On the plant improvement of Solar-driven ORC-based power unit for domestic micro-cogeneration

Abstract

The integration of flat solar thermal collectors and ORC-based power units ensures the simultaneous production of heat and electric power in domestic applications (CHP). Thus, such micro-cogenerative solutions ensure to reduce the use of fossil fuels and CO2 in a sector that accounted for about 30% of global energy-related CO2 emissions in 2021, considering both residential and commercial buildings. The significant variability of solar power during the day and domestic hot water (DHW) demand pushes the unit to work in off-design and transient conditions frequently. Therefore, to maximize the energy recovery of the unit, the design phase and the components selection assume a crucial role. The critical component is the expander and, generally, volumetric machines are adopted for their operating flexibility and robustness when working with highly variable hot sources. Among the volumetric expanders adopted for this kind of application, scroll machines present higher performance and reliability than other positive displacement technologies. Nevertheless, similarly to the other volumetric expanders, such devices still have room for improvement. Indeed, in many applications, they are obtained by reversing scroll compressors. Hence, to participate in the improvement of these machines, a comprehensive model was developed following an integrated zero and mono-dimensional (0-1D) thermo-fluid-dynamic approach. The model was validated through experimental data measures during a comprehensive testing campaign performed on a fully instrumented solar-driven ORC-based power unit which employs a 1 kW scroll expander as mechanical power at design. Once the scroll expander model was validated, it was theoretically optimized in terms of working fluid flow rate aspiration (volumetric efficiency), indicated performances and mechanical power produced. Subsequently, the benefits of the optimized machine to the recovery unit performances have been estimated.