Usually, three groups of fluids are distinguished: dry (positive slope), isentropic (zero slope) and wet (negative slope) fluids. For low temperature organic Rankine cycle systems and particularly with low power output levels, wet fluids like water, methanol or ethanol are not suitable. This category of fluids requires a superheat in order to avoid moisture after the expansion process. Fortunately, there are dry and isentropic fluids that do not exhibit excessive moisture after the expansion of the vapor through the expansion device. When isentropic fluids are used, isentropic expansions of saturated organic vapors result in saturated or superheated vapor regions so that erosion of the blades is avoided. Dry fluids with high positive slopes yield low efficiencies. In order to overcome this drawback, a regenerative heat exchanger should be used in parallel with the evaporator and condenser to raise the temperature of the liquid entering the evaporator. An ideal working fluid for ORCs is likely to have a vertical vapor saturated line and a vertical liquid saturated line so that all the heat input transfer should take place during the phase change and the heat rejection at the condenser occurs at the minimum temperature [1, 2, 6]. Fig.1 shows the T-s diagram for an ideal working fluid.

ik

T [K] [15]

s [kJ/kg. K]

Fig. 1: T-s diagram for the ideal working fluid

dODP: Ozone depletion potential, relative to R-11; eGWP: Global warming potential, relative to CO2. n. a: non available