Operation of HELISOL®5A in a parabolic trough test loop

Abstract

Polydimethylsiloxanes (PDMS / silicone oils) are commonly used as heat transfer fluids (HTF) at temperatures up to 400 °C in various industries, but the applications in large scale parabolic trough collector fields for thermal power generation has not been established due to higher prices compared to the commonly used mixture of diphenyl oxide and biphenyl. This paper describes the first system prototype demonstration in operational environment (loop scale) of a new silicone-based heat transfer fluid called HELISOL® 5A available at a competitive price level compared to the state of the art. Technical details of the parabolic trough test loop operated with HELISOL® 5A are presented. Solar operation at the state-of-the-art temperature of 400 °C for 150 h and at 425 °C for 480 h, demonstrated HELISOL® 5A’s loop scale functionality in analogy to DIN 51528. The tolerance of HELISOL® 5A to temperatures above 425 °C was demonstrated by an operation period of 50 h at 450 °C. The degradation of the HTF is determined by the degree of cross-linking between macromolecules. Based on accompanying HTF analysis correlated with sample-based gas analysis, no measurable HTF degradation after 480  h at 425 °C was found. Only the stress test at 450 °C reveals an onset of moderate thermal degradation reaching 0.1 % mole fraction of cross-linked molecules. The heat transfer performance of HELISOL® 5A was confirmed at 400 °C by solar blind infrared absorber tube temperature measurements indicating a steady temperature increase of 0.2  K/m along the receiver tubes at 6.2 kg/s mass flow.