In arid regions that face water scarcity, solar distillation offers hope by meeting the increasing need for clean drinking water. This study investigates the integration of a hot water storage system, heated by a flat plate solar collector, into a hemispherical solar still. Placing the storage tank below the still's absorber enhances heat input and efficiently stores excess daytime thermal energy. Conducted in Bouismail from December 2022 to October 2023, the study yielded significant results: the modified solar still outperformed the conventional one, with production increases of 157% in winter and 207% in summer. Moreover, the modified still demonstrated remarkable efficiency improvement in summer, reaching 37.42% compared to 20.38% for the conventional still. The orientation of the fins within the storage tank, with respect to the hot water entrance orifice, significantly impacted water production, with alterations of the angle resulting in decreases of up to 25%. Increasing saltwater depth led to reduced distilled water output, with declines of up to 37.08% for the modified still. Economic analysis showed a slightly higher cost per liter of water produced by the modified still ($0.1897) compared to the simple one ($0.1446).

  • Integration of hot water storage with a solar collector boosted hemispherical solar still efficiency.

  • The modified solar still achieves 157% production increase in winter and 207% in summer, surpassing the conventional model.

  • The modified design reaches 37.42% efficiency in summer.

  • Tank fin orientation influences water production.

  • Economic analysis reveals a slightly higher per-liter cost for the modified solar still.

A

area (m2)

a

accuracy of the measuring instrument

abs

absorber

ai

stands for interest per year ($)

Air

humid air

Cp

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)

hfg

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)

md

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)

Tamb

ambient temperature (°C)

u

standard uncertainty

UL

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

Ust

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

w

water

WP

average productivity of still per year ($)

Wv

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

ηg

efficiency (%)

Subscripts

a

atmosphere

abs

solar still absorber

AHSS

modified hemispherical solar still

C

solar still transparent cover

Cv

convective

Cw

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:
      formula
      (1)
    • - Water:
      formula
      (2)
    • - Absorber:
      formula
      (3)

  • (2) The hot water tank:
    formula
    (4)

The heat transfer from transparent cover to atmospheric (), basin water to transparent cover (), absorber to transparent cover (), absorber to basin water to (), storage water to absorber (), and storage water to atmospheric () are as follows:
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)
The useful heat (Qu) delivered by the solar collector is (Kalogirou 2009)
formula
(10)
Solar energy absorbed for the transparent cover, water, absorber, collector glass cover, and collector absorber are as follows (Yahia Mahammed et al. 2019):
formula
(11)
formula
(12)
formula
(13)

The heat transfer coefficients are defined as follows:

Convective heat transfer is (AbdEl-Rady-Zeid et al. 2024)
formula
(14)
Evaporative heat transfer is (AbdEl-Rady-Zeid et al. 2024)
formula
(15)
where and are the partial pressure for water and transparent cover according to (AbdEl-Rady-Zeid et al. 2024)
formula
(16)
formula
(17)
Convective transfer coefficient between transparent cover of the still and the atmosphere and glass cover of the collector and atmosphere is (Kalogirou 2009)
formula
(18)
where Wv is the wind velocity.
Radiative heat transfer coefficient between the water and transparent cover in the solar still is (Kalogirou 2009)
formula
(19)
Radiative heat transfer coefficient between the absorber and glass and glass cover and atmosphere is (Kalogirou 2009)
formula
(20)
formula
(21)

Experimental setup

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 hemispherical solar still consists of a circular basin with a diameter of 500 mm and a height of 40 mm, made of galvanized sheet with a thickness of 2 mm. The bottom of the basin is painted in matte black to ensure maximum absorption of solar radiation. For the salty water inlet, a hole was cut in the side of the basin and supplied with an 8 mm diameter pipe. To minimize heat losses, the basin side and bottom were insulated with a 50-mm-thick layer of polyurethane foam. The dome-shaped cover of the basin is made of 3- mm-thick plexiglass with a diameter of 400 mm. For distilled water collection, a circular ring of U-shaped cross-section made of copper was placed at the bottom of the acrylic cover, and a wiper was placed on the inner face of the hemispherical transparent cover to accelerate the collection of fresh water and clean the inner surface from water droplets, which increased the transmissibility of solar radiation. The wiper is driven by an electric motor powered by an electrical system. The electrical system consists of four photovoltaic panels (the panel characteristics are shown on Table 1), a battery (100 Ah and 12 V), and a solar charge controller, making the system completely autonomous. To preserve electrical energy, the windshield wipers are activated for a set duration within a specific time interval. For this purpose, a control system consisting of an Arduino Mega and two relays was used. To control it, a C ++ program was written and uploaded to the Arduino. This program involves activating the relay for 3 s during a 3-min period. When the relay is activated, the wiper motor is powered, allowing the wipers to operate for 3 s. Once the duration has elapsed, the relay is deactivated, cutting off the power supply to the motor and stopping the wipers. The relay remains deactivated for a 3-min period, and then it reactivates (Figure 1).
Table 1

Photovoltaic panel characteristics

Max rated power (W) 60 
Tolerance of output (%) ±10 
Open circuit voltage (V) 14.70 
Voltage at Pmax (V) 17.4 
Short circuit current (A) 5.83 
Maximum power voltage (V) 12.30 
Maximum power current (A) 4.88 
Maximum system voltage (V) 1,000 
Maximum series fuse rating (A) 12 
Max rated power (W) 60 
Tolerance of output (%) ±10 
Open circuit voltage (V) 14.70 
Voltage at Pmax (V) 17.4 
Short circuit current (A) 5.83 
Maximum power voltage (V) 12.30 
Maximum power current (A) 4.88 
Maximum system voltage (V) 1,000 
Maximum series fuse rating (A) 12 
Figure 1

Electrical and controller systems.

Figure 1

Electrical and controller systems.

Close modal
Several modifications were made to the first still to obtain the second one. The first modification was the addition of three rectangular fins on the outside face of the absorber. Several modifications were made to the first still to obtain the second one. The first modification involved adding three rectangular fins to the outside face of the absorber. The first fin (350 mm × 50 mm × 3 mm) was placed in the middle of the absorber, while the other two (260 mm × 50 mm × 3 mm) were symmetrically positioned 135 mm apart from the first one (Figure 2). The storage tank is made of galvanized sheet metal with a thickness of 2 mm, a diameter of 400 mm, and a height of 460 mm. This tank is thermally insulated with a 50-mm-thick layer of polyurethane foam. The tank is connected to the flat solar collector by 12 mm diameter tubes with a total length of 3,020 mm. The water inside the tank is heated by a flat solar collector. A flat solar collector with dimensions of 2,000 mm in length and 1,000 mm in width is used. Its transparent cover is made of 4-mm-thick tempered glass, providing sturdy protection while allowing optimal transmission of solar radiation. The absorber is made of aluminum with a thickness of 0.4 mm, carefully coated with a selective black paint for maximum absorption of solar energy. The collector of the solar collector consists of seven copper tubes with a diameter of 12 mm, promoting efficient heat transfer to the heat transfer fluid. To minimize heat losses, there is a 20-mm-thick lateral insulation of glass wool. In addition, a 40-mm-thick bottom insulation of glass wool is added. The collector is also equipped with a protective tray, with lateral sides made of profiled aluminum sheet and a bottom made of galvanized steel with a thickness of 0.5 mm. These features optimize the efficiency of the flat solar collector while ensuring its durability and protection against external elements (Figures 3 and 4).
Figure 2

Photographic view of the modified cylindrical basin.

Figure 2

Photographic view of the modified cylindrical basin.

Close modal
Figure 3

Schematic view of the experimental step.

Figure 3

Schematic view of the experimental step.

Close modal
Figure 4

Photograph view of the experimental bench.

Figure 4

Photograph view of the experimental bench.

Close modal

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.

Experimental proceedings and measurements

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.

In the experiments, it is assumed that the data are uniformly distributed. The standard uncertainty of this type is evaluated as follows (Dumka & Mishra 2020):
formula
(23)
where a represents the accuracy of the measuring instrument. Table 2 provides information on the type, accuracy, range, and standard uncertainty of the measuring instruments.
Table 2

Type, accuracy, range, and standard uncertainties of measuring devices

InstrumentTypeAccuracyRangeStandard uncertainty
Thermocouple Type K ±0.1 −100 to 500 °C 0.069 °C 
Pyranometer Kipp & Zonen (CMP3) ±0.05 0 to 2,000 W/m2 0.043 W/m2 
Scale Kern PCB ±0.1 g 0 to 10,000 g 0.069 g 
Anemometer Thies Clima ±0.5 m/s 0.7 to 40 m/s 0.34 m/s 
InstrumentTypeAccuracyRangeStandard uncertainty
Thermocouple Type K ±0.1 −100 to 500 °C 0.069 °C 
Pyranometer Kipp & Zonen (CMP3) ±0.05 0 to 2,000 W/m2 0.043 W/m2 
Scale Kern PCB ±0.1 g 0 to 10,000 g 0.069 g 
Anemometer Thies Clima ±0.5 m/s 0.7 to 40 m/s 0.34 m/s 

Still productivity

The hourly distillate productivity of the modified solar still is defined as (Yahia Mahammed et al. 2019)
formula
(24)
where is the he,wc is the evaporative heat coefficient between water and transparent cover, Tw and Tc are the temperature of water transparent cover, respectively, and hfg is latent heat of vaporization.

Still thermal efficiency

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):
    formula
    (25)
    where ηg is the thermal efficiency of the system, hfg 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):
    formula
    (26)
    where Qex is the total energy supplied by hot water in the storage tank.
    formula
    (27)

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

Depending on the distilled water's production cost and its applicability, the best financial return on investment can be achieved. The following is an economic analysis of the two stills (Kerfah et al. 2015):
formula
(28)
where CREF is the capital recovery factor, j is the life expectancy, which is 15 years, and ai stands for interest per year (a = 8%).
The following equation is used to determine the first yearly cost:
formula
(29)
where CSC is the cost of the still's capital.
The following formula is used to calculate annual salvage value:
formula
(30)
where SY is the system's salvage value (20% of the still's capital cost) and SIFF is the factor of sinking funds.
formula
(31)
About 15% of the initial annual cost represents the operational cost of maintenance.
formula
(32)
The following equation can be used to calculate the annual cost of the distillation system:
formula
(33)
Distilled water costs can be calculated as follows:
formula
(34)

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

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

Solar radiation, ambient temperature, and wind velocity variations

Solar radiation variation

Figure 5 illustrates the hourly variation of solar radiation incident on both the solar still and the flat plate solar collector over the course of two typical days. This comprehensive visual representation provides insights into the dynamic nature of solar radiation. As seen in the figure, solar radiations consistently rise from sunrise, culminating at their maximum values at solar noon, and subsequently diminish as the day progresses, reaching their minimum levels at sunset.
Figure 5

Hourly variation of global radiation on typical day of winter and summer.

Figure 5

Hourly variation of global radiation on typical day of winter and summer.

Close modal

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.

Ambient temperature and wind velocity variation

In Figure 6, we present the hourly variations in ambient temperature and wind velocity for the two experimental days, namely, winter and summer days. Several key observations can be drawn from the data, providing valuable insights into the environmental conditions at the experimentation site.
Figure 6

Hourly variation of ambient temperature and wind velocity on typical day of winter and summer.

Figure 6

Hourly variation of ambient temperature and wind velocity on typical day of winter and summer.

Close modal

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.

Comparative analysis of temperature variances in solar stills

Figures 7 and 8 provide a comprehensive illustration of the hourly temperature variations of different components within the two solar stills across the two experimental days. Several key observations can be made from this detailed temperature data, shedding light on the dynamic thermal behavior of the systems.
Figure 7

Hourly variation of the temperatures of the absorber and water inside the solar stills.

Figure 7

Hourly variation of the temperatures of the absorber and water inside the solar stills.

Close modal
Figure 8

Hourly variation of the temperature of air inside the solar stills.

Figure 8

Hourly variation of the temperature of air inside the solar stills.

Close modal

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.

Figure 9 illustrates the hourly variation of the water temperature in the tank storage on winter and summer experimental days. It is noteworthy that during the daytime, solar energy was absorbed by the flat plate solar collector absorber and then transferred by conduction to the water, contributing to an increase in its temperature. Some of this energy led to the heating of the water in the solar still through the solar still absorber. This is why the initial increase in water temperature is not high at the beginning of the day. Subsequently, when the temperature of the saltwater became sufficiently hot, the heat exchange between the stored water and the solar still absorber decreased. As a result, the majority of the absorbed energy by the flat plate solar collector was stored in the water, leading to a rapid temperature increase between 11:00 and 13:45 in winter and 10:40 and 15:10 in summer. The maximum water storage temperature reached 56.22 °C at 13:45 in winter and 72.63 °C at 15:10 in summer. In the afternoon, solar energy decreased, causing a reduction in saltwater temperature on one hand. On the other hand, the temperature of the stored water remained high; therefore, some of the energy stored in the water was transferred to the saltwater, resulting in a decrease in the storage water temperature. This decrease becomes more significant at night when the ambient temperature drops.
Figure 9

Hourly variation of the e water temperature in the storage tank.

Figure 9

Hourly variation of the e water temperature in the storage tank.

Close modal

Comparative analysis of hourly cumulative production in solar stills

In Figure 10, we present a comprehensive analysis of the hourly cumulative production of two distinct solar stills throughout our experimental days. Notably, it is evident that the modified solar still consistently outperforms the simple solar still in terms of freshwater production, regardless of the season.
Figure 10

Hourly variation of cumulated production of the stills.

Figure 10

Hourly variation of cumulated production of the stills.

Close modal

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.

Comparative analysis of daily energy efficiency in solar stills

Figure 11 presents the daily thermal efficiency of the conventional hemispherical solar still and the modified hemispherical solar still. In the winter season, the conventional hemispherical solar still demonstrates a thermal efficiency of 19.10%, while the modified hemispherical solar still exhibits a lower efficiency at 15.26%. This suggests that the increase in production of the modified still does not compensate for the energy provided by the storage tank. In contrast, during the summer season, there is a substantial difference in thermal efficiency between the two stills. The conventional hemispherical solar still achieves an efficiency of 20.38%, whereas the modified hemispherical solar still shows a higher efficiency at 37.42%. This increase in efficiency for the modified still in summer could be attributed to the high amount of energy stored as sensible heat and used to heat the saltwater at night
Figure 11

Daily thermal efficiency of stills.

Figure 11

Daily thermal efficiency of stills.

Close modal

Effect of the fins orientation on water production of the modified hemispherical solar still

To investigate the influence of fin orientation on the performance of the modified solar still, the orientation of the fins was adjusted by modifying the angle between the orifice through which hot water enters the storage tank and the fin's surface. Specifically, experiments were conducted using three different angles: 0°, 60°, and 90° (Figure 12).
Figure 12

Schematic view of fins inclined relative to the hot water inlet.

Figure 12

Schematic view of fins inclined relative to the hot water inlet.

Close modal
As illustrated in Figure 13, it becomes evident that increasing the angle between the hot water inlet and the fins results in a noticeable reduction in daily production. This reduction can be attributed to the increased angle diminishing the effective heat exchange area, consequently reducing the amount of heat transferred from the hot water in the storage tank to the solar still absorber. In fact, we observed a decrease in production of approximately 23 and 25% when the angle was set to 60° and 90°, respectively, in comparison to when the angle was 0°.
Figure 13

Effect of fins orientation on the daily production of the modified hemispherical solar still.

Figure 13

Effect of fins orientation on the daily production of the modified hemispherical solar still.

Close modal

Effect of saltwater depth on the daily water production of the stills

Figure 14 illustrates the effect of saltwater depth on the daily water production of the two stills. It is noteworthy that in both stills, an increase in saltwater depth results in a reduction in distillate production. This phenomenon occurs because the greater quantity of water in the stills requires more energy to raise its temperature. However, the available energy is nearly constant (solar energy during the same period), leading to a decrease in the attained temperatures. Consequently, this reduction in temperatures diminishes the amount of evaporated water, and, hence, the quantity of distillate.
Figure 14

Effect of the salt water depth on the daily distillate water production.

Figure 14

Effect of the salt water depth on the daily distillate water production.

Close modal

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.

Investigating the current study in comparison to existing related works

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.

Table 3

Comparative analysis of the present study and related works on improvements in hemispherical solar stills

Author nameSystem specificationProduction, L/m2/dayGain, %Season
Present study Hemispherical system with hot water storage tank connected to flat plat solar collector 2.417 157 Winter 
Present study Hemispherical system with hot water storage tank connected to flat plat solar collector 6.998 207 Summer 
Ismail (2009)  Simple transportable hemispherical solar 5.7 – Summer 
Attia et al. (2022a)  Hemispherical solar still using internal aluminum foil sheet as reflector 4.1 22.21 – 
Attia et al. (2021b)  Phosphate pellets as energy storage materials 6.85 47.9 Spring 
Sathyamurthy et al. (2023)  Hemispherical distiller with nanoparticles paraffin wax 8.3 71.13 Summer 
Bellatreche et al. (2021)  Cylindrical parabolic collector + sand 6.5 91 Summer 
Farghaly et al. (2023)  Evacuated tube collector B
Evacuated tube collector + parabolic trough reflectors C 
0.986
1.150 
82.26
112.57 

– 
Negi et al. (2021)  Tilted wick solar + flat plate collector 3.99 38.11 Spring 
Author nameSystem specificationProduction, L/m2/dayGain, %Season
Present study Hemispherical system with hot water storage tank connected to flat plat solar collector 2.417 157 Winter 
Present study Hemispherical system with hot water storage tank connected to flat plat solar collector 6.998 207 Summer 
Ismail (2009)  Simple transportable hemispherical solar 5.7 – Summer 
Attia et al. (2022a)  Hemispherical solar still using internal aluminum foil sheet as reflector 4.1 22.21 – 
Attia et al. (2021b)  Phosphate pellets as energy storage materials 6.85 47.9 Spring 
Sathyamurthy et al. (2023)  Hemispherical distiller with nanoparticles paraffin wax 8.3 71.13 Summer 
Bellatreche et al. (2021)  Cylindrical parabolic collector + sand 6.5 91 Summer 
Farghaly et al. (2023)  Evacuated tube collector B
Evacuated tube collector + parabolic trough reflectors C 
0.986
1.150 
82.26
112.57 

– 
Negi et al. (2021)  Tilted wick solar + flat plate collector 3.99 38.11 Spring 

Economic evaluation

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).

Table 4

Summary of the cost evaluation results of the stills

CREF ($)CSC ($)FYC ($)YSV ($)YMC ($)YC ($)PPL ($)
Modified hemispherical solar still 0.0022 420.1014 49.0803 7.7361 7.3620 48.7062 0.1897 
Conventional hemispherical solar still 0.0022 101.6328 11.8737 1.8715 1.7811 11.7832 0.1446 
CREF ($)CSC ($)FYC ($)YSV ($)YMC ($)YC ($)PPL ($)
Modified hemispherical solar still 0.0022 420.1014 49.0803 7.7361 7.3620 48.7062 0.1897 
Conventional hemispherical solar still 0.0022 101.6328 11.8737 1.8715 1.7811 11.7832 0.1446 

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.

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