Performances and cost assessment of an air cooled two-stage rankine cycle for large power plants operating with different working fluids

Abstract : Steam/water rankine cycles are mainly used as energy conversion systems (ECS) for large power plants. Due to the low density of steam at low pressure, the turbine size is generally very big. Furthemore water cooling is almost always preferred as cooling option since it provides the best energy efficiency as compared to air cooling. Nevertheless, water use can be strengthened in the future due to environmental constraints(withdrawal or heat release limitations). In the present work, a two-stage rankine cycle (TSRC) is considered, combining a steam/water cycle and organic rankine cycle (ORC) cooled by an air cooled condenser (ACC). The backpressure of the steam cycle is limited and the heat which remains in the steam is transferred to an ORC through a condenser-boiler. Due to the organic fluid high density, it is possible to reduce the installation size. Furthermore, the organic fluid is likely to provide additional power when ambient temperatures are low. With those two advantages it is expected for dry cooling to become more cost effective. The paper reports the methodology that has been used in order to optimize a design in terms of fluid selection, component sizing and efficienty. Preliminay cost analyses related to the different working fluids and to local ambient conditions are given.