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

- A domestic solar combined heat & power (S-CHP) system is optimised for maximum electrical output in the UK.
- The S-CHP system comprises a solar collector array, an ORC engine and a working fluid buffer vessel.
- A working fluid and evaporation temperature optimisation are performed for an annual operation period.
- A single-stage and a two-stage collector array configuration are compared.
- An optimum annual-average electrical power of 122 W for the two-stage configuration is reported.

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-m2 non-concentrated 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 kW h 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 saturation 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], thus highlighting the gains possible when using optimal system configurations and fluids and suggesting that significant further improvements may be possible. A performance and simple cost comparison with stand-alone, side-by-side PV and solar-thermal heating systems is presented.