The ORC fluid can be classified into three categories according to the temperature-entropy (T – s) diagrams. It is noteworthy that for some kinds of fluids, the derivative of temperature with respect to entropy on the saturation vapor curve may change from positive value to negative value, e. g. dL of R123 on the saturation vapor curve is positive when T is smaller than 150°C while negative at higher temperature ranges. In this case, dry fluids are generally named for the positive dL in practical operation temperature range from the cold side to the hot side. And wet fluids would have negative dL on the saturation vapor curve. Meanwhile, isentropic fluids have approximately infinite value of dL (nearly vertical curve).
The working fluids of dry or isentropic type are more appropriate for ORC systems. The reason is that dry or isentropic fluids are superheated after isentropic expansion, thereby eliminating the concerns of impingement of liquid droplets on the turbine blades and making the superheated apparatus unnecessary . Based on this consideration, five dry fluids are selected in the analysis. They are R113, R123, R245fa, pentane and butane. Some of properties of these fluids are listed in table 1. The optimal FPC proportion and the overall collector efficiency are related to the latent heat and heat capacity in saturation liquid states as discussed in Section 5.3.