I. What are the classifications of energy storage systems?
In the analysis of energy storage processes, the portion of objects or space demarcated to define the research object is called the energy storage system. Currently, existing energy storage systems are mainly divided into five categories: mechanical energy storage, electrical energy storage, electrochemical energy storage, thermal energy storage, and chemical energy storage.
II. Advantages and disadvantages of various energy storage systems
1. Mechanical energy storage: mainly includes pumped hydro storage, compressed air storage, and flywheel energy storage.
(1) Pumped storage: During periods of low grid activity, excess electricity is used as a liquid energy medium to pump water from a low-lying reservoir to a higher-lying reservoir. During peak grid activity, the water in the higher-lying reservoir flows back to the lower reservoir to drive turbine generators to generate electricity. The efficiency is generally around 75%, commonly known as "4 in, 3 out," and it has daily regulation capabilities, used for peak shaving and backup. Disadvantages: Difficult site selection, extremely dependent on terrain; long investment cycle; high losses, including pumping losses and line losses.
(2) Compressed air energy storage: This utilizes surplus electricity generated during off-peak periods in the power system. An electric motor drives an air compressor to compress air into a large-capacity, sealed underground storage chamber. When the system's power generation is insufficient, the compressed air is mixed with oil or natural gas through a heat exchanger and burned, then fed into a gas turbine to generate electricity. Compressed air storage also has peak-shaving capabilities and is suitable for large-scale wind farms because the mechanical work generated by wind energy can directly drive the compressor, reducing intermediate steps in converting it into electricity and thus improving efficiency. Disadvantage: Lower efficiency.
(3) Flywheel energy storage: This method uses a high-speed rotating flywheel to store energy in the form of kinetic energy. When energy is needed, the flywheel slows down and releases the stored energy. Disadvantages: The energy density is not high enough, and the self-discharge rate is high. If charging stops, the energy will be depleted within a few to tens of hours.
2. Electrical energy storage: mainly includes supercapacitor energy storage and superconducting energy storage.
(1) Supercapacitor energy storage: It uses a double-layer structure composed of porous activated carbon electrodes and electrolytes to achieve ultra-large capacity. It has short charging time, long service life, good temperature characteristics, energy saving and environmental protection. Disadvantages: Compared with batteries, its energy density results in relatively low energy storage for the same weight, which directly leads to poor battery life. It relies on the emergence of new materials, such as graphene.
(2) Superconducting energy storage: This is a device that stores electrical energy by utilizing the zero resistance characteristic of superconductors. A superconducting energy storage system generally consists of four main parts: a superconducting coil, a cryogenic system, a power regulation system, and a monitoring system. Disadvantages: The high cost of superconducting energy storage (materials and cryogenic refrigeration system) greatly limits its application. Reliability and economic constraints further hinder its commercial application.
3. Electrochemical energy storage: mainly includes lead-acid batteries, lithium-ion batteries, sodium-sulfur batteries and flow batteries.
(1) Lead-acid battery: It is a type of battery whose electrodes are mainly made of lead and its oxides, and whose electrolyte is sulfuric acid solution. It is widely used, has a cycle life of about 1000 cycles, and an efficiency of 80%-90%, making it cost-effective. Its disadvantage is that its usable capacity will decrease if it is deeply, rapidly, or discharged at high power.
(2) Lithium-ion batteries: These are a type of battery that uses lithium metal or lithium alloy as the negative electrode material and a non-aqueous electrolyte solution. They can cycle up to 5,000 times or more, have a fast response, and are the most energy-efficient practical batteries. Disadvantages include high price (4 yuan/Wh), overcharging leading to overheating and combustion, and other safety issues requiring charging protection.
(3) Sodium-sulfur battery: This is a secondary battery that uses metallic sodium as the negative electrode, sulfur as the positive electrode, and a ceramic tube as the electrolyte membrane. It can achieve 4500 cycles, with a discharge time of 6-7 hours, a cycle efficiency of 75%, high energy density, and fast response time. Disadvantages: Because it uses liquid sodium, it operates at high temperatures and is prone to combustion.
(4) Flow battery: A high-performance battery that uses separate positive and negative electrolytes that circulate independently. It can store energy for hours to days, with a capacity of up to MW. Disadvantages: The battery is too large; the battery is too sensitive to ambient temperature; it is expensive and the system is complex.
4. Thermal energy storage
In thermal energy storage systems, thermal energy is stored in a medium within an insulated container. When needed, it is converted back into electrical energy, or it can be used directly without further conversion. Thermal energy storage is further divided into sensible heat storage and latent heat storage. Thermal energy storage can store a large amount of heat, so it can be used in renewable energy power generation. Its drawbacks include the need for various high-temperature chemical working fluids, which limits its application scenarios.
5. Chemical energy storage
Using hydrogen or synthetic natural gas as a secondary energy source, hydrogen can be produced from surplus electricity. It can be used directly as an energy carrier or reacted with carbon dioxide to produce synthetic natural gas (methane). Besides power generation, hydrogen or synthetic natural gas can be used in other applications such as transportation. However, its overall lifecycle efficiency is relatively low, with hydrogen production efficiency at only 40% and synthetic natural gas efficiency at less than 35%.
