Spectral characterisation and long-term performance analysis of various commercial Heat Transfer Fluids (HTF) as Direct-Absorption Filters for CPV-T beam-splitting applications

Abstract

Hybrid concentrated photovoltaic – thermal systems (CPV-T) provide simultaneous supply of electrical and thermal energy, using solar cells with cooling systems to avoid high cell temperatures that decrease the system electrical conversion efficiency. Heat transfer fluids arranged in front of the cell, acting as selective beam-splitting filters, may represent a feasible alternative to absorb unwanted solar radiation, preventing the cell from overheating and directly generating usable thermal output. The cooling efficiency and the temperature output of the liquid depend on optical transmittance as well as chemical and physical stability. A research study for the most suitable commercial heat transfer fluid for a direct-absorption beam-splitting CPV-T system is conducted in this paper, analysing the effects of high temperature and exposure to UV light on the optical transmittance of the fluid under accelerated lifetime test conditions.Optical transmittance of 18 different commercial heat transfer fluids has been measured. The most promising liquids to serve as Direct-Absorption Filters selected for accelerated tests include Duratherm600, DurathermG, industrial Propylene Glycol (PG), pink-dyed PG, and Royco782. Long-term degradation tests include low temperature test at 75°C, high temperature test at 150°C, and UV light exposure. Results from the accelerated tests show that the optimum fluid for our application is the industrial grade Propylene Glycol adapted with a chemically-inert red dye such as Oil Red 235 inorganic dye. Propylene Glycol is a liquid used in food processing, with a cost of A$2.50/kg.