Thermal stability of n-pentane, cyclo-pentane and toluene as working fluids in organic Rankine engines

In this paper we investigate the thermal stability of three representative hydrocarbons used as working fluids in organic Rankine cycles (ORC): n-pentane, cyclo-pentane and toluene. The experimental used method is a “static” one, based on the recording of the pressure during the permanence of the fluid sample at constant temperature and on the measure of the differences in the vapour pressure in comparison with the reference values for the virgin pure fluid. The sample container and the circuit are in stainless steel. The decomposition was estimated by a parameter based on the mean value of differences in the sub-atmospheric vapour pressures at selected temperatures. The increase of the vapour pressure resulting from even a small thermal decomposition (more noticeable at sub-atmospheric pressures) is an important indicator as it may anyway have great effects on the engine in which the fluid works. The adopted procedure is relatively simple and may be applied to different fluids and to different families of fluids and the results are significant in the comparison and in the relative classification of working fluids with regards to their thermal stability. As to the three hydrocarbons considered, the cyclo-pentane results thermally very stable: at 350 °C, after eighty hours, the rate of decomposition is about twenty times lower than the corresponding value for n-pentane. The toluene sample remains remarkably stable at higher temperatures, confirming its potential application in the so-called high temperature Rankine cycles: at 400 °C the rate of decomposition is the same of that of n-pentane at 315 °C and of that of cyclo-pentane at 375 °C.