A Strategy for the Design of ORC Multi-Stage Axial Turbines for Geothermal Energy Conversion

Abstract

Abstract Geothermal energy is one of the most attractive renewable sources, largely available across US and Europe, but featuring medium-low enthalpy level. Such kind of energy source can be effectively exploited by resorting to Organic Rankine Cycles (ORCs), namely phase-change cycles based on organic fluids, whose thermo-physical properties better match with the low heat grade of the geothermal fluid. Nowadays, ORCs stand out as a mature energy conversion technology, whose performance and tecno-economic optimization greatly rely on the efficiency, reliability, and cost of the turbine. The use of organic compounds implies a peculiar aerodynamic design, where the low specific work and the low speed of sound often lead to turbines featuring one or a few transonic/supersonic stages. For this reason, ORC turbines were historically characterized by lower efficiency and rangeability with respect to steam turbines. This work presents a strategy to design multi-stage axial turbines for ORC power systems devoted to geothermal energy conversion. Starting from a set of turbine operating conditions, several turbine flow paths are optimized at preliminary level, considering alternative configurations in terms of angular speed, number of stages, and size. A peculiar case study is then investigated in detail, also resorting to throughflow modeling, with the aim of integrating the preliminary sizing with the blade spanwise design. Finally, an assessment of the off-design turbine operation is also presented and discussed, so to characterize the impact of the seasonal variation on the turbine operation and efficiency. Despite the peculiar features of the organic fluids, the systematic optimization of the turbine allows to achieve high performance at both design and off-design conditions.