Life time analysis of thermal oil used as heat transfer fluid in CSP power plant

Abstract

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.

High temperature stability of a commercial terphenyl-based thermal oil

“…in accordance with the reaction mechanism proposed by Schneider and Morgan [9], [13] , where the first step consists of a hydrogen atom elimination from a cyclohexyl ring:…”

“…Also considering the thermogravimetric analysis discussed in a previous paragraph, the increase in dynamic viscosity and decrease in volatility of the thermally stresses oil can be explained considering a partial polymerization of the T66 components, actually already reported in the scientific literature [9], [21] . In fact, if a hydrogen atom is removed from the second or third aromatic position, the resulting cyclohexyl radical ring (-C6-H10) may open forming a double C-C bond [9] (Eq. 12) that can be the precursor of polymerization processes.…”

“…The condensable products formed at high temperature (3 hours at 430°C) were analyzed by gas chromatography with mass spectroscopy detection. The formation of the detected aromatic and hydrogenated aromatic compounds can be due to the cracking of one terphenyl ring, although no data were found in the scientific literature about this mechanism, and only a generic degradation into "light" molecular weight species is reported [1], [9], [13] (where "light" stands for compounds between diphenyl (excluded) and benzene (included)), especially above 350°C, with a degradation path that could be either parallel or consecutive to the one of hydrogen formation [13] .…”

“…There are several types of heat transfer oils available on the market, such as coolants, chiller fluids, antifreezes and refrigerants; they are used to provide cooling within machinery, process equipment or combustion engines. Nowadays, they are mainly used in high temperature process applications, in a bulk fluid operating temperature range between approximately 120 °C and 400 °C and, in particular, as heat transfer fluids in solar trough CSP and hybrid nuclear-CSP applications [9] [10] . However, one serious drawback to be taken into account is that oils are unstable if temperatures are increased above the rated temperature set-points, causing the oil molecules breakdown and possible oxidation processes.…”

Pressure drops, heat transfer coefficient, costs and power block design for direct storage parabolic trough power plants running molten salts

“…In these calculations the value of the energy consumed to heat the solar field when using MS is discounted from the total value of produced energy. In addition, the degradation of the TO that occurs in conventional plants is also taken into account [26]. As such, the cost of oil exchanges every 5 years has been included in the OPEX.…”