Dynamic response of a micro-scale ORC-based power unit fed by solar flat panels

In the residential sector, micro-cogeneration can be performed with organic Rankine cycle (ORC)-based power unit (ORC-bPU) bottoming flat-plate solar thermal collectors. Usually, the thermal energy recovered is stored inside a Thermal Energy Storage (TES) reservoir as hot water (HW) at a temperature of 90-110 C, which can behave as hot source for the mentioned ORC-bPU. In this case, when the solar source is absent, the ORC unit is driven by TES whose thermal energy progressively decreases as the ORC-bPU power production going on. Hence, to set optimal operating strategies is fundamental to characterize the dynamic response of the ORC-bPU. A knowledge gap is observed in literature about dynamic characterization of micro-ORC-bPU driven exclusively by TES reservoir. Hence, this study presents an extensive experimental activity on a small ORC-bPU to characterize its transient behaviour defining novel aspects about operating strategies to increase the daily electricity production, simultaneously satisfying the domestic direct thermal needs. The time response of the unit was assessed when severe step-variations of the main regulating variable (Hot source and Working fluid flow rate) were considered. Results shows that the time required to reach 98 % of the steady final value are equal to 232 s and 190 s if step change of WF and HW are performed, respectively. Concerning the best operating strategy, a proper reduction of the WF mass flow rate allows to achieve a beneficial decrease of the cooling rate of the hot water, (-0.005 C/s). This ensures to maximize the operating time of the unit and, therefore, the electricity production.