Evaluation of operation control parameters of a residential solar organic Rankine cycle with effects of expander-generator coupling

Cagri Kutlu; Mehmet Tahir Erdinc; Amin Ehtiwesh; Michele Bottarelli; Yuehong Su; Saffa Riffat

Authors

Cagri Kutlu School of Built Environment, Engineering and Computing, Leeds Beckett University, Leeds LS2 8AG, UK https://orcid.org/0000-0002-8462-0420
Mehmet Tahir Erdinc Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, NG7 2RD, UK; Department of Mechanical Engineering, Tarsus University, 33400, Mersin, Turkey https://orcid.org/0000-0003-2201-2937
Amin Ehtiwesh Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, NG7 2RD, UK
Michele Bottarelli Department of Engineering, University of Ferrara, 44122, Ferrara, Italy
Yuehong Su Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, NG7 2RD, UK https://orcid.org/0000-0002-6616-7626
Saffa Riffat Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, NG7 2RD, UK https://orcid.org/0000-0002-3911-0851

Keywords:

Solar ORC, Scroll expander, Transient response, Operation control

Abstract

Solar-powered organic Rankine cycles (s-ORCs) are promising technologies for converting solar radiation into electrical energy, offering a viable alternative to conventional photovoltaics. However, the intermittent nature of solar radiation presents operational challenges, as these systems cannot regulate their heat input and rely entirely on solar energy availability. This study investigates the transient performance of a small-scale s-ORC system without thermal energy storage, focusing on how operational parameters influence system behaviour under variable solar conditions. A simulation model is developed, integrating effectiveness–NTU-based heat exchanger models and a validated expander–generator coupling sub-model that accounts for real-world responses such as torque balance, rotational speed, and internal leakage. Key control parameters, including the flow rates of the refrigerant, cooling water, and thermal oil, are examined to assess their influence on overall performance. The results show that solar heat input is the dominant factor affecting system efficiency, followed by the cooling water flow rate, which has a more significant impact than thermal oil flow. Based on a one-day simulation, the expander’s volumetric efficiency was found to average around 60% due to leakage losses, and the expander predominantly operated under off-design conditions. The generator efficiency varied between 49% and 58%, with lower rotational speeds resulting in better conversion efficiency.