Abstract:Power-to-gas is a chemical energy storage technology that converts electrical energy into combustible gas with high energy density. Combined with subsurface energy storage, it is expected to meet the demand for a large capacity of energy storage, and it will become one of the most effective energy storage technologies in the future. Power-to-gas based subsurface energy storage includes four stages of hydrogen production from electrolysis, methanation, geological storage of carbon dioxide (CO₂), and geological storage of synthetic methane (CH₄). Since the development of power-to-gas based geological energy storage technology is at a preliminary stage and involves multiple technical aspects, the overall economic analysis is relatively rare. It is necessary to carry out an economic analysis and further explore the application potential of this technology in China. Based on a large amount of literature research, a technical and economic analysis of electrolysis and methanation are firstly carried out, and then the technical and economic analysis of the geological storage of CH₄ and CO₂ are carried out respectively, and finally the whole process is statistically analyzed according to the estimated results under different assumptions and conditions. The results are compared with pumped hydro storage and compressed air energy storage, and the technical cost of power-to-gas based geological energy storage is clarified. The application opportunities and challenges of this technology in China are pointed out. It is found that alkaline electrolysis (AEL) currently has advantages at lower investment costs, which is about 7850 yuan/kW, the investment cost of polymer electrolyte membrane (PEM) is expected to reduce the investment cost to about half of the current AEL in 2050. The biological methanation and isothermal catalytic methanation technology is still in the experimental and demonstration stage. The adiabatic fixed-bed methanation technology has more technical advantages because of its more mature commercial applications. It is expected that the cost will be reduced to the current 50%～60% in 2050. The cost of geological CH₄ storage fluctuates greatly, because the cost is affected by different reservoirs and various operating parameters. However, based on published data, the investment cost of energy storage in aquifers is equivalent to that of energy storage in depleted oil or gas fields, which fluctuates in the range of 1.88～3.30 yuan/m³. The investment cost of geological CH₄ storage in salt caverns is about twice that of the other two geological reservoirs. CO₂ geological storage costs varies widely, except for individual high-cost cases, the storage cost is usually less than 53.38 yuan/t. At present, the opportunities and challenges for the application of power-to-gas based geological energy storage in China coexist. However, the innovations of electrolysis and methanation technology in the future, the optimizations of geological energy storage efficiency, mode, site selection, the improvements of integration method with existing gas storage sites, natural gas pipeline network, power grid, and the impacts of low-carbon policy, will enhance the possibility of applying power-to-gas based geological energy storage technology in China.