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SUPERCRITICAL CO2 CYCLE FOR FLEXIBLE AND SUSTAINABLE SUPPORT TO THE ELECTRICITY SYSTEM

Periodic Reporting for period 2 - sCO2-Flex (SUPERCRITICAL CO2 CYCLE FOR FLEXIBLE AND SUSTAINABLE SUPPORT TO THE ELECTRICITY SYSTEM)

Reporting period: 2019-07-01 to 2021-06-30

sCO2-Flex consortium addresses the challenge of flexibility by developing and validating (at simulation level the global cycle and at relevant environment boiler, heat exchanger (HX) and turbomachinery) the scalable/modular design of a 25 MWe Brayton cycle using supercritical CO2, able to increase the operational flexibility and the efficiency of conventional power plants

sCO2-Flex developed and optimized the design of a 25MWe sCO2 Brayton cycle and of its main components able to meet long-term flexibility requirements, enabling entire load range optimization with fast load changes, fast start-ups and shut-downs, while reducing environmental impacts and focusing on cost-effectiveness. The project, bringing the sCO2 cycle to TRL6, will pave the way to future demonstration projects (from 2020) and to commercialization of the technology (from 2025). Ambitious exploitation and dissemination activities will be set up to ensure proper market uptake.

The last two years of the project had as main objectives, the final design and testing of the main components, the adjustment of the control strategies to improve the flexibility of the cycle, the architecture and the economic evaluation of the 25 MWe design. Additional work on the social acceptability, as well as first reflections on the improvement of the environmental impact of the sCO2 cycles have also taken place.
1.Works and results on the boiler and materials
The design of the boiler was finalized for the selected cycle. A pulverized coalTT-shape fired boiler was selected. The duct system and auxiliary components of the designed boiler were also included. This arrangement is suitable for both 25 MWe and 100 MWe boilers. Additionally, to meet emission limits, a SCR should be taken into an account. To increase flexibility, high temperature recuperator bypass is used for the last heat transfer surface of the pressure system, which respectively corresponds to the economizer in conventional steam boilers.
Several materials were selected for each major component and have been tested in the project after hundreds of hours spent in industrial- or research-grade CO2, under severe conditions. The experimental campaign focused on corrosion by means of two parallel testing campaigns.

2. Works and results on the turbomachines
UDE conducted a series of investigations to understand the effect of CO2 properties on the compressor design and performance. A fundamental study over a NACA airfoil was conducted. A design tool was created using geometry and performance correlations that are generated for fluids obeying the ideal gas law.
Shaft lines, compressors and turbine design have been finalized. Regarding, the main compressor, this has been fully designed till the production drawings in order to proceed with the production of the prototype compressor. The main compressor, with the nominal power of 5.3MW has been produced and has been installed and commissioned in a dedicated test bed located in BH campus where this has been tested in all the conditions from the design point, very close to the critical point. For the other compressor and the turbine, the design and analysis has been performed till the preliminary phase that anyway, allowed to assess all the topic related to this turbomachinery.

3. Works and results on the Heat Exchangers
Fives Cryo manufactured two stainless steel BPFHE prototypes for thermal-hydraulic testing on the SCARLETT bench at USTUTT. Fatigue tests on BPFHE specimens also showed that the brazed joints can be preserved and thus that this type of HX can be integrated into sCO2 cycles. A reliable BPFHE design was then proposed for the 2 recuperators for implementation in a 25 MWe s-CO2 power cycle.
In addition, BPCHE prototypes with different channel shapes were built for testing in the CVR loop.

4. Works and results on the Cycle flexibility
Two numerical codes have been developed and validated against the results obtained during the project on component design and experimental campaign. Steady state analysis showed that the part-load range attainable with the designed sCO2 power plant is quite large (from 20% to 100% of nominal load) and wider than current power plants.
Dynamic analysis confirmed that the plant can be controlled in a stable way throughout the entire operating range, and it can provide fast load changes required for primary frequency control.
However, annual simulations carried out for Spain and Denmark have demonstrated that the adoption of wet&dry heat rejection units can substantially reduce the LCOE in hot climates with an acceptable water consumption. Final LCOE is competitive with current USC power plants and lower than small size biomass fired conventional systems.

5. Technological validation and applications
Previous models were consolidated according to the final inputs provided by the partners on main cycle components, confirming an overall system efficiency of 37% (efficiency already higher than the average efficiency of current power plants and which offers great prospects for improvement with equipment improvements).
Based on the design of the plant’s main components and system-level modelling, a detailed design was produced, leading to a refined list of equipment, PID and plant layout.
The implementation of two alternative carbon capture technologies,was investigated from the technological and LCA points of view: results showed higher costs and technological complexity for the first solution, but significantly better environmental performances.
The viability of replicating/transferring the technological solutions developed to Waste Heat Recovery, CSP and biomass applications was also investigated.

6. Works and results on the Cycle Financial, Environmental and Social Impact
CAPEX of a 25 MWe plant was compared to a reference plant. The costs were also extrapolated to 100 MWe. Overall, the plant’s performance is significantly better than for a water/steam plant, with comparable CAPEX.
Environmental analysis compared a reference plant with the sCO2-Flex plant. sCO2-Flex plant achieves an 8% decrease in greenhouse gases emissions; however it uses more noble materials than its counterpart and is comparable to wind turbines on that criterion.
Preliminary risk assessment for the deployment of the sCO2-Flex technology was conducted, including the identification of the main stakeholders and the main societal risks. Impact in labour market has been pointed out as main potential risk. This analysis has been completed with a set of guidelines to integrate social impact in energy research projects.
The supercritical CO2 cycle developed in sCO2-Flex shows promising efficiency, flexibility and environmental footprint for sCO2 cycles. The expected gain in component compactness has been widely confirmed by the project’s developments, leading to the competitiveness of the selected sCO2 cycle versus its counterpart in terms of CAPEX. As the sCO2-Flex plant’s efficiency is better than the average European plants’, and as most greenhouse gas emissions occur in operation, the plant offers a lower environmental impact regarding this indicator.
The next step towards the commercial availability of a full-scale sCO2 cycle would be the construction and testing of a medium-scale demonstrator (> 10 MWe), in order to confirm the actual behaviour of every component in the relevant environment and be able to test new equipment and layouts.