Development of an Air-based Open Loop Building-integrated Photovoltaic/thermal System Model

This paper develops convective heat transfer coefficients for several different BIPV/T system configurations using Computational Fluid Dynamics models. The results for one of the CFD cases are validated with experimental data from a BIPV/T installation. The second part of the paper describes a model which is used to generate annual performance data for the system, including thermal and electrical energy production. Framing elements increase the turbulence which in turns increases the convective heat transfer coefficients. The optimal airspeed in the channel is a function of its length, weather conditions and the final required exit air temperature. INTRODUCTION Building-Integrated Photovoltaic/Thermal (BIPV/T) systems produce electrical and thermal energy thus having the potential to collect more energy per unit area than individual building-integrated PV systems and solar air collectors. In open loop air-based BIPV/T systems, the heated air can be used for different purposes such as space heating, domestic hot water heating and clothes drying. BIPV/T systems consist of three layers. The outer layer consists of PV modules, the middle layer is an air channel, and the back layer is an insulated surface. By using PV modules on the outer layer, the need for traditional roofing materials such as asphalt shingles is eliminated. The framing that supports the PV modules also serves to increase turbulence in the air channel, which promotes heat transfer to the air. The waste heat from the PV modules is transferred to the airstream. Another benefit is that the PV modules are cooled, which leads to improved performance. Bazilian and Prasad (2002), Eicker (2003), Charron and Athienitis (2006) have shown that most existing models assume the following: a) convective heat transfer correlations developed for ducts or pipes which inherently assume constant heat flux across the boundaries, constant temperature, heating symmetry and constant cross sections, b) the effect of moisture content in the air and turbulence caused by framing are ignored. Previous work by Chen et al. (2007) and Candanedo et al. (2007) both used the upwind scheme with control volumes where the energy balance equations are solved. Computational Fluid Dynamics (CFD) simulations for a BIPV/T façade configuration has been performed with the use of a constant reference temperature for determination of the convective heat transfer coefficients (Liao et al., 2007). Determination of convective heat transfer coefficients (CHTC) for a BIPV/T façade has also been performed using Fluent (Bloem, 2004; Bloem, 2008), however the values of such coefficients were not reported. He determined the CHTCs for specific designs based on CFD simulations. Finally, the thermal network model described in detail allows whole-year simulations to be performed in order to properly characterize the collector. CFD SIMULATION The commercially available CFD program, FLUENT (2006), was used as a tool to reproduce the fluid and heat transfer phenomena for a BIPV/T configuration. By postprocessing the thermal and fluid dynamics data, the CHTCs for different geometries were computed. Figure 1 shows the PV modules and the framing used for mounting the modules in the Northern Light Canadian Solar Decathlon House. Experimental thermal data indicate that heat transfer in such BIPV/T applications does not vary significantly along the width of the channel Candanedo et al. (2008). Therefore, it is concluded that a 2D CFD model can represent the behaviour of the system. Figure 1. Photo of the BIPV-T installation found at the Northern Light Canadian Solar Decathlon 2005 house. Geometry and Meshing The geometry under study consisted of a 2.84 m long by 0.04 m high air gap between the top plate (PV) and the bottom representing the roofing material. The simulation also included coupling with a solid zone attached to the back representing insulation which Eleventh International IBPSA Conference Glasgow, Scotland July 27-30, 2009