Influence of PV/T waste heat on water productivity and electricity generation of solar stills using heat pipes and thermoelectric generator: An experimental study and environmental analysis

Highlights

• The combination of PV and solar still using heat pipe and TEG was assessed.

• Energy, exergy and electrical efficiencies of the system was improved.

• Water and electrical generation of the SS-HP-WT was increase by 55.3% and 20.9%.

• CPL and CPP of the SS-HP-WT were about 0.042 $/L and 0.061 $/kWh.

Abstract

In recent years, numerous studies have been performed to improve the evaporation rate of solar stills and the electrical power of the photovoltaic module. In this paper, an experimental study of modified single slope solar still integrated with a photovoltaic panel, heat pipes and thermoelectric generator to enhance the performance of solar still is presented and compared with conventional solar stills. Five heat pipes were proposed and installed below a photovoltaic module to transfer the generated waste heat to the saline water of a solar still. Two thermoelectric generators were installed between the photovoltaic panel (hot sides) and the cooling water block (cold sides) to use the generated waste heat. The saline water flows to the cooling water block and then to the evaporation zones of the heat pipes to increase the water temperature and enhance the electrical power of the photovoltaic panel and thermoelectric generator. Results showed that the highest hourly power generation of the solar panel of conventional solar still (CSS), solar still by water cooling and thermoelectric generators (SS-WT), solar still by heat pipes (SS-HP) and solar still by heat pipes, water cooling and thermoelectric generators (SS-HP-WT) was about 68 W, 69 W, 73 W and 75 W, respectively. Also, the daily water productivity of CSS, SS-WT, SS-HP and SS-HP-WT was about 748 ml/m², 832 ml/m², 1058 ml/m² and 1162 ml/m², respectively. Moreover, CPL of CSS, SS-WT, SS-HP and SS-HP-WT were about 0.042 $/L, 0.098 $/L, 0.077 $/L, 0.084 $/L, respectively. In addition, cost per power production of conventional photovoltaic, SS-WT, SS-HP and SS-HP-WT were about 0.061 $/kWh, 0.141 $/kWh, 0.128 $/kWh and 0.147$/kWh, respectively.

Introduction

In recent years, the acceptability of freshwater is one of the most fundamental challenges in different regions around the globe. Safe drinking water is essential in many sectors such as agriculture [1], domestic [2], household and industries as it plays a critical role, especially in arid and remote areas. One of the cheapest and most energy-efficient ways to purify water is utilising solar energy as a permanent source of energy using solar stills as a sustainable, inexpensive and environmentally friendly technique [3], [4], [5]. The origin of solar still is based on evaporation and condensation operations, so to have drinking water from brackish water by solar still, a major problem is its low yield [6], as a result, to improve the efficiency of solar stills, several numerical and experimental studies have been performed [7], [8], [9], [10], [11], [12], [13]. Also, many studies have used machine learning methods to forecast the performance of the systems [14], [15]. Moreover, different techniques such as heat pipes [16], thermoelectric heating and cooling [17], photovoltaic thermal [18], [19], solar collectors [20] and different insulations have been utilized to enhance the productivity of solar desalination [21].

The waste heat of the photovoltaic panels can be used to increase the water temperature of solar stills and enhance the system’s electrical efficiency. An experimental study on the performance of integration of photovoltaic panels with single slope solar still was conducted by Hassan and Elbar [22]. Their results improved the efficiency of the solar desalination system by 10 % in comparison with traditional solar still. Kabeel et al. [23] investigated the impact of cover cooling, photovoltaic module and phase change material on the performance of solar desalination. They found that the water productivity of the solar still increased by 36.25 %. Abdullah et al. [24] improved the efficiency of the solar still using a photovoltaic module, electrical heaters, phase change material and a wick corrugated absorber. The results illustrated that the freshwater yield of an electrical heater, which was supplied with a photovoltaic (PV) module, and phase change material with nanoparticles were improved by 150 % and 122 %, respectively. Experimental and numerical analysis to increase the performance of the stepped solar still using photovoltaic thermal (PV/T) has been done by Naroei et al. [25]. The results showed that coupling PV/T with stepped solar still improved the productivity and thermal efficiency by about 20 % and 2 times higher than conventional ones. Parsa et al. [26] experimentally investigated active solar still by a photovoltaic panel. Hourly efficiency and total productivity at high altitudes are enhanced by about 27.8 % and 42.5 %, respectively. In another research, photovoltaic thermal (PV/T)-compound parabolic concentrator (CPC) collectors and water-based nanofluids integrated with double solar still coupled with a heat exchanger was analytically performed by Sahota et al. [27]. According to their results, the enhancement of heat transfer of PV/T-CPC was reported by 46.6 %.

Thermoelectric modules have a significant effect on the water productivity of solar desalination systems. The numerical study of solar still in conjunction with thermoelectric cooling to enhance the productivity of water was reported by Esfe and Toghraie [28]. The results of their work showed that the application of the thermoelectric with the Peltier effect on the glass of solar still increased water production from 15 % to 62 %. In another research, Shoeibi et al. investigated the performance of a solar still using simultaneous thermoelectric heating and cooling [29]. The results of their study showed that the efficiency of modified solar still in comparison with the conventional system was increased by about 76.4 %. In regards to solar still with thermoelectric generator (TEG), Shafii et al. [30] investigated the temperature difference between water vapor and cold ambient air to generate electricity and noticed that creating forced convection by a propeller fan can lead to a 14 % increase in daily water. Shoeibi et al. [31] assessed the effect of various nanoparticles and thermoelectric modules on the performance of solar still. They showed that the highest and lowest water output of the device were achieved in solar still with MWCNT/water and TiO2/water nanofluids, which were about 11.57 % and 4.66 %, respectively. Rahbar et al. [32] used thermoelectric cooling to improve the performance of the solar still. They investigated the influence of the hot side of the thermoelectric which was attached under the basin to increase the water vapor, and the result of their work showed that the maximum exergy efficiency of modified solar still was enhanced by 25 %. In another experimental study with solar still and thermoelectric Shoeibi et. al [33] were able to increase the water productivity of solar still using simultaneous thermoelectric cold and hot sides. Their result indicated that the energy and exergy efficiencies of the solar still increased by 11.2 % and 45.7 %, respectively. Nazari et al. [34] enhanced the productivity of single slope solar still by employing thermoelectric cooling channels and copper oxide nanofluids. The results revealed that the highest enhancement values of productivity, energy and exergy efficiencies were reported to be 81 %, 80.6 % and 112.5 %, respectively. In an experimental study to utilize thermoelectric cooling, Esfahani et al. [35] used portable active solar still. The experiments were carried out in winter days and the average daily productivity of solar still increased to 1.2 L/m².

Heat pipes are suitable devices to transfer heat from a high-temperature source into low-temperature applications [36]. Rastegar et al. [37] considered thermal waste energy of the exhaust chimney by heat pipe to enhance the performance of double slope solar still. They reported that the energy and exergy efficiencies of the system using heat pipe were enhanced by 65.5 % and 41 % higher than the traditional system. Rahbar and Esfahani [38] performed an experimental study on a portable solar still by heat pipes and thermoelectric cooling. They improved the temperature difference between evaporation and condensing zones by thermoelectric cooling and heat pipes instead of a heat sink to cool one side of the thermoelectric module. Their results indicated the highest daily efficiency of the solar desalination system was improved by 7 %. The effects of external heat storage and heat pipes with phase change material on the performance of solar still evaluated by Faegh and Shafii [39]. The results showed that the water productivity of modified solar still was increased by 86 % in comparison with solar still without phase change material (PCM). The impact of using thermosyphon heat pipes and vacuum glass to increase the performance of a solar still was reported by Jahangiri Mamouri et al. [40]. Results showed that the water production rate and system efficiency was about 1.02 kg/m²h and 22.9 %, respectively. Fallahzadeh et al. [41] evaluated the impact of single-spiral solar collectors and closed-loop pulsating heat pipes on the productivity of solar still. Their results showed that the productivity of modified solar still was about 2230 ml/m²day. Table 1 shows the comparison of various techniques used in solar still.

In recent years many studies have been performed on increasing the productivity of solar still and electrical power of photovoltaic modules. Coupling of photovoltaic thermal system with solar still can enhance the water temperature by using the waste heat of a photovoltaic panel and simultaneous increase the electrical power of the photovoltaic panel. Also, installing the thermoelectric generator under the photovoltaic module can reduce panel temperature and increase the electrical power of TEG and PV/T at the same time. To the best of the authors’ knowledge, the use of waste heat of photovoltaic panels to increase the productivity of solar still by heat pipes, electrical generation of a thermoelectric generator, and electrical generation of photovoltaic have not been performed and assessed yet. As such, the impacts of combining PV/T, solar still and thermoelectric generator on the electrical and thermal efficiency of the system were evaluated in this study. Moreover, at the same time, finding the cost per litre (CPL) of water generation cost per power production (CPP) of PV and TEG in the solar energy systems was studied. In addition, profit-cost ratio, environmental parameters based on exergy and energy, exergoeconomic based on energy and exergy, enviroeconomic, and exergoenviroeconomic were evaluated.

The rest of the paper is organized as follows. Section 2 explains the photovoltaic panel/solar still system and equipment specification. Section 3 contains the mathematical background of energy, exergy and uncertainty analysis. Section 4 contains the environmental, exergoenvironmental and cost analysis of system. Section 5 compares and discusses the results of different experimental setups. Section 6 summarizes the general and specific results of this study.