Impact of molecular structure of working fluids on performance of organic Rankine cycles (ORCs)

Abstract

Organic Rankine cycle (ORC) power plants are the most mature and economically competitive technology used for converting low- and medium-temperature heat to electricity, especially for renewable energy sources such as geothermal and solar thermal. This paper analyzes the performance of four types of ORC plants (subcritical and supercritical cycles, both with and without heat recuperator) optimized for maximum utilization or exergetic efficiency. The analysis was performed for 13 working fluids, heat source temperatures from 100 to 230 °C, and for cycles with either a wet cooling tower or an air cooled condenser. The primary objective of this work was to investigate the effect of the molecular structure of the working fluids on the efficiency of ORC plants. Using group contribution methods, a number of thermodynamic properties of working fluids can be easily evaluated based on their molecular structures. We show that using two of such properties, namely, the reduced ideal gas heat capacity C⁰_p/R and the critical temperature of the working fluid, one can accurately predict a number of ORC performance metrics including: (1) maximum utilization efficiency of the ORC plant and the heat source temperature at which it is achieved, (2) sensitivity of utilization efficiency to changes in the heat source temperature, and (3) increase in both thermal efficiency and heat source outlet temperature due to addition of heat recuperator.