Thermally-integrated CO2 cycles for MW-scale power generation and storage

Abstract

In an energy scenario driven by Renewable Energy Sources (RES), where more and more bulky quantities of RES should be introduced on the grid, the role of energy storage systems is crucial. Further to already available electric storage technologies (mostly based on batteries), it will be mandatory to have grid flexible large scale energy storages able to operate ramp-up/down with large capacity, whose behaviour/management should be as much similar as possible to traditional power plants (also to guarantee specific grid services like grid frequency regulation via rotating inertia etc.) which are currently used to instantaneously regulate the grid. At this purpose, Pumped Thermal Energy Storage (PTES) offers GWh scale storage without geographical constraints, at reasonable costs, and implementing power and heat pump cycles integrated with thermal energy storage (TES) solutions. The peculiar features of supercritical CO2 (sCO2) make it the ideal candidate to act as working fluid for large scale PTES applications. sCO2 cycles are indeed fully compatible with the temperature range of TES hot storage sources and sCO2 has already been used in commercial HP solutions (even targeting high temperature HPs). In addition, sCO2 allows energy storage to embody a compact design as well, making the whole PTES footprint smaller compared to technologies based on other working fluids. Nevertheless, sCO2 based PTES solutions cannot achieve significant Round Trip Efficiency (RTE): a possible solution to such a limitation is represented by the exploitation of freely available heat sources (like thermal RES or waste heat), which could increase the COP of the charging cycle and, at the end, the electrical-based RTE, therefore in the so-called Thermally Integrated Pumped Thermal Energy Storage (TI-PTES). In the framework of the PRIN 2022 project ECO-SEARCHERS, specific cycle layouts of interest are studied and compared on a thermodynamic basis: the most promising solution is presented in this paper. Finally, the test rig that will be used for laboratory-scale validation of a small size radial bladeless turbine is described, providing a first glance to the technical challenges in realizing and managing sCO2 cycles in practice.