Working fluid selection and electrical performance optimisation of a domestic solar-ORC combined heat and power system for year-round operation in the UK

In this paper, we examine the electrical power-generation potential of a domestic-scale solar combined heating and power (S-CHP) system featuring an organic Rankine cycle (ORC) engine and a 15-m solar-thermal collector array. The system is simulated with a range of organic working fluids and its performance is optimised for operation in the UK climate. The findings are applicable to similar geographical locations with significant cloud coverage, a low solar resource and limited installation areas. A key feature of the system’s design is the implementation of fixed fluid flow-rates during operation in order to avoid penalties in the performance of components suffered at part-load. Steady operation under varying solar irradiance conditions is provided by way of a working-fluid buffer vessel at the evaporator outlet, which is maintained at the evaporation temperature and pressure of the ORC. By incorporating a two-stage solar collector/evaporator configuration, a maximum net annual electrical work output of 1070 kWh yr−1 (continuous average power of 122 W) and a solar-to-electrical efficiency of 6.3% is reported with HFC-245ca as the working fluid at an optimal evaporation temperature of 126 ◦C (corresponding to an evaporation pressure of 16.2 bar). This is equivalent to ∼ 32% of the electricity demand of a typical/average UK home, and represents an improvement of more than 50% over a recent effort by the same authors based on an earlier S-CHP system configuration and HFC-245fa as the working fluid [1]. A performance and simple cost comparison with standalone, side-by-side PV and solar-thermal heating systems is presented.