Thermal stability and material compatibility for hexamethyldisiloxane as the working fluids of organic Rankine cycle

Abstract

The organic Rankine cycle (ORC) is one of the most suitable methods to generate electricity for renewable energy and industrial waste heat recovery systems. High temperature ORCs have recently attracted much interest because of the improved theoretical thermal efficiency. For high temperature ORCs, the working fluids need high critical temperature, low flammability and good thermal stability. Siloxanes are considered as the promising working fluids for high temperature ORCs in previous studies because of their high critical temperature, low flammability and environmental friendliness. The thermal stability and material compatibility of working fluid is the primary limitation for the working fluid selection at high temperatures. However, the results about thermal stability and material compatibility of siloxanes are scarce in previous studies. In this paper, the thermal stability and material compatibility of siloxanes were studied with hexamethyldisiloxane (MM) as a representative fluid. A test system and an experimental method were designed to identify the compositions of decomposition products. The masses of liquid samples were measured before and after tests to confirm the mass fractions of gaseous and liquid compositions in decomposition products. Liquid decomposition products, such as octamethyltrisiloxane (MDM) and tetramethylsilane, were found to be the main decomposition products of MM. The possible decomposition paths of MM were analyzed by the results of decomposition product composition analysis. The decomposition ratios of MM at 220, 240, 260, 280 and 300°C were measured after decomposition for 24 h. The decomposition could be detected at 240°C, which was lower than the critical temperature of MM. Thus, the thermal stability of MM should be emphasized for supercritical ORCs if the decomposition of MM is unacceptable in ORC systems. The air was confirmed to have big effects on MM decomposition and could change the reaction paths of MM decomposition. Thus, air dissolved in the liquid working fluid should be removed before filling the working fluid into ORC systems. The material compatibility of MM with metal samples was measured experimentally. 304 stainless steel and copper were chosen as test metal materials and the metal samples were processed into the shapes for the tensile experiments. The experimental temperature was 220°C, which was considered as a thermal stable temperature for MM. The material compatibility experimental periods in this study were 10 days, which were long enough in previous studies. The experimental results showed that the mass and hardness relative changes in both metal samples were very small and could be negligible. However, the tensile strength changes in the copper samples were obviously bigger than those in 304 stainless steel samples. Thus, 304 stainless steel has better compatibility with MM than copper as the metal material in systems at 220°C. The material compatibility of n-pentane with metal samples at 220°C was also measured using the same test system and method to be a comparison. The results showed that the tensile strength changes in metal samples with n-pentane were much bigger than those with MM. Thus, MM exhibited better compatibility with metal materials than n-pentane at 220°C as ORC working fluids.