Techno-economic Analysis of Hydrocarbon-CO 2  Binary Mixtures in Heat Pump-based Thermal Energy Storages

The transition to renewable energy increases the demand for efficient energy storage technologies, such as Carnot batteries (CB), to balance supply and demand. This study investigates the heat pump for CB charging starting at ambient temperature and operating with CO2-containing fluid mixtures. Thermodynamic optimization of operating conditions is performed within the pressure range of commercially available compressors and hot storage temperatures of 363.15-403.15 K. Butane-CO₂ and isobutane-CO₂ mixtures containing 15-20% CO₂ achieve an optimal coefficient of performance of approximately 3.6, corresponding to a 70 K hot-storage temperature lift. Pentane-CO₂ and isopentane-CO₂ mixtures reach 403.15 K but operate with lower efficiency. Exergy analysis shows that while mixtures reduce heatexchanger irreversibility, throttling losses increase with higher CO₂ content, with relative exergy destruction in the expansion valve reaching ~50% at 45% CO₂. Although increasing CO₂ mitigates flammability, it penalizes performance and raises system costs. Economic evaluation indicates that butane-CO₂ and isobutane-CO₂ mixtures require specific investment costs (SIC) of 1200-1400 $ kW⁻¹ and a levelized cost of heat (LCOH) of 0.16-0.18 $ kWh⁻¹ at 900 operating hours per year. Both SIC and LCOH are more sensitive to mixture composition than to storage temperature.