Rapid Screening of Fluids for Chemical Stability in Organic Rankine Cycle Applications

Abstract

While the Carnot cycle is the basic idealized thermodynamic cycle for energy transfer, it is generally impractical and uneconomical to implement. Although less thermodynamically efficient than the Carnot cycle, the Rankine cycle (and heat engines based upon it) is practical and adaptable. Typically, water is used as the working fluid in this cycle. A difficulty that arises with the use of water is the need to superheat the water to prevent turbine blade erosion. Organic working fluids, on the other hand, can be used at lower temperatures and do not require superheating. This results in a practical increase in efficiency over the use of the cycle with water as the working fluid. In these organic Rankine cycle (ORC) applications, the choice of working fluid is critical since the fluid must have not only thermophysical properties that match the application but also adequate chemical stability at the desired working temperature. In this paper, we present the application of simple and relatively rapid ampule testing techniques by which the chemical stability of potential working fluids may be assessed. The method allows the determination of the decomposition reaction rate constant of simple fluids at the temperature and pressure of interest. The fluids studied were pentanes, which are being considered for application in parabolic solar collectors. Results show that the thermal stability decreases in the following order: n-pentane > 2-methylbutane > 2,2-dimethylpropane and benzene ≫ toluene.