Impact of Ageing on Thermal Efficiency of Solar Thermal Collectors

Abstract

Today it is common practice to calculate the performance of solar thermal systems or solar collectors based on the results of a thermal performance test carried out with a new solar collector. However, for an integral assessment of solar thermal collectors and systems, it is necessary to consider the change of performance with time due to degradation effects. Degradation effects like a reduced transmittance due to deposits on the transparent collector cover or a reduction in the absorption coefficient of the absorber due to high temperature oxidation mechanisms may result in decreased collector efficiency. Up to now it is not possible to take these degradation effects into account, systematically.

In order to determine the extent of ageing and its impact on the thermal performance of solar thermal collectors, thermal efficiency tests have been carried out with 15 collectors, both in new and used state. The collectors were exposed for a period of 3 years to normal weather conditions. During that time the collectors were not filled with a heat transfer fluid. Hence the conditions were similar to the operating state “stagnation” where no usable heat is removed from the collector and very high absorber temperatures are reached. After the exposure the thermal performance of the collectors was determined again. Based on the collector efficiency parameters determined for the new and the exposed (aged) collectors, the fractional energy savings were calculated for a typical solar domestic hot water system.