Advancements in turbomachinery for thermomechanical energy storage

This thesis addresses the critical need for advanced energy storage solutions to support the increasing penetration of variable renewable energy sources like solar and wind power. While Thermo-Mechanical Energy Storage systems offer potential for long-duration storage, their competitiveness is often hindered by the high cost and performance limitations of conventional turbomachinery. This research focuses on advancing unconventional turbomachinery concepts, two-phase turbines and reversible turbomachinery, to enhance the feasibility and cost effectiveness of Thermo-Mechanical Energy Storage systems. Two-phase-turbines are a promising technology to improve the efficiency of Liquid Air Energy Storage systems, by reducing throttling losses in the cryogenic region, thus increasing the liquid yield. Reversible turbomachines, a key technology that helped cement pumped hydro as the leading energy storage technology, are studied in the context of compressible flows, with possible applications to Pumped Thermal Energy Storage and Compressed Air Energy Storage systems. 

The work employs a multifaceted approach encompassing numerical analysis, techno-economic assessment, and advanced modeling and optimization techniques. Computational Fluid Dynamics is utilized to investigate complex flow phenomena, particularly in two-phase turbines where a simplified barotropic model is shown to be effective for predicting performance and guiding design  improvements. Techno-economic analysis was used to compare various Pumped Thermal Energy Storage configurations, highlighting the potential cost reductions achievable through reversible turbomachinery and identifying optimal operating conditions and working fluids, suggesting liquid storage may be preferable for larger scales. 

Furthermore, the thesis advances non-dimensional modeling frameworks for radial turbomachinery, providing more accurate design point efficiency predictions crucial for system level optimization, especially for smaller, modular storage systems. The feasibility of applying reversible concepts to radial machines is explored through detailed numerical studies, demonstrating that existing compressor designs can  operate effectively in turbine mode with minor modifications. Finally, adjoint-based optimization is applied for the first time to design variable geometry components, specifically vaned diffusers, and applied to reversible machines. The results showcase significant potential for enhancing performance across multiple operating points, as required by reversible operation in Pumped Thermal Energy Storage systems. 

Key conclusions indicate that while cost optimization may lead to system designs with round-trip efficiencies lower than the maximum technical potential, reversible turbomachinery, potentially combined with variable geometry, offers a viable path to reduce the capital cost of Thermo-Mechanical Energy Storage. The research provides valuable insights and tools for the design and analysis of next generation of two-phase and reversible turbomachinery, tailored for energy storage applications, recommending experimental  validation and further system integration studies as crucial next steps.