Evaluation of the absorber temperature frequency function valid for evacuated flat plate collectors

The degradation of performances for solar thermal collectors is linked to the decrease of the absorber efficiency caused by diffusion process, strongly dependent on temperature. The Standard procedure for qualification of solar absorbers surface durability defines the absorber temperature frequency function as one of the necessary parameters to estimate the failure time for collectors, but it only refers to applications at low temperatures like domestic hot water production (DHW). Because of this deficiency of the Standard, in order to make aging predictions for selective solar absorbers mounted on Evacuated Flat Plate Collectors (EFPCs) used for Mid-Temperature applications (with stagnation temperature over 573 K), in this paper a procedure to reconstruct their specific temperature frequency function, considering the high thermal efficiency and stagnation temperature, is provided. A dynamic simulation model of a small plant with a TVP-Solar EFPC is implemented in Simulink environment to obtain the yearly absorber temperature history in operating conditions and experimental data are collected to obtain the temperature trend of the EFPC under stagnation. The absorber temperature frequency function valid for EFPCs is presented for different operating temperatures considering that stagnation occurs for 30 days every year, as the Standard prescribes. 30 days of stagnation in a year is an unrealistic assumption for thermal collectors used for industrial application and, in order to give an indication in this sense, the variation of aging parameters for different stagnation periods (fallowing periods) is also presented. Using the simulation model for service-time, experimental data for stagnation and assuming the exact fallowing period for the solar plant, a greater accuracy of EFPC failure time evaluation can be obtained. (C) 2022 The Authors. Published by Elsevier Ltd.