1. Introduction
Solar energy is an eco-friendly renewable energy source that can produce energy in two forms, i.e., heat and electricity, and is commonly used in photovoltaic (PV) and solar thermal system individually [
1]. However, a PV system has a low energy conversion efficiency, and it therefore faces problems such as the decrease in efficiency and aging of the cell due to the rise in temperature of the solar cell [
2]. Thus, it is important to release heat of the solar cell in a PV system. A photovoltaic thermal (PVT) system is a combination of the PV and solar thermal systems that not only produces heat and electric energy at the same time but also prevents the reduction of PV efficiency through heat absorption of the heat medium [
3]. The performance of the PVT system is influenced by various design parameters, such as mass flow rate, number of glass covers, diameter and thickness of absorbing tube, and thermal conductivity of heat medium [
4].
In order to improve the performance of the PVT system, several studies on the flow change of the heat medium and structural improvement of the collector have been conducted. Sopian et al. [
5] conducted a steady-state analysis of the performance of single-pass and double-pass combined PVT collectors. The simulation results show that the performance of the double-pass collector is excellent and that the double-pass collector produces more thermal energy and has a higher cooling effect on the solar cell than the single-pass collector. Bambrook and Sproul [
6] confirmed the thermal and electrical efficiencies of varying air mass flow rates using a PVT air system. Their results showed that the mass flow rate of air tends to increase thermal efficiency as the flow rate increases in the range of 0.013 to 0.115 kg/sm
2 of mass flow rate. In addition, at high flow rates, even though cooling of the PVT models was greatly enhanced, the electrical efficiency was not increased significantly. Similarly, Chow et al. [
7] evaluated a PVT collector with the presence of a glass cover from the thermodynamic perspective. They observed that the glazed PVT system was suitable for maximizing the total energy output, and the unglazed system had excellent electrical efficiency because the glass cover is disadvantageous in terms of electricity. In addition, Ibrahim et al. [
8] designed various absorbers of PVT systems and studied the efficiency of each type through simulation. Values of 50.12% for thermal efficiency of the spiral flow design and 11.98% for cell efficiency were obtained.
However, most of the heat mediums used in many studies were conventional fluids such as water and air. These common fluids have lower thermal conductivity than solid metals, which limits the maximum efficiency of the existing system. Therefore, it is necessary to enhance efficiency by improving the heat transfer characteristics of the heat medium [
9]. This problem can be improved by manufacturing nanofluids using nano-powder material that can be produced relatively easily through recent advances in nanotechnology [
10].
A nanofluid is formed by the dispersion of nanosized powder particles with high thermal conductivity in a conventional fluid. It has a large transfer area, high dispersion stability and high thermal conductivity coefficient [
11]. In addition, even if a small quantity of nanoparticles is added at a volume ratio of less than 1%, the thermal conductivity increases by about 10%, and the convective heat transfer characteristics increase by up to 30% [
12]. Therefore, several studies using nanofluids in PVT systems have recently been reported. Ghadiri et al. [
13] conducted an experimental study comparing the efficiency of the PVT systems using Fe
3O
4-water in concentrations of 1 wt%, 3 wt%, and distilled water and found that the overall efficiency of the 3 wt% Fe
3O
4-water fluid improved by approximately 76%. Mahammad et al. [
14] confirmed the effects on thermal and electrical efficiency of the PVT systems using SiO
2-water nanofluids and pure water as coolants. Their study showed that the overall energy efficiency of PVT systems using SiO
2-water nanofluids in 1 and 3 wt% concentrations was increased by 3.6% and 7.9%, respectively, compared to pure water. Karami and Rahimi [
15] showed that when the concentration of nanoparticles was 0.1 wt%, Boehmite nanofluids had better cooling performance than water and that the electrical efficiency was 20.57% and 37.67% in linear and spiral channels, respectively.
Various studies on the heat transfer properties of nanofluids have confirmed their availability as a thermal medium for heat transfer, but due to the difference in size and concentration of nanofluids, more research is needed to achieve efficiency using nanofluids as a thermal medium in solar energy systems [
16].
A sun trackable PVT system was constructed in this study. In order to evaluate the efficiency of the PVT system, the comparative experiments were conducted using water and nanofluids (CuO-water and Al2O3-water) as heating medium. Thermal and electrical efficiencies of the PVT system were investigated on comparison with heating medium. Outlet temperature of heating medium and surface temperature of the PVT system were predicted with computational fluid dynamics (CFD) simulation analysis and compared with measured values.
4. Conclusions
In order to evaluate the efficiency of a PVT system, a comparative experiment was conducted using nanofluid. Among the tested media, the total efficiency and energy accumulation efficiency of the PVT system using CuO-water nanofluid as a heating medium were 51.22% and 72.58%, respectively, which were higher than those using Al2O3-water nanofluid. However, in the case of electrical efficiency, the efficiency tended to decrease due to rise in temperature in all heat mediums; hence, the difference in efficiency was not significant. The surface temperature difference in the case of using water and nanofluid was 0.6 °C to a maximum of 2 °C; thus, the difference in the amount of electricity produced was not high. However, the PVT system had a higher overall energy efficiency than the solar system. This is a positive result because the improvement of the thermal energy alone can sufficiently increase the overall performance of the system. Therefore, using a nanofluid as a heating medium of the PVT system is expected to improve the efficiency sufficiently compared to the existing heating medium.
Additionally, the proposed PVT model for simulation analysis was confirmed by comparing the experimental and predicted values. The actual measured surface temperature was influenced by the outside air of the collector, which may have led to the difference between the predicted and actual values. The outlet temperature of the heating medium was predicted more accurately than the surface temperature.
Since it is suggested that the flow rate affects the efficiency of the PVT system, the critical flow rate of each nanofluid will be investigated in future study. Moreover, the surface temperature of the PVT system would be more exactly predicted by considering additional outside wind speed with air temperature and radiation in the CFD simulation.