A cobalt oxides-based redox energy storage system hybridised with a supercritical CO2 Brayton cycle

S. Wu, C. Zhou, E. Doroodchi, B. Moghtaderi
The University of Newcastle,
Australia

Keywords: energy storage, thermochemical, metal oxide, CO2 Brayton

Summary:

The study proposes an innovative grid-scale energy storage system which consists of a CO3O4/CoO thermochemical redox cycle and a supercritical CO2 Brayton power cycle. The redox cycle has an ability to store and release energy via its reduction and oxidation reactions, respectively. This thermochemical energy storage method is featured with a high operating temperature, a significantly large energy storage density and the generation of a high value-added by-product - oxygen. In the supercritical CO2 Brayton power cycle, CO2 is compressed to a supercritical state using off-peak/surplus electricity and stored in a high-pressure storage tank. During peak periods, the stored CO2 participates in the oxidation reaction cycle, absorbing most of the reaction heat, and subsequently expands in a CO2 turbine for power generation. In this way, the supercritical CO2 Brayton cycle can work at an upper temperature near the oxide transition point with a high energy conversion efficiency. The energy storage performance of the proposed hybrid system is examined thermodynamically using the simulation package - Aspen Plus v10. The results show that the proposed system can achieve a maximum round-trip efficiency of around 50% with an energy storage density of 50 kWh/m3. The obtained round-trip efficiency is found to be 10-20% higher than those of the conventional thermochemical energy storage cycles integrated with a steam Rankine or air Brayton cycle. This is partly due to the energy saving realised by compressing pure CO2 instead of air. Meanwhile, the oxygen production is found to reach 202 Nm3 per MWh. The sensitive analysis demonstrates that the round-trip efficiency is mostly sensitive to the isentropic efficiency of the turbine and turbine inlet temperature, followed by the supercritical CO2 pressure.