Assessment of the Dynamic Behaviour of an Organic Rankine Cycle (ORC)-Based Carnot Battery System

Long-term energy storage technologies are fundamental to fully exploit renewable sources characterized by high variability. The Organic Rankine Cycle (ORC)-based Carnot Battery system represents an emerging technology as a long-term energy storage device thanks to its lower cost, high capacity and absence of geographical restrictions. Indeed, compared to electrical energy storage Carnot Batteries ensure a lower cost and longer storage duration. In particular, the integration of a Heat Pump with an ORC-based power unit allows to achieve higher power-to-power efficiency values. Indeed, ORC-based power units ensure to exploit low thermal heat source typical of these units. In Carnot Batteries, ORC units are driven by a Thermal Energy Storage (TES) system whose temperature is often limited to 60-150 °C when pressurized water is used. Despite the huge effort in the analysis of Carnot Battery benefits, few works are observed in literature about the of ORC-based unit section model-based improvement and the assessment of dynamic behaviour. This knowledge is fundamental to develop reliable operating control strategies to ensure ORC-based power unit works always in proper conditions. To perform this assessment, a complete thermo-fluid-dynamic model of the ORC-based unit section was developed in GT-SuiteTM environment and validated on experimental values collected in the literature. Once validated, the model was adopted as a software-platform to observe possible plant improvements. In particular, a Sliding Rotary Vane Pump was introduced as a replacement for the baseline conditions and a fluid receiver was installed upstream the pump itself with the aim of reducing the expander outlet pressure, thus increasing the power production. The numerical analysis shows that these plant modifications ensure to double the power production, boosting the plant efficiency from 2.9 up to 5.6%. Finally, the benefits were observed under dynamic conditions with sudden and large variations in hot water temperature. With an appropriate modification of the pump speed, the power production varies in a range between 1.5 and 3.5 kW with an efficiency between 4 and 6%, confirming the operability robustness of the optimized plant.