OBJECTIVES AND TRL
The project pursues the following main objectives related to dispatchable renewable energy production and storage, converted through sCO2 cycles:

1. TECHNO-ECONOMIC ASSESSMENT OF THE sCO2-PTES CYCLES FOR ENERGY STORAGE
Optimised layouts for 1-10MWe sCO2 cycles will be defined, targeting, for the first time, plant layouts specifically developed for energy storage through a power-to-heat-to-power (PHP) concept, which, thanks to the properties of sCO2, can potentially reach round trip efficiencies higher than 75%; moreover, the sCO2 utilization promises significant reduction in CAPEX and will enhance plant flexibility. Depending on the location and application, this system can be integrated with heat sources at high temperature (such as CSP) or at low temperature (for low-T WHR), enhancing the efficiency of the system. Joule-Brayton PTES already shows
interesting results for energy and power density ([kWh/m3] and [kW/m3]), having higher values than other thermo-mechanical storage such as CAES and LAES [Olympios 2021], and also in economic terms of energy and power capital costs ([$/kWh] and [$/kW]), resulting possibly more competitive than electro-chemical batteries [McTigue 2020]. The application to sCO2 can confirm and enhance these results, together with the possibility of harvesting external thermal sources where available.

2. SUSTAINABLE ECO-DESIGN
With reference to EF 2.0 indicator impact assessment method, the eco-design aims to drive the technical development towards the most environmental-friendly solutions. The goal is to achieve an overall environmental footprint for the proposed system between PV and wind technologies (e.g. 1.5 – 12 mPts/MWh and 10 – 30 CO2eq/kWh of Global Warming Potential), which would pave the way for the ecological competitiveness over the current technological level of renewables, including geothermal, PV solar, wind and biogas. With reference to comparable storage systems, such as Solar Integrated Thermo-Electric Storage, the goal of the present proposal in terms of environmental impact points is to keep it below an average of 15 – 25 Pts/day [Fiaschi Manfrida 2020].

3. DESIGN AND CFD MODELING OF NOVEL RADIAL TURBOMACHINERY FOR SUPERCRITICAL CO2
sCO2 power systems of medium capacity (1-10 MW) feature turbomachinery of radial architecture, which best fit with the low volumetric flow rates resulting by the adoption of CO2 in supercritical state. The supercritical conditions have several other implications on the design and operation of the machines, namely: (1) the non-ideal thermodynamics of the fluid close to the critical point; (2) the onset of phase change close to the critical point, whose effect must be explicitly considered for the operation and structural integrity of the machines; (3) the high density of the fluid flowing at high speed in the high temperature components, which leads to structural, performance, and technology issues (large blade sections, severe leakage flows, high-cost materials). These difficulties have been successfully faced in the EU H2020 projects to which POLIMI and UNIGE are participating, resulting in dedicated modelling/design techniques and experimentally-validated machine prototypes, which can now be considered of TRL 5. This project will take a step forward by pursuing optimal designs of the turbomachinery specifically tailored for the need of this energy storage system. Moreover, in this proposal a novel high-temperature sCO2 compressor and a novel near-critical sCO2 turbine
will be studied for the first time. Thanks to the level of fidelity of the developed techniques, new-concepts will be explored and multiple optimizations will be performed, considering structural, operation, and technology issues, to reach TRL 3 for these novel machines.

4. EXPERIMENTAL VALIDATION OF BLADELESS MACHINES FOR CO2 APPLICATIONS
For the first time, bladeless turbomachinery will be investigated for sCO2 cycles, leveraging on their ability to handle low volumetric flows, ideals for low-size plant applications. Furthermore, the UNIGE Tesla ultra-efficient concept [SIT patent 2022] will be applied and run at a laboratory scale (~1 kWe) for the first time, promising efficiency aligned with conventional bladed machinery (80%+ at CFD level w/o leakages, 50%+ experimentally), but retaining simplicity and flexibility of Tesla machines. An existing bladeless expander prototype, owned by UNIGE, will be upgraded and tested into one experimental CO2 loop (by UNIFI), allowing the advancement of this technology up to TRL 4.