Brayton-Based Carnot Batteries: Design, Management, and Performance Assessment Assisted by Off-Design and Dynamic Analysis

The increasing need to reduce CO2 emissions is accelerating the adoption of renewable energy sources in the power generation system. However, solar and wind power are intermittent and unpredictable, requiring the development of efficient energy storage technologies to balance supply and demand. Among large-scale energy storage solutions, Carnot batteries have emerged as particularly promising. This technology consists of storing electrical energy in the form of thermal energy, employing thermodynamic processes for both the charging and discharging phases. This study concentrated on Brayton cyclebased configurations, examining significant aspects that are still lacking in the literature. Initially, an off-design analysis was conducted for a Brayton-based Carnot battery. A detailed off-design model was created by incorporating the part-load behaviour of all its components. With this model, a comprehensive energy and exergy analysis was performed to identify the main sources of losses and suggest possible layout modifications to improve efficiency. Specifically, switching from the current external charging system to an internal one could potentially increase round-trip efficiency by 10 percentage points. This possibility was explored in the second phase of the study, where a novel thermal storage system was developed, featuring direct electrical charging and convective discharging. This system is composed of a set of storage elements, each made up of an electric heater surrounded by a storage material matrix. The effects of cross-sectional shape and void fraction were examined through computational fluid dynamics analysis. The next stage of the study examined the dynamic behaviour of a Brayton-based Carnot battery under transient conditions and developed its control system. These aspects remain unexamined in the literature, but they are crucial for determining the potential future roles of these systems in power grids. The system dynamics were compared with that of a conventional gas turbine, revealing that, despite fundamental differences in control systems, Brayton-based Carnot batteries exhibit response times and behaviour during transients similar to those of conventional gas turbines. Although further research is required on the impact of transients on system performance, this preliminary analysis indicates that these systems have the potential to function as fast-responsive plants during sharp increases or decreases in power demand. In conclusion, a hybrid energy storage system was proposed by integrating a Carnot battery with rapid-response devices, such as lithium-ion batteries and flywheels. This system offers flexible power regulation, enabling the Carnot battery to function with slower adjustments. This approach helps to minimise wear and tear on the Carnot battery, thereby prolonging its lifespan.