Model-based optimisation of a reversible heat pump-Organic Rankine cycle Carnot battery for residential applications

Reversible Carnot Batteries are emerging as alternative solutions for renewable energy storage, thanks to their scalability, high energy density, and suitability for seasonal energy storage. Such systems are becoming increasingly competitive for daily energy storage compared to conventional lithium-ion batteries due to their capability to be integrated into multi-energy systems and their lower environmental impact over the entire life cycle. Reversible Carnot Batteries (R-CBs), based on the integration of Heat Pump (HP) and Organic Rankine Cycle (ORC)-based units allows to further reduce the investment costs as the same components are shared. Nevertheless, there is a notable lack of plant analysis of this system when applied on a domestic scale, as well as an assessment of the energy autonomy of the integrated photovoltaic (PV) and R-CB storage system. Indeed, while the literature is rich in thermodynamic and economic analyses, it lacks comprehensive models that consider plant features such as the metallic mass of heat exchangers, piping layout, expander permeability, and mass charge of the working fluid. To fill this gap, a thermo-fluid dynamical model of a reversible Heat Pump–Organic Rankine Cycle (HP–ORC) Carnot Battery for residential applications has been developed and optimized under photovoltaic (PV)-driven operating conditions. The model combines zero-dimensional and one-dimensional approaches to accurately capture both thermodynamic performance and system-level dynamics, including heat exchanger thermal inertia, piping layout, and working fluid mass distribution. Validation against experimental data shows good agreement, with relative errors in the range of 3–6 %. The system is analyzed under winter conditions in central Italy, considering a 60 m 2 PV installation. Results indicate that a thermal energy storage (TES) volume of 8000 L ensures the best trade-off between storage temperature (up to 100 °C) and thermal inertia, in line with those currently adopted in similar applications found in the literature. During discharge, increasing the hot water flow rate from 20 to 30 L·min⁻ 1 results in the power production of a 3 kW ORC unit growing, thus storing up to 4 kWh of electric energy. This ensures that all daily load requirements are fulfilled, with a residual amount of about 1.3 kWh. Another novel aspect highlighted by this work is the assessment of the impact of expander speed on system energy performance. It was observed that reducing the expander speed from 1900 RPM to 1000 RPM increases operating time and improves energy utilization, enabling full load coverage with residual stored energy of 1.5 kWh and peak ORC efficiency close to 7 %. Moreover, the Round-Trip Efficiency (RTE) and exergy efficiency increase by a factor of 2 (34 % and 26 %, respectively) compared to the baseline condition. Therefore, these results open the way for a reversible application of the ORC-based unit as a heat pump to store thermal energy during periods of low electrical energy demand, serving as an alternative to electrochemical storage within batteries.