Liquid CO2 and Liquid Air Energy Storage Systems: A Thermodynamic Analysis

Energy storage is a key factor to confer a technological foundation to the concept of energy transition from fossil fuels to renewables. Their solar dependency (direct radiation, wind, biomass, hydro, etc. horizontal ellipsis ) makes storage a requirement to match the supply and demand, with fulfillment being another key factor. Recently, the most attention is directed toward the direct electrical storage inside batteries, probably driven by interest in the transportation sector, which today is the main focus in the transition path. On the contrary, for the generation of electrical energy and, more generally, for industrial sectors whose CO2 emissions are defined as hard-to-abate, electrical storage is not a feasible answer to many political and non-technological concerns. Therefore, other storage methods must be considered to address excess electricity, the most characteristics of which being both the capacity and rate of charging/delivering. Among the efforts under consideration, the liquid storage of gases at ambient conditions is certainly an interesting option. This is the case with air and CO2. The paper focused on the storage of CO2 in liquid form, comparing its performance with those of air liquefaction, which well-studied in the literature. The paper proposed a novel plant layout design for a liquid CO2 energy storage system that can improve the round-trip efficiency by up to 57%. The system was also compared to a liquid air energy storage unit considering a state-of-the-art level of technology for components, showing better efficiency but lower energy density. Finally, a sensitivity analysis was used to discuss the most relevant variables for a plant design. Particular focus was devoted to the discharging time of the plant, one of the most relevant variables that matches the energy demand.