Session: 09-01 Compressed / Liquid Air Energy Storage

Submission Number: 175434

Conceptualization and Optimization of a Three-Shaft Turbomachinery Architecture for Adiabatic Compressed Air Energy Storage Enabling Enhanced Operational Flexibility 

Long-duration thermo-mechanical energy storage can stabilize grids with high renewable penetration and leverage existing supply chain and manufacturing expertise related to fossil fuelled plants. Despite these advantages, the adoption of such systems has lagged due to high capital expenditure and poor efficiency at part-load and varying state of charge.

To address these shortcomings, we propose and optimize a new Adiabatic Compressed Air Energy Storage (A-CAES) architecture, enabling high performance across loads and state of charge conditions (storage pressure from 40 bar to 100 bar with extended operation from 20 bar to 150 bar). The goals are to maintain the main-stage turbomachinery at peak-efficiency conditions across the entire operating envelope, and to maximise the round-trip efficiency (RTE) of the energy storage system. The proposed system uses a three-shaft powertrain with a main high-pressure-ratio machine operating at fixed corrected mass flow and pressure ratio in both charge (main compressor) and discharge (main turbine) mode. The compressor and turbine have the same design density ratio, opening the possibility to use reversible turbomachinery. Flexibility is provided by two auxiliary stages: (i) a variable-speed low-pressure (LP) unit governing system flow, and (ii) a high-pressure (HP) module that can switch between three units: a variable-speed compressor, a variable-geometry (nozzle-guided) turbine, and a throttle valve. The HP unit and its operating conditions are selected to match the underground store pressure while preserving the main machine’s fixed corrected conditions. Compressor and turbine performance maps, including the HP variable-nozzle turbine, are validated against literature experimental and design data. Throttling and recirculation valves are employed to avoid operation behind surge or choke limits for each component. The system design parameters are selected trough a multi-point thermodynamic optimization, spanning a grid of different loads and states of charge. Thereafter, a design point is selected for a multi-objective techno-economic optimization, constructing a Pareto front between capital expenditure and electric RTE.  

Off-design simulations indicate a peak electric RTE of 75% (averaged across a full cycle), remaining ≥68% down to 45% of rated mass flow rate across a storage-pressure window of 40 bar to 100 bar, while keeping the main machine at fixed corrected conditions. Compared with a fully variable-speed train, the required variable-speed drive capacity is reduced by over 60%. Benchmarking versus a three-stage CAES baseline with uniform compressor pressure ratios suggests a significant capital expenditure reduction, with reversible turbomachinery providing incremental savings. The higher main-stage pressure ratio increases energy-storage density and enables the use of molten salts (in addition to thermal oil), lowering thermal-storage cost.

By holding the main stage on-design and shifting flexibility to a reconfigurable HP module and a variable-speed LP unit, the proposed three-shaft A-CAES achieves high efficiency over wide load and state of charge ranges with reduced variable speed drive requirements compared to a single shaft configuration. This architecture offers a practical path to lower capital expenditure and improved part-load performance of long-duration storage, reducing its deployment barriers.

Presenting Author: Simone Parisi Technical University of Denmark

Presenting Author Biography: Simone Parisi is a postdoctoral researcher at the Technical University of Denmark (DTU). His research interests include the design and optimization of turbomachinery and energy systems, with applications to energy storage, renewable heat to power conversion, and heat pumps.

Authors:

Simone Parisi Technical University of Denmark
Bartosz G. Kątski Technical University of Denmark
Roberto Agromayor Technical University of Denmark
Fredrik Haglind Technical University of Denmark