The Partially Evaporated Organic Rankine Cycle

The Organic Rankine Cycle (ORC) offers a great potential to mitigate the effects of climate change due to its ability to enable low-temperature heat sources for power generation. This thesis investigates a modification of the ORC, which is characterized by a not fully evaporated working fluid prior to the expansion step: the partially evaporated ORC (PEORC). The partial evaporation of the working fluid allows for a better match between heat source and working fluid and thus a higher heat source utilization. First, the influence of the applied lubricant oil on the properties of the refrigerant-oil mixture is experimentally investigated. The collected data is then used to develop a model to describe refrigerant-oil mixtures, allowing the computation of the liquid and vapor phase composition. This allows for the experimental evaluation of the expansion in a twin-screw expander for partly evaporated and pure vapor refrigerant. The results show, that independent of the type of expansion, with decreasing rotational speeds the internal leakage rises, which in turn has a detrimental effect on the isentropic efficiency. The collected experimental data is then used to extend a semi-empirical expander model to describe the expansion of two-phase working fluid. In the next step, ORC and PEORC are compared experimentally for a wide range of heat source conditions. The experimental data confirms the theoretical assumption that the PEORC is able to utilize the heat source to a greater extend than the ORC and thus shows higher net system efficiencies. The last step of this thesis is an economical comparison of ORC and PEORC for a geothermal use case. Both systems are optimized in terms of net present value (NPV), which results in two designs with similar investment costs of about 7 million €. After 20 years of project life time the PEORC achieves a NPV of 13.4 million € and outperforms the ORC by 37 %.