Simulation of supercritical carbon dioxide cycles for electrothermal energy storage

Abstract

An up to date synopsis of the existing electrothermal energy storage systems is presented in this thesis. The different concepts are structured accordingly to the working fluid and particularly emphasis is laid on systems that utilize transcritical carbon dioxide cycles. Further a brief overview of thermodynamic gas power cycles and the advantages of carbon dioxide as a working fluid are described. Also an introduction to organic Rankine cycles is given and the disadvantages and advantages over steam cycles are compared. Additionally the used thermal energy storage system is presented and the heat transfer between the media sand and carbon dioxide is analysed. A plethora of electrothermal energy storage systems are calculated in the process simulation software IPSEpro. First simulations feature simple Brayton and re-compression cycles, while also the utilization of waste heat and the usage of an isothermal expander during the charge cycle are proposed. Furthermore the integration of a carbon dioxide transcritical organic Rankine cycle to improve the efficiency of the system is examined. Main findings include that the best round trip efficiency is provided by systems with an isothermal expander which are site-independent, while the usage of waste heat also provides high efficiency values but makes the plant site-dependent. It was shown that the limiting factor to achieve an electrothermal energy storage system with high round trip efficiency are the high exergy losses in the thermal energy storage unit.