The potential role of organic bottoming Rankine cycles in steam power stations

Firstly it is observed that in a large number of existing steam power plants the energy potential of the cooling medium is not fully utilized owing to turbine limitation in exhaust volume flow-handling capability. A method is proposed by which a fraction of the low-pressure steam is extracted and fed to an auxiliary organic Rankine cycle (ORC) module of small capacity which, besides being perfectly suited to exploiting even the coldest cooling agent, improves the working conditions of the main turbine by reducing its exhaust volume flow. Through the implementation of an appropriate computer program the performance of a typical power station supplemented with an ORC system is analysed for different cooling situations. Alternatively, as an obvious reference option, the performance of the same plant is evaluated under the assumption that the turbine is provided with an additional exhaust section. The characteristics of the ORC module are then considered. Working fluid selection within the new classes of ambient friendly refrigerants is discussed. Particular attention is devoted to turbine optimization, leading to high-efficiency low-stress two- and three-stage turbine configurations. With reference to the extended geothermal experience in the use of low-temperature ORC conversion systems a preliminary economic analysis is performed, giving encouraging indications about the potential viability of the proposed method.