Systematic testing of hybrid PV-thermal (PVT) solar collectors in steady-state and dynamic outdoor conditions

Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of 8 min is measured for a commercial PVT module, compared to <2 min for a flat plate solar-thermal collector. It is also concluded that the use of a lumped, first order dynamic model to represent the thermal mass of the PVT collector is not appropriate under certain irradiation regimes and may lead to inaccurate predictions of the system performance. This paper outlines a procedure for the testing and characterisation of solar collectors, provides valuable steady-state and dynamic performance characterisation data for various PVT collector designs, and also provides a framework for the application of this data in a system model to provide annual performance predictions in a range of geographical settings.