Long duration energy storage systems with 8-12 hours of capacity are one of the best options to reduce the increasing curtailment of renewable energy, being able to provide intra-day storage. Among those systems, Carnot batteries operating with CO₂ can be a promising solution due to the relatively high round trip efficiencies (up to 60%), being site-independent, including the possibility to store and sell both cold and hot thermal power in addition to electricity. In this work, the detailed sizing of the main components of a CO₂ Carnot battery is proposed: in particular, an interesting and promising feature of the system is represented by in the adoption of the same heat exchangers during the charging and the discharging phase, while the cold and hot storage systems are inspired by the commercial solution proposed by Echogen Power Systems. Specifically, the hot storage consists of two heat transfer fluid loops: a pressurized water loop and a diathermic oil loop, requiring two different insulated tanks, whereas the cold storage is based on an ice slurry tank. A routine in MATLAB has been developed to properly design the system and optimize its main variables to maximize the round-trip efficiency and minimize the storage costs, but also including the calculation of the levelized cost of storage. The battery is simulated with a cold storage at 0°C and hot storage between 71.6°C and 293.7°C, with the cycle maximum pressure of 250 bar and a round-trip efficiency of 54.6%. The specific capital cost of the system is 2033 €/kW_el,ch, with the largest share being the storage systems. The levelized cost of storage is estimated around 0.1 and 0.3 €/kWh, depending on the electricity selling price, and a relatively large internal temperature difference in the hot storage (20°C) is suggested for these systems.

The European Commission promotes technologies that support economic growth decoupled from the use of fossil fuels and promote carbon emissions mitigation. Its objectives for 2050 include the achievement of greenhouse gas neutrality, energy transition towards renewable and sustainable sources (e.g. solar, wind, hydro, etc.) and replacement of fossil fuels and feedstocks. These ambitious goals should be achieved by promoting research and technological development in clean energy, industrial efficiency and innovative technologies. One promising technology addressing these challenges is a large-scale CO₂-based electrothermal energy and geological storage system. When excess renewable energy is available, the system acts as a heat pump, storing electrical energy in the form of heat at two temperature levels and as mechanical energy in CO₂ injected in geological formations. The trigeneration system allows flexible coverage of electricity, heating or cooling demand, compensating for the temporary mismatch between renewable energy supply and demand. The use of CO₂ as a working fluid makes the system potentially integrable in carbon capture, utilisation and storage (CCUS) processes. The pressure (30-200 bar) and low-temperature conditions of transcritical CO₂ cycles are compatible with the transport of captured CO₂ under supercritical conditions. This work analyses different integration options for transporting and conditioning CO₂ captured in stationary sources to conditions suitable for injection in geological formations. Options for transport as liquid CO₂ in storage tanks and supercritical CO₂ by pipeline are considered. Different scenarios where the CEEGS system could complement the process are studied, and the cycle behaviour and impact are evaluated. CEEGS system is positioned as suitable technology in the process of using and conditioning the captured CO₂ to transport as well as geological injection conditions. The energy storage concept of CEEGS can present strong synergies with the implementation process of carbon capture plants, allowing the captured CO₂ to be conditioned through a cycle of energy storage and power production from renewable sources with roundtrip efficiencies higher than 50%, and the flexible coverage of electrical or thermal demands (heating and cooling). Renewable energy could be used for the energy needs of the entire process of conditioning and transporting the captured CO₂.

The management of electrical energy represents a significant challenge that must be overcome, given that electricity must be consumed immediately upon production. In this regard, an innovative energy storage solution is proposed, employing a heat pump with a supercritical Brayton cycle using pure CO₂ and CO₂–based mixtures to enhance the system's performance. This analysis encompasses a techno-economic, energetic, entropic and exergetic study, with consideration given to the levelized cost of storage (LCOS).

This study is concerned with the effect of binary mixtures based on pure CO₂ on the round-trip efficiency and the levelized cost of storage (LCOS), taking into account the irreversibilities associated with each component of the cycle. The methodology employed in the calculation of the plant performance entails the optimisation of the parameters of work of the components within the cycle under study. The simulations presented here are based on a code developed in MatLab. It employs a Python wrapper that enables access to a database of the working fluid's thermodynamic properties in REFPROP.

Based on exergetic and entropy analysis of the cycle studied, a comparison between pure supercritical carbon dioxide and sCO₂ mixtures is carried out. The initial results indicate that the blends result in a lower LCOS compared to the standard fluid in the cycle studied.

The management of electrical energy represents a significant challenge that must be overcome, given that electricity must be consumed immediately upon production. In this regard, an innovative energy storage solution is proposed, employing a heat pump with a supercritical Brayton cycle using pure CO₂ and CO₂–based mixtures to enhance the system's performance. This analysis encompasses a techno-economic, energetic, entropic and exergetic study, with consideration given to the levelized cost of storage (LCOS).

This study is concerned with the effect of binary mixtures based on pure CO₂ on the round-trip efficiency and the levelized cost of storage (LCOS), taking into account the irreversibilities associated with each component of the cycle. The methodology employed in the calculation of the plant performance entails the optimisation of the parameters of work of the components within the cycle under study. The simulations presented here are based on a code developed in MatLab. It employs a Python wrapper that enables access to a database of the working fluid's thermodynamic properties in REFPROP.

Based on exergetic and entropy analysis of the cycle studied, a comparison between pure supercritical carbon dioxide and sCO₂ mixtures is carried out. The initial results indicate that the blends result in a lower LCOS compared to the standard fluid in the cycle studied.