The impact of the corrugated absorber shape on the performance of a hemispherical solar still for water desalination: an experimental study

Solar stills utilization has received great interest in arid isolated regions. In this study, experimental approaches have been investigated to prove the impact of the corrugated absorber shape on the hemispherical solar still performance. Four geometric corrugated shapes including present flat, triangular, semi-circular, square forms were tested for the corrugated absorber. Five hemispherical solar distillers have been fabricated and tested. The first distiller includes flat absorber (HSD-FA), the second distiller includes square corrugated absorber (HSDSA), the third distiller includes semi-circular corrugated absorber (HSD-SCA), the fourth distiller includes triangular corrugated absorber (HSDTA), and the fifth distiller is conventional (CHSD). The first and second distillers were compared with the CHSD on the first day. On the second day, the third and fourth distillers were compared with the CHSD as well. Experimental results obtained showed that, the total freshwater yield of the HSD-FA, HSDSA, HSDSCA and HSDTA were improved by 16.67, 27.08, 39.58 and 48.96%, respectively, compared over that of the CHSD. These results prove that the use of the triangular corrugated absorber shaped provides the highest thermal efficiency, and the optimum corrugated absorber shape is the triangular corrugated absorber shape.


Introduction
Solar distillation systems have been used for years to turn salt water, contaminated water, and even liquid wastes into freshwater. In comparison with other basin types, the hemispherical basin solar still has the greatest area for solar radiation absorption Benoudina et al. 2021;. The main drawback of solar stills is their limited production. To address this issue, numerous initiatives have been made to enhance the thermal efficiency of solar stills (Chandrika et al. 2021 Feb 24;Prasad et al. 2021). Using a corrugated absorber board to increase the area of solar radiation absorption is one method. Increased heat transfer rates from the absorption plate to the aquarium water result from increased solar radiation absorption area, which boosts production (Hadi et al. , 2021c).

Literature survey
In order to increase the efficiency of distillation basins, we shall discuss some of the earlier research that addressed the usage of corrugated absorbent plates. The effectiveness of modified SS with corrugated absorber, wick material, and internal reflectors was studied by Omara et al. (2016). According to the results, the modified SS and traditional SS are respectively efficient to about 59% and 33%. El-Sebaii and Shalaby (2014) conducted a simulation and analysis of the single basin SS with corrugated performance. According to the simulated outcomes, the corrugated basin increased production by 24% in comparison with the flat basin. A tubular solar still's performance using a v-corrugated absorber and wick materials was investigated by Kabeel et al. in (2021). According to the findings, the typical tubular tube only achieves an average efficiency of 35% despite employing v-corrugated absorber and wick materials. Katekar and Deshmukh (2021) examined the yield of a single slope SS with stepped-corrugated absorber. When compared to a normal basin, the results found that the stepped-corrugated absorber enhanced productivity, energy and exergy efficiency by 147.93, 259.61 and 418.61 percent, respectively. Abdullah et al. (2021) investigated the impact of wick, PCM blended with CuO nanoparticles, flat and corrugated absorbers, and electric heaters for increased yield in a solar still. According to the findings, the use of corrugated absorbent, PCM combined with CuO nanoparticles, and electric heaters produced the highest yield of SS, with an improvement of 180% when compared to other improvements. Elsheikh et al. (2021) numerically studied by neural network model the productivity of the stepped SS with a copper corrugated absorber. Results indicated the productivity of the stepped with a copper corrugated absorber enhanced about 128% compared with that of conventional stepped SS. Omara et al. (2015) investigated the performance of the SS utilizing a variety of techniques (corrugated absorbers, wick, an external condenser, and internal reflectors). When compared to CSS, the data showed that these techniques increased productivity by 180 percent. Hafs et al. (2021) studied numerically (Comsol Multiphysics software model) a desalination system by solar still with of different absorbent surfaces shapes (flat, rectangular, triangular and spherical) and PCM as the thermal energy storage system. Based on the simulation results, the corrugated surface with rectangular ripples is the best and improves pure water productivity by 109%. In an inclined single basin SS, Hansen and Murugave (Hansen and Murugave 2017) investigated the geometry of new absorber plate configurations (flat, grooved, and fin absorbers form). They found that the overall production was around 5210 ml/day when solar still coupled with fin absorber shape is 74.5% more productive. The thermal and energy efficiencies of tubular solar stills with flat and semi-circular corrugated absorber were studied by Elshamy and El-Said (2018). According to the results, thermal and exergy efficiencies were improved by 25.9% and 23.7% percent, respectively. Additionally, it was demonstrated that using a semi-circular corrugated absorber as opposed to a flat absorber enhanced yield by 26.47 percent. Shalaby et al. (2016) designed the single slope SS with v-corrugated absorbers plate built-in PCM. According to the findings, the daily productivity of the still with the PCM integrated into the v-corrugated absorbers plate is 12% higher than that of the v-corrugated still without the PCM. In order to increase the efficiency of hemispherical solar stills, Attia et al. (2021a) (Hadi et al. 2021d) examined at the usage of several tray metals, including iron, zinc, and copper. When compared to CHSS, they discovered that using copper trays increase production by 53.125 percent, using zinc trays by 31.25 percent, and using iron trays by 14.6 percent. Attia et al. (2021a)   investigated the use of different basin metals (zinc and copper) to improve the performance of single slope SS. When compared to CSS, they found that employing copper basin enhances production by 34.63 percent while using zinc basin boosts output by 18.21 percent. A desalination system featuring a solar still and a corrugated fiber cement absorber was presented by Cardoso et al. in (2022). A stated 1971 mL/m 2 productivity rate. After being evaluated in a lab using the distillate from the trial, it was determined that the desalination system can generate drinkable water that meets WHO requirements. Swellam et al. (2023) evaluated the thermo-economic performance of a hemispheric solar still that was experimentally investigated and fitted with four different configurations. The daily production increased by ratios of 11%, 25.7%, 34.1%, and 39.6% in the four cases, respectively. The best water productivity and thermal performance were displayed in the fourth example. It had a 45% thermal efficiency and 4737.5 mL/ m 2 daily productivity. A model for forecasting the performance of the solar still was created by Mohsenzadeh et al. (2022) (Mohsenzadeh et al. 2022). The model was able to produce results that were within 5.2% of the experimental results. At salinity and water depth of 3% wt and 10 mm, the model's thermal efficiency and daily yield were 19.2% and 2.52 L/m2, respectively. A model with a 6% variation was developed by Lisboa et al. (2022). The major purpose of the model was to evaluate how solar stills performed under various operating and design conditions. Hollow coper fins were utilized by Javad Raji Asadabadi and Sheikholeslami (2022) to boost the output of a pyramid distiller. Given that 5.33 L/m 2 of generated water was produced, the distiller's water productivity was 62.5% higher than that of a traditional distiller (Asadabadi and Sheikholeslami 2022). Darabi et al. (2022) introduced a single slope distiller modified with tilt wick and reflectors while, the evaporation exhaust was diminished by an external condenser. The proposal produced 1.71 L/m 2 of water at a thermal efficiency and cost of 46.13% and 0.041 $/L, respectively. Finally, the most current developments in the creation of vertical solar stills were reviewed by Abbaspour et al. in (2022). According to the authors, a multi-effect vertical solar still may produce water at a rate of 5.78 L/m 2 h.

Aim and novelty of the current study
A variety of solar distiller designs, including typical single slope, tubular, pyramid, stepped, and ad wick type solar distillers, were inferred from the prior survey. Numerous academics have made numerous improvements to traditional solar distillers with tremendous results. Additionally, certain designs require additional components like flat plate collectors, condensers, etc. in order to increase the daily productivity and overall distilled production. However, the sum of their costs will be substantial. In addition, some designs had high distillation yields, but the daily energy efficiency was rather low because the area exposed to solar irradiance increased. Therefore, considering utilizing a hemispherical distiller to increase the condensation surface area could be a useful technique for enhancing still yield. The hemispherical distiller has a larger surface area than a single slope solar distiller. The hemispherical cover is thereafter thoroughly exposed to the outside, lowering its temperature and enhancing condensation. The integration of different corrugated absorber geometries and thermal economic analysis has not been addressed in previous studies. Consequently, the novelty of the presented study is based on: first, a comparison of types of corrugated absorbers (flat, square, semi-circular and triangular). Second: Conducting thermal and economic analyzes of the corrugated absorber types. According to experimental trials, the use of a hemispherical distiller with a basin corrugated absorber is aimed at improving water productivity at a low-cost. To achieve the following objectives, four hemispherical distiller models were designed, manufactured, and examined.
• Measurement of the water production of the proposed hemispherical distiller configurations. • Economic analysis of hemispherical distiller productivity configurations. • Comparison of the water harvest of HSD-FA, HSDSA, HSDSCA and HSDTA with CHSD. • Comparison of thermal, thermodynamic, and economic performance of HSD-FA, HSDSA, HSDSCA and HSDTA with CHSD.

Hemispherical solar still
Hemispherical solar still consists of a transparent acrylic cover and a circular water basin. The salt water in the basin absorbs thermal solar heat that is transmitted through the transparent acrylic cover. Brine water heats up and evaporates due to the thermal process. The condensed water flows down the sides of the cap by the effect of gravity and is then collected. Five different configurations were designed, manufactured and tested in El Oued City, Algeria (Latitude of 33.3683° N, Longitude of 6.8674° E). The system process was started, in a May 2022, at 07:00 and the data were collected hourly during the day until 18:00. The proposed hemispherical solar still system is displayed in Fig. 1 as consisting of a transparent acrylic cover and a basin made of black-colored wood with a diameter of 40 cm and a thickness of 3 mm. It is designed to pour salt water into this basin.
The water basin has diameter and height of 36 cm and 4 cm, respectively. However, the bottom of each trough is different; silicon layer, flat plate, square corrugated, semi-circular corrugated, triangular corrugated, for increase the surface area subjected to the solar radiation as presented in Fig. 2. Figure 2 illustrates the geometry and the dimensions of the different corrugated absorber shape. Five hemispherical solar distillers were designed and manufactured. The first still includes flat plate absorber (HSDFA), the second distiller includes square corrugated absorber (HSDSA), the third distiller includes semi-circular corrugated absorber (HSDSCA), the fourth distiller includes triangular corrugated absorber (HSDTA), and the fifth distiller is conventional distiller (CHSD). The first and second distillers were compared with the CHSD on the first day. On the second day, the third and fourth distillers were compared with the CHSD as well. Figure 3 shows the pictorial view of the experimental setup actual. The hemispherical distillers were maintained at 1.5 cm water levels.

System process and collection of data
Water depth of the basin, in all the testing cases, was kept fixed at 1.5 cm. The operational procedures, as shown in Fig. 3, involved installing a thermocouple (K-type) wire into each device after saltwater charging. Hourly records were kept of the volume of distilled water used and the temperature distribution throughout the apparatus (water basin, absorber, and inner and outer surfaces of the glass transparent cover). Additionally, data on the local weather conditions, including solar intensity and ambient temperature, were gathered. All the data that have been recorded are subjected to an uncertainty analysis. The digital thermometer, the calibrated flask, and the solar power meter are the sources of error. The accuracy and uncertainty of the measuring equipment are displayed in Table 1. The uncertainties in the experimental outputs are influenced by the errors in the preliminary measurements. Experimental error analysis was conducted using the procedure presented by Holman modeling as follows (Holman 2012): Let the result X be a given function of the independent variables V 1 , V 2 , V 3 ,… V n .
Let W R be the uncertainty in the result and W 1 , W 2 , W 3 ,.… W n be the uncertainties in the independently variables V 1 , V 2 , V 3 ,.…y n , respectively. W R can be computed as given (Cardoso et al. 2022); The relative error (RE) is represented as follows:

Results and discussions
The solar distiller with a hemispherical shape and five distinct corrugated basins, as described previously, were compared with CHSD in El-Oued, south Algeria. The key objective of the current study is to inspect the circular corrugated basin usage in improving the hemispherical solar distillers' performance. Therefore, a comparative analytical study of different corrugated basin configurations of a hemispherical distiller and CHSD has been investigated through the freshwater productivity and the freshwater distillate cost. According to the previous articles, such   , the performance and productivity of the hemispherical solar stills are measured through several variables. These variables are the total solar intensity per hour, the climate temperature with time, the temperature variation per hour for the salt water, outer glass transparent cover, and inner glass transparent cover. Figures 4 presents the variation of the total solar intensity per hour and the climate temperature with time for the different examination days for the solar distiller with a hemispherical shape, with five different basin configurations, in May 2022 from 07:00 to 18:00. It can be observed that the solar irradiation and the climate temperature rise steadily till 13:00, and later it decreases till 18:00. Also, the solar intensity was 654 W/m 2 on average and 1001 W/m 2 at its highest point. Additionally, the highest and average climate temperatures were 46.0 °C and 38.9 °C , respectively.
For CHSD, HSDFA, HSDSA, HSDSCA, and HSDTA, Fig. 5 displays the temperature variation per hour for the salt water, outer glass transparent cover, and inner glass transparent cover. The temperatures of the salt water and the glass transparent cover of the HSDTA are the highest compared to the other hemispherical solar distiller designs. However, CHSD has the lowest salt water and glass transparent cover temperatures. These variances, in the water temperatures of the five hemispherical configurations, are as a result of a greater water surface area and area for heat transfer exposed to the incident radiation for the HSDTA as compared to the other hemispherical solar distiller designs that collect higher incident solar energy and convert it into thermal energy. It is obtained that the average and highest water temperatures of the HSDTA were reached to be 57.2 °C and 67.0 °C , respectively, however, they were found to be 47.5 °C and 56.0 °C , respectively, for the CHSD. According to the results shown in Fig. 6, HSDFA, HSDSA, HSDSCA and HSDTA show a significant increase in the mean temperature of the salt water compared to CHSD. The percentage increase values for HSDFA, HSDSA, HSDSCA and HSDTA are 3. 16, 8.1, 17.4 and 20.4, respectively. Freshwater productivity variations through time for CHSD, HSDFA, HSDSA, HSDSCA, and HSDTA at a salt water 1.50 cm depth inside the solar still are depicted in Fig. 6. According to the findings, Fig. 6 shows that the freshwater output per hour started out at low levels in the morning at the outset of the solar still and gradually increased with time, reaching its peak productivity at 13:00, after which it gradually decreased until late in the day. According to Fig. 5d, thermal energy storage occurred from 8:00 till 13:00. As a result, the water productivity was maximized at 13:00 and decreased gradually due to heat transfer exchange.
It is clearly observed, as represented in Fig. 7, that at all periods the freshwater productivities of HSDFA, HSDSA, HSDSCA and HSDTA were much higher than that of CHSD. This is the effect of used corrugated copper basin, which had better water harvests than CHSD because of its outstanding thermal properties and large surface exposed to incident radiation. In contrast to the other corrugated basin designs, HSDTA had the highest rates of distilled vapor generation and freshwater productivity. The average relative productivity of HSDFA, HSDSA, HSDSCA and HSDTA are 1.19, 1.36, 1.57 and 1.67, respectively.
The relative freshwater productivity per hour (λ) for each distiller design is: A histogram graph of the daily total freshwater production for the five proposed hemispherical distillers is shown in Fig. 8. The relative freshwater productivity per day for HSDFA, HSDSA, HSDSCA, and HSDTA in comparison to CHSD is shown in Fig. 9. However, it is stated that at a constant saltwater depth throughout the day cycle, the hemispherical distillers with corrugated basins boosted the freshwater harvest by 16.67%, 27.08%, 39.58%, and 48.96% over the CHSD (480 mL/day) for HSDFA, HSDSA, HSDSCA, and HSDTA, respectively.
The relative freshwater productivity per day (β) for each distiller design is: Freshwater productivity per hour of each hemispherical solar distiller Freshwater productivity per hour of CHSD Fig. 3 The pictorial view of the experimental setup   Comparison of daily productivity of present work with previous works Table 2 contrasts the yield enhancement for our study and other modified solar stills (corrugated absorber shapes). The data shown here demonstrate the value of (5) = Total fresh wate rproductivity per day of each hemispherical solar distiller Total fresh water productivity perday of CHSD the corrugated absorber shapes as an increase in yield for this solar still is noteworthy. Compared to several other solar still designs with a corrugated basin, this Tabulated exhibits the largest improvement in yield (48.96% from HSDTA).

Economic evaluation
The enhancement percentage in the daily productivity due to the use of corrugated absorber shapes The average productivity values of the hemispherical solar stills (CHSD, HSD-FA, HSDSA, HSDSCA, and HSDTA) over the duration of testing are shown in Table 3. The improvement of HSDTA is 48.96% more for CHSD, as indicated in Table 3, and is the greatest for solar stills other HSD-FA, HSDSA, and HSDTA. These findings demonstrate that the corrugated absorber efficiently uses the energy used in the heating and evaporation of salt water. As a result, it uses less energy than CHSD.
d. Temperature variation per hour between the internal and external glasses.

Conclusion
In this study, trials were performed to enhance the thermoeconomic performance and productivity of hemispherical solar distiller. Five experimental setups (conventional, flat absorber, triangular corrugated absorber, semi-circular corrugated absorber, square corrugated absorber) were constructed and tested. Based on the experimental results, the following conclusions were drawn: 1. Increased heat transfer between the corrugation absorber and salt water is caused by the increased contact surface area between the plate and the corrugated absorber. Salt water in the basin is heated more effectively when the temperature of the corrugation absorber is higher. Solar radiation is better absorbed by corrugated absorbers.
2. In a hemispherical solar distiller with a flat absorber, square corrugated absorber, semi-circular corrugated absorber, and triangular corrugated absorber, the average productivity is 5.60 kg/m 2 /day, 6.10 kg/m 2 /day, 6.70 kg/m 2 /day, and 7.15 kg/m 2 /day, respectively. Contrarily, CHSD produces 4.80 kg/m 2 /day on average. 3. Improvement in productivity from HSD-FA, HSDSA, HSDSCA and HSDTA are 16.67, 27.08, 39.58 and 48.96%, respectively, compared of CHSD. 4. These results prove that the use of the triangular corrugated absorber shaped provides the highest thermal efficiency, and the optimum corrugated absorber shape is the triangular corrugated absorber shape.
The limitation of the current study is the climate conditions and the hemispherical distiller configurations used in the absorber. However, for a future scope of the presented work, wick or PCM could be added and investigated for an attempt to further improving productivity.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). No funding used in this paper.

Conflict of interest No competing interests.
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