Comparison Between the Experimental and the Extrapolated Stagnation Temperature – a Data Base Evaluation
Abstract
Well-designed liquid heating Flat Plate Collectors (FPC) or Evacuated Tube Collectors (ETC) could be used to provide heat at medium temperature levels for industrial processes. According to the most recent ISO 9806:2013 testing standard, the efficiency curve of a collector is constructed by a weighted least square fitting (WLS) based on at least four observations at temperatures spaced over the operating temperature range of the collector. This could lead to poor efficiency predictions at higher operating temperatures, since in this region the fit is not backed up by experimental data. To improve results in the extrapolated area without any additional experimental effort an extended observation fitting could be used, including the collector’s stagnation temperature as an additional node point. Two databases have been evaluated. One database was provided by the National Renewable Energy Centre in Spain (CENER), the other one by the Institute for Solar Technologies (SPF) in Switzerland. It has been shown that in the case of conventional FPCs the extrapolated stagnation temperature from the conventional 8-point fitting constantly underestimates the measured stagnation temperature. In the case of ETCs, on the other hand, the conventional approach leads to an overestimation.
Keywords
Iam, Factorization, Collector testing, Medium temperature collectors, Industrial heat