In order to make full use of renewable energy and improve the reliability and efficiency of the power grid, various energy storage technologies have been rapidly developed.
Mechanical grid energy storage mainly includes pumped hydro storage, compressed air energy storage, and flywheel energy storage. Among them, pumped hydro storage requires using surplus electricity during low-demand periods to pump water from a low-altitude reservoir to a high-altitude reservoir, and during peak demand periods, the water flows back to the lower reservoir to drive a turbine and generate electricity. Therefore, pumped hydro storage has a certain reliance on the topographical environment, with an efficiency generally ranging from 65% to 75% and a maximum of 80% to 85%. However, it has daily regulation capabilities and is suitable for supporting nuclear power plants, large-scale wind power generation, and ultra-large-scale solar photovoltaic power generation, among others.
Electrical grid energy storage technologies mainly include supercapacitor energy storage and superconductor energy storage. Supercapacitor energy storage, due to its low energy density, is generally suitable for use in combination with other energy storage methods. It can be used in microgrids in conjunction with energy storage batteries and can also be used for starting and accelerating electric vehicles. Superconductor energy storage is currently mostly experimental and further technological breakthroughs are needed.
Electrochemical grid energy storage mainly includes lead-acid batteries, lithium-ion batteries, sodium-sulfur batteries, and flow batteries.
Lead-acid batteries are currently the most widely used in the world, with a cycle life of about 1000 times, an efficiency of 80% to 90%, and a high cost-performance ratio. They are commonly used as accident or backup power sources in power systems.
Lithium-ion batteries are mainly used in portable mobile devices. They have an efficiency of over 95%, a discharge time of several hours, a cycle count of 5000 times or more, and a rapid response.
Sodium-sulfur batteries require the battery operating temperature to be maintained above 300 degrees Celsius to ensure that the electrodes are in a molten state. The cycle period can reach 4500 times, the discharge time is 6-7 hours, the round-trip efficiency is 75%, and it has high energy density and fast response time.
Flow batteries store energy in electroactive species dissolved in liquid electrolytes, while the liquid electrolyte is stored in external tanks. The electrolyte stored in the tank is pumped into the battery stack and converted into electrochemical energy through electrodes and membranes, or converted back into electrical energy.
In thermal energy storage systems, heat energy is stored in a medium in an insulated container and can be converted back to electricity when needed, or directly utilized without conversion. Thermal energy storage can be divided into sensible heat storage and latent heat storage. The heat stored in thermal energy storage can be significant, making it suitable for use in renewable energy generation.
Chemical grid energy storage utilizes hydrogen or synthetic natural gas as carriers of secondary energy. Excess electricity is used to produce hydrogen, which can be used directly as an energy carrier, or it can react with carbon dioxide to produce synthetic natural gas (methane). Hydrogen or methane, as energy carriers, can store a large amount of energy, reaching the TWh level, and have a long storage time. In addition to power generation, hydrogen or synthetic natural gas can be used in other applications such as transportation. Germany is keen on promoting this technology and has demonstration projects in operation. The disadvantage of chemical storage is its low overall efficiency, with hydrogen production efficiency only at 40% and synthetic natural gas efficiency less than 35%.
Among the five types of grid energy storage technologies, a comprehensive comparison based on the characteristics of various energy storage technologies is usually conducted to select the appropriate technology. The selectable indicators mainly include energy density, power density, response time, energy storage efficiency, equipment lifespan or charge-discharge times, technological maturity, economic factors (investment cost, operation and maintenance expenses), as well as safety and environmental considerations. Based on the purpose and requirements of the application, the type of energy storage, installation location, capacity, and various technologies are selected accordingly.