Performance evaluation of a novel high drop composite power/storage system for low-temperature thermal energy utilization and storage

With the increasingly severe global climate issues and high targets for carbon emissions, the efficient and rational utilization, as well as storage, of low-temperature thermal energy such as solar energy, geothermal energy and industrial waste heat is crucial for achieving energy conservation and emission reduction targets. In order to solve the problems of high additional energy consumption, limited geographical environment, and significant transmission losses in traditional gravity energy storage systems, this paper first proposes a high drop composite power/storage (HD-CPS) system, which can achieve efficient utilization of low-temperature thermal energy for power generation while further storing the utilized thermal energy by thermal-to-gravitational energy potential conversion. In this paper, a series of performance evaluation indicators for the “energy charging-power generation” stage and the “energy discharging-power generation” stage were established. And the carbon reduction benefit, working fluid selection, and system performance were analyzed. The results indicated that the carbon reduction benefit of this system was 1.22, approximately 8 times that of conventional large-scale pumped storage systems (a typical gravity energy storage system), reflecting the superiority of this system in carbon reduction. To select the optimal working fluid, the defined total energy storage efficiency was taken as the optimization objective, which concluded that R1234ze(Z) was superior to others. The total energy storage efficiency of R1234ze(Z) was about 28%-86% higher than other working fluids such as R236ea and R141b at the cold source temperature of 10°C. And the maximum thermal efficiency of the R1234ze(Z) system can reach 13.77% during the “energy charging-power generation” stage. In conclusion, the proposed HD-CPS system has a positive effect on broadening the utilization and storage forms of low-temperature thermal energy, which has great application potential in high drop buildings located in industrial parks or large giant mining ships that contain low-temperature heat sources and high drop building structures, and the low-temperature heat sources can come from industrial waste heat, solar energy, geothermal energy, etc.