Off-design of a lab scale SCO2 based thermally integrated pumped thermal energy storage

In this paper, an off-design analysis of a lab-scale cycle sCO2 Thermally Integrated - Pumped Thermal Energy Storage (TI-PTES), based on Brayton type cycle, is presented. Off-design conditions arise due to load-schedule following, or due to variations in storage tank temperatures (e.g. for incomplete charging/discharging of the TES) or due to the temperature of the external heat source integrated in the cycle or the ambient temperature. The goal of this off-design analysis is to identify the potential operating envelope for the off-design temperatures in the cycle. To that end, off-design envelope for the off-design of ambient temperature and the integrated heat source is evaluated. The off-design envelope is mapped for the rotational speed of the hot machines in the charging and discharging cycles of the TI-PTES. Based on the off-design envelope, mitigation strategies have been proposed with the target to bring the storage temperatures to design point. Inventory control for the heat transfer fluid of TES has been proposed for all the temperature related off-design conditions, as it is a less complex strategy than controlling the mass flow rate of the CO2 loop. The operational range of hot turbomachines, storage temperatures, pressure ratios, powers and performance of the cycles have been evaluated for the selected off-design cases.