## Abstract

The use of solar energy for increasing the amount of water heating inside the solar still unit provided an important basis for various process designs in this research. Concerning the wide use of solar energy, solar panels have been used for heating water through thermal elements. In the first setup, a cylindrical parabolic collector (CPC) was used together with 300-watt and 500-watt solar panels, the results of which were then compared with each other. Based on the results the system best performed when more powerful solar panels and a longer CPC device were used. In the second setup, solar heating was achieved by direct use of solar still units. According to comparative results between the two experimental setups, the use of direct thermal elements in the solar still unit provided better performance compared with the use of indirect thermal energy. The highest amounts of freshwater were 3.679 kg and 3.945 kg per day in the first and the second setups, respectively.

## INTRODUCTION

Water is one of the most valuable materials on Earth, on which depends on the survival of all living organisms. Since most of the water on the planet has been salty for many years, many scientists have come up with solutions for seawater desalination. Researchers have always taken ways of enhancing solar still productivity into consideration by examining the effect of various parameters.

The water depth in the basin is one of the most important parameters affecting the amount of desalinated water (Tiwari & Tiwari 2006). Absorbent materials are significantly effective in improving the thermal performance of a solar still (Abdallah et al. 2009).

Solar distillation is a promising alternative to provide the freshwater that is part of the essential needs of people (Omara et al. 2013). An investigation of the economic appraisal of the solar still has suggested that the water production cost is comparable to that reported by previous researchers (Ayoub & Malaeb 2014).

The system efficiency might decrease with increasing the volume of water (Altarawneh et al. 2017). Mathematical modeling of solar desalination and optimization of solar still structure has been performed (Feilizadeh et al. 2017).

The results have shown that the use of sand or steel fibers in the basin, as well as steam cooling, will increase the temperature of saline water compared with the reference (Hassan & Abo-Elfadl 2017). The reduction of the boiling point of saline water may also increase the system efficiency by employing a vacuum pump in the solar still (Morad et al. 2017).

The daily amount of distillation is affected by the water depth and fan speed of the solar still (Omara et al. 2017). The presence of an agitator in the solar still has improved the heat transfer and increased the amount of distillation per day (Rajaseenivasan et al. 2017).

Black rubber can also be used as a solar energy absorber in the basin (Sathyamurthy et al. 2017). The growth of population and industry contributes to a great demand for freshwater (Sellami et al. 2017). Economic evaluations and estimates indicate that such a system can be operated in most remote areas (Al-harahsheh et al. 2018).

The investigation of the functions of a parabolic trough collector (PTC) with a double slope on the solar still has revealed that it increases the temperature, thereby increasing the efficiency of the system (Fathy et al. 2018). According to investigations, thickening of absorbent materials and water flow may affect the performance of the solar still (Hou et al. 2018).

The number of heat transfers between the light absorber plate and the water inside the basin would be increased by adding fins on the plate (Jani & Modi 2018). The forced displacement by the DC fan increases the evaporation of saline water and thus increases the system efficiency (Nayi & Modi 2018). Lack of drinking water would be one of the most influential threats to the future of humankind, as many freshwater sources would have been exploited by then (Selvaraj & Natarajan 2018).

This paper investigated two types of desalination process designs by the solar water desalination method. In designing the first process, the energy was supplied from solar panels and a CPC that was used outside the saline water basin containing a water rotation system once every hour. Moreover in the second method, the energy was generated from solar plates directly used in the basin by thermal elements. Finally, the results of these two processes were compared.

## METHODS

For data collection and measurement, a laboratory was used at Yasuj, Iran. The geographical range is 30°40′06″N and 51°35′17″E.

### Experimental setup 1

In this experimental setup, 300 W and 500 W solar panels were used along with two DC pumps with constant discharge rates of 1 l/min and 10 l/min. The water height in the basin was 25 mm. The basin was made of aluminum with a bottom area of 0.5 m*1 m, which was insulated with insulating glass wool. Because of the high absorption of black dye, the bottom of the aluminum basin was painted black. The height of the basin from the surface of the desalination plant was 0.5 m. The barrel was located on a metal frame. The angle of the glass was 30° horizontally. The glass which was used in this process was 5 mm thick. Figure 1 shows the technical form of this process.

Figure 1

Schematic diagram of experimental setup 1.

Figure 1

Schematic diagram of experimental setup 1.

In this process, the CPC was used with stainless steel of 1 m*1 m, as well as 1 m*2 m. The pipe was located at the center of the CPC device with a diameter of 1 cm. The charge controller device was employed to get the output voltage from the solar panels to the desired and standard level. To test water heating by the thermal element, 250 W and 400 W elements were used in various experiments.

In addition, three valves were used to open or close water flow. The volume of water in the 25 mm basin was 12.5 litres. During the procedure, 25 litres of water were processed. Half of the 25 litres of saline water were heated during the sweetening process in the basin, and the other half were heated in the CPC device. Within 1 hour, the water in the solar still basin was being distilled, and the sweetened water was removed from the process.

The water in the water heater was heated by the thermal element in the solar panels for 47 minutes. Then, it was passed through the CPC device by the DC pump with a constant discharge rate of 1 l/min and was stored in the insulated tank. The saline water in the basin was removed from the basin every 1 hour by a constant discharge rate of the DC pump at 10 l/min entering the water heater. After evacuating the basin, the water in the insulated tank entered the basin on the solar still unit.

This saline water cycle lasted from 9 am to 5 pm, once every hour. Almost half of the saline water was heated, and the other half was being sweetened. Since the ambient temperature increases the efficiency of solar panels, the effect of reducing the temperature of the solar panels to 25 °C was measured in some experiments. The cooling of the solar panels was performed by spraying cold water over these solar areas.

### Experimental setup 2

In this experimental setup, 300 W and 500 W solar panels were used to supply the energy needed for boiling water. Furthermore, 250 W and 400 W thermal elements were used to heat the water in the basin. The schematic process is shown in Figure 2.

Figure 2

Schematic diagram of experimental setup 2.

Figure 2

Schematic diagram of experimental setup 2.

The basin was made of aluminum, which was completely painted black for enhancing the absorption of solar energy. Glass wool insulation was used to prevent energy loss. The thickness of the glass cover was 5 mm. The system had a double slope glass cover. The bottom of the basin had dimensions of 0.5 m*1 m. The thermal element in the process was embedded in the saline water in the basin, which constantly caused more basin warming, leading to an increase in the amount of distilled water during the process. Throughout the tests, the charge controller was used to maximize solar panel power output, as well as to cut off the charge after a full battery charge.

Various conditions were investigated and tested in each experimental setup, which is described through different experimental cases in Table 1.

Table 1

Details of case studies in setup 1 and setup 2

Experimental caseSolar panelCPCCooling panelThermal elementBatteryCharge controller
Case 1 300 W 1 m – 250 W 2*12V*150Ah 10 A
Case 2 300 W 2 m – 250 W 2*12V*150Ah 10 A
Case 3 300 W 2 m ✓ 250 W 2*12V*150Ah 10 A
Case 4 500 W 1 m – 400 W 4*12V*150Ah 20 A
Case 5 500 W 2 m – 400 W 4*12V*150Ah 20 A
Case 6 500 W 2 m ✓ 400 W 4*12V*150Ah 20 A
Case 7 300 W – – 250 W 2*12V*150Ah 10 A
Case 8 500 W – – 400 W 4*12V*150Ah 20 A
Experimental caseSolar panelCPCCooling panelThermal elementBatteryCharge controller
Case 1 300 W 1 m – 250 W 2*12V*150Ah 10 A
Case 2 300 W 2 m – 250 W 2*12V*150Ah 10 A
Case 3 300 W 2 m ✓ 250 W 2*12V*150Ah 10 A
Case 4 500 W 1 m – 400 W 4*12V*150Ah 20 A
Case 5 500 W 2 m – 400 W 4*12V*150Ah 20 A
Case 6 500 W 2 m ✓ 400 W 4*12V*150Ah 20 A
Case 7 300 W – – 250 W 2*12V*150Ah 10 A
Case 8 500 W – – 400 W 4*12V*150Ah 20 A

### Experimental measurement

The experimental data for each of the setups were measured on June 2018 in Iran at Yasuj city. In the course of the experiments, the amount of water purified during different hours between 9 am and 5 pm, glass cover temperature, ambient temperature, water temperature, solar radiation, and wind speed were measured using different measuring devices. The experimental items related to temperature were measured by a digital thermometer (TPM-10). A solarmeter (TES 1333R) was used to measure the amount of intensity at different times. The value of wind speed in the surroundings was obtained by a digital anemometer (GM 8902). The measurement of the amount of freshwater during different hours was also obtained by a calibrated cylinder.

Table 2 shows type, accuracies and errors of the measuring devices (solarmeter, anemometer and thermometer).

Table 2

Accuracies and errors of measuring devices

DeviceAccuracyError (%)
Solarmeter ±10 W/m2
Anemometer ±0.1 m/s
Thermometer ±1 °C 0.3
Calibrated cylinder ±1 ml
DeviceAccuracyError (%)
Solarmeter ±10 W/m2
Anemometer ±0.1 m/s
Thermometer ±1 °C 0.3
Calibrated cylinder ±1 ml

## RESULTS AND DISCUSSION

Experimental measurements were carried out between 9 am and 5 pm for two different experimental setups. Experiments were conducted in June 2018. The results indicated an increase in the amount of water sweetened by the use of solar panels with higher energy production. The first setup test results also showed the effect of using the CPC device and solar panel cooling.

### Solar still temperature

Figure 3(a) and 3(b) represent the values of ambient temperature, water temperature, glass cover temperature, and solar intensity on two different days. Figure 3(a) shows the first experimental setup on the first day of June, and Figure 3(b) shows the second experimental setup on June 8. The amount of received solar energy was gradually increasing from 9 am and reached its maximum after 3 hours at 12 pm. Afterward, the amount of solar power gradually decreased. The maximum and minimum amounts of energy received in the first setup were 1,062 W/m2 and 439 W/m2, respectively.

Figure 3

Hourly solar intensity, water temperature, ambient temperature and glass cover temperature (a) on June 1, 2018, in experimental setup 1 and (b) on June 8, 2018, in experimental setup 2.

Figure 3

Hourly solar intensity, water temperature, ambient temperature and glass cover temperature (a) on June 1, 2018, in experimental setup 1 and (b) on June 8, 2018, in experimental setup 2.

In the second setup, the maximum and minimum amounts of received energy were 1,059 W/m2 and 435 W/m2, respectively. Examining the temperature variations in Figure 3(a) and 3(b) shows that the water and glass cover temperatures were rising from 9 am onwards, and reached their peak at 1 pm. The reason for the temperature changes from 9 am to 5 pm during the various experiments lies in the use of solar energy to generate heat energy by solar panels and the CPC device. The amount of energy absorbed by the thermal element was increased as a result of using solar panels, thereby raising the saline water temperature, leading to an increase in the amount of sweetening. The maximum difference between water temperature and ambient temperature was 33.8 °C and 35.7 °C in Figure 3(a) and 3(b), respectively.

Table 3 provides information on wind speed, solar intensity, and ambient temperature during testing days. Cases 1 to 8 were tested on June 1 to 8, respectively.

Table 3

Wind speed, solar intensity and ambient temperature during the test days

Time
9:0010:0011:0012:0013:0014:0015:0016:0017:00
June 1 Wind speed (m/s) 0.5 0.8 0.7 1.2 0.5 0.6 0.9 1.3
Solar intensity (W/m2785 929 1,023 1,062 1,038 957 824 647 439
Ambient temperature (°C) 35.1 36.5 38.2 40.8 42.7 44 43.4 42.2 37
June 2 Wind speed (m/s) 1.1 0.5 0.7 0.9 0.6 1.2 0.9
Solar intensity (W/m2785 929 1,023 1,062 1,038 957 824 647 439
Ambient temperature (°C) 35.1 36.5 38.3 40.9 42.8 44.2 43.5 42.2 37.2
June 3 Wind speed (m/s) 1.1 0.5 0.8 0.6 0.9 1.2 0.6 0.7
Solar intensity (W/m2784 929 1,023 1,061 1,038 957 824 647 439
Ambient temperature (°C) 35.2 36.6 38.4 41 43 44.3 43.6 42.3 37.4
June 4 Wind speed (m/s) 0.9 0.8 1.2 0.5 0.5 0.7 0.9 1.1
Solar intensity (W/m2783 928 1,022 1,061 1,038 957 824 647 439
Ambient temperature (°C) 35.2 36.7 38.4 41 43 44.3 43.8 42.4 37.4
June 5 Wind speed (m/s) 1.4 0.5 0.8 0.9 1.3 0.7 0.6 0.5 0.9
Solar intensity (W/m2782 928 1,022 1,060 1,038 958 825 648 440
Ambient temperature (°C) 35.4 36.8 38.5 41.2 43.2 44.4 43.9 42.4 37.5
June 6 Wind speed (m/s) 0.6 0.5 0.7 1.2 1.3 0.6 0.7 0.5
Solar intensity (W/m2782 927 1,022 1,060 1,037 958 825 648 440
Ambient temperature (°C) 35.6 37 38.6 41.3 43.3 44.6 44 42.5 37.6
June 7 Wind speed (m/s) 0.9 1.4 0.6 0.5 0.5 0.8 1.2 0.5
Solar intensity (W/m2780 926 1,021 1,060 1,037 958 825 648 440
Ambient temperature (°C) 35.7 37.1 38.8 41.4 43.4 44.6 44 42.5 37.8
June 8 Wind speed (m/s) 1.1 0.5 0.7 0.9 1.8 2.1 2.4 1.9
Solar intensity (W/m2779 926 1,021 1,060 1,037 956 823 645 439
Ambient temperature (°C) 35.8 37.2 38.9 41.5 43.4 44.7 44.1 41.9 38
Time
9:0010:0011:0012:0013:0014:0015:0016:0017:00
June 1 Wind speed (m/s) 0.5 0.8 0.7 1.2 0.5 0.6 0.9 1.3
Solar intensity (W/m2785 929 1,023 1,062 1,038 957 824 647 439
Ambient temperature (°C) 35.1 36.5 38.2 40.8 42.7 44 43.4 42.2 37
June 2 Wind speed (m/s) 1.1 0.5 0.7 0.9 0.6 1.2 0.9
Solar intensity (W/m2785 929 1,023 1,062 1,038 957 824 647 439
Ambient temperature (°C) 35.1 36.5 38.3 40.9 42.8 44.2 43.5 42.2 37.2
June 3 Wind speed (m/s) 1.1 0.5 0.8 0.6 0.9 1.2 0.6 0.7
Solar intensity (W/m2784 929 1,023 1,061 1,038 957 824 647 439
Ambient temperature (°C) 35.2 36.6 38.4 41 43 44.3 43.6 42.3 37.4
June 4 Wind speed (m/s) 0.9 0.8 1.2 0.5 0.5 0.7 0.9 1.1
Solar intensity (W/m2783 928 1,022 1,061 1,038 957 824 647 439
Ambient temperature (°C) 35.2 36.7 38.4 41 43 44.3 43.8 42.4 37.4
June 5 Wind speed (m/s) 1.4 0.5 0.8 0.9 1.3 0.7 0.6 0.5 0.9
Solar intensity (W/m2782 928 1,022 1,060 1,038 958 825 648 440
Ambient temperature (°C) 35.4 36.8 38.5 41.2 43.2 44.4 43.9 42.4 37.5
June 6 Wind speed (m/s) 0.6 0.5 0.7 1.2 1.3 0.6 0.7 0.5
Solar intensity (W/m2782 927 1,022 1,060 1,037 958 825 648 440
Ambient temperature (°C) 35.6 37 38.6 41.3 43.3 44.6 44 42.5 37.6
June 7 Wind speed (m/s) 0.9 1.4 0.6 0.5 0.5 0.8 1.2 0.5
Solar intensity (W/m2780 926 1,021 1,060 1,037 958 825 648 440
Ambient temperature (°C) 35.7 37.1 38.8 41.4 43.4 44.6 44 42.5 37.8
June 8 Wind speed (m/s) 1.1 0.5 0.7 0.9 1.8 2.1 2.4 1.9
Solar intensity (W/m2779 926 1,021 1,060 1,037 956 823 645 439
Ambient temperature (°C) 35.8 37.2 38.9 41.5 43.4 44.7 44.1 41.9 38

Figure 4(a) and 4(b) show water temperature at different times of the day in various experimental cases. Figure 4(a) shows the water temperature in Cases 1, 2, 3, and 7. Cases 1, 2, and 3 are related to the first setup, and Case 7 is related to the second setup. The results show that the water temperature in Case 7 of the second setup was greater than the water temperature in Cases 1, 2, and 3 related to the first setup during different times of the experiment.

Figure 4

Hourly water temperature for (a) Case 1, Case 2, Case 3 and Case 7 and (b) Case 4, Case 5, Case 6 and Case 8.

Figure 4

Hourly water temperature for (a) Case 1, Case 2, Case 3 and Case 7 and (b) Case 4, Case 5, Case 6 and Case 8.

In addition, it was found that the use of direct solar energy and a thermal element in a solar still unit would increase the water temperature compared with those in the first setup. Figure 4(b) indicates the water temperature in the first and second setups. The experiments shown in this figure were related to Cases 4, 5, and 6 in the first setup and Case 8 in the second setup. Again, the results of this graph show that the direct use of a thermal element in the basin would increase the water temperature compared with the first setup.

Since the use of solar panels, CPCs, and cooling units does not continuously absorb energy from solar panels of various sizes into saline water in a solar still unit, the water temperature was less than that obtained directly from solar panels in the basin. However, by comparing Case 7 in Figure 4(a) with Case 4 in Figure 4(b), it was concluded that the power of 300-watt solar panels directly in the second setup was less than 500-watt solar panels in the first setup, raising the water temperature.

The comparison of temperatures between Cases 1, 2, and 3 shows that using a 2 m CPC device caused the saline water temperatures to increase compared with a 1 m CPC. In Case 3, the use of solar panel cooling also increased the saline water temperature of Cases 1 and 2.

Figure 5(a) and 5(b) show the glass cover temperature at different times of the day. The results presented in these two figures are for Cases 1 to 6 of the first setup, and Cases 7 and 8 were related to the second setup. In Figure 5(a), the results of Cases 1, 2, 3, and 7 are compared. In Figure 5(b), the results of cases 4, 5, 6, and 8 are compared.

Figure 5

Hourly glass temperature for (a) Case 1, Case 2, Case 3 and Case 7 and (b) Case 4, Case 5, Case 6 and Case 8.

Figure 5

Hourly glass temperature for (a) Case 1, Case 2, Case 3 and Case 7 and (b) Case 4, Case 5, Case 6 and Case 8.

Figures 4(a), 4(b), 5(a) and 5(b) show the highest temperature at 1 pm. The highest temperature was 76.5 °C, 80.2 °C, 58.7 °C, and 60.5 °C in Figures 4(a), 4(b), 5(a) and 5(b), respectively.

### Freshwater production

Figure 6(a) and 6(b) represent the duration of times spent in the experimental setups. In Figure 6(a), the results of Cases 1, 2, 3, and 7 are compared. Figure 6(b) compares the results of Cases 4, 5, 6, and 8. Furthermore, Figure 6(a) indicates the direct effect of using a thermal element in the solar still unit in the second setup experiments compared with the amount of water depleted using solar panels and CPC devices in the second setup.

Figure 6

Hourly water production for (a) Case 1, Case 2, Case 3 and Case 7 and (b) Case 4, Case 5, Case 6 and Case 8.

Figure 6

Hourly water production for (a) Case 1, Case 2, Case 3 and Case 7 and (b) Case 4, Case 5, Case 6 and Case 8.

As shown in Figure 6(a), the use of a 2 m CPC device resulted in more sweetened water compared with the use of a 1 m CPC device. In addition, the cooling of solar panels led to an increase in the amount of water because of an increase in the efficiency of the solar panels. The highest amount of water sweetened in Figure 6(a) was related to Case 7 under conditions of using a 300 W solar panel and the direct supply of heat energy in the solar still unit. The maximum amount of freshwater was 0.635 kg at 2 pm.

Figure 6(b) compares Cases 4, 5, 6, and 8, and the amount of sweetened water shown in Case 8. Case 8 refers to the amount of water consumed by 500 W solar panels and the energy supply of the thermal element in the basin. Case 8 was related to the second setup, and Cases 4, 5, and 6 were related to the first setup. Like Figure 6(a), the amount of drained water in Figure 6(b) reached its highest value, i.e., 0.746 kg, at 2 pm. The amount of sweetened water was rising from 9 am and peaked at 2 pm, while it dropped after 2 pm.

Figure 7 represents the amount of freshwater accumulated in a system during the process from 9 am to 5 pm. All experimental cases for both experimental setups and the solar still unit without a solar panel and CPC device are shown in this chart. The lowest and highest amounts of water (2.594 and 3.945 kg) were related to Case 1 and Case 8, respectively.

Figure 7

Accumulated water production for different cases and solar still unit without solar panel and CPC device.

Figure 7

Accumulated water production for different cases and solar still unit without solar panel and CPC device.

A comparison between the experimental setups and the solar panel and CPC device solar panel reflects an increase in the amount of water consumed using the solar panel, the CPC device, as well as the cooling of the solar panels during the tests. The use of CPC devices and solar panels increased the amount of solar energy absorbed. Increased saline water temperature resulted in the evaporation of more saline water and, consequently, an increase in freshwater content.

Figure 8 depicts the amount of sweetened water during a day in the various experimental cases, as well as the solar still unit without a solar panel and CPC device. The lowest amount of water was related to the solar still unit without a solar panel and CPC device. As shown in this figure, the amount of water increased gradually with an increase in solar panel power and the use of a larger CPC device, as well as solar panel cooling. Due to the proper use of solar energy, the temperature of saline water was raised, increasing the evaporation of saline water.

Figure 8

Daily production of freshwater for different cases and solar still unit without solar panel and CPC device.

Figure 8

Daily production of freshwater for different cases and solar still unit without solar panel and CPC device.

## ECONOMIC ANALYSIS

First in economic analysis is the ‘first annual cost’ for every experimental case (Rashidi et al. 2017):
where CRF is the capital recovery factor and is defined as follows. In this equation P is the capital cost of experimental cases (Rashidi et al. 2017).
where i is the interest rate of bank lending (20% in Iran), and n is the life of experimental cases (Rashidi et al. 2017).

n = 10 years

ASV (the first annual salvage value) for experimental cases is defined as follows (Rashidi et al. 2017):
where S is the salvage value of experimental cases, and in this equation SSF is a sinking fund factor (Rashidi et al. 2017).
S and SSF are defined as follows (Rashidi et al. 2017):

AMC (annual maintenance cost) in the economic analysis section is defined as follows (Rashidi et al. 2017):
AC (total annual cost) of experimental cases is calculated based on the following equation (Rashidi et al. 2017):
The amount of CPL (cost per litre) related to the freshwater output in different experimental cases is calculated as follows (Rashidi et al. 2017):

M is the mean annual freshwater production by different experimental cases (Rashidi et al. 2017).

Table 4 presents the results of the full economic analysis of the different experimental cases.

Table 4

Economic analysis for different experimental cases in setup 1 and setup 2

TypeinP($)CRFFACSSSFASVAMCACMCPL Case 1 0.2 10 71 0.238 16.898 14.2 0.038 0.5396 2.5347 18.8931 946.81 0.0199 Case 2 0.2 10 74 0.238 17.612 14.8 0.038 0.5624 2.6418 19.6914 1,022.365 0.0192 Case 3 0.2 10 75 0.238 17.85 15 0.038 0.57 2.6775 19.9575 1,109.965 0.0179 Case 4 0.2 10 93 0.238 22.134 18.6 0.038 0.7068 3.3201 24.7473 1,164.35 0.0212 Case 5 0.2 10 96 0.238 22.848 19.2 0.038 0.7296 3.4272 25.5456 1,259.25 0.0202 Case 6 0.2 10 97 0.238 23.086 19.4 0.038 0.7372 3.4629 25.8117 1,342.835 0.0192 Case 7 0.2 10 64 0.238 15.232 12.8 0.038 0.4864 2.2848 17.0304 1,204.5 0.0141 Case 8 0.2 10 86 0.238 20.468 17.2 0.038 0.6536 3.0702 22.8846 1,439.925 0.0158 TypeinP($)CRFFACSSSFASVAMCACMCPL
Case 1 0.2 10 71 0.238 16.898 14.2 0.038 0.5396 2.5347 18.8931 946.81 0.0199
Case 2 0.2 10 74 0.238 17.612 14.8 0.038 0.5624 2.6418 19.6914 1,022.365 0.0192
Case 3 0.2 10 75 0.238 17.85 15 0.038 0.57 2.6775 19.9575 1,109.965 0.0179
Case 4 0.2 10 93 0.238 22.134 18.6 0.038 0.7068 3.3201 24.7473 1,164.35 0.0212
Case 5 0.2 10 96 0.238 22.848 19.2 0.038 0.7296 3.4272 25.5456 1,259.25 0.0202
Case 6 0.2 10 97 0.238 23.086 19.4 0.038 0.7372 3.4629 25.8117 1,342.835 0.0192
Case 7 0.2 10 64 0.238 15.232 12.8 0.038 0.4864 2.2848 17.0304 1,204.5 0.0141
Case 8 0.2 10 86 0.238 20.468 17.2 0.038 0.6536 3.0702 22.8846 1,439.925 0.0158

According to the results of the economic analysis, the lowest cost to produce a litre of water in these experiments is Cases 7 and 8. Consequently, the direct use of solar energy generated by solar panels in the field of solar desalination is more cost-effective than the indirect method. The highest amount of freshwater was related to Case 8 (3.945 kg), which was obtained in 0.5 m2 of solar still basin. In this case, the price per litre of freshwater was 0.0158.

## CONCLUSIONS

This paper investigates and compares the direct and indirect use of solar energy for solar water desalination. Two different processes were designed to compare the effect of direct and indirect use of solar energy. In the first process (i.e., the indirect use of solar energy), solar panels and CPCs were used for further water heating in the basin. In the second process (i.e., the direct use of solar energy), the solar energy absorbed by solar panels caused the thermal element in the solar still to heat up and heat the water in the basin. Overall, the results indicate that the direct use of solar energy results in more desalinated water than the indirect use of solar energy. In these two experimental setups, 300 W and 500 W solar panels are used. The maximum and minimum amounts of freshwater were related to Case 8 in the second process (i.e., 3.945 kg) and Case 1 in the first process (i.e., 2.594 kg). The results of the economic analysis show that the lowest CPL is related to experimental setup 2 (Cases 7 and 8). The CPL values were 0.0141 and 0.0158 for Cases 7 and 8, respectively.

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