Based on the different forms of energy storage, energy storage technologies can be divided into five categories: mechanical energy storage technology, electrochemical energy storage technology, electrical energy storage technology, chemical energy storage technology, and thermal energy storage technology.
◇Electrical energy storage technology
◇Chemical energy storage technology
Mechanical energy storage technology
As is well known, various forms of kinetic and potential energy exist in nature, such as flowing water, natural wind, tides, and waves; human activities also generate a great deal of kinetic and potential energy, such as moving people, vehicles, ships, and fluids. All of these energies, both those generated in nature and those generated by human activities, are renewable energy sources. Mechanical energy is the sum of kinetic and potential energy, a physical quantity representing the state of motion and altitude of an object. The kinetic and potential energy of an object can be converted into each other; in the process of mutual conversion between kinetic and potential energy, the total amount of mechanical energy remains constant, that is, mechanical energy is conserved.
Mechanical energy storage is a technology that converts energy into mechanical energy for storage and then converts it back into electrical energy when needed. Common mechanical energy storage methods include pumped hydro storage, compressed air storage, and flywheel energy storage. Mechanical energy storage technologies typically have high power density, fast response capabilities, and long service life, making them suitable for grid regulation and emergency power supply. Their storage time and scale vary depending on the specific technology, ranging from minutes to days, and can meet diverse energy storage needs.
Pumped hydro storage：
Pumped hydro storage is currently the most widely used large-scale energy storage technology. It uses electricity to pump water from a low-level reservoir to a high-level reservoir, storing its potential energy. During peak electricity demand periods, the water is released to generate electricity through turbines. This method has a relatively high conversion efficiency (typically 70%–85%), is suitable for regulating peak-valley differences in the power grid, and offers large storage capacity and stable operation.
Pumped hydro storage is often used to support renewable energy grid integration, balance supply and demand fluctuations, and possesses long storage duration and strong reserve capacity. Its principle is shown in Figure 1-1.
Compressed air energy storage：
Compressed air energy storage involves compressing air using an electrically driven compressor and storing it in underground caverns, tanks, or pressure vessels. When electricity demand increases, the stored compressed air is released, heated, and used to drive a turbine to generate electricity. Compressed air energy storage typically offers large-scale, long-term energy storage capabilities, with efficiencies generally ranging from 50% to 70%. These efficiencies can be further improved when combined with heat recovery technologies. It is suitable for integration with large-scale renewable energy power plants to enhance grid flexibility and stability.
Flywheel energy storage：
Flywheel energy storage uses a motor to drive a flywheel at high speed, converting electrical energy into kinetic energy for storage. When needed, the flywheel uses a generator to convert the kinetic energy back into electrical energy. Flywheel energy storage technology is known for its extremely fast response speed (typically in the millisecond range) and high cycle life (up to hundreds of thousands of cycles), making it suitable for short-term, high-power energy storage scenarios, such as grid frequency regulation and uninterruptible power supplies (UPS). Flywheel energy storage typically has a high conversion efficiency, reaching 85%–95%, but its storage time is relatively short, usually used to balance short-term power fluctuations. Figure 1-2 shows the schematic diagram of a flywheel energy storage power system.
Electrochemical energy storage technology
Electrochemical energy storage is a technology that converts electrical energy into chemical energy through electrochemical reactions, storing it and then converting it back into electrical energy when needed. Its core is the storage and release of energy through the charging and discharging process of batteries. Electrochemical energy storage technology has advantages such as fast response speed, high efficiency, flexible installation, and modular design, making it suitable for scenarios such as renewable energy grid-connected frequency regulation, peak-valley regulation, and emergency power supply. Currently, mainstream electrochemical energy storage technologies include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, sodium-ion batteries, and flow batteries, each with its unique performance, application scenarios, and development potential. With the increasing proportion of renewable energy, electrochemical energy storage plays a crucial role in the global energy structure transformation and is an important guarantee for achieving a clean, low-carbon, and safe energy system.
Lead-acid Batteries：
Lead-acid batteries are a long-established and widely used electrochemical energy storage technology. Their principle involves using lead and its oxides as positive and negative electrode materials, and sulfuric acid aqueous solution as the electrolyte, to charge and discharge through an electrochemical reaction. Lead-acid batteries have advantages such as low production cost, mature technology, high reliability, and strong resistance to overcharge and over-discharge, and are widely used in automotive starting batteries, backup power supplies, and energy storage systems. However, lead-acid batteries have low energy density, limited cycle life, and contain toxic lead, which can pollute the environment if improperly disposed of. Despite this, lead-acid batteries still occupy a significant position in certain fields, especially in cost-sensitive applications. In the future, environmentally friendly recycling and performance improvement of lead-acid batteries will be key directions for the development of this technology.
Nickel-Metal Hydride (NiMH) Batteries：
NiMH batteries are an electrochemical energy storage technology that uses nickel hydroxide as the positive electrode and nickel hydride as the negative electrode. They offer advantages such as high energy density, environmental friendliness, and long cycle life. Compared to traditional batteries, NiMH batteries do not have the chemical hazards associated with cadmium and molybdenum, making them more environmentally friendly. Therefore, they are widely used in power tools, hybrid vehicles, and portable electronic devices. They also have high charge-discharge efficiency and can operate stably in various environments. A key characteristic of nickel batteries is their strong overcharge and over-discharge tolerance, making them excellent in applications requiring frequent charge and discharge. Although the rise of lithium-ion batteries in recent years has led to a decline in the market share of NiMH batteries, they still maintain a place in specific application areas.
Lithium-ion Batteries：
Lithium-ion batteries are an electrochemical energy storage technology that achieves charging and discharging through the insertion and extraction of lithium ions between the positive and negative electrodes. Lithium's light weight and high energy density have led to the widespread application of lithium-ion batteries in portable electronic devices, electric vehicles, and renewable energy storage. Lithium-ion batteries offer advantages such as long cycle life and no memory effect, but they also have certain safety concerns, such as thermal runaway caused by overcharging and over-discharging. With technological advancements, the safety and electrochemical performance of lithium-ion batteries have continuously improved, while costs have decreased, making them one of the most widely used energy storage batteries on the market today. In the future, the development of technologies such as solid-state electrolytes and silicon-based anodes is expected to further improve the electrochemical performance and safety of lithium-ion batteries.
Sodium-ion Batteries：
Sodium-ion batteries are a rapidly developing new electrochemical energy storage technology in recent years. Their working principle is similar to that of lithium-ion batteries, where lithium ions intercalate and deintercalate between the positive and negative electrodes for charging and discharging. The advantages of sodium-ion batteries lie in the abundance and low cost of sodium resources, and their independence from lithium resource limitations, making them particularly suitable for large-scale energy storage applications. Although their energy density is lower than that of lithium-ion batteries, sodium-ion batteries exhibit good performance in terms of cycle stability, low-temperature performance, and safety, showing great promise for future development. Currently, research on sodium-ion batteries focuses on improving energy density, extending cycle life, and reducing manufacturing costs. With continuous technological advancements, sodium-ion batteries are expected to become one of the important solutions for large-scale energy storage in the future.
Flow battery：
Flow batteries are an electrochemical energy storage technology where the electrolyte is stored in an external tank. Their principle involves the storage and release of energy through electrochemical reactions between two different electrolytes within the battery. A significant characteristic of flow batteries is their independently adjustable energy and power output; the storage capacity can be expanded by increasing the amount of electrolyte stored, making them particularly suitable for large-scale, long-term energy storage applications. Common types of flow batteries include vanadium redox flow batteries and zinc/bromine flow batteries. Flow batteries offer long lifespan, good safety, and environmental friendliness, but their initial investment is high, and the battery system is complex. With technological advancements, the potential of flow batteries in large-scale energy storage is gradually emerging, especially in renewable energy grid integration and grid regulation applications.