Optimisation of a high-efficiency solar-driven organic Rankine cycle for applications in the built environment

Abstract

Energy security, pollution and sustainability are major challenges presently facing the international community, in response to which increasing quantities of renewable energy are to be generated in the urban environment. Consequently, recent years have seen a strong increase in the uptake of solar technologies in the building sector. In this work, the potential of a solar combined heat and power (CHP) system based on an organic Rankine cycle (ORC) engine is investigated in a domestic setting. Unlike previous studies that focus on the optimisation of the ORC subsystem, this study performs a complete system optimisation considering both the design parameters of the solar collector array and the ORC engine simultaneously. Firstly, we present thermodynamic models of different collectors, including flat-plate and evacuated-tube designs, coupled to a non-recuperative sub-critical ORC architecture that delivers power and hot water by using thermal energy rejected from the engine. Optimisation of the complete system is first conducted, aimed at identifying operating conditions for which the power output is maximised. Then, hourly dynamic simulations of the optimised system configurations are performed to complete the system sizing. Results are presented of: (i) dynamic 3-D simulations of the solar collectors together with a thermal energy storage tank, and (ii) of an optimisation analysis to identify the most suitable working fluids for the ORC engine, in which the configuration and operational constraints of the collector array are considered. The best performing working fluids (R245fa and R1233zd) are then chosen for a whole-system annual simulation in a southern European climate. The system configuration combining an evacuated-tube collector array and an ORC engine is found to be best-suited for electricity prioritisation, delivering an electrical output of 3,605 kWh/year from a 60 m2 collector array. In addition, the system supplies 13,175 kWh/year in the form of domestic hot water, which is equivalent to more than 6 times the average annual household demand. A brief cost analysis and comparison with photovoltaic (PV) systems is also performed, where despite the lower PV investment cost per kWel, the levelised energy costs of the different systems are found to be similar if the economic value of the thermal output is taken into account. Finally, a discussion of the modelled solar-CHP systems results shows how these could be used for real applications and extended to other locations.