Advancing hybrid liquid air energy storage systems for enhanced performance and operational flexibility

With the increasing penetration of renewable energy sources, the demand for effective and scalable energy storage solutions has become critical. While batteries face economic limitations at long durations, thermo-mechanical energy storage technologies such as liquid air energy storage offer promising alternatives due to their high energy density, low cost, and lack of geographical constraints. However, their deployment remains limited by high costs and modest round-trip efficiency.

This work aims to enhance the competitiveness of liquid air energy storage by improving cost-effectiveness, thermodynamic efficiency, and operational flexibility in both standalone and hybrid configurations, thereby contributing to the advancement of this energy storage technology towards viable large-scale deployment.

The research combines steady-state thermodynamic modelling with non-dimensional turbomachinery models validated against experimental data. The developed models enable robust characterisation of system behaviour in part-load and off-design operation and ensure the feasibility of the proposed control strategies. Hybrid system concepts, including thermal integration with concentrated solar power and coupling with pumped thermal energy storage are developed and evaluated. Additionally, a hybrid configuration is analysed in which pumped thermal storage is combined with liquid air energy storage. A new arrangement of this hybrid system is proposed that supports the application of an inventory-control operating strategy. Finally, a comprehensive techno-economic optimisation framework is formulated to integrate performance, cost, and operational flexibility to identify optimum design parameters across a domain of storage integration scenarios.

The results show that combining liquid air energy storage with concentrated solar power is highly beneficial, as it not only improves the efficiency of the storage cycle but also enhances the overall performance of the solar plant. It is further demonstrated that existing components of liquid air energy storage can be repurposed to operate as a self-sufficient engine cycle driven by surplus solar heat, thereby providing an effective means of increasing dispatchability and operational flexibility of the analysed solar-aided energy storage system. In the assessment of thermal storage technologies, packed-bed cold storage is found to be less expensive than liquid-based alternatives; however, the overall system capital cost remains of a similar order, while the round-trip efficiency is reduced. The reliance of cold storage on system performance is reduced for the analysed pumpedthermal liquid air energy storage system, which allows for size reduction of cold storage and increase in the achievable system energy density. Moreover, this hybrid system is optimised for operational flexibility, leading to a marked improvement in part-load performance compared with conventional liquid air energy storage designs.

The findings reveal that, while stand-alone thermo-mechanical systems may not outperform batteries in short-term financial metrics, they can deliver higher net present values, supporting their role in long-duration, value-driven applications. Round-trip efficiency alone is insufficient as an optimisation target; practical design requires balancing efficiency, cost, and operational flexibility. Moreover, storage flexibility and the feasible operating range can be significantly extended without compromising rated performance by considering off-design operation already at the system conceptualisation stage. Overall, these insights provide design principles for advancing thermo-mechanical storage technologies towards large-scale, real-world deployment.