Design of organic Rankine cycle power systems accounting for expander performance

Organic Rankine cycle power systems have recently emerged as promising solutions for waste heat recovery in low- and medium-size power plants. Their performance and economic feasibility strongly depend on the expander. Its design process and efficiency estimation are particularly challenging due to the peculiar physical properties of the working fluid and the gasdynamic phenomena occurring in the machine. Unlike steam Rankine and Brayton engines, organic Rankine cycle expanders have to deal with small enthalpy drops and large expansion ratios. These features yield turbine designs with few highly-loaded stages in supersonic flow regimes. This paper proposes a design method where the conventional cycle analysis is combined with calculations of the maximum expander performance using a validated mean-line design tool. The high computational cost of the turbine optimization is tackled building a model which gives the optimal preliminary design of the turbine as a function of the cycle conditions. This allows to estimate the optimal expander performance for each operating condition of interest. The test case is the preliminary design of an organic Rankine cycle turbogenerator to increase the overall energy efficiency of an offshore platform. The analysis of the results obtained using a constant turbine efficiency and the method proposed in this paper indicates a maximum reduction of the expander performance of 10% points for pressure ratios between 10 and 35. This work also demonstrates that this approach can support the plant designer on deciding the optimal size of the organic Rankine cycle unit when multiple exhaust gas streams are available.