Thermo-economic examination of ocean heat-assisted pumped thermal energy storage system using transcritical CO2 cycles

Abstract

In response to the challenges posed by the growth of renewable energy electricity to the security of the power grid, energy storage technology has rapidly developed in recent years. Pumped thermal energy has been recognized as an attractive massive energy storage technology with the superiority being independent of site. An ocean heat-assisted pumped thermal energy storage system using transcritical CO2 cycles is proposed in this study. State-of-the-art thermo-economic models are developed and described to simulate the system processes and conduct cost estimates. The originality of this system lies in the ability to effectively improve its efficiency by configuring seawater at different depths at the low temperature end of the system. Research has found that, if the seawater used in the charge process always comes from the surface of the ocean, the system efficiencies are 48.1 %, 61.3 % and 66.4 % respectively when the depth of seawater during the discharge process are 50 m, 100 m and 1000 m. When the seawater employed in the entire system comes from 1000 m underwater, the efficiency of the system is only 53.5 %. In addition, for the 50 MW/10 h system, the minimum value of levelized cost of storage is 0.143 $/kWh. The uncertainty analysis of cost for the proposed system is carried out using the Monte Carlo method. Results show that with the charge/discharge duration ratio rising from 4 h/10 h to 10 h/4h, the total capital cost falls from 245 ± 76 M$ to 102 ± 34 M$ and the levelized cost of storage reaches the minimum value when this ratio is 1. The larger the energy storage capacity, the lower the levelized cost of storage. In summary, the system has shown satisfactory results in both thermodynamics and economic performance, and further research is worthwhile in the future.