Hydrogen monitoring in the heat transfer fluid of parabolic trough plants

Abstract

The hydrogen formation of diphenyl oxide (DPO) / biphenyl (BP) based heat transfer fluids (HTFs) can cause undesirably high concentrations of the gas in the HTF system of solar thermal parabolic trough plants. Hydrogen permeates from the HTF into the vacuum insulation of the heat collecting elements (HCEs). Excessive hydrogen concentrations in the HTF cause early saturation of the getter materials in the HCEs and consequently to high thermal losses when the heat insulation effect of the vacuum gets lost. In order to avoid a “hydrogen problem” the concentration of the gas has to be monitored and controlled in the HTF. In this study hydrogen analysis has been performed with HTF samples that were collected including all dissolved gases using special steel cylinders that were directly connected to the HTF system. The samples were analysed off-line at ambient conditions in a lab applying a combination of pressure measurement and compositional analysis with gas chromatography. The results indicate that under regular operating conditions hydrogen can exceed the specified limit more than hundredfold. Decompression tests in a CSP plant also indicate that hydrogen concentrations can be lowered using ready available components of a parabolic trough plant. Hence, extremely high hydrogen levels can be avoided by sufficient nitrogen exchange.