Systematic Solar-Collector Testing in Steady-State and Dynamic Conditions

Abstract

In order to predict accurately the performance of solar-thermal or hybrid PVT systems, it is necessary that the steady-state and dynamic performance of the collectors is understood. This work focuses on the testing and detailed characterization of nonconcentrating PVT collectors based on the testing procedure specified in the European standard EN 12975-2. Three different types of PVT collectors were tested in Cyprus under outdoor conditions similar to those specified in the standard. Amongst other results, we show that that poor thermal contact between the laminate and the copper absorber can lead to a significant deterioration in thermal performance and that a glass cover improves the thermal performance by reducing losses as expected, but causes electrical losses that vary with the glass transmittance and the incident angle. It is found that the reduction in electrical efficiency at large solar incidence angles is more significant than that due to elevated temperatures representative of water heating applications. Dynamic tests are performed by imposing a step change in incident irradiance in order to quantify the collector time constant and effective heat capacity. A time constant of ~8 min is found for a commercial PVT module, which compares to <2 min for a flat plate solar collector. The PVT collector time constant is found to be very sensitive to the thermal contact between the PV layer and the absorber, which may vary according to the quality of construction, and also to the operating flow rate.