Experimental Daily Energy Performance of Flat Plate and Evacuated Tube Solar Collectors

New comparative tests on different types of solar collectors are presented in this paper. Tests have been performed at the solar energy conversion laboratory of the University of Padova. Two standard glazed flat plate collectors and one evacuated tube collector are installed in parallel; the evacuated collector is a direct flow through type with external CPC (compound parabolic concentrator) reflectors. The present test rig allows to make measurements on the flat plate, on the evacuated collector or on both simultaneously, by simply acting on the valves to modify the circuit. In this paper measurements of the performance of the evacuated tube collector and flat plate collectors working at the same conditions are reported. Efficiency in stationary conditions is measured following the standard EN 12975-2 [1] and it is compared with the input/output curves measured for an entire day. The main purpose of the present work is to characterize and to compare the daily energy performance of the two types of collectors. An effective mean for describing and analyzing the daily performance is the so called input/output diagram, in which the collected solar energy is plotted against the daily incident solar radiation. Test runs have been performed in several conditions to reproduce different conventional uses (hot water, space heating, solar cooling).