A Hybrid Photovoltaic–Solar Desalination System: Recovery of Panel Waste Heat to Improve Water Evaporation in Desalination Process

Freshwater scarcity is one of the most critical global challenges of current century, as increasing pressure on renewable water resources has intensified the need for efficient and sustainable technologies for water production [1]. In many arid and semi‑arid regions, limited access to clean water, together with the high cost and environmental impacts of fossil‑fuel‑based desalination systems, has driven strong interest toward solar‑powered freshwater production. Among the existing technologies, passive solar stills have gained considerable attention owing to their simple design, low operating cost, and suitability for remote or off‑grid communities [2], [3]. Nevertheless, their relatively low distillate output and restricted thermal efficiency remain significant obstacles to widespread practical application. Recently, hybridization of solar stills with emerging renewable technologies has been suggested as a promising strategy to improve their performance. One particularly effective approach involves integrating photovoltaic (PV) panels with solar distillation units, enabling simultaneous electricity generation and recovery of the panels’ waste heat to accelerate water evaporation [4]. This combined configuration not only enhances the thermal behavior of the desalination system and increases freshwater productivity but also improves the electrical efficiency of PV modules through direct cooling. Therefore, the development of PV‑assisted solar desalination systems offers a high‑potential, economical, and multi‑functional solution for addressing global freshwater shortages while promoting low‑carbon and sustainable energy utilization.

The simplest and most traditional form of this technology is the single slope or double slope basin solar still, in which solar radiation directly heats saline water in an insulated basin and the generated vapor condenses on a cooler glass surface [5]. The main limitation of these systems is their relatively low efficiency and freshwater productivity per unit area. Extensive research has been conducted to overcome this limitation, mainly focusing on increasing the evaporation surface area, improving heat transfer, and reducing thermal losses. The design of stepped solar stills is one of the effective approaches in this regard. In this configuration, saline water flows over a series of blackened steps, which increases the effective surface exposed to solar radiation and prolongs the residence time of the water [6]. Another strategy involves the use of enhanced thermal absorber materials, additional fins inside the basin, or corrugated absorber plates to increase the rate of heat transfer and improve the evaporation process [7]. Gawande et al. [8] investigated the performance of a stepped solar still at different water depths. In their study, additional factors affecting the desalination and distillation processes, such as climatic conditions and other operational parameters, were also examined. The experiments were conducted at three different water depths. Furthermore, several design parameters influencing the system performance were analyzed, including insulation thickness, glass cover area, packing density of the cover materials, absorber plate geometry, conditioned absorbing materials applied to the basin surface, and the inclination angles of the solar still. Omara et al. [9] attempted to enhance the productivity and efficiency of a stepped solar still by incorporating internal mirrors within the system to increase the absorption of solar radiation. El-Samadony and Kabeel [10] reported that the freshwater productivity of a stepped solar still equipped with a condenser, and with both a condenser and a reflector, was approximately 66% and 161% higher, respectively, than that of a conventional solar still. Kabeel et al. [11] reported that increasing the inlet water temperature can enhance freshwater productivity by up to 65% in the stepped solar still. Sharon et al. [12] investigated the stepped solar still. Their comparative analysis revealed that the annual average freshwater productivity of the stepped inclined solar still was approximately 19.76% higher than that of a conventional basin-type solar still. Muftah et al. [13] investigated the performance of a stepped solar still. In order to enhance the efficiency of the stepped solar still, several modifications were implemented, including the use of internal and external reflectors, different absorber materials, and an external condenser. Abujazar et al. [14] investigated the performance of stepped inclined solar stills. In this system, an inclined tray was used as part of the distillation unit. Experimental data were collected from the system and used to validate their simulation model. They calculated hourly radiative and convective heat transfer coefficients, evaporation rate, freshwater productivity, and overall efficiency. The results of their study indicated that the hourly freshwater production was about 605 mL, while the daily efficiency was approximately 28%. Tarahomi et al. [15] used the sponge layer to improve the performance of the stepped solar still. They concluded that the amount of freshwater produced in one day by the stepped solar still modified by the sponge layer is 35.5% higher than the conventional one. Zarei et al. [16] integrated the thermal energy storage unit and solar water preheater with the stepped solar still. They found that the daily freshwater production for the conventional solar still was 3.262 L and for the modified solar still with solar water preheater, Phase Change Material (PCM), and fins was 3.806 L. Mosahebi et al. [17] studied the performance of a stepped solar still equipped with internal reflector and concave steps. They found that the amounts of freshwater produced by the stepped solar stills with the flat steps and the modified concave steps are 466 and 621 mL/m$^2$, respectively.

Many multifunctional systems are based on photovoltaic/thermal (PV/T) technology. As mentioned earlier, PV cells convert only a small portion of the solar spectrum into electricity, while a significant fraction of the absorbed solar energy is dissipated as heat. This excess heat not only reduces the electrical efficiency of the PV cells with every degree Celsius increase in temperature, but also imposes long-term thermal stress on the modules. The fundamental concept of PV/T systems is to recover this waste heat by using a cooling fluid (water or air) that flows behind the PV cells. In this way, two useful outputs are simultaneously produced: electricity (with higher efficiency due to cooling) and thermal energy. Numerous studies have focused on optimizing the performance of such systems. For example, Chen et al. [18] investigated concentrated PV/T collectors and reported that electrical and thermal efficiencies of approximately 25% and 45% can be achieved. Similarly, Elsafi [19], in an analysis of a concentrated PV/T device, highlighted its potential for the cost effective production of both electricity and freshwater. The performance of these systems strongly depends on the design of the heat exchanger, the properties of the working fluid, and the intensity of solar radiation. Dev and Tiwari [20] conducted experiments on an active hybrid PV/T system. The system consisted of a single‑slope solar still with an area of 1 m$^2$, covered with a compact cover and inclined at an angle of 30° relative to the ground. This unit was integrated with two flat‑plate collectors (FPC) having a total area of 4 m$^2$, each inclined at 45° with respect to the ground. A 75 W PV module was installed beneath one of the FPC to supply electricity for a DC motor, which was used to circulate water forcibly from the solar collector to the basin. During non‑sunny hours, the pump was turned off to prevent reverse circulation and subsequent heat loss. It was observed that the productivity of the hybrid active solar still was about 3.5 times higher than that of the passive solar still at a water depth of 5 cm during nine hours of pump operation. The maximum and minimum freshwater yields were reported to be 7.223 kg and 2.006 kg, respectively. Kumar and Tiwari [21] compared PV/T active solar still and passive solar still under different climatic conditions during various months of the year. The highest freshwater production in the warmest month was reported as 7.22 kg/m$^2$ for the hybrid active solar still and 2.26 kg/m$^2$ for the passive solar still. In the coldest month, the freshwater yield for active solar still and passive solar still was 1.96 kg/m$^2$ and 0.825 kg/m$^2$, respectively. Their results indicated that the productivity during the warm season was approximately 3.3 times higher than that in the cold season. Gaur and Tiwari [22] optimized the performance of a PV/T system integrated with several FPC. This system, referred to as a PV/T‑FPC system, was fully covered with a semi‑transparent PV module to generate direct electricity, which was used to operate the water pump. The PV /thermal collectors were connected in series. Each FPC contains ten tubes, providing a total surface area of 2 m$^2$, and is installed at an inclination angle of 45° with respect to the ground. At the lower section of all FPC units, a semi‑transparent PV module with dimensions of 1.2 × 0.27 m is integrated. The electrical output of the PV panel is 37 W, which is sufficient to operate the DC pump throughout the daytime. The system proposed by Al-Nimr and Al-Ammari [23] is a single‑slope basin solar still in which a PV/T cell is installed beneath the basin, while a finned condenser is mounted on one of its side walls. In addition, a reflective mirror is installed on the wall to increase the reflection of solar radiation toward the basin. The proposed system is environmentally friendly and is capable of simultaneously producing distilled water and electricity. In addition, a low‑power fan powered by a portion of the electricity generated by the PV cell can be used to enhance the removal of water vapor from the evaporation chamber to the condensation chamber, particularly during midday. Ashtiani et al. [24] designed a solar cogeneration system for freshwater and electricity production. Their results showed that despite the PVT collector, the daily efficiency of the solar still increases to 34.8% that indicates improvement of 13.9% in comparison with the passive solar still.

The constructed system for the simultaneous production of freshwater and electricity was designed in such a way that freshwater is produced using a passive stepped solar still, while electricity is generated by PV panels that serve as the bottom surface of the basin in each step. To conduct the experiments and enable a more accurate comparison of the data, two similar devices were constructed with identical dimensions, number of steps, basin volume, and desalination chamber size. The only difference between them was the bottom surface of each basin. In one device, a dark-colored plate was used to maximize the absorption of solar energy, while in the other device; PV panels in direct contact with the water were used. A schematic of the dimensions of the solar desalination unit and its frame is shown in Figure 1. In this study, a glass chamber mounted on a metal frame was used to construct the solar desalination unit. The dimensions of the glass chamber, which serves as the solar still, were considered to be 70 cm in width, 80 cm in length, 70 cm in rear height, and 15 cm in front height.

The steps used in the solar desalination unit are shown in Figure 2. Inside the glass chamber, another stepped metallic platform was constructed to accommodate the water basins. The width of each step was 20 cm, the length was 50 cm, and the height of each step was 10 cm, with a total of three steps. These dimensions were selected based on the size and dimensions of the PV panel used in the experiments. The edges of the panels were enclosed with transparent glass sheets with a height of 5 cm, forming the basin of the solar still. The joints were sealed and waterproofed using aquarium adhesive. Similarly, another basin with the same dimensions was constructed using glass sheets. To ensure both devices appear visually similar, the bottom of this basin was covered with black-colored glass. The experiments were conducted simultaneously in two configurations: basins without PV panels and basins with PV panels. In one desalination chamber, three basins made of glass sheets with a black-coated bottom were placed on the steps, while in the other chamber, PV panels enclosed with glass sheets were installed on the steps. The volume of water entering the two chambers is controlled by a valve. In both configurations, water enters the first basin from the top and is transferred to the next basin through the outlet located at the bottom of each basin. The outlets are arranged alternately in a spiral pattern to guide the flow through all basins. Excess water is discharged outside the desalination chamber through a hose. In addition, a butterfly valve was used to more precisely control the inlet water flow rate to the basins. In this experiment, the inlet water is in direct contact with the PV panels in order to cool them, increase their efficiency, and enhance the evaporation rate. After some time, the water inside the basins evaporates due to the heat generated within the desalination chamber by solar radiation. The produced vapor rises and contacts the inner surface of the glass cover of the chamber. Because of the temperature difference between the inner surface of the glass and the outside environment, condensation occurs and the vapor turns into water droplets. Due to the slope of the glass cover, the formed droplets move downward along the inclined surface and are collected at the bottom of the chamber. To remove the produced freshwater, a hole is provided at the bottom of the desalination chamber. The collected droplets exit through this hole and are transferred via a hose to a sealed container to prevent evaporation from the container and avoid affecting the accuracy of the experimental data.

The experiment was conducted during October and November in the city of Semnan. No rainfall occurred during the experimental period, and the tests were carried out under sunny weather conditions. Although the sky was slightly cloudy during some hours, the duration was short and therefore had no significant effect on the experimental process.

For the experiment, two devices with the same number of steps and identical basin dimensions were constructed. Each device was placed separately inside a glass chamber that served as the main body of the solar desalination unit. In the basins of one device, a dark coating was used at the bottom to maximize the absorption of solar radiation, while in the other device PV panels were used, with the water being in direct contact with the panels. The inlet water flow to the basins was regulated using a butterfly valve so that the incoming water volume was neither too high nor too low. If the inlet flow were too high, the water could leave the desalination chamber before reaching the evaporation stage. Conversely, if the flow were too low, the basins might become dry. A controlled and intermittent water flow also helps cool the surface of the PV panels, which, according to previous studies, can improve their efficiency.

All basins are equipped with overflow outlets that connect them to one another, allowing excess water to flow sequentially between the basins. The excess water from the last basin is discharged through an outlet and transferred outside the desalination chamber. The evaporated water, after striking the inclined glass surface at the top of the chamber, cools due to the temperature difference between the outer glass surface and the air inside the desalination chamber. As a result, condensation occurs and the condensed water is directed through a designated outlet into a container for collecting the produced freshwater.

Every hour, several parameters were measured, including the glass surface temperature, inlet and outlet water temperatures, ambient temperature, solar radiation intensity, wind speed, air humidity, and the amount of freshwater produced.

The economic evaluation of solar desalination systems has been presented by Maghsoudian et al. [25]. The main factors involved in the cost analysis of a solar desalination system are as follows:

If $P$ is considered as the initial investment cost of the system and CRF as the capital recovery factor, the fixed annual cost (FAC) of the system can be calculated using the following relation [26]:

where, $i$ represents the bank interest rate (20% in Iran) and $n$ denotes the useful lifetime of the system (10 years considered in this study). The salvage value of the system, $S$, is assumed to be 201% of the cost of the reusable components that remain after the end of the system's lifetime. Sinking fund factor (SSF), annual salvage value (ASV), annual maintenance and operational cost (AMC), annual cost (AC), and cost per liter (CPL) are calculated by:

where, $M$ is the average annual productivity of the device.

In general, the exergy efficiency of solar desalination systems can be calculated as follows [25]:

The output exergy of the system can be expressed as follows [25]:

where, $T_\mathrm{a}$ and $T_\mathrm{w}$ represent the ambient temperature and the water temperature, respectively, expressed in degrees Celsius. The total input exergy of passive solar desalination systems consists of the received solar energy, which includes the total solar radiation incident on the surface of the saline water basin of the device. Accordingly, the input exergy of the system can be expressed as follows [25]:

where, $T_\mathrm{s}$ is the effective sun temperature (℃), $T_\mathrm{a}$ is the ambient temperature (℃), $I$ is the solar irradiance (W/m$^2$), and $A$ is the area of the solar-receiving surface (m$^2$). The effective temperature of the sun is taken as 6000 K.

The energy efficiency of solar desalination systems can be calculated as follows [25]:

where, $h_\mathrm{fg}$ is the latent heat of vaporization of water at the mean ambient temperature, with units of kJ/kg. In this study, the value of $h_\mathrm{fg}$ is taken as 2260 kJ/kg; the term $\dot{m}$ denotes the distillate production rate in kg/s; $A_\mathrm{b}$ represents the basin area of the solar still in square meters; and $I$ is the solar irradiance incident on the still, expressed in W/m$^2$.

The standard uncertainty of measuring devices can be expressed by the following relation [25]:

where, $a$ represents the accuracy of the measuring instrument and $u$ denotes the standard uncertainty. In this study, due to the experimental conditions and the characteristics of the measuring instruments, the uncertainty is considered to be of Type B. Uncertainty of the measuring instruments can be found in Table 1.

As mentioned in the previous sections, two stepped solar stills with identical dimensions were constructed. In one of them, a black plate was installed at the bottom of the basin to maximize solar energy absorption, while in the other system a PV panel was used as the basin floor. Figure 3 shows the variations of ambient temperature, inlet water temperature, and outlet water temperature at different hours during three testing days for both systems with and without PV. Figure 3 illustrates the variations of ambient temperature, inlet water temperature, and outlet water temperature for the stepped solar stills with and without the PV panel during three different days. On all three days, as solar radiation increases toward noon, the inlet and outlet water temperatures as well as the ambient air temperature increase; afterward, with the reduction of solar radiation, these temperatures show a decreasing trend. The comparison of the results shows that the outlet water temperature in both systems is higher than the inlet water temperature and the ambient temperature. However, differences in performance between the two systems can be observed at certain hours. In general, the system equipped with a PV panel, in addition to generating electrical energy, provides better thermal performance (higher outlet water temperature) compared with the system without PV. The multilayer structure of the PV panel (glass, silicon cells, and back layers), despite producing electricity (with a certain efficiency), can also act as a thermal absorbing surface. Due to its higher mass, it can store more heat than a simple plate and transfer it to the water. Consequently, the combination of electricity generation and heat absorption in the PV panel may cause the outlet water temperature in some conditions to be even higher than that of the system without PV. On 5 November 2024 ( Figure 3a), when solar radiation was more favorable, the outlet water temperature in both systems reached its maximum values (36.6 ℃ and 34.2 ℃). In the PV assisted system, this temperature is higher than that of the system without PV due to the additional heat absorbed by the PV panel. On 7 and 8 December 2024 ( Figure 3b and Figure 3c), the ambient temperature dropped significantly (about 12–18 ℃), which led to a reduction in the maximum outlet water temperature of the PV assisted system to 35.4 ℃ and 27.5 ℃, respectively. Nevertheless, the system equipped with the PV panel still maintained its temperature advantage compared with the system without PV.

Figure 4 shows the variations of ambient temperature, glass surface temperature, and wind speed for the two systems at different hours of the test days. On days with favorable solar radiation, the system equipped with the PV panel, despite electrical power extraction, was able to maintain the internal temperature (water and humid air) at a level where the glass surface temperature—and consequently the outlet water temperature—remained close to or slightly higher than that of the system without PV. This observation is consistent with the results presented in Figure 3. The maximum difference in glass surface temperature between the two systems on the test days was 1.7 ℃, 1.9 ℃, and 0.9 ℃, respectively. On colder days or under higher wind conditions, e.g. 7 and 8 December, both systems were affected by increased wind speed and reduced ambient temperature, leading to a decrease in the glass surface temperature. However, if the glass surface temperature in the PV equipped system does not fall significantly behind that of the system without PV, it indicates that the PV panel, acting as a thermal absorbing surface, has been able to partially compensate for the negative effects of wind and low ambient temperature. Overall, the graphs in the figure confirm that the thermal behavior of both systems is influenced by the combined effects of ambient temperature and wind speed. Nevertheless, the system equipped with the PV panel, despite electrical energy extraction, demonstrates acceptable and competitive thermal performance compared with the system without PV, particularly in terms of glass surface temperature and outlet water temperature (in conjunction with the results shown in Figure 3).

Figure 5 shows the variations of solar radiation intensity and freshwater production during different hours of the test days for the two systems. On all three test days, as solar radiation increased toward midday, the freshwater production rate in both systems increased, and then decreased as the radiation declined. On the warmer day (5 November 2024) and during periods of higher solar radiation (with a maximum value of 572 W/m$^2$), the system equipped with the PV panel produced 2.45 times more freshwater than the system without PV. On the colder days (7 and 8 December 2024), with lower solar radiation intensity, the absolute freshwater production in both systems decreased; however, the PV assisted system still maintained its relative advantage, producing about three times more freshwater than the system without PV. Therefore, for the same level of received solar radiation, the system equipped with a PV panel demonstrates a higher evaporation–distillation efficiency, while simultaneously generating electrical power.

Figure 6 shows the variations of current intensity and output voltage under different resistances on 5 November, 7 December, and 8 December 2024. As observed, as the output voltage increases, the current intensity decreases, following an approximately uniform downward trend. However, at a resistance of 160 $\Omega$, the current suddenly drops. This indicates that, for the considered time period, a resistance of 160 $\Omega$ represents an optimal value.

This behavior is also observed in the power–voltage curves of the PV panel, shown in Figure 7. As illustrated in this figure, as the output voltage increases, the output power also increases, following an approximately uniform upward trend. However, at a resistance of 160 $\Omega$, a turning point occurs and the power suddenly decreases.

Figure 8 compares the energy efficiency at different hours of the experiment conducted on 7 December 2024, along with the solar radiation intensity and the amount of freshwater produced for the systems with and without a PV panel. In both graphs, it can be observed that the system equipped with a PV panel exhibits higher energy efficiency compared with the system without the panel. The main reason for this behavior is the energy storage capability of the PV panel and the resulting greater thermal inertia in the system, especially during the afternoon hours and when solar radiation decreases. This helps maintain freshwater production and increases the overall desalination efficiency. At noon, the freshwater production of the system equipped with the PV panel is more than twice that of the conventional system. However, as shown in Figure 8, during the afternoon hours, with the reduction of solar radiation, both the freshwater production and the system efficiency decrease.

Figure 9 compares the temperature difference between the inlet and outlet saline water for 5 November and 8 December 2024. As observed, the temperature difference increases from the beginning of the process until around noon and then decreases afterward. In both test days, the temperature difference between the inlet and outlet water in the system equipped with the PV panel is significantly higher than that of the simple system without PV. This greater temperature difference leads to increased evaporation of saline water, resulting in higher freshwater production and improved efficiency of the PV assisted system.

In this study, two passive stepped solar stills were experimentally investigated during several days of the cold season to evaluate the effect of integrating a PV panel beneath the desalination chamber. One system employed a conventional dark absorber plate, while the other utilized a PV panel in direct contact with the saline water. The experimental results showed that the PV assisted system consistently produced higher outlet water temperatures and a larger temperature difference between the inlet and outlet saline water compared with the conventional system. This enhanced thermal behavior increased the evaporation rate and significantly improved freshwater production. Despite variations in solar radiation, ambient temperature, and wind speed during the test days, the PV based configuration maintained a clear performance advantage. The results also indicated that the PV panel operated at a relatively lower temperature due to direct contact with water, which helped improve its electrical performance while simultaneously transferring heat to the saline water. This dual function enabled the system to generate electricity while enhancing the desalination process. Under certain operating hours, the freshwater productivity and energy efficiency of the PV integrated system were more than three times higher than those of the conventional stepped still. Overall, integrating PV panels beneath the basin of stepped solar stills can effectively improve both thermal and electrical performance, offering a promising approach for simultaneous freshwater and electricity production, particularly in regions with limited water resources.