OBJECTIVES AND TRL
The project pursued 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 1–10 MWe sCO2 cycles were defined, targeting, for the first time, plant layouts specifically developed for energy storage through a power-to-heat-to-power (PHP) concept. Thanks to the properties of sCO2, these systems potentially reached round trip efficiencies higher than 75%. Moreover, the use of sCO2 promised a significant reduction in CAPEX and enhanced plant flexibility.

Depending on the location and application, this system was integrated with high-temperature heat sources (such as CSP) or low-temperature sources (for low-T WHR), enhancing overall system efficiency. Joule-Brayton PTES already showed interesting results in terms of energy and power density ([kWh/m³] and [kW/m³]), exhibiting higher values than other thermo-mechanical storage technologies such as CAES and LAES [Olympios 2021]. It also demonstrated competitive economic performance in terms of energy and power capital costs ([$/kWh] and [$/kW]), potentially outperforming electrochemical batteries [McTigue 2020].

The application to sCO2 confirmed and enhanced these results, while also enabling the harvesting of external thermal sources where available.

2. SUSTAINABLE ECO-DESIGN
With reference to the EF 2.0 indicator impact assessment method, the eco-design approach aimed to drive technical development toward the most environmentally friendly solutions. The objective was to achieve an overall environmental footprint for the proposed system between those of PV and wind technologies (e.g., 1.5–12 mPts/MWh and 10–30 CO₂eq/kWh of Global Warming Potential).

This positioned the system toward ecological competitiveness with existing renewable technologies, including geothermal, PV solar, wind, and biogas. With reference to comparable storage systems, such as Solar Integrated Thermo-Electric Storage, the proposal aimed to maintain environmental impact points 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) featured turbomachinery with radial architecture, which best suited the low volumetric flow rates resulting from the use of CO2 in a supercritical state. The supercritical conditions introduced several implications for machine design and operation, including:
(1) non-ideal thermodynamics near the critical point;
(2) the onset of phase change near the critical point, requiring explicit consideration for operation and structural integrity;
(3) high fluid density at high temperatures and velocities, leading to structural, performance, and technological challenges (e.g., large blade sections, severe leakage flows, and the need for high-cost materials).

These challenges were successfully addressed in EU H2020 projects involving POLIMI and UNIGE, resulting in dedicated modelling and design techniques, as well as experimentally validated machine prototypes reaching TRL 5.

The project advanced this work by pursuing optimal turbomachinery designs tailored to the requirements of this energy storage system. Additionally, a novel high-temperature sCO2 compressor and a novel near-critical sCO2 turbine were studied for the first time. Leveraging high-fidelity modelling techniques, new concepts were explored and multiple optimizations were performed, considering structural, operational, and technological constraints, achieving TRL 3 for these novel machines.

4. EXPERIMENTAL VALIDATION OF BLADELESS MACHINES FOR CO2 APPLICATIONS
For the first time, bladeless turbomachinery was investigated for sCO2 cycles, leveraging its capability to handle low volumetric flow rates, making it suitable for small-scale plant applications. Furthermore, the UNIGE Tesla ultra-efficient concept [SIT patent 2022] was applied and tested at laboratory scale (~1 kWe) for the first time.

This concept demonstrated efficiency levels aligned with conventional bladed machinery (80%+ at CFD level without leakages and 50%+ experimentally), while maintaining the simplicity and flexibility of Tesla machines. An existing bladeless expander prototype, owned by UNIGE, was upgraded and tested within a CO2 experimental loop (developed by UNIFI), advancing this technology to TRL 4.


The following are the publications fully or partially contributed by the project: