Compressibility factor as evaluation parameter of expansion processes in Organic Rankine Cycles

In the conversion of low temperature heat sources into electricity using an Organic Rankine Cycle system the working fluid selection is a major design choice to maximize the overall performance. The placement of the power cycle on a T-s diagram depends on the fluid critical temperature. Several studies have shown that the power output can be maximized by using fluids with critical temperatures similar or lower than the inlet temperature of the heat source, which allow a better temperature profile match between the heat source and the working fluid. However, the choice of a fluid having a specific critical temperature also influences the fluid properties in the expansion process over a given temperature interval, as shown by the generalized compressibility chart. The aim of this paper is providing a better insight into the results of optimized ORCs through the analysis of the compressibility factor in the expansion process. To this purpose the real enthalpy change in the expansion process is regarded as two separate terms associated with temperature and pressure drops, respectively. Starting from the analysis of different expansion processes in optimized cycles a correlation is obtained between the compressibility factor and the ratio between real enthalpy change and the enthalpy change term associated with temperature drop. Thus, the ratio between the former and the latter can be directly evaluated from the simple knowledge of pressure and temperature values along the expansion process and the observation of the compressibility chart.