In order to better predict efficiencies of PVT collectors, we extend the standard Hottel-Whillier collector model to collectors with an additional PV layer and distinguishable ambient temperatures at the front and the back side of the collector. The model is then included in a discrete model of a roof-integrated collector field mounted with a ventilated back side. As an example of application of the new model, we analyse the effect of different sizes of the air gap between the collector field and the roof on the efficiency of the PVT collector field.

SPF Institute for Solar Technology