Brayton Cycle Using s-CO2 Mixtures as Working Fluid for Pumped Thermal Energy Storage: Exergy and Cost Analysis

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 Recompression Brayton cycle (RBC) utilizing pure s-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 s-CO2 on the round-trip efficiency and LCOS, considering the irreversibilities associated with each component of the cycle. The methodology employed in the calculation of the plant performance entails the optimization of the parameters of work of the components within the cycle under study. The present study has utilised simulations which have been based on code developed in the programming languages MatLab and Python. The findings of the present study demonstrate that the mixture gives rise to a lower LCOS in comparison to pure s-CO2 (151.11 $/MWh). The lowest LCOS is observed in CO2/Xe, with a value of 138.26 $/MWh. Furthermore, mixtures such as CO2/Kr and CO2/H2S have been observed to exhibit LCOS values of 142.13 $/MWh and 146.64 $/MWh, respectively. In addition, the round-trip efficiency of pure s-CO2 is estimated to be approximately 57.4%, which is outperformed by CO2/Kr and CO2/H2S mixtures, which achieve round-trip efficiencies of 59.7% and 58.3%, respectively. However, when employing a CO2/Xe mixture within the cycle, a round-trip efficiency analogous to that of pure s-CO2 is realised.