Enhanced water production in hemispherical solar stills via solar collector and thermal storage integration: experimental validation

A

area (m²)

a

accuracy of the measuring instrument

abs

absorber

ai

stands for interest per year ($)

Air

humid air

C_p

Specific heat (J/kg °C)

CREF

capital recovery factor ($)

CSC

cost of the still's capital ($)

FYC

first year cost ($)

h

heat transfer coefficient (W m⁻² °C⁻¹)

h_fg

latent heat of vaporization (J kg⁻¹)

I(t)

solar irradiation (w m⁻² day⁻¹)

j

life expectancy (year)

hourly distillate productivity (kg s⁻¹)

m_d

distillate production (kg day⁻¹)

P

saturation partial pressure (Pa)

PPL

price of produced water per liter ($)

Q

heat (W)

SIFF

factor of sinking funds ($)

SY

the system's salvage value ($)

T

temperature (°C)

T_amb

ambient temperature (°C)

u

standard uncertainty

U_L

heat losses of solar collector (W m⁻² °C)

U_st

heat losses of storage tank (W m⁻² °C)

w

water

WP

average productivity of still per year ($)

W_v

wind velocity (m s⁻¹)

YC

cost of distillation system per year ($)

YMC

operational cost of maintenance ($)

YSV

salvage value per year ($)

Greek symbols

absorptivity

ε

emissivity

transmissivity

Stefan–Boltzmann constant

η_g

efficiency (%)

Subscripts

a

atmosphere

abs

solar still absorber

AHSS

modified hemispherical solar still

C

solar still transparent cover

C_v

convective

C_w

collector water

e

evaporative

r

radiative

SHSS

simple hemispherical solar still

sky

sky

Sw

storage water

Water scarcity is a critical global issue affecting millions, particularly those with limited access to clean water. One solution to this challenge is the use of low-cost, environmentally friendly hemispherical solar stills, although their low efficiency limits their adoption. Researchers have proposed improvements to increase production by increasing evaporation, condensation, or both. Methods to increase evaporation include increasing solar radiation, using reflectors, or changing the absorber shape to increase absorption surface. In addition, reducing the condensation surface temperature can increase condensation, producing more clean water.

In a study by Modi et al. (2020), a solar still with a parabolic reflector was developed and tested. The researchers found that the daily yield of distilled water increased as the water mass in the basin increased, reaching up to 8.2596 L/m² with 5 L of water. The daily average efficiency of the still also increased with water mass, reaching up to 39.06% with 5 L. When the concentrator is placed to concentrate solar radiation on the underside of the absorber, allowing radiation incidence on both sides, productivity increased by 50.4% compared to that of the conventional hemispherical still (Attia et al. 2022b).

The development of the hemispherical solar still targets increased absorber surface and modification of its nature, yielding significant results. Attia et al. (2021a) utilized iron fins, optimizing at 7 cm spacing and 2 cm length, enhancing heat absorption. Modified absorber shapes, like v-corrugated basins and reversed solar collectors, improved production by up to 68.82% (Kabeel et al. 2022c). Wick-based designs, transporting water from basin to surface, demonstrated notable improvements: yellow- and black-dyed flax fibers increased yield by 29.7 and 39.6%, respectively (Kabeel et al. 2022a). Savithiri et al. (2022) found that rubber and wick materials, with increased thickness, provided 46.94 and 40.81% yield increases compared to conventional hemispherical solar stills. Finally, Attia et al. (2021) studied a hemispherical solar still using black-painted metal trays (copper, zinc, iron). Results showed that iron, zinc, and copper trays enhanced accumulative yield by 1.17, 31.25, and 53.125%, respectively, compared to the conventional still.

Another proposed improvement for a hemispherical still designed for salt rejection is the use of a low vacuum and solar heat localization. This method enhances efficiency by improving evaporation and capillary water circulation, resulting in a daily production of 4.3 L of distilled water with a 35.6% efficiency (Mohsenzadeh et al. 2022).

The sensible storage of heat can improve the production of hemispherical solar stills. By adding a layer of material that can absorb excess heat during the day and release it slowly at night, heat loss is reduced, and the still's overall performance is improved. Sensible storage can also reduce the start-up time and provide a more stable output of freshwater production. Several studies have used this technique. Attia et al. (2022f) compared the use of rock salt balls with different gap spacings to the conventional still and found that the optimal arrangement was with a 4 cm gap spacing. This resulted in a 49% improvement in freshwater production and a 60.10% enhancement in daily energy efficiency. Similarly, Attia et al. (2022g) added spherical rock salt balls to saline water. The largest salt balls produced the most freshwater, reducing production costs by 28.83%. Kabeel et al. (2022b) used phosphate grains as an energy storage material and found that a concentration of 30 g/L was optimal for the highest performance. This yielded 57.9% more freshwater and 23% higher daily efficiency. Finally, Beggas et al. (2023) used aluminum waste to improve the still's thermal conductivity and achieved a 48.19% improvement in freshwater production.

The excess heat produced during periods of high solar radiation can be stored as latent heat and used during periods of low solar radiation, especially at night. One innovative approach to this is the use of phase change materials (PCMs) in the basin of hemispherical solar stills. Sathyamurthy et al. (2023) proposed the use of paraffin wax as a PCM in a hemispherical still, resulting in a 27.84% improvement in solar still production compared to conventional hemispherical stills. In another study, Abdelgaied et al. (2022) found that the improvement made to the hemispherical distiller was a 29.17% improvement compared to the conventional distiller.

Using nanoparticles in a solar still basin can increase the rate of evaporation, resulting in higher production of distilled water. With their high surface area-to-volume ratio and efficient solar radiation absorption, nanoparticles make solar stills more efficient. Abdelgaied et al. (2022) studied the thermo-economic performance of a modified hemispherical solar still with copper oxide (CuO) nanoparticles in the basin. The results showed that the usage of CuO/water nanofluid increased productivity by 60.41% compared to the conventional still.

It is also possible to combine two or more of the techniques mentioned above to further improve the performance of the hemispherical distiller. Various techniques can be used to improve the performance of hemispherical solar distillers. Attia et al. (2022c) found that the use of copper trays with reflective mirrors was the most effective option, resulting in a 104.3% increase in cumulative production to 9.5 L/m²/day. Sharshir et al. (2023) modified the distiller's basin with a V-corrugated basin, a black cotton wick, and a PCM (sheep fat), leading to a daily productivity of 4,737.5 mL/m² and a thermal efficiency of 45%. Hemispherical distillers with nanoparticle paraffin wax were found to be the most effective by Sathyamurthy et al. (2023), producing 8.3 L/m²/day, with a 71.13% increase in daily yield compared to conventional hemispherical still. Abdelgaied et al. (2022) found that using paraffin wax as a PCM and copper oxide nanoparticles in the hemispherical solar still resulted in an 80.20% increase in production. Attia et al. (2022e) proposed a hemispherical still that incorporates extended hollow cylindrical fins filled with PCMs in the basin of hemispherical solar distillers, which resulted in an 80.4% increase in productivity and an improved daily efficiency from 39.6 to 71%. In addition, the use of reflective mirrors and sand grains, as observed in the study by Attia et al. (2022d), and the combination of reflective mirrors and high thermal conductivity metal trays, as found by Attia et al. (2022c), can significantly improve the performance of hemispherical solar stills. Attia et al. (2023) investigated the impact of reflective aluminum foil, zinc, and copper sheets on the efficiency of hemispherical solar stills and found productivity gains of 8.67, 29.08, and 42.35%, respectively. Furthermore, the use of aluminum foil and phosphate granules increased productivity by 30.61%, while using zinc and copper metal sheets with phosphate granules increased productivity by 49.96 and 62.24%, respectively, compared to a traditional solar still coated with black silicon. Finally, Attia et al. (2022h) enhanced freshwater production of hemispherical solar distillers by increasing vaporization surface area using iron trays and wicks, resulting in an 83.12% increase in productivity when using v-corrugated tray with wick materials.

Increasing the condensation leads to an increase in the amount of distilled water, which can be obtained by decreasing the temperature of the transparent cover. It is noteworthy that Arunkumar et al. (2012) proposed a hemispherical solar still with a glass top cover and water flowing over it to increase the daily yield. The results showed that the efficiency of the still increased from 34 to 42%.

Combining the increase of evaporation and condensation simultaneously can lead to the best improvement in the production of a hemispherical solar still. On this basis, Attia et al. (2021c) proposed a hemispherical still with the addition of CuO nanoparticles to the basin water and the use of water film glass cooling technology. The experimental results showed that the modified hemispherical solar still with CuO–water-based nanofluid at a 0.3% volume fraction produced the highest accumulative yield of distilled water at the lowest cost. The daily accumulative yield was 6.80 L/m²/day, which represents a 76.6% improvement over the conventional distiller.

The purpose of this study is to experimentally investigate the effect of adding a hot water storage tank heated by a flat solar collector. This tank is placed directly beneath the absorber of the distiller. This technique allows for an increase in the amount of heat received by the distiller and for the excess heat produced during the day to be stored and transferred to the distiller at night, which significantly improves its performance. In addition, the effect of the orientation of the fins under the absorber and the depth of the saltwater on daily production is investigated.

The subsequent analysis relies on the following assumptions (Matrawy et al. 2015).

• One-dimensional heat transfers through the absorber, water depth, and transparent cover thickness.

• The temperatures in the longitudinal axis are constant for the transparent cover, basin water, and absorber hot water tank.

• Neglect the temperature gradient through the absorber thick, water depth, and transparent cover thickness.

• All properties are independent of the temperature.

According to the mechanisms of heat transfer through modified hemispherical solar still, the energy balance is as follows:

• (1) The solar still:

• • - Transparent cover:

    

    (1)
  • - Water:

    

    (2)
  • - Absorber:

    

    (3)

• (2) The hot water tank:

  

  (4)

The heat transfer coefficients are defined as follows:

Two hemispherical stills were manufactured to carry out the tests. The first one is a conventional hemispherical still used as a reference, and the second one is modified to be able to add a hot water storage tank above it.

The aim of adding the flat plate solar collector is to heat the water inside the storage tank using solar energy. This hot water serves as an additional energy source that helps increase the amount of water evaporated and, consequently, the quantity of distilled water produced by the distiller. At the same time, the presence of the storage tank enables the retention of hot water at high temperatures, allowing the distiller to be powered even in the absence of solar radiation, especially at night when ambient temperatures are low. This increases the distiller's production during nighttime, unlike a simple distiller.

The experiments were conducted at the Solar Equipment Development Unit in Bouismail, Algeria (36.6428° N latitude and 2.6900° E longitude) from December 2022 to October 2023. The water depth in the two stills was initially set to 3 cm at the beginning of the test.

To comprehensively assess the behavior of the two solar stills under varying weather conditions, it was decided to analyze their performance during two typical days. The first typical day was representative of a winter scenario, characterized by unfavorable conditions, such as low solar radiation and a short-day duration. Conversely, the performance of the two stills during a summer typical day was investigated, where favorable conditions, including high solar radiation and an extended day duration, were encountered. The winter typical day was selected as 28 December 2022, while 12 July 2023 was chosen for the summer typical day. Both of these days were subject to exceptionally clear and sunny weather conditions.

Some type K thermocouples were implanted in different locations to measure the temperature of various components of the still, including the absorber, saltwater, humid air, hot water in the tank, etc. Solar radiation was measured using a pyranometer of type Kipp & Zonen (CMP3). The data were collected using a Fluke Hydra data logger. The amount of produced water was automatically collected and weighed using a Kern 10000-0.1 scale connected to a computer. The wind velocity was measured using an anemometer of type Theis Clima (4.3515.51.105). All data were collected every 15 min.

An energy analysis has been developed to investigate and evaluate the suggested systems based on the first principle of thermodynamics. This study involves estimating the average energy efficiency of the proposed distillation system over the course of a day, computed using the formula below:

• - Conventional solar still (Kerfah et al. 2017):

  

  (25)

  where η_g is the thermal efficiency of the system, h_fg is the latent heat of vaporization md is a total productivity, A is a projected surface area of hemispherical still, and I is the solar intensity.
• - Modified hemispherical soar still (Muraleedharan et al. 2019; AbdEl-Rady-Zeid et al. 2024):

  

  (26)

  where Q_ex is the total energy supplied by hot water in the storage tank.

  

  (27)

m_ws is the water mass in storage tank (40 L), T_sd is the average temperature of water in storage tank at the beginning of the day, T_ed is the average temperature of water in storage tank at the end of the day, and C is the thermal capacity of water.

Here, WP represents the distillation system's annual average productivity (kg).

Prices for various materials are determined based on the Algerian market.

The presented measurements draw focus to the maximum solar radiation values in both winter and summer conditions. Throughout winter, the solar collector records a peak of 936.70 W/m², while the solar still receives 496.63 W/m² (12:00 TSV). In summer, these values experience a substantial increase, with the solar collector reaching a maximum of 844.20 W/m², and the solar still registering 737.20 W/m² (12:00 TSV).

Furthermore, there is a remarkable 94% increase in solar radiation during summer compared to winter. This striking contrast reinforces the significant seasonal variations and underscores the untapped potential of solar energy generation during the summer months.

First, it is evident that ambient temperature exhibits a clear diurnal pattern for both seasons. The temperatures increase during the daytime, primarily influenced by solar radiation, and decrease during the night. This cyclic pattern is a direct result of the solar heating effect on the site.

Secondly, a notable contrast emerges when comparing the two seasons. Summer temperatures consistently surpass those observed during winter. In winter, ambient temperatures vary between a low of 10.04 °C at 6:00 local time and a high of 21.22 °C at 13:00 local time. However, summer temperatures exhibit a wider range, fluctuating between 27.9 °C at 4:00 and a scorching 38.94 °C at 10:00. This distinction emphasizes the significant seasonal variability in temperature.

Shifting our focus to wind velocity, it becomes evident that the wind patterns at the experimentation site are characterized by irregularity during both winter and summer. This irregularity can be attributed to the experimental site's proximity to the sea, located just 600 m away. The sea's influence on wind patterns, including the presence of coastal breezes and turbulence, plays a crucial role in shaping the observed wind velocity dynamics. Specifically, during winter, wind velocities range from a minimum of 0.38 m/s to a maximum of 4.68 m/s, showcasing the site's susceptibility to variable wind speeds even in the absence of extreme weather events. In contrast, during summer, wind velocities exhibit a similar irregular pattern, with values ranging between 0.62 and 5.22 m/s.

First and foremost, it is evident that the temperature variations closely mirror the fluctuations in solar radiation. During daylight hours, the solar radiation is absorbed by the bottom of the solar still, significantly elevating its temperature. A portion of this absorbed energy is effectively transferred to the water, causing a gradual increase in water temperature. This continuous heat transfer process results in water gradually evaporating. In addition, the water contributes to warming the adjacent air. However, during nighttime when solar radiation is absent, the components of the solar stills lose heat to the cooler ambient environment, leading to a decline in temperature.

Notably, the temperatures of the absorber and water within the solar stills are quite close to each other. This proximity in temperature can be attributed to the use of aluminum as the absorber material, which boasts excellent thermal conductivity properties, facilitating efficient heat transfer between the two.

Furthermore, the presence of the flat plate solar collector in the modified solar still notably increases the amount of energy transferred to the still during the day, elevating the temperature of all components when compared to the conventional solar still. During the night, the presence of the water tank helps retain heat within the system, providing a continual source of thermal energy to the still. As a result, the temperatures of the modified solar still components consistently exceed those of the conventional solar still throughout the day, regardless of the season.

Interestingly, the maximum temperatures of all components within the modified solar still are achieved earlier in the day compared to the conventional solar still. This discrepancy can be attributed to the presence of the flat plate collector and the hot water tank, which provide more energy to the solar still. Finally, a distinct seasonal trend emerges from the data, with higher temperatures recorded in summer compared to winter. In winter, the absorber, water, and air reach maximum temperatures of 53.76, 53.41, and 52.23 °C, respectively, for the modified hemispherical solar still, while the simple hemispherical solar still records 38.15, 38.07, and 40.65 °C, respectively. In contrast, during summer, the modified solar still reaches maximum temperatures of 76.54 °C for the absorber, 74.54 °C for the water, and 70.90 °C for the air, while the simple solar still achieves temperatures of 56.47 °C for the absorber, 56.18 °C for the water, and 59.21 °C for the air.

In the winter season, a remarkable production trend becomes evident as both stills reach their highest production levels during daylight hours. Moreover, during the summer season, the modified solar still demonstrates its superiority in nighttime production due to its substantial heat supply stored in the storage tank as sensible heat in the water. Impressively, nighttime production accounts for approximately 54% of the total daily production during the summer months. Notably, the modified solar still's nighttime production surpasses that of the simple solar still by 29% during winter and an astonishing 110% during summer.

When examining daily production amounts, the modified solar still consistently produces higher yields compared to the simple solar still. Specifically, during winter day, the modified solar still achieves a daily production rate of 2.147 kg/m², whereas the simple solar still yields only 0.836 kg/m². This represents a substantial 157% increase in daily production during winter. In contrast, during summer day, the disparity in daily production is even more pronounced, with the modified solar still producing 6.998 kg/m² compared to the 2.279 kg/m² of the simple solar still, resulting in an impressive 207% increase in daily production during summer.

An increase in saltwater depth by 1 cm (from 2.5 to 3.5 cm) induces a decrease in production from 1.542 to 1.198 for the simple hemispherical still and from 5.083 to 3.708 for the modified hemispherical still. This results in a reduction in production for the simple and modified hemispherical stills by 28.71 and 37.08%, respectively.

In Table 3, the production rates and gain percentages of various hemispherical solar still studies are compared. In this study, exceptional results are achieved with a hemispherical system connected to a hot water storage tank and a flat plate solar collector. During winter, a production rate of 2.417 L/m²/day is attained, accompanied by a 157% gain, while in summer, production increases to 6.998 L/m²/day with a 207% gain. In the study by Ismail (2009), a production rate of 5.7 l/m²/day is achieved in summer with a portable design. Attia et al. (2022a) employed an aluminum foil sheet as a reflector, resulting in a production rate of 4.1 L/m²/day with a 22.21% gain. Finally, in the study by Attia et al. (2021b), phosphate pellets were used as energy storage, yielding a production rate of 6.85 L/m²/day and a 47.9% gain in spring.

In Table 4, we present a summary of the cost evaluation results for the modified hemispherical solar still and the conventional hemispherical solar still, with various financial parameters, including CREF, CSC, first yearly cost (FYC), annual salvage value (YSV), annual cost of maintenance (YMC), annual cost of the distillation system (YC), and the cost of producing one unit of distilled water (PPL).

The modified hemispherical solar still exhibits a CREF of 0.0022, indicating a favorable capital recovery factor. However, its higher CSC of $420.1014 and FYC of $49.0803 are noteworthy, implying relatively substantial initial costs. On the positive side, the YSV of $7.7361 suggests a partial recovery of investment over time. In addition, YMC and the overall YC are higher at $7.3620 and $48.7062, respectively. The PPL is $0.1897.

In contrast, the conventional hemispherical solar still demonstrates similar CREF and FYC values of $0.0022 and $101.6328, respectively, with a notably lower CSC of $11.8737. The YSV of $1.8715, although lower than that of the active still, suggests some potential for cost recovery. YMC and the YC are also lower at $1.7811 and $11.7832, respectively. The PPL in the conventional still is $0.1446.

The primary objective of this study is to experimentally explore the influence of integrating a hot water storage system, heated through the utilization of a flat solar collector, into a hemispherical solar still. Specifically, this storage tank is strategically positioned directly beneath the distiller's absorber. This innovative approach serves the dual purpose of increasing the heat input to the hemispherical solar still and efficiently storing excess daytime thermal energy for subsequent nighttime utilization.

Two hemispherical solar stills were fabricated and tested under the same conditions in Bouismail, Algeria, during the period from December 2022 to October 2023. Drawing upon the data collected and the in-depth discussions documented in these experiments, the following findings can be ascertained:

• - The addition of the flat plate solar collector increases the amount of heat, leading to an increase in the hemispherical solar still's temperature and, consequently, the amount of evaporated water. This results in an augmentation of fresh water production in both seasons.

• - The production of the modified hemispherical solar still was higher at night, especially during summer, due to the high quantities of heat stored during the daytime in the water as sensible heat, which is then used at night to heat the water in the hemispherical solar still.

• - Night time production of the modified hemispherical solar still represents 18 and 54% of the daily production during winter and summer days, respectively. During winter days, the modified hemispherical solar still demonstrates a 29% increase in night time production compared to the simple hemispherical solar still, while in the summer, this improvement is even more significant, reaching an impressive 110%.

• - The daily production of the modified and simple hemispherical solar stills was 2.147 and 0.836 kg/m² on a winter day, and 6.998 and 2.279 kg/m² on a summer day, respectively. This represents a production increase of 157 and 207% for the modified hemispherical still compared to the simple one during winter and summer days, respectively.

• - The orientation of the fins in the storage tank has a significant effect on the daily production of the modified solar still. In fact, increasing the angle between the entrance hole for the hot water and the fins by 60° and 90° leads to a decrease in the water production of the modified solar still by 23 and 25%.

• - A rise in saltwater depth leads to a decrease in distillate water production in both hemispherical solar stills. Transitioning from 2.5 to 3.5 cm, the production decreases by 28.71% for the simple hemispherical still and 37.08% for the modified hemispherical still.

• - By adding a water storage tank connected to a flat plate solar collector and a hemispherical solar still, the efficiency of the solar still is improved during summer days. The efficiency of the modified solar still is 15.26 and 37.42% compared to that of the simple hemispherical solar still, which is 19.10 and 20.38% during winter and summer days, respectively.

• - The price of the water per liter produced by the modified and the simple hemispherical solar stills was $0.1897 and $0.1446, respectively.

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

The authors declare there is no conflict.