Performance analysis of tubular solar still with different water depths on corrugated and flat absorbers

TSS

Tubular solar still

RSM

Response surface methodology

NPs

Nanoparticles

CuO

Copper oxide

LSTM

Long short-term memory

PCM

Phase change material

SS

Solar still

D_w

Water depth

m_e

Mass of water accumulated

L_fg

Latent heat of vaporization

I(t)

Solar radiation

A

Area of solar still

ANOVA

Analysis of variance

The need for fresh water is vital for better human civilization and the ecosystem. Around two-thirds of the Earth's surface is covered with seawater and two-thirds with land mass. The amount of water available for drinking is almost 3%, which is very low for meeting the present demand and utilization. Most of the water on the surface, such as rivers, lakes, and water streams, is mainly affected by discharging industrial waste. Due to the population overuse of surface and groundwater resources and economic development, the water shortage problem increases along with urbanization (Kannan et al. 2022). Fresh water is utilized in our daily life, and it is used in various tannery industries, for food production, electrical power generation, and sanitation purposes. However, the present utilization of fresh water has made several advancements. Several desalination plants have been developed to meet the global water shortage, where the requirement for water is high. Thermal and membrane processes are widely used techniques in desalinating sea water. The demand for desalination is projected to grow rapidly. Due to the increasing energy crisis, the use of renewable energy is gaining more attention (Ahmadi et al. 2020; Khorampoor et al. 2020; Menni et al. 2020). A recent study revealed that the use of renewable energy in desalinating water is only 1% (Sathyamurthy et al. 2017; Kabeel et al. 2019; Elsheikh et al. 2022; Sharshir et al. 2019).

To effectively improve potable water production and thermal efficiency with simultaneous energy storage, Kabeel et al. (2020) used paraffin wax filled in copper tubes in the basin. The potable water production from a tubular solar still (TSS) with copper tubes and phase change material (PCM)-filled was found to be 9.05 L/m², and it was 52.37% higher than a conventional flat absorber solar still (SS) without PCM in copper tubes. There was a significant improvement in the daily efficiency of about 72.7% from the TSS with PCM-filled copper tubes. This was 115.5% higher than the SS without PCM. Experiments on the TSS using a black sponge at different thicknesses and densities were experimentally analyzed by Abdelgaied et al. (2021b). The thickness of the floating sponge varied in the range of 20, 30, and 40 mm, whereas the density of the sponge varied in the range of 16, 22, and 30 kg/m³. It was reported that the increase in the density of the sponge leads to a decrease in cumulative yield and daily efficiency of the TSS. In addition, an increase in the density of the porous sponge leads to a reduction in porosity, which leads to the entrapment of evaporated water from the floating sponge. The average daily yield of fresh water produced using 16 kg/m³ was found to be 5.37 kg/m², whereas the yield of fresh water obtained using 20 kg/m³ was found to be 3.77 kg/m².

Abdelgaied et al. (2021a) used hollow circular and square fins to enhance the thermal performance of TSS as a heat storage medium. In the second phase of the experiment, hollow fins were filled with PCM to store the heat energy during the sunshine hours and dissipate the heat during the night hours. With the addition of hollow square and circular fins in the absorber, the daily yield of fresh water was enhanced, and it was found to be 6.11 and 5.52 L/m², respectively, which is 47.2 and 33% higher than the traditional SS without fins. The improvement in the water produced using circular fins was the higher circumferential area compared to the circumferential area of a square cross-section that was immersed in the water. Furthermore, the influence of PCM in the hollow circular fins enhanced the potable water produced by 90.1% compared to the TSS. The effect of the reversed solar collector on the hemispherical SS using a v-corrugated absorber was experimentally analyzed by Kabeel et al. (2022) for enhancing fresh water production. Along with the incoming solar radiation transmitting through the transparent cover, the radiation was reflected using the parabolic reflector to heat the bottom of the basin. Using the reversed solar collector, the cumulative fresh water produced from the hemispherical SS was found to be 7.85 kg/m², and it was higher by about 68.82% than the hemispherical SS using a flat absorber. It was also reported that using the reverse solar concentrating effect with the corrugated absorber, the cost per liter (CPL) of water produced was reduced to about 27.12% compared to the hemispherical SS using a flat absorber.

The energy and exergy approaches to the finned-type TSS were experimentally and theoretically analyzed by Sathyamurthy et al. (2022). The theoretical and experimental yield results obtained from the TSS with and without fins were in good agreement. It was reported that the thermal efficiency was improved by 69.9% compared to the TSS without fins in the absorber. Similarly, the fresh water produced from the TSS using fins in the absorber was enhanced by 53.08%. The analysis revealed that the cover material of TSS exhibits similar irreversibility using a finned and flat absorber. However, on comparing the irreversibility of water, the irreversibility is lower using a finned absorber. Comparing the exergy efficiency of TSS, the absorber with fins in the TSS was higher compared to the exergy efficiency of TSS using a flat absorber. Abdullah et al. (2022) used a convex-shaped absorber in a conventional solar still (CSS) to enhance fresh water production. Along with the convex-shaped absorber with wick material, PCM with nanoparticles was added beneath the absorber. The height of the convex absorber varied from 5 to 20 cm. Cotton and jute cloth was used as wick material to increase the rate of evaporation. Silver (Ag) nanoparticles with a maximum concentration of 2.5% were added to black paint. In addition to the convex absorber, the Ag NPs were added at a concentration of 2.5% in paraffin wax. The fresh water production and thermal efficiency of CSS using a convex absorber were optimized at 15 cm using jute cloth as wick material. It was reported that the absorber coating with Ag NPs in black paint improved the fresh water production by 72%. With the addition of PCM with Ag NPs along with wick in a convex absorber, the fresh water produced was enhanced to about 112%. Sharshir et al. (2020) enhanced the thermal performance of a pyramidal SS using v-corrugated and flat absorbers and compared the energy, exergy, and economic analysis of pyramidal SS using a flat absorber. Along with the v-corrugation, they used wick material and CuO nanofluids in the basin to enhance the rate of evaporation. Results showed that the CuO nanofluid and wick material in the v-corrugated absorber enhanced the fresh water production by 72.95%, while the SS with wick material alone in the corrugated absorber SS enhanced by 45% compared to the flat absorber pyramidal SS. The energy and exergy efficiency of pyramidal SS with v-corrugated absorber, CuO nanofluids, and wick material was enhanced to about 60.5 and 93%, respectively. The CPL of water produced was reduced by 28% compared to the flat absorber pyramidal SS.

The experimental and numerical investigation of v-corrugated absorber TSS with wick material on TSS was analyzed by Kabeel et al. (2021). Black cloth (jute) material was used in the corrugated absorber, and half of the ‘v’ is filled with sea water. The jute cloth placed in the corrugated absorber gets wet. The wettability of jute cloth depends on the capillary effect, which enhances the evaporation rate. Results exhibited that the use of a v-corrugated absorber improved the average daily efficiency from 35 to 51.4%, and there was an improvement of about 44.82% in the daily fresh water produced. There was a deviation of about 2.5% between the excremental and predicted values of fresh water and temperatures. The empirical modeling for the prediction of fresh water production using a long short-term memory neural network for the stepped SS with a v-corrugated absorber was done by Elsheikh et al. (2021). It was reported that the stepped SS with a v-corrugated absorber enhanced fresh water production by 128% more than the traditional SS. The simulation of the model used for predicting the fresh water used was more accurate. It was further concluded that the modified stepped SS with a v-corrugated absorber can be commercialized for remote locations with a huge demand for fresh water.

Abdelaziz et al. (2021) used wick material and nanofluid in the v-corrugated absorber in TSS for fresh water production. Furthermore, nanoparticles were added to paraffin wax and used as energy storage beneath the v-corrugated absorber. Results showed that the proposed modification enhanced thermal and exergy efficiency to about 82.16 and 221.8%, respectively. Similarly, there was a reduction in the cost of fresh water production by 22.47% compared to the TSS without any modification. Shalaby et al. (2016) used a v-corrugated absorber in a single slope SS and added paraffin wax as a heat storage medium beneath the basin. Thirteen copper tubes are placed in the heat storage, and vents are provided in the copper tubes. The copper tubes take a higher volume of wax during melting. The water mass in the conventional and modified basins of SS varied from 25 to 35 kg, while the PCM mass was kept constant. It was reported that using wick material in the corrugated absorber and PCM improves daylight productivity, while nighttime productivity decreases. With PCM alone in the corrugated absorber, daylight productivity was lower, while nighttime productivity increased. Since the wick material wetted in the v-corrugated absorber was directly exposed to the incoming solar radiation, a significant improvement in the fresh water during the daytime is higher. However, the daily efficiency reduced from 37.1 to 23.2% on increasing water mass in the v-corrugated absorber and PCM as energy storage. On adding wick material to the v-corrugated absorber and PCM as energy storage, the daily efficiency of SS was found to be 34.8% for the water mass of 25 kg. Thakur et al. (2022) used candle soot carbon nanoparticles dispersed in PCM (paraffin wax) and studied the thermal performance of TSS. The concentration of candle soot nanoparticles was limited to 0.3%, and it is dispersed in paraffin wax, and the same is filled in tubes. It was reported that the daily fresh water produced by the paraffin wax with 0.3% of candle soot nanoparticle was improved by about 87.85% compared to the SS without paraffin wax. TSS using eggshell powder as an energy storage medium for improving the potable water at different water depths was experimentally analyzed by Thakur & Sathyamurthy (2022). The impact of eggshell as energy storage improved the fresh water production by about 67.64% compared to the SS without energy storage at the lowest water depth. Similarly, the thermal efficiency of the TSS with eggshell as energy storage for a water depth of 20, 15, and 10 mm was found to be 36, 42, and 48%, respectively.

From the review of literature, it is identified that the water depth in the SS plays a significant role in thermal performance, and in the TSS, with a corrugated absorber, the influence of water depths has not been widely studied. Because of the v-corrugated absorber, the amount of surface area of water exposed to the incoming transmitted solar radiation is increased. The triangular water trough holds a small amount of water to evaporate, and in a flat absorber SS, bulk water mass will be available, and the energy requirement for evaporating water would be higher. In the present experimental investigation, the effect of water depth on v-corrugated and flat absorbers of tubular cover SS is analyzed and compared. The water depths in the basin of the v-corrugated and flat absorber vary from 5 to 15 mm. The thermal performance, hourly yield, daily efficiency, and daily yield from the v-corrugated and flat absorber of TSS on various water depths are compared and analyzed. Furthermore, the yield of fresh water produced from the TSS using flat and v-corrugated absorbers is predicted using the response surface methodology (RSM) technique for various operating conditions such as solar radiation, water depth, and water temperature. Utilizing RSM, the regression equation is created to predict the reactions of the yield of fresh water produced from the SS using v-corrugated and flat absorbers.

Using a 1-mm-thick mild steel sheet, the flat and v-corrugated absorbers are fabricated in the same area. The breadth and length of the absorber are 0.27 and 0.57 m, respectively. The maximum capacity of the water that can be filled inside the absorber of SS is 2.3 L, and the height of the absorber is 0.15 cm. The bottom and side walls of the absorber materials are packed using insulation material, and the heat loss from the absorber is avoided. The tubular cover is made of acrylic material with transparency of 0.88, and the thickness of the tube is 2 mm. The length and diameter of the acrylic tube are 0.6 and 0.3 m, respectively. The absorbers are placed in the axial position of the tube, and hinge support is provided to the absorber. Gaps are provided between the absorber wall and the tube surface, so that the water droplets gliding to the bottom of the tube are not affected by the mix of water droplets again falling in the basin where the water is placed. Using the powder coating method, the black paint is coated in the flat and corrugated absorber to improve the absorptivity of the solar radiation and to avoid the formation of rust particles due to the continuous interaction of water with the absorber. The corrugation of the mild steel plate is formed by keeping it in a V-shape with an angle of 60°. The V-corrugation improves the free surface area of water with solar radiation for enhanced evaporation.

The fed water is continuously supplied into the basin of the corrugated and flat absorber of the TSS, and the thermal performance is continuously recorded. Water from one well, located in the industrial area of Tamil Nadu, was chosen as the water source for feed in the still. Fed water is separately stored in a plastic tank with a capacity of 30 L. After evaporation of water from the surface, the vapor gets accumulated inside the circular enclosure and condensed in the inner cover surface. The condensed water droplets are taken into the calibrated flask. The flow control valves are provided between the SS's inlet and the storage tank. Suitable measuring instruments are used to assess the thermal performance of SS. The thermal performance factors include solar radiation, wind velocity, ambient, cover, basin and water temperature, and fresh water yield. On an hourly basis, the temperatures are recorded using a J-type thermocouple, while the wind velocity and solar radiation are measured using an anemometer and a solar power meter. The experiments are conducted for three different water depths, namely 0.05, 0.1, and 0.15 cm, for the modification proposed in the SS, and the same is compared with the SS with a flat absorber. All experiments were carried out between 9:00 AM till 6:00 PM in May 2021.

The measuring instruments are subjected to errors, and the errors that occurred during the experimentation using these measuring instruments are summarized in Table 1. The errors that occurred during the experiments are the measurement of solar radiation, wind velocity, different temperatures (cover, basin, water, and ambient), and fresh water collecting jar. Based on the observed reading and mathematical correlation, the uncertainty associated with solar radiation, wind velocity, temperature sensor, and fresh water collection were estimated as 3.5, 5.7, 1.5, and 2.7%, respectively.

Continuous evaporation of water placed in the basin increases the cover temperature as the vapor accumulates in the inner cover surface, and the solar radiation interacts with the outer cover. The maximum cover temperature using water depths of 5, 10, and 15 mm from TSS with a corrugated absorber is recorded as 46, 45, and 43 °C, respectively. The higher the evaporative heat transfer in the circular enclosure, the higher the maximum cover temperature attained. The major driving force for fresh water yield is the water-to-cover temperature difference. From the experimentation, it is found that the temperature difference between water and cover for a water depth of 5, 10, and 15 mm using a corrugated absorber is found to be 4–10, 1–10, and 1–10 °C, respectively, whereas the temperature difference for TSS using a flat absorber is found to be 1–8, 1–7, and 1–5 °C, respectively. Similarly, the average temperature difference between water and cover using a corrugated absorber is estimated as 7.2, 6.9, and 6.6 °C, while the TSS with a flat absorber is estimated as 4.7, 4.2, and 3.1 °C for 5, 10, and 15 mm water depths, respectively.

Using Equation (1), the daily efficiency of the SS with flat and v-corrugated absorbers is calculated. From the observed values, it is found that the daily energy efficiency of SS using the v-corrugated absorber for 5, 10, and 15 mm is calculated as 48.5, 44.7, and 44.3%, respectively. The daily efficiency is improved by about 51.85, 56.2, and 68.38% for 5, 10, and 15 mm using the TSS with a v-corrugated absorber compared to the TSS using a flat absorber.

Figure 9(b) and 9(e) shows the prediction of hourly yield from the TSS with a corrugated absorber using water temperature and solar radiation as input variables. It is seen that the yield from the TSS with a corrugated absorber increases with increased water temperature. Similarly, the prediction of yield using water temperature and water depth as input variables is plotted in Figure 9(c) and 9(f). From Figure 9(c) and 9(f), it can be inferred that the temperature of the water is a critical parameter for the higher rate of evaporation as the temperature of the water is higher at a lower water depth.

In water quality analysis, the present study explored three major parameters: total dissolved solids (TDS, mg/l), pH and electrical conductivity (EC, mS/cm). The feedwater from the open well is fed into the TSS, and its parameters like TDS using TDS meter, EC using EC meter, and pH using pH meter were measured before and after the distillation. Before distillation, the TDS of feedwater was 625 mg/l, which reached only 7 mg/l after the desalination. Before water treatment, the EC was 517 mS/cm and it reached 34 mS/cm. In addition, pH values of feedwater before and after desalination were 6.3 and 7.3, respectively. All the collected fresh water after desalination was in the range described by the WHO for drinking standards after adding the suitable minerals in the distilled water.

Solar radiation, water depth, and water temperature are the three most important variables to consider when optimizing fresh water yield from an SS. The present study deals with the experimental analysis of TSS with a v-corrugated absorber, and the influence of water depth is analyzed for the climatic condition of Coimbatore, India. Based on the experimental analysis, the following conclusions have arrived:

• The peak water temperatures recorded for the TSS with a flat absorber using water depths of 5, 10, and 15 mm are 53, 52, and 51 °C, whereas, for the similar condition using a v-corrugated absorber in the TSS, the temperatures are enhanced to about 57, 56, and 53 °C, respectively. The increase in the temperature of the water using a v-corrugated absorber is the higher exposure area of the water surface and the corrugated surface, which simultaneously increases the rate of evaporation from the water surface.

• The peak cover temperature from the TSS using a corrugated absorber is higher compared to the cover temperature of TSS using a flat absorber. The increase in the temperature of cover using a corrugated surface is completely due to the higher vapor temperature attained in the circular enclosure over the entire circumferential area of the circular cover. The peak temperatures of the cover were recorded as 46, 45, and 43 °C for 5, 10, and 15 mm water depths, respectively, using a v-corrugated absorber.

• The peak hourly yield from the SS using 5 mm water depth from the TSS using v-corrugated and flat absorbers is recorded as 0.75 and 0.44 kg, respectively, and in both cases, the yield decreases with an increase in depth of water from the SS using v-corrugated and flat absorbers. Utilizing a v-corrugated absorber with 5 mm water depth, the hourly yield increased to around 70.45% when compared to a flat absorber in TSS.

• The mean water temperatures calculated using a v-corrugated absorber at depths of 5, 10, and 15 mm are found to be 49.3, 48.1, and 47.6 °C, respectively; meanwhile, using a flat absorber for the same water depths, the mean water temperatures are found to be 44.5, 44.2, and 44.1 °C, respectively.

• The TSS with a v-corrugated absorber results in cumulative yields of 4.35, 3.86, and 3.83 kg for water depths of 5, 10, and 15 mm, respectively.

• The average daily efficiencies from the SS using v-corrugated and flat absorbers are found to be 48.5, 44.7, and 44.3% for water depths of 5, 10, and 15 mm, respectively, which is comparatively higher than the SS using a flat absorber.

• The RSM technique shows that the accuracy of predicting the yield using solar radiation, water temperature, and water depth is more reliable.

From the analysis, it is found that the use of hybrid composite nanoparticles in PCMs improves the rate of fresh water produced from the SS along with the v-corrugated absorber in the basin of TSS. In addition to latent heat energy storage, parabolic concentrators can also be used to focus the incoming solar radiation to heat the basin for a higher rate of evaporation.

There is no funding received for the research work carried out.

Conceptualization, methodology, resources, formal analysis, writing – original draft preparation, review and editing, supervision and investigation were carried out by R.S. and A.E.K.; Writing – review and editing were carried out by A.R.P. and A.K.T.. All authors have read and agreed to the published version of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.