Heat Removal Factor in Flat Plate Solar Collectors: Indoor Test Method

This paper presents a couple of methods to evaluate the heat removal factor FR of flat plate solar collectors, as well as a parametric study of the FR against the tilt angle β, and (Ti − Ta)/G, and its effects on the a0-factor (FRτα) and the a1-factor (FRULmin). The proposed methods were based on indoor flow calorimetry. The first method considers the ratio of the actual useful heat to the maximum useful heat. The second takes into account the slopes of the family of efficiency curves (FRULmin) according to ANSI/ASHRAE 93-2010, and the minimum overall heat loss coefficient, ULmin. In both methods, a feedback temperature control at collector inclinations from horizontal to vertical allows the inlet temperature and the emulating of the solar radiation to be established by electrical heating. The performance of the methods was determined in terms of the uncertainty of the FR. Method 1 allowed a three-fold improved precision compared to Method 2; however, this implied a more detailed experimental setup. According to the first method, the effects of the tilt angle β, and the (Ti − Ta)/G, on the a0-factor were considerable, since FR is directly proportional to the a0-factor. The changes in (Ti − Ta)/G caused an average change in FR of 32% The FR shows almost linear behavior for inclinations from horizontal to vertical with a 14.5% change. The effects of β on the a1-factor were not considerable, due to the compensation between the increase in FR and the decrease in ULmin as β increased.