MODELLING OF HYBRID PV-THERMAL SYSTEMS

Systems involving solar thermal collection into a heat store have a complex dynamical behaviour, due to the influence of storage temperature on solar collector fluid inlet temperature, and on the other side the influence of load patterns on the varia-tion of heat storage temperature. The photovoltaic part of a hybrid PVT (PV-thermal) system depends on the complex tem-perature behaviour only through the temperature dependence of collector performance, which is weaker than that of the ther-mal collection part [1]. This paper explores the dynamical behaviour of a number of PVT systems by use of the newly updated time simulation software NSES [2]. Details of the model is described in refs. [4] and more specifically in [5]. The collector model includes solar radiation capture through multiple layers of glazing, mediators and absorbers, with con-sideration of reflection and emittance. The absorbed radiation is converted to electricity with an efficiency that depends on temperature through a quadratic relationship determined from data for various photovoltaic materials. The further use of non-converted solar energy for heat production is achieved by thermalisation of electrons not converted to electricity, in either of the layers, followed by heat transfer by conduction and convection to a working fluid either passing behind the absorber or in front of it (typically water or air). The heat fluid circuit exchanges heat with a thermal store, from which building loads are covered through a secondary heat circuit. The NSES model simulates the behaviour of such systems by an hourly time-step calculation using hourly data sets such as the Danish Reference Year [7] data for solar input (from which radiation upon tilted surfaces is calculated), wind speeds (a f