Thermo-economic evaluation of ORCs for various working fluids

An inclusive component-level technical and economic assessment procedure for the general design and operating strategy of Organic Rankine Cycles (ORC) for use across major application categories (waste heat recovery, solar thermal, geothermal) and sub-MW scales can be an important tool for leveraging the cost-effective deployment of low and medium temperature power cycles. Previous analyses and design approaches tended to focus on thermodynamic efficiency rather than financial performance. To bridge this gap, a general thermo-economic optimization of sub 500 kWe ORCs is developed using a 7-dimensional design space with minimum investment cost per unit of nameplate electricity production as an objective function. Parameters used include working fluid, heat source temperature, pinch in condenser, boiler (HEX) and regenerator, expander inlet pressure and air cooled condenser area. Optimized power block configurations are presented for the application of ORCs with waste or “free” heat sources and solar heat input for power scales of 5, 50 and 500 kWe to facilitate rapid selection of design parameters across a wide range of thermal regimes. While R152a yields the lowest cost ORCs in the case of the former, isopentane is found to be more cost effective in the latter case for heat source temperatures between 125 and 275 °C.