Comparative study on hydrogen issues of biphenyl/diphenyl oxide and polydimethylsiloxane heat transfer fluids

Abstract

The formation of hydrogen from new and several used qualities of eutectic biphenyl (BP) and diphenyl oxide (DPO) mixture was investigated with an established lab experiment in the range of 380 – 410 °C and 410 – 430 °C respectively. Kinetic evaluations show that hydrogen formation increases not only with temperature but also with time. Field samples extracted after 5 and 10 years’ operating time confirm an increase of hydrogen formation with operating. A thermal aging study at 430 °C for 480 hours with a series of samples composed of new BP/DPO each doped with a degradation product indicates that the formation of phenol is the mayor driving factor for steadily increase with time. The impact of hydrogen formation on the concentration in the fluids of parabolic trough systems was checked with a simplified model that takes volumetric dimensions, residence time at maximum temperature, back mixing with fluid from overflow vessels and some assumed hydrogen removal step into account. New BP/DPO at 400 °C and polydimethylsiloxane (PDMS) at 425 °C is compared. After initially higher formation of hydrogen, in the long term low hydrogen formation is reported for PDMS. Although PDMS is considered at significantly higher operating temperature lower hydrogen concentrations are to be expected for this fluid after a starting-up phase in the model system. Accordingly, hydrogen control with PDMS is expected to be easier with PDMS fluids like HELISOL® 5A as no steady increase of effort on hydrogen removal will be required.