REVCO2

Développement et optimisation d’un cycle de Brayton au CO2
supercritique REVersible pour la récupération de chaleur fatale

Supercritical CO2 (sCO2 ) Brayton cycles have emerged as a promising solution for achieving high efficiency and increased flexibility across various applications. The numerous benefits of sCO2 power cycles include high cycle efficiency, compact equipment size and their applicability not only to the next generation nuclear reactors but also to the exploitation of industrial waste heat sources. In project REVCO2, four research laboratories (CETHIL, Lafset, LMFA and LUSAC) will combine their expertise to fully develop, that is including sub-systems design and optimization, a versatile reversible sCO2 Brayton cycle targeted to harvest industrial waste heat. The following technological locks impairing the large spread of this technology will be addressed: (i) the unsteady behavior of the system when subjected to an intermittent heat source (as it is often the case in the context of waste heat recovery); (ii) the efficient design of heat transfer subsystem in the thermodynamic critical region; (iii) the overall optimal design of turbomachinery for a reversible sCO2 Brayton cycle that could be used to store heat in periods of grid electricity over-production and (iv) the design of efficient heat storage when the cycle is used in reverse. Together with the other participants, CETHIL will identify relevant operating conditions for the reversible sCO2 Brayton cycle covering a wide range of industrial applications: high temperature heat-pump upgrading process waste heat at high discharge temperature (> 200°C), Power-to-Heat-to-Power configurations to take advantage from grid price variations to deliver electricity and heat to the process at the lowest cost while providing flexibility of service to the grid.

Given this context, Lafset will concentrate on the design, optimisation and experimental testing of the heat exchange components with emphasis on the complex thermal behavior close to the critical point of CO2. CO2-based mixtures (with propane, for example) will also be considered to reduce operating pressures while maintaining optimum performance. Together with the partners, LUSAC will focus on the energy management and storage of the system for the direct and reverse Brayton cycle. Energy storage and conversion strategies will be analyzed to ensure efficient energy management of the system. During this phase of the project, innovative storage technologies will be investigated to define an optimal design of a thermal battery heat storage. Experimental investigations will be conducted for integration in the system and measurements will be used to validate the numerical model and improve the efficiency. LMFA will concentrate on global design of the turbomachinery stages. The supercritical compressor and turbine stages will be first designed using a tailored first-principle turbomachinery design tool. In a second validation stage, high fidelity simulations will be used through a specifically designed Lattice Boltzmann method (LBM) suited for compressible and supercritical flows of real gases. Together with the Reynolds-Averaged Navier Stokes (RANS) approach, these three different levels of fidelity will be combined in a multi-fidelity optimization procedure to optimize the overall design of the turbomachinery stages. Once the full design available, a perturbation analysis of the entire thermodynamic cycle will be considered by all REVCO2 partners to gather data on the system behavior away from the nominal conditions and in the reverse case. From these data, a performance map covering different operating temperatures and partial-load operations will be generated by CETHIL based on a machine learning approach. The reduced model will be used to optimise the design of the complete system in conditions representative of the targeted applications: thermo-economic criteria estimated from an advanced exergy analysis will be minimized.