A comparative assessment of innovative hemispherical solar distillers with cylindrical and conical fins: an experimental approach

ABSTRACT The present study aims to reach the best design of the fins installed on the basin absorber surface that achieves the highest performance of the hemispherical solar distillers. The fins represent one of the most important improvement techniques utilized to increase the evaporation rates inside the solar distillers, by increasing the heat transfer area between the absorption surface and the basin water. However, some designs of the fins have negative effects due to the shading problem, which causes a reduction of the absorption rates of solar rays. Therefore, the current study aims to find the best design of the fins that achieve the least shading effects and the highest performance of hemispherical solar distillers. To achieve this idea, two types of fins (cylindrical and conical) with a diameter of 1.5 cm and a height of 3 cm, with different gap distances of 3, 3.75, and 4.5 cm were studied. To achieve these, three hemispherical solar stills were tested under the same weather conditions, first is a conventional hemispherical still without fins (CHSWF) which is the reference distiller. The second is the hemispherical distiller with cylindrical fins (CyFHS), and the third is the hemispherical distiller with conical fins (CoFHS). The results show that the hemispherical distiller with cylindrical fins (CyFHS) productivity values are 5.55, 6.00, and 6.70 kg/m2, respectively, while the hemispherical distiller with conical fins (CoFHS) productivity values are 6.20, 6.65, and 7.15 kg/m2 at the gap distances of 3, 3.75, and 4.5 cm, respectively, compared to 4.20 kg/m2 produced by CHSWF. The results showed that the use of conical fins with a gap distance of 4.5 cm represents the optimal fin design that achieves the least shading and the highest performance of hemispherical solar distillers, with improvement rates of 70.24% compared to reference distiller (CHSWF).


Introduction
Recently, as a result of population growth, urbanization, industrialization, and climatic changes, water scarcity, and pollution have become one of the most serious issues facing all countries of the world, especially developing countries in the Middle East and North Africa (Attia et al. 2021).The sea contains 97% of the world's salt water, the polar region contains 2% of the world's water in the form of ice, and the remaining 1% of the water is available to humans and animals (Essa, Abd Elaziz, and Elsheikh 2020).Therefore, it is necessary to use brackish water and seawater treatment techniques to produce drinking water (Attia et al. 2021).Solar stills are one of the solar desalination technologies, which are characterized by their simplicity and ease of maintenance (Attia et al. 2021).However, it is defective in its low productivity (Attia et al. 2021), so many studies were conducted, all of which aim to improve the productivity of solar stills by incorporating energy storage material (Prasad et al. 2021), nano-particles (Benoudina et al. 2021), mirrors (Chandrika et al. 2021), nanofuid (Bellila et al. 2021), with the solar stills.Manokar and Winston (2017) investigated the differences between a single-basin solar distiller made of acrylic with aluminum fins and single basin solar distiller made of galvanized iron.The results showed that a single-basin acrylic solar distiller with aluminum finned has daily productivity of 2640 mL/m 2 /day, while single slope solar distiller made of galvanized iron has daily productivity of 2340 mL/m 2 /day.Attia et al. (2021) used iron fins of various lengths (1, 2, and 3 cm) and spacing in the absorber plate of 5 and 7 cm to increase the yield of a hemispherical distiller.The results showed that using fins in the absorber plate with a length of 2 cm and spacing of 7 cm improves performance by 56.73%.Vaithilingam et al. (2022) compared acrylic stainless steel with copper fins to acrylic stainless steel without copper fins.The acrylic solar distiller with copper fin produced a maximum daily output of 5.08 kg, according to the findings.The maximum daily output of the acrylic solar distiller without copper fins was 3.75 kg.At different water depths, Rajaseenivasan and Srithar (2016) investigated theoretical and experimental SS performance with circular and square hollow fins covered by wick (1, 2, 3, and 4 cm).The results showed that the experimental analysis and theoretical values were in good agreement.Still yielded 4.55 and 4.25 kg/m 2 per day in square fins with and without wick covering, and 4.27 and 3.99 kg/m 2 in circular fins with and without wick covering, respectively, compared to 3.16 kg/m 2 in the conventional still.Total CO 2 emission mitigation ranges from 5.6 to 36.6 t for a still with square fin and wick covered, with a life span of 5-30 years.The effect of fins and internal reflectors on single slope solar distiller performance was investigated experimentally by Bataineh and Abbas (2020).
Internal reflectors performed better in the winter due to their high efficiency.The daily average increase in productivity on conventional still, still with fins, still with internal reflector, and still with fins and internal reflector, respectively, was 3.6428, 3.6445, 4.0965, and 4.6085 L/day/m 2 .Tuly et al. (2021) evaluated modified conventional stills for (fins, PCM, External condenser, and wick materials) cases.When compared to other stills, a solar still with an external condenser produced extremely high productivity (3.07 L/m 2 ).The productivity of a modified still with an external condenser was 24.79% higher than finned stills and 9.76% higher than conventional stills.The effect of fins on tubular solar still productivity was investigated experimentally by Sathyamurthy et al. (2021).Fins improved productivity; it was discovered that adding fins increased productivity by 0.65 L/m 2 , compared to 0.55 L/m 2 for traditional tubular solar stills.External fins on passive solar still were investigated by Mohaisen, Esfahani, and Ayani (2021).External fins improved distillate by 92.3%, according to the researchers.Its research revealed that these stills are cost-effective; the cost per liter for stills with external fins is 0.007 $/l, while the cost for still without fins is 0.0117 $/l.Panchal et al. (2020) looked into the effect of fins (vertical and inclined (30°)) on the productivity of traditional solar panels.Windchime pipes were recycled into fins.According to the findings, the inclined finned solar distiller produced (2.322 L/m 2 ), the vertical finned solar distiller produced (2.375 L/m 2 ), and traditional SS produced (1.873 L/m 2 ).A pyramid-shaped solar distiller with hollow copper fins and PCM produced more energy than a pyramid-shaped solar distiller with or without hollow fins, according to Kabeel et al. (2020).Stills with hollow copper fins and no PCM produced 5.75 L/m 2 , while ordinary pyramid-shaped stills with no modifications produced 4.02 L/m 2 , Shmroukh and Ookawara (2020) found that a stepped solar distiller with fins and inner and outer reflectors produced higher yields than a stepped solar distiller without any modifications.The first one had a productivity of 8.285 kg/m 2 , which was 129% higher than plain stepped stills without any modifications (3.615 kg/m 2 ).Attia et al. (2022) investigated the exergy/thermal analysis of finned in acrylic solar stills at various saltwater depths (1 cm, 2 cm, and 3 cm).The exergy efficiency of the finned acrylic SS at 1 cm, 2 cm, and 3 cm is 3.83, 3.22, and 2.7%, respectively, according to the findings, while the thermal efficiency is 42.54, 37.92, and 31.2%.To improve hemispherical distiller performance, Attia et al. (2021) used phosphate pellets with two concentrations of 1 and 2%.The use of phosphate pellets as a storage medium improved the hemispherical distiller's performance, according to the findings.When compared to a traditional hemispherical distiller, distiller productivity increased by 33.7 and 47.9% for 1 and 2% of phosphate pellets, respectively.Also, some previous studies have used reasonable thermal energy storage materials of indeterminate shapes, such as gravel (Attia et al. 2021), quartz rock (Murugavel et al. 2021), graphite (Attia et al. 2021), red brick coated with cement (Kabeel et al. 2019), sand (Attia, Kabeel, and Abdelgaied 2021), phosphate (Kabeel et al. 2022), wick convex absorber (Bacha et al. 2023), waste aluminum (Beggas et al. 2023), corrugated basin (Kabeel et al. 2022), flax fibers (Kabeel et al. 2022), rock salt balls (Attia et al. 2022), hollow fins (Attia et al. 2022), extended iron fins (Kabeel et al. 2022), three effective modifications (Abdelgaied et al. 2022), nanofluids and PCM (Abdelgaied et al. 2022), nano-coated and PCM (Abdelgaied and Kabeel 2021), and hollow fins with PCM (Abdelgaied et al. 2021) to improves the yield of hemispherical solar distillers.
The present study aims to reach the best design of the fins installed on the basin absorber surface, that achieves the highest performance of the hemispherical solar distillers.The hemispherical solar distillers are characterized by having a large area of receiving and condensing, so incorporating the fins with the absorber surface is very interesting to improve the thermal performance and then increase the evaporation rates within hemispherical solar distillers.By reviewing previous studies, it was found that the use of fins has a positive effect as it improves thermal performance and distillate production due to increased heat transfer areas.But some designs of the fins have negative effects due to the shading problem, which causes a reduce the absorption rates of solar rays.Therefore, the current study aims to find the best design of the fins that achieve the least shading effects and the highest performance of hemispherical solar distillers.To achieve this idea, the absorber surface is made from a cement layer with two types of fins (cylindrical and conical fins) with a diameter of 1.5 cm and a height of 3 cm at different gap spacing (3, 3.75, and 4.5 cm) were suggested in this manuscript to obtain the best configuration of fins that achieves the highest productivity.To achieve these, three hemispherical solar stills were tested under the same weather conditions, first is a conventional hemispherical still without fins (CHSWF) which is the reference distiller.The second is the hemispherical distiller with cylindrical fins (CyFHS), and the third is the hemispherical distiller with conical fins (CoFHS).In May 2022, the experimental study was carried out in the geographical conditions of El Oued, Algeria (33.3683°N, 6.8674° E).

Test-rig construction
In the present study, three hemispherical solar stills that have the same dimensions were built in this experiment work.One has conical fins, another has cylindrical fins, and the third is still a simple hemispheric as a reference.The diameter and height of the circular basin of the hemispherical solar stills are 36 cm and 4 cm, respectively.Temperature differences between the basin and the atmosphere cause heat transmission loss from the still basin edges and bottom.To address this issue, it is insulated with a black silicon material.The basin top is encased in a 3 cm thick transparent hemispherical cover.Figure 1 shows a photograph of a hemispherical distiller.
For the three hemispherical solar distillers, a 1-cm-thick cement layer was made at the bottom of the basin to obtain better thermal conductivity, in addition to being painted black to better absorb solar radiation.In order to increase the area of solar radiation absorption and heat transfer between the absorption surface and the basin water to improve the heat transfer, cylindrical and conical cement fins with a height of 3 cm and a diameter of 1.5 cm and different spacings (3, 3.75, and 4.5 cm) were manufactured and fixed on the basin cement layer.These fins are made of cement (the same material as the absorbent surface of the three distillers basin).During the experiments, the basin water depth is kept constant at 3 cm along the test days (maximum height of the fins).Table 1 lists the properties of the cement.
Figure 2 shows the photographic view of the cylindrical and conical fins utilized in the present experimental work which have the same height and diameter of 3 cm and 1.5 cm, respectively.

Experimental setup
To obtain the best fin configurations that achieve the lowest shading effects and highest performance of hemispherical solar stills.The three modules are tested in the same environmental conditions.Figure 3 shows the photo of the experimental setup.In the present work, the first distiller is the CHSWF which represents the reference hemispherical still devoid of the fins.The second distiller is the cylindrical finned hemispherical still (CyFHS) with spacing remains unchanged at 3, 3.75, and 4.5 cm (CyFHS for test case 1, CyFHS for test case 2, and CyFHS for test case 3).The third distiller is the conical finned hemispherical still (CoFHS) with a spacing of 3, 3.75, and 4.5 cm (CoFHS for test case 1, CoFHS for test case 2, and CoFHS for test case 3) were tested for 3 days in a row as shown in Table 2. Figure 4 illustrates the different cases of test configurations.On three sunny days in May 2022, the cylindrical finned still and conical finned still of energy and exergy performance were investigated for the level of the basin saltwater at 3 cm.The data were extracted for 60 min, starting at 7:00 a.m. and ending at 6:00 p.m.

Uncertainty analysis
Table 3 summarizes the specifications of the measuring instruments used in the present experimental work.Based on the accuracy, measuring ranges, and standard uncertainty of the measuring instruments shown in Table 3, the uncertainties in the cumulative yield and thermal efficiency were calculated using the calculation steps presented by (Holman 2012) which reached 1.45% and 2.28%, respectively.

Results and discussion
Experiments were carried out in three consecutive days in May 2022 at the University of El Oued in Algeria.The readings were taken hourly from 7:00 a.m. to 6:00 p.m.The effectiveness of hemispheric solar is still influenced by the solar intensity and ambient temperatures.As a result, hourly readings of the experiment's solar intensity and ambient temperature are required.During experimentation test hours, Figure 5 depicts the variation in ambient temperatures and solar intensity.The solar intensity increases until midday on each of the three test days, then gradually decreases as time passes until it reaches the lowest point of sunset.Around 1:00 p.m., the highest recorded temperature of ambient was also reached.This means test locations are subjected to the same climatic and weather conditions, and they can be compared more precisely.
The performance of the hemispherical solar still (CHSWF) is compared to that of the cylindrical fins hemispherical solar still (CyFHS) and the conical fins hemispherical still (CoFHS) with spacing remains unchanged at 3, 3.75, and 4.5 cm.The maximum temperature of the water basin appears to increase as the distance between fins evolves.At about 2:00 p.m., the maximum water basin temperatures of CHSWF, CyFHS (Case 1), CoFHS (Case 1), CyFHS (Case 2), CoFHS (Case 2), CyFHS (Case 3), and CoFHS (Case 3) reached 55, 58, 61, 58, 65, 65, and 67°C, respectively.These results of the basin water temperature along the day from 7:00 a.m. to 6:00 p.m. presented that the utilization of conical fins with gap spacing of 4.5 cm (CoFHS-Case 3) represented the optimal fin design.
From 7:00 a.m. to 6:00 p.m. on May 13, 14, and 15, Figure 7 shows the change in yield for various configurations.The hourly variance of collected water production rises as the fin spacing of the cylindrical and conical hemispherical continues will be increased.The daily cumulative distillate yields for CHSWF, CyFHS (Case 1), CoFHS (Case 1), CyFHS (Case 2), CoFHS (Case 2), CyFHS (Case 3), and CoFHS (Case 3) are 4.2, 5.55, 6.2, 6, 6.65, 6.7, and 7.15 L/m 2 /day, respectively.The results of cumulative productivity presented that the utilization of conical fins with a gap spacing of 4.5 cm represents the best configuration that achieved the highest productivity.This is mainly due to the reduction of the shading effect of the conical fins addition to the shading effect will decrease with an increase in the gap distance between the conical fins.
The accumulated daily yield for the three test configurations is shown in Figure 8, with the maximum daily yield of 7.15 L/m 2 /day achieved when the fins spacing is 4.5 cm in CoFHS (Case 3). Figure 9 shows the daily yield enhancement due to using CyFHS and CoFHS at different fins spacing, it is seen that the maximum daily yield increase of 70.24% compared to CHSWF is achieved when fins spacing is 4.5 cm in CoFHS (Case 3).These are mainly because of the reduction of the shading effect of the conical fins addition the shading effect will decrease with an increase in the gap distance between the conical fins.
Based on the fin spacing, Figure 10 depicts the efficiency of CyFHS and CoFHS.The average efficiency of hemispherical distillers CyFHS and CoFHS increases as the fins spacing increases because the efficiency of hemispherical distillers is dependent on yield and water temperature.CHSWF, CyFHS (Case 1), CoFHS (Case 1), CyFHS (Case 2), CoFHS (Case 2), CyFHS (Case 3), and CoFHS (Case 3) have average efficiency ratings of 37.15, 48.95, 54.47, 52.95, 58.33, 58.86 and 62.72%, respectively.Figure 11 shows an enhancement in daily efficiency due to using CyFHS and CoFHS at different fins spacing, it is seen that the maximum daily efficiency enhancement of 68.08.% compared to CHSWF is achieved when fins spacing is 4.5 cm in CoFHS (Case 3).

Comparison of present results with similar published studies
Table 5 presents a comparison between the rates of improvement in the daily productivity of solar distillers as a result of using the fins of the current study with previous studies.It is clear from the results of the comparison that the use of conical fins achieved the highest rates of improvement in the productivity of the solar stills.Where the improvement rates for previous studies ranged between 9.76% when using rectangular fins (Tuly et al. 2021) to 56.73% when using iron fins (Attia et al. 2021), while in the current study when using conical fins with a gap distance of 4.5 cm, the improvement rates in productivity reached 62.72%.

Conclusions
The current experimental study aims to reach the best design of the fins, which achieves the highest performance of the hemispherical solar stills.Whereas the use of fins has a positive effect in terms of increasing the heat transfer area between the basin water and the absorption surfaces, improving thermal performance, and then increasing freshwater productivity.However, there are some designs of fins that have negative effects due to the problem of shading, which causes a reduction in the absorption rates of solar radiation.Therefore, the current study aims to find the best fin design that achieves the lowest shade effect and the highest performance for hemispherical solar stills.To achieve this idea, the absorbent cement bed with two types of fins (cylindrical and conical fins) with a diameter of 1.5 cm and a height of 3 cm at different gap spacings (3, 3.75, 4.5 cm) was proposed in this manuscript to obtain the best fin configuration that achieves the highest yield.To achieve these, three hemispherical solar stills were tested under the same weather conditions, the first being a conventional hemispherical wiyhout fins (CHSWF).The second is a cylindrical fins hemispherical solar still (CyFHS), and the third is a conical fins hemispherical solar still (CoFHS).Results prove that cylindrical and conical fins are important in performance enhancement as follows: • Due to the shading effect in solar distillers, the effect of cylindrical and conical fins on performance is dependent on their spacing.• CyFHS have respective collective yields of 5.55, 6.00, and 6.70 L/m 2 /day; but CoFHS has respective collective yields of 6.20, 6.65, and 7.15 L/m 2 /day for gap spacings of 3, 3.75, and 4.5 cm, respectively.• CyFHS has respective daily thermal efficiencies of 48.95, 52.95, and 58.85%; but CoFHS has respective daily thermal efficiencies of 6.20, 6.65, and 7.15% for gap spacings of 3, 3.75, and 4.5 cm, respectively.• Using the conical fins with a gap spacing of 4.5 cm represents the best configuration of CoFHS which improves the accumulative productivity and thermal efficiency with rates up to 70.24 and 68.06% as compared to the reference distiller.
In future works, we recommend studying the best conical fin dimensions that yield the highest performance for solar distillers.We also recommend conducting a comparative study to determine the best materials used in the manufacture of conical fins that achieve the highest performance of solar distillers.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
No funding used in this paper.

Figure 3 .
Figure 3. Photo of the experimental setup.

Figure 6
Figure 6 depicts a change in water temperature for the three distinct configurations of CyFHS and CoFHS on May 13, 14, and 15.

Figure 4 .
Figure 4. Photo of different cases of test configuration.

Figure 5 .
Figure 5. Solar intensity and temperature throughout a day.

Figure 6 .
Figure 6.Hourly variation of water temperature throughout the day for CHSWF, CyFHS, and CoFHS test configurations.

Figure 8 .
Figure 8. Daily yield for different cases of CyFHS and CoFHS.

Figure 9 .
Figure 9. Daily yield enhancement for different cases of CyFHS and CoFHS.

Figure 10 .
Figure 10.The efficiency for different cases of CyFHS and CoFHS.

Figure 11 .
Figure 11.The enhancement in daily efficiency for different cases of CyFHS and CoFHS.
1) Cylindrical Finned Hemispherical Still set at 3 cm gap spacing CyFHS (Case 2) Cylindrical Finned Hemispherical Still set at 3.75 cm gap spacing CyFHS (Case 3) Cylindrical Finned Hemispherical Still set at 4.5 cm gap spacing CoFHS (Case 1) Conical Finned Hemispherical Still set at 3 cm gap spacing CoFHS (Case 2) Conical Finned Hemispherical Still set at 3.75 cm gap spacing CoFHS (Case 3) Conical Finned Hemispherical Still set at 4.5 cm gap spacing

Table 2 .
The different test configurations.

Table 3 .
Instrument errors and range.

Table 4 .
Cumulative yield comparisons of hemispherical solar distillers with cylindrical and conical fins with spacing 3, 3.75, and 4.5 cm.

Table 5 .
Comparison of the productivity improvement of the current study with previously published studies.