Photovoltaic (PV) systems have witnessed exceptional development in the last two decades, where it has been shown that PVs may absorb more than 75% of the insolation, however, only limited percentage can be transformed into electricity (7-24%). The remaining energy is released mostly as waste heat in the cells. Overheating may also cause damage to adhesive seals, delamination and non-homogeneous temperatures. Therefore, PV/Thermal (PV/T) systems are a mechanism that can address these issues by keeping the PV cell temperature at the operating range improving efficiency to acceptable levels, as well as producing heat and electricity simultaneously. In this study, PV/T air systems are considered.
There are three main challenges to overcome with PV/T air systems; 1) the fan power requirement, 2) extreme weather temperature, 3), and the poor heat capacity of air, which leads to poor thermal performance, compared to other coolants such as water. The aim of this research is to address these challenges developing an efficient and affordable PV/T air system. To achieve this, eleven objectives have been suggested where appropriate several different solution methods are utilised. The CFD software of COMSOL Multiphysics and Matlab are used in this study.
The main findings of this research can be divided into three parts. The first part evaluates the performance of the standard PV system utilising theoretical and numerical methods. This system is considered as a reference for subsequent models. The results shows that the convection currents in inclined and horizontal surfaces are weaker relative to the vertical surface. The increase of the PV length enhances heat transfer rate up to length (2L). However, after this length, the PV temperature increases and convective heat transfer coefficients are reduced regardless of the inclination of the PV system. In the case of the horizontal surface, the convective heat transfer rate is lower, especially at the bottom surface of the PV system. It can also be concluded that the effect of inclination appears in the laminar region (short length) and dissipates after this region.
The second part numerically and experimentally evaluates the performance of the multi-pass solar air heaters. The impacts of flow configurations on the thermal performance of a solar heater system are investigated. Recycled aluminium cans (RAC) have been utilised as turbulators with a double pass single duct solar air collector. CFD results of the models A, B, and C reveal that model C offers a greater thermal performance of 5.4% and 6.5%, respectively, compared to A and B. Furthermore, an outdoor experiment is performed based on these results. The experimental setup is examined for three configurations of model C, namely, solar air heater (SAH) without RAC model C-I, model C-II and model C-III. A good agreement between model C and the experimental data and model C-III has the best thermal efficiency of 60.2%.
The third part, the combination of the two systems from these two parts are evaluated. Firstly, a design optimisation process is performed for different multi-pass PV/T air collectors considering three steps to obtain optimal design. The steps are the selection of design parameters, preliminary parametric studies for the five models (model 1, 2, 3, 4 and 5) and employ model 4 in the optimisation process. The key results from this optimisation demonstrate explicitly the compromise that must be accepted between the conflicting objectives of thermal and electrical efficiencies or the fan power consumption and electrical power generation. It can be concluded that the use of optimisation has contributed clearly in improving both the electrical and thermal performance for finned and plain mode
