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Abstract. The present work describes stability testing of hydrogenated terphenyl (HT), thermal oil available in the market and considered as a potential HTF for CSP power plants. Before ageing tests, hydrogenated terphenyl was compared to Biphenyl/diphenyl oxide (DPO) at the initial state, which is the most commonly used HTF in CSP plants (SEGS VI and ANDASOL I) and included as a comparison material in the NREL HTF requirements. The (HT) stability tests were performed in sealed ampoules (stainless steel) under inert gas blanket in the range of temperature between 250°C and 350°C (max temperature of HT) for 500 hrs. After ageing, many investigations were made to track the thermal oil behavior after extended time over a range of temperature, such as chemical composition, flash point, viscosity, acid value…. Laboratory testing indicated that the hydrogenated terphenyl (HT) is stable after ageing process at a temperature of about 250°C. Nevertheless, it has shown signs of serious thermal cracking at elevated temperature which is reflected by low flash point temperature. Therefore, the system must be purged effectively to purge the volatile decomposition products.

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