New classification of dry and isentropic working fluids based on the subcooled liquid region in temperature-entropy (T-s) diagram

The traditional classification of working fluid, which is based on the characteristics of saturated vapor curve in temperature-entropy (T-s) diagram, focuses more on the output power of organic Rankine cycle (ORC). The performance of an ORC is greatly influenced by pump power consumption, which is closely related to the subcooled region and saturated liquid curve of the working fluid. This paper proposes a new classification method of working fluids, which simultaneously considers the curved triangle area in subcooled region (A1), the slope of saturated liquid curve (k), and the pump power consumption (wp). It is discovered that dry working fluids have a larger area of curved triangle in subcooled region and higher pump power consumption compared to isentropic working fluids. Among all the dry or isentropic working fluids studied, halogenated hydrocarbons have the steepest slope of saturated liquid curve. The more silicon in siloxane, the larger the values of A1 and k, however, the smaller the value of wp. On this basis, the ratio of the curved triangle area in subcooled region (A1) to the curved triangle area in superheated region (A2), which is (A1:A2), is used to evaluate the working fluid characteristics. Additionally, the ratio of the net output power (wnet) to the condenser load (qc), which is (wnet:qc), is used to evaluate the ORC performance. It can be determined that D5 (decamethylcyclopentasiloxane) and MD4M (tetradecamethylhexasiloxane) may be the essential working fluids for future ORC design and operation.