1. Introduction
Solar energy is a renewable and clean energy, and the demand for solar power has increased worldwide because of the need to reduce greenhouse gas emissions [
1]. Solar power generation through solar cells, however, has low energy conversion efficiency, and electrical efficiency is reduced as a result of electric performance due to the increase of the cell’s temperature [
2]. As an alternative, a photovoltaic thermal (PVT) system combined with a photovoltaic (PV) module and a solar thermal collector has been proposed, which circulates the fluid to lower the temperature of the PV cell and produces heat and electricity simultaneously [
3]. Combining the two systems has the advantage of reducing the installation area compared to separately installing the PV module and the solar thermal collector [
4]. Since the 1970s, a considerable number of studies on PVT technology have been conducted, and many international developments have emerged from these studies [
5].
Tripanagnostopoulos et al. [
6] constructed a hybrid PVT system and carried out a study using air as a circulating medium, based on two types of the PV modules. The results showed that the total efficiency of this system can be improved through the circulation medium. Aste et al. [
7] compared the daily and annual energy yields of the standard PV module and the PVT collector. This suggested that although the electricity production of the PVT system is slightly lower than that of the standard PV module, PVT technology has a high overall efficiency due to producing thermal energy at the same time. Prakash [
8] conducted a transient analysis of the PVT collector by spilling water/air under a solar cell, and reported the thermal efficiency of air as the heating medium. Chow et al. [
9] evaluated PVT collectors from a thermodynamic point of view, depending on the presence or absence of a glass cover above the PV module. They reported that the glazed PVT system was more suitable for maximizing the total energy output. A CPV (concentrated photovoltaic) system is also useful for utilizing solar energy, but it is important to include an appropriate cooling system for the effective operation of the CPV system, because it increases the operating temperature of the solar cell and reduces the electrical efficiency, in turn decreasing the lifetime of the solar cell [
10]. Sharaf et al. [
11] defined the CPVT (concentrated photovoltaic thermal) as a hybrid system of a CPV and PVT, which concentrates solar radiation and converts it into useful electric and thermal energy. CPVT collectors have been reported to increase the intensity of solar radiation using concentrator optics, unlike conventional PVT collectors. Coventry [
12] showed that the heat loss of the CPVT does not increase as the operating temperature increases, due to a decrease in surface area. However, the CPVT is more costly than the flat-plate PVT module because of the additional expense of high-efficiency solar cells for large scale electricity production, solar tracking drive, etc. [
13].
The overall performance of the PVT system is affected by many factors relating to the collection of heat and electrical energy [
14]. The key to designing a solar collector’s cooling system is to control the flow rate of the working fluid, which can directly affect the operating temperature and the performance of the system in terms of the overall power output [
15]. Bambrook and Sproul [
16] changed the mass flow rate of air through the PVT system, confirming an increase of the thermal and electrical efficiencies, with an increasing flow rate. Na et al. [
17] analyzed the performance of the conical solar concentrator system according to the mass flow rate of the working fluid. The results showed that thermal efficiency tends to improve with an increasing flow rate, but decreases when the optimum flow rate is exceeded.
Most solar systems use working fluids such as water, air, and ethylene glycol. Because these working fluids have low thermal conductivity, the maximum efficiency that can be obtained in existing systems is limited. Therefore, an efficiency improvement of the solar system is needed via an increase of the heat transfer characteristics of the working fluid. Nanofluids, which can be used as the working fluid (and which have exceptional heat transfer characteristics) were presented by Choi and Eastman [
18]. These nanofluids have nanoscale particles, which were distributed in base fluids such as water and ethylene glycol. Lee et al. [
19] manufactured oxide nanofluids, which have improved thermal conductivity while containing small amounts of nanoparticles (CuO, Al
2O
3). Asirvatham et al. [
20] conducted a study on the forced convection heat transfer of nanofluid, using a low volume concentration of nanoparticles with water (CuO/water). The results showed that the convection heat transfer coefficient was improved, even at a low concentration of nanoparticles. They reported that the presence of suspended nanoparticles contributed to an improvement of the thermal conductivity of the nanofluid. Nassan et al. [
21] confirmed an improvement of the thermal properties of nanofluids (CuO/water, Al
2O
3/water). It was also reported that the convective heat transfer coefficient of CuO/water was increased more than that of Al
2O
3/water at the same concentration.
Nanofluids are generally made with metals and metallic oxide nanoparticles, and many studies have been conducted to apply them to various fields (systems for heating and cooling, solar water heating, thermal storage, etc.) by improving the thermal conductivity of conventional heat transfer fluids [
22]. Lu et al. [
23] studied the thermal performance of an evacuated tubular solar collector with nanofluid (CuO/water). By applying the nanofluid, the value of the heat transfer coefficient was improved slightly, with an increase of the heat flux. They reported that the value of the heat transfer coefficient was changed according to the concentration of the nanoparticles, and the mass concentration corresponding to the optimal heat transfer improvement was 1.2%. Kang et al. [
24] conducted an economic analysis of flat-plate and U-tube solar collectors with nanofluid (Al
2O
3/water). The flat-plate and U-tube solar collectors using nanofluid with a nanoparticle size of 20 nm and a concentration of 1.0 vol% had 14.8% and 10.7% higher thermal efficiency values, respectively, compared with utilizing water as the working fluid. Karami and Rahimi [
25] performed experiments on the cooling performance of channels using Boehmite (AlOOH∙xH
2O) nanofluid for the PV module. The results showed that the nanofluid had better cooling performance than water with the highest electrical efficiency at 0.1 wt% concentration. Waeil et al. [
26] conducted an experimental study on the PVT system using nanofluid (SiC/water). As a result, the thermal conductivity of the nanofluid at a concentration of 3 wt% was improved to 8.2% in the temperature range of 25 °C to 60 °C. The overall efficiency of the PVT system was about 88.9%, which was high compared to the PV system. Shmani et al. [
27] studied a PVT system with various types of nanofluids (SiO
2, TiO
2, SiC). Using an SiC nanofluid, the highest thermal and electrical efficiencies were 81.73% and 13.52%, respectively. Many studies on the heat transfer characteristics of nanofluids have been carried out, but there is a limit to how accurately one can predict the trend of heat transfer enhancement. There are also many variables in the application of nanofluids, so more theoretical and experimental studies are needed [
28].
This study was carried out to improve the efficiency of a PVT system, with a nanofluid as the working fluid, at an optimum flow rate. The role of the working fluid is important in increasing the efficiency of the PVT system. Therefore, the electrical and thermal efficiencies of the PVT system were compared and analyzed with various flow rates of water as the working fluid. In addition, the thermal and electrical efficiencies of the PVT system were investigated using water and nanofluids (CuO/water, Al2O3/water) as working fluids at an optimum flow rate. Nanofluids, for application in the PVT system, are used after surfactant is added for dispersion stability, and the thermal conductivity according to the concentration of surfactant was measured. Additionally, the effects on the performance of the PVT system were investigated using water and nanofluids as working fluids.
4. Conclusions
This study compared thermal efficiency based on the flow rate of a PVT system using water as the working fluid. Based on the results of the thermal efficiency analysis, an optimum flow rate of 3 L/min was deduced. At a 3 L/min rate, the thermal, electrical, overall, and energy saving efficiencies were 23.59%, 12.80%, 36.39%, and 57.28%, respectively, which were the highest among the other flow rates. The efficiency of a nanofluid-based PVT system was also analyzed, with an optimal flow rate of 3 L/min.
The nanofluids used in the study were manufactured by dispersing the CuO and Al2O3 nanoparticles at 0.05 wt% concentration into water. For dispersion stability, nanofluids with the highest thermal conductivity were selected and added to the surfactant CTAB or AG in various concentrations. All thermal conductivities of the nanofluids were higher than those of water, and it was confirmed that there was a significant difference in thermal conductivity between nanofluids and water. The nanofluids with the highest thermal conductivity were chosen and applied to the PVT system as working fluids. The selected nanofluids were CuO + AG (1/2 times the amount of nanoparticles) and Al2O3 + CTAB (1/10 times the CMC concentration). The thermal conductivities of CuO/water and Al2O3/water as nanofluids were 0.901 W/m·°C and 0.829 W/m·°C, respectively. The PVT systems using nanofluids showed a significant increase in thermal efficiency compared to the water-based system, but the difference in electrical efficiency was not significant. The CuO/water and Al2O3/water-based PVT systems showed higher efficiency than the water-based system. Thus, the heat transfer was improved by nanofluids with high thermal conductivity. The reason that the difference in electrical efficiency was small was because the reduction of the surface temperature was small, and the area of the PV module used in the study was very small, such that the efficiency of the cell was not greatly changed due to the thermal characteristics of the cell.
Through this study, it was confirmed that the heat transfer characteristics were improved by adding nanoparticles to the base fluid. It was also suggested that application of nanofluids improves system efficiency through increased heat transfer. However, the PVT system, which was fixed in one direction, showed no significant temperature change on the surface of the cell, and the maximum temperature of the heating medium was not high. Therefore, in order to further investigate the effect of nanofluids on the efficiency, research should be conducted on tracking and concentrating of PVT systems in which a high cell temperature could be induced to obtain a high degree of thermal energy, with a reduction of the temperature and an improvement in efficiency.