Effects of the thermal capacitance on the daily thermal performance of a CPC: An experimental investigation

The availability of reliable models for the prediction of the daily thermal behaviour of thermal solar collectors is of great relevance for a correct design of solar thermal systems. The most significant design parameters of a solar collector are outlet temperature and useful energy collection, both calculated on the base of the efficiency. The last, in turn, is commonly available on the base of experimental correlations obtained with the Steady State Testing method proposed by EN 12975-2. In these steady state tests, through the respect of very strict requirements, the contributions due to the thermal inertia of the solar collector becomes negligible. However, in its daily behaviour, it undergoes variable weather conditions with both regular and sharp changes of solar radiation. For these conditions the thermal inertia contributions become relevant in the correct prediction of its thermal behaviour. EN 12975-2 in Annex K suggests a modified equation for the prediction of the useful energy collection, based on a modification of the efficiency equation. A term dependent on the time derivative of the mean inlet/outlet water temperature is added. In this paper we examine experimentally the role played by this term for a Compound Parabolic Concentrator solar thermal collector.