Off-Design Analysis of ORC-Based Carnot Batteries: Performance Assessment and Insights into Components Design

Carnot batteries are a promising energy storage technology that converts surplus renewable electricity into thermal energy and subsequently back into electricity to balance energy supply and demand. A comprehensive understanding of their off-design and part-load performance is critical, not only to evaluate their potential under real-world conditions but also to obtain useful information for the design and operation of their system components, namely the turbomachinery and heat exchangers. While the off-design analysis of standalone ORC systems is well-documented, studies addressing ORC-based Carnot Batteries are scarce, in particular for system architectures with heat exchangers shared between charging and discharging phases. This work aims to fill this knowledge gap, providing insights into the performance and operational flexibility of Carnot Batteries under realistic, variable conditions. In this work off-design and part-load simulations are conducted to assess system performance under fluctuating renewable electricity inputs, variable thermal power availability and demand, hours of discharge and integration of heat during discharge. In particular, the heat exchangers are simulated by adopting a discretized approach to account for the variation in working fluid thermodynamic properties. Referenced correlations are employed to compute the heat exchangers heat transfer coefficients and pressure drops. On the other hand, turbomachinery (ORC turbine and HP compressor) is simulated by means of performance maps and guidelines are provided on the optimal design of these components for such applications. Results show that ORC-based Carnot Batteries are able to operate efficiently in a wide range of conditions, with the main limitation related to activation of anti-surge loop of compressors for low electricity availability in charging mode. The findings contribute to the development of ORC-based Carnot Batteries, providing valuable insights into the design and optimization of their key components, helping to understand the potential of this solution against other energy storage technologies under off-design conditions.