Wind analysis over a building roof in the presence of photovoltaics

Abstract

Solar power generation using photovoltaic (PV) panels is one of the most common sources of alternative energy. Compared to pitched roofs, the installation of building integrated PV panels on flat roofs offers the possibility to incline and orient the photovoltaic array in such a way as to maximize diurnal insolation hence achieving better system performance and higher energy output. Apart from their main function of generating electrical energy from solar radiation, PV panels create secondary effects to the building structure. PV panels offer cooling effects to the top-most surface of a building through shading, and also through the effects on the thermal convective heat transfer coefficient (CHTC) which results from the interference of the wind velocity profile. The latter has seldom been addressed in the literature. This main focus of this project is to test the hypothesis of whether the CHTC between roofs with PV panels and those without cause any significant difference in building energy performance. This quantity is known to have an impact on the thermal transmittance coefficient (U-value) of roof structures which has a direct impact on the energy consumption of heating, ventilation, and air conditioning (HVAC) equipment which is used to maintain the building envelope in suitable thermal comfort. For the purpose of this study, several wind tunnel experiments were performed. These experiments consisted of studying the behaviour of flow and CHTC on a flat plate surface. The influence of inclined panels on flow and CHTC was analysed. These observations were also compared to numerical analysis using computational fluid dynamics (CFD). Building energy simulation tools were used to model a building envelope and perform simulations using CHTC results obtained from CFD simulations. The main scope of the building energy simulations was to quantify the energy demand of a building without PV panels and compare it to that of a building with PV panels. From these results, the effects of PV panels on building energy performance due to external CHTC could be quantified. It was found out that the presences of PV panels force the flow of air to accelerate and reach higher velocity beneath the panels. Higher velocity resulted in improving the CHTC of localized areas of the building roof. Using building energy simulations, the increase of CHTC in buildings with PV panels resulted in a reduction of approximately 2% in the total energy consumption for heating and cooling of the topmost floor of a building. This study was limited to a constant wind velocity and energy consumption results were based on this wind velocity. The obtained results offer a research direction for future studies. Future work should include the analysis of wind transient effects to investigate whether or not similar results are obtained.