High-Temperature Thermal Energy Storage: Process Synthesis, Material Selection, and Optimal Integration with a Power Plant

High-temperature thermal storage (HTTS), particularly when integrated with steam-driven power plants, offers a solution to balance temporal mismatches between the energy supply and demand. However, a simple HTTS design is inefficient and sometimes is infeasible, as the design does not achieve the same quality of discharged energy due to the loss of degree-of-superheating. More complex HTTS design and operation face challenges due to time-varying generation, interconnected storage cycles, the presence of nonlinear phase changes for both latent and sensible heat transfer, and the need for optimal selection of storage medium. We present a systematic optimization approach, utilizing an HTTS process superstructure representation and a mixed-integer nonlinear programming (MINLP) model, to address these challenges. This approach allows for the synthesis and integration of optimal HTTS designs, considering various process configurations and molten salts as storage medium. Case studies for HTTS design and integration with natural gas combined cycle (NGCC) power plants demonstrate that charging and discharging conditions significantly influence the round-trip efficiency (RTE) and the levelized cost of energy storage (LCOS). While integrating HTTS with steam turbines can achieve a high RTE, it increases the LCOS and the complexity of the HTTS design. Nonetheless, HTTS offers lower LCOS compared to stand-alone thermal storage systems. Among design options, a three-cycle HTTS system, utilizing liquid sensible heat, latent heat, and vapor sensible heat, achieves the highest RTE but may not always be cost-optimal. The selection of molten salt further affects RTE and LCOS, highlighting the trade-offs between the efficiency- and economics-based design objectives. The findings underscore the potential of HTTS in enhancing the ramping capacity and distributed storage capabilities of steam power plants, but emphasize that technological and economic feasibility must guide integration strategies.