Performance improvement of ORC by breaking the barrier of ambient pressure

Abstract

Decreasing the condensation pressure of organic Rankine cycle (ORC), which is usually limited by ambient pressure, can greatly increase the system efficiency. In this paper, the condensation pressure is extended from traditional ambient condensation to vacuum condensation. Liquefied natural gas (LNG) is used as the cold sink to generate vacuum condensation pressures between 0.001 MPa and 0.1 MPa. Nine substances with no or low flammability are chosen as the working fluids. The system main parameters are optimized with the objective of maximum exergy efficiency. The results show that the exergy efficiency of R227ea increases from 18.87% to 26.03% as the condensation pressure decreases from 0.1 MPa to 0.001 MPa. For some substances, however, their efficiencies turn to decrease as the condensation pressure is approaching 0.001 MPa and their optimal condensation pressures are different. The working fluids share the same optimal condensation temperature of around −90 °C under the studied conditions. R227ea is a generally good substance because of its higher critical temperature to match with heat source and lower triple point pressure (temperature) to reach lower condensation pressure (temperature). The condensation pressure always has opposite effect on exergy destructions of condenser and steam generator, as well as on direct expansion turbine and ORC turbine.