Optimization-based state-of-charge management strategies for supercritical CO2 Brayton cycle pumped thermal energy storage systems

We present a study concerning the state-of-charge (SoC) management strategies for pumped thermal electrical energy storage (PTES) systems. The particular system under study is a recuperative Brayton Cycle PTES with supercritical CO2 as the working fluid and uses molten salt and water as hot and cold side thermal storage reservoirs. The charging and discharging cycles, including the turbomachinery, heat exchangers, and two-tank thermal storage units are modelled using Aspen HYSYS, considering variable speed operating characteristics of the turbomachines. An in-cycle SoC management strategy is proposed to maintain equal charging and discharging capacities between the hot and cold side thermal storage reservoirs, whereas a cycle-to-cycle SoC management strategy is used to constrain the PTES operating envelope for charge/discharge power and duration given operational objectives. The model is used in several case-studies to demonstrate the SoC management strategies. The case study results showed that, given an electricity price profile, the algorithm can determine feasible charge/discharge profiles while maximizing the operational profit. Additionally, if the PTES system is integrated with a wind farm, it enables the wind farm to provide dispatchable power. The round-trip efficiencies of the system is within the range of 35-60 % and in certain scenarios with increased part-load operation, such as the wind farm integration scenario, the average efficiency is observed to be 46.5 %. The SoC of both tanks displayed a negligible deviation of 0.24 % after five days of operation, including operation under part load conditions. The findings provide a new avenue for revenue stacking via flexible operations and can help accelerate the adoption of PTES systems.