperature while compared to the rectangular flat absorber.
Kabeel et al. (2021) increased the surface area of exposure to solar radiation with the water placed in the TSS using corrugated absorber and wick material. Thermal modeling analysis was carried out to assess the performance of the proposed modification in TSS. Results showed that the corrugated fins on the absorber increased the potable water produced by 44.82% than conventional TSS. The daily potable water produced from the corrugated absorber TSS and flat absorber TSS was found to be 6.01 and 4.15 L/m2 respectively. According to the experimental and numerical results, the largest variances were found to be within the limits (2.5 %). Also, there is a significant improvement of about 46.86% in the daily efficiency was observed using corrugated absorber and wick material in TSS.
Abdelaziz et al. (2021) proposed five different configurations on TSS to augment the potable water produced. SS with modifications such as corrugated absorber, corrugated absorber with wick material, corrugated absorber with wick material, and carbon black nanoparticles in working fluid and corrugated absorber with wick material, NePCM (carbon black nanoparticles in PCM), and carbon black nanoparticles were proposed. Results show that the use of wick material in corrugated absorber improves the potable water produced by 30.3% and 16.2 % than the use of flat absorber and corrugated absorber without wick material. Furthermore, the influence of using carbon black nanoparticles in PCM and nanofluids as working fluid in corrugated absorber solar still, the potable water produced was improved to about 88.84% than the conventional TSS. Economical aspects revealed that the CPL was reduced to about 22.47% using corrugated absorber, CB nanofluids, NePCM and wick material in the basin as compared to TSS with the flat absorber.
The effect of thickness of water in the conventional solar still using sensible energy storage was experimentally investigated by Kabeel et al. (2018). Along with the sensible heat storage material, jute cloth was knitted to augment fresh water production. It was reported that the production of fresh water is fully dependent on characteristics like sensible energy mass and basin depth. With the use of jute cloth woven in the sensible heat energy storage, the augmentation of fresh water production was found as 25% than solar still without jute cloth on energy storage at the lowest water thickness of 0.02 m. The cumulative yield from solar still without any energy storage was recorded as 2.5 kg/m2 while the solar still equipped with energy storage and energy storage with jute cloth were 5.5 and 5.9 kg/m2 respectively. The increase in the freshwater was completely due to the higher capillary effect exhibited by the jute cloth on the energy storage material placed in the basin. Employing low-cost heat storage materials, Samuel et al. (2016) were able to augment the performance of basin solar still productivity by 22.73 %. Their findings also revealed that the potable water produced from basin-type solar still was improved from 2.2 to 2.7 L/m2 (a 21.7 % increase). The properties of various sensible heat energy storage materials are compared and presented in Table. 1.
It is relatively simple to add sensible or latent heat energy storage material in conventional solar still as the basin area is higher, but it is difficult to place energy storage material in tubular solar still. The area of condensing cover is closer to the exposure area of the basin. In this study, the effect of waste eggshells was used as an energy storage material in the basin of TSS to improve the potable water produced. Three different water layer thicknesses were chosen namely 10, 15, and 20 mm to assess the performance of solar still. Furthermore, the effect of energy storage material on energy efficiency was analyzed in comparison to TSS without energy storage.
Table 1
Properties of sensible heat energy storing material
Property | Material |
Pebbles (Kedida et al. (2019)) | Gravel | Graphite (Kabeel et al. (2018)) | Marbles (Čáchová et al. (2016)) | Mild steel scraps (Murugavel et al. (2010)) | Cement (Murugavel et al. (2010)) | Red bricks (Murugavel et al. (2010)) | Sand (El-Sebaii et al. (2009)) | Aluminium (Murugavel & Srithar (2011)) |
Thermal conductivity (W/m K) | 2.5 | 0.9# | 190 | 2.995 | 50# | 0.3# | 0.77# | - | 205# |
Specific heat capacity (J/ kg K) | 880 | - | 706.9# | 609# | 465# | 780 | 840 | 798 | 850 |
Density (kg/m3) | 2700 | 1680# | 2260# | 2750# | 7850# | 1440# | 1500# | 1500# | 2710# |
# Experimental value measured for the present work |