Comparison of Enhanced Organic Rankine Cycles for Geothermal Power Units

Binary cycles have drawn the attention as a technical solution for the geothermal power production. This attention is mainly due to the huge potential of medium-low temperature geothermal sources, typically exploited by means of a binary cycle, and the relevance of the environmental concern, which can be conveniently dealt with by means of a closed cycle. The binary cycle has been therefore the object of an extended research activity, in order to attain higher plant performance. A crucial matter is the improvement of the heat introduction process. For a given geothermal fluid in liquid state, i.e. for a variable temperature heat source, in a conventional ORC the working fluid evaporation process is responsible for an important second law loss: removal of this loss allows greater power and possibly higher cycle efficiency to be attained. Aim of the present paper is to investigate and compare recently proposed technical solutions based on the current technology, which do not entail considerable operating risk or relevant investment; they can however lead to an improvement in plant performance and economics. The selected cycle options were dealt with in the open literature, and try to reduce the heat introduction second law loss: in the first one, the so called OFC, this loss is strongly reduced, because heat is introduced in the cycle when the working fluid is in liquid phase, but a dissipative flash process is then required. In the second one, the so called Pinch Point Smoother, this loss is reduced because the working fluid heating curve is smoothed by means of a flow split, which allows a fraction of the working fluid flow to evaporate at a pressure lower than the pressure of the main flow, but mechanical recompression is then required to inject the separated flow fraction into the turbine. The result of comparison may depend both on the temperature level of thermal sources involved and on the working fluid selected: the present paper will discuss several examples, representative of geothermal applications, and try to assess whether the adoption of these solutions can be convenient for geothermal exploitation.