Part load operation of kilo-Watt scale thermally integrated supercritical CO2 pumped thermal energy storage

Pumped thermal energy storage (PTES), a sub-category of Carnot battery technologies has attracted attention in recent years, mainly because of its ability to use grid arbitrage to convert electricity to be stored as heat (charging), which can be again converted to electric power(discharging). The benefit offered by the heat as the storage medium is the direct supply of heat for industrial or district heating purposes (heat as output) as well as the integration of heat sources to support the cycle performance (heat as input) usually in the charging cycle. The latter configuration is referred to as thermally integrated PTES(TI-PTES). A key advantage of TI-PTES is that it allows the integrated heat to be stored together with electricity during charging and used at a latter time of high demand while removing the constraint of cold storage especially for Brayton cycle-based TI-PTES plants as well. Leveraging on such a concept, the EU SCO2OP-TES project aims at demonstrating a sCO2-based TI-PTES up to TRL 5 through a lab-scale demonstration plant using industrial waste heat as integrated heat source; the charging and discharging cycles are sCO2-based Brayton heat pump and power cycles respectively, adopting radial turbomachinery for both the hot compressor(charging) and the hot turbine(discharging). Along with the mean line design and CFD analysis of the sCO2 turbomachines, this study explores the influence of their part load operation on the SCO2OPTES TI-PTES demonstration plant. The study elaborates on the effect of partial electric loads considered for hot compressor (high-temperature turbomachine for the charging) and for hot turbine (high-temperature turbomachine for discharging) on the waste heat recovery unit in the charging cycle, thermal energy storage heat exchanger, recuperator and cooler in the discharging cycle. Separate and combined performance of the charging and the discharging cycles as a result of variations in thermodynamics and mass flows is evaluated. The part load scenario shows an exceedingly small off design envelop for the charging cycle (mostly limited by the hot compressor operating envelope), while for the discharging cycle, the part load operation benefits from a broader operational range as well as allowable mitigation space. Based on such results preliminary operational constraints for the proper controllability of the cycles are suggested.