Building integrated solar concentrators for façade applications
In this study a novel building façade integrated photovoltaic/thermal (BIPVT) collector was designed to produce both electricity and hot water. Based on the shortcomings of concepts presented in the literature, a simple robust façade integrated collector using flat plate reflectors was developed, modelled and tested both experimentally and theoretically. In developing the BIPVT collector, an optical study comparing façade integrated collectors incorporated with parabolic and flat plate reflectors was carried out using a non-imaging ray tracing analysis and experiments. It concluded that, flat plate reflectors rather than parabolic reflectors for façade applications offer significant potential. It was shown that flat reflectors avoid uneven illumination profiles and hence the need for special photovoltaic absorber materials. Subsequently a generalised expression for the concentration ratio of the proposed collector was determined. Furthermore, the absence of natural convection heat transfer relationships for façade integrated solar concentrators was realized. To address this, a computational fluid dynamics analysis was carried out to deduce a relationship, and this was validated by an experimental test rig. The relationship showed that the heat transfer, expressed in terms on the Nusselt number, is strongly dependent on the Rayleigh number and the aspect ratio (A/H), and can be expressed in the form Nu=a Rab (A/H)c. Having characterised the heat loss, ways of improving the thermal transport properties of the heat transfer media were examined. For this study multi-walled carbon nanotube (MWCNT) nanofluids were examined. It was found that these impaired the turbulent forced convection heat transfer coefficient, needed more pumping power compared to water and would not be suitable for use in a BIPVT collector. Finally, a conceptual collector was designed and analysed using a one-dimensional thermal network. This numerical model was coupled with the generalised geometric relationship for the concentration ratio and was validated with an outdoor experiment. Using the model, a number of parameters affecting the collector performance were identified. It was found that when the thermal conductance between the silicon cells and thermal absorber was doubled, the combined efficiency of the collector was improved by 10%. Increasing the flow rate of the working fluid only marginally improved the efficiency; however, increasing the number of cooling channels across the absorber improved it significantly, due to increasing fin efficiency. It was also found that, the power generated by the collectors was strongly dependant on the reflectance of the reflectors. In summary, the results show the potential opportunity for concentrating BIPVT collectors in façade applications.