Comparative study of an innovative coldly integrated pumped thermal electricity storage system: Thermo-economic assessment and multi-objective optimization

Energy storage is an important means of solving the instability of renewable energy sources. As a novel energystorage technology, thermally integrated pumped thermal electricity storage systems have gained considerable attention owing to their capacity to enhance the power-to-power efficiency of pumped thermal electricity storage systems by integrating low-grade heat sources. However, previous studies have predominantly focused on the integration of low-grade heat sources during the charging phase. In this paper, a novel coldly integrated pumped thermal electricity storage system that integrates liquefied natural gas on the discharge side is proposed. Seawater and R1233zd(E) were utilised as the system's heat source and working fluid, respectively. The performance of the proposed system was compared with that of a previous thermally integrated pumped thermal electricity storage system that used geothermal energy as the heat source on the charge side. System models were constructed in MATLAB, and thermodynamic and economic comparative analyses were conducted. The Genetic Algorithm was adopted for the multi-objective optimisation of the two systems. The results indicate that, within given temperature ranges for thermal and cold storage temperatures, the proposed system exhibits a higher power-to-power efficiency and has a lower levelised cost of storage compared to the existing system. The optimal power-to-power efficiency and levelised cost of storage solutions obtained via multi-objective optimisation were 1.45, 0.297 $center dot kWh- 1 and 0.807, 0.411 $center dot kWh- 1 for the coldly integrated and thermally integrated pumped thermal electricity storage systems, respectively. The proposed system outperformed the thermally integrated pumped thermal electricity storage system under comparison in terms of thermodynamic and economic performance. The findings of this study shed light on the design and performance of coldly integrated pumped thermal electricity storage systems.