Transient infrared thermography heat loss measurements on parabolic trough receivers under laboratory conditions

Abstract

This paper describes a transient infrared thermography heat loss measurement method for parabolic trough receivers and its development under laboratory conditions. The method is designed for future field measurements and represents a complementary approach to steady-state heat loss measurement methods. The receiver specific heat loss is determined by applying a thermal excitation to the absorber tube and by measuring both the absorber and glass temperature response signals with infrared pyrometers. These transient signals are processed to derive a mean absorber temperature, a mean glass temperature, an amplitude ratio and a phase shift at a mean air temperature. Temperature signals are used to identify receiver thermal properties with the help of a numerical heat transfer model coupled to a parameter identification algorithm. Parabolic trough receivers with distinct absorber coating and annulus properties have been tested with two different transient excitation profiles, i.e. sinusoidal and ramp-and-hold signals. The identified receiver thermal properties have been compared to steady-state heat loss experiments conducted with DLR THERMOREC test bench. The observed specific heat loss deviations between steady-state and transient laboratory measurements respectively range from 2% to 4% for evacuated receivers with selective coating and from 1% to 9% for receivers with black painted absorbers.