If ventilation should be applied as an overheating mechanism in polymeric collectors, it is necessary to limit heat losses when the collector is operative. Hence a flap at the top of the collector frame was introduced, which should be opened when a critical temperature in the collector/of the absorber is reached (Fig. 1). Due to the high thermal expansion coefficient of polymeric materials (~10-4 K-1), the temperature of the absorber itself and its longitudinal

expansion can be used to trigger the opening of the flap. This is a simple, self-controlled mechanism, which will also work during power failure.