Detailed description: five technical routes for grid energy storage

Because of the gap and variability of renewable energy generation and the continuous improvement of its permeability, it has a certain degree of impact on the normal operation and dispatching of the grid, in order to utilize as much renewable energy as possible and improve the reliability of the grid operation. And efficiency, various energy storage technologies have developed rapidly.

Electrical energy storage technology is divided into five categories: mechanical energy storage, electrical energy storage technology, electrochemical energy storage technology, thermal energy storage technology and chemical energy storage technology. At present, the highest proportion of electric energy storage capacity in the world power grid is pumped storage. Its total installed capacity has reached 127GW, accounting for 99% of the total energy storage capacity, followed by compressed air energy storage, with a total installed capacity of 440MW, ranking third. The sodium-sulfur battery has a total capacity of 316 MW.

Power grid technology

1. Mechanical energy storage

Mechanical energy storage mainly includes pumped storage, compressed air energy storage and flywheel energy storage. The pumped storage needs to use the excess power as the liquid energy medium when the grid is low, and pump it from the low-lying reservoir to the high-lying reservoir. During the peak load, the water in the high-lying reservoir returns to the lower reservoir to promote the turbine generator to generate electricity. Therefore, the pumped storage has a certain dependence on the terrain environment, and the efficiency is generally 65%-75%, and the highest can reach 80%-85%. However, it has daily adjustment capability and is suitable for nuclear power plants, large-scale wind power generation, and ultra-large-scale solar photovoltaic power generation.

2, electrical energy storage

Electrical energy storage technology mainly includes supercapacitor energy storage and superconducting energy storage. Among them, supercapacitor energy storage is generally suitable for use with other energy storage methods because of its low energy density, and can be used in microgrid and energy storage battery. Combined use, it can also be used for the start-up acceleration of electric vehicles; superconducting energy storage is mostly experimental, and the technology needs further breakthrough.

3. Electrochemical energy storage

Electrochemical energy storage mainly includes lead-acid batteries, lithium-ion batteries, sodium-sulfur batteries and flow batteries.

(1) Lead-acid batteries are currently the most widely used in the world, with a cycle life of up to 1000 times, efficiency of 80%-90%, high cost performance, and often used in power system accident power or backup power, but if deep, fast At high power discharge, the available capacity will decrease. It is characterized by low energy density and short life. Lead-acid batteries can increase their cycle life by adding super-active carbon materials to the negative plates of lead-acid batteries this year.

(2) Lithium battery is mainly used in portable mobile devices, its efficiency can reach more than 95%, discharge time can reach several hours, the number of cycles can reach 5000 times or more, and the response is fast, he is the most energy-efficient in battery. In recent years, the technology has been continuously upgraded. There are many applications for positive and negative materials. However, there are high prices (4 yuan/ wh), and overcharge can cause safety problems such as heat generation and combustion, and charging protection is required. With the research and development of lithium batteries at home and abroad, its performance has been continuously improved, and the cost has also dropped significantly. It is expected to be more widely used in the future.

(3) Sodium-sulfur battery, the anode is composed of liquid sulfur, the cathode is composed of liquid sodium, and the beta-aluminum tube with ceramic material is isolated. The operating temperature of the battery needs to be kept above 300 degrees Celsius to ensure that the electrode is in a molten state and the cycle can be reached. 4500 times, discharge time 6-7 hours, cycle round-trip efficiency 75%, high energy density, fast response time. At present, more than 200 such energy storage power stations have been built in Japan, Germany, France, and the United States, mainly for load leveling, peak shifting, and improved power quality. However, because of the use of liquid sodium, it operates at high temperatures and is easy to burn.

(4) The energy storage battery can be stored in an electroactive species dissolved in a liquid electrolyte, and the liquid electrolyte is stored in a tank outside the battery, and the electrolyte stored in the tank is pumped into the battery stack through the electrode and A film that converts electrical energy into electrochemical energy or converts electrochemical energy into electrical energy. The power and energy of the battery are irrelevant. The energy stored depends on the size of the storage tank, so it can store energy for hours to days, up to MW. This battery has multiple systems, such as iron-chromium system, zinc-bromine system, sodium polysulfide bromine system and all-vanadium system. Among them, all-vanadium redox flow battery is the most concerned, and it is the main technical development direction. It is characterized by mature technology. Long life, the number of cycles can exceed 10,000 times, but the energy density and power density are lower than lithium batteries, and the response time is not fast.

4. Thermal energy storage

In a thermal energy storage system, thermal energy is stored in the medium of the insulated container, converted back to electrical energy when needed, or directly used instead of being converted back to electrical energy. Thermal energy storage is divided into sensible heat storage and latent heat storage. The heat storage can store a large amount of heat, so it can be used to generate electricity from renewable sources.

5. Chemical energy storage

Chemical energy storage is the use of hydrogen or synthetic natural gas as a secondary energy carrier. Using excess electricity to produce hydrogen, it can directly use hydrogen as a carrier of energy, or it can react with carbon dioxide to synthesize natural gas (methane) because of hydrogen or Methane as an energy carrier can store a large amount of energy, up to TWh level, and it takes a long time to store. In addition to hydrogen or synthetic natural gas, it can be used for power generation, and other uses such as transportation. Germany is keen to promote this technology and has demonstration projects put into operation. The disadvantage of chemical energy storage is that its full cycle efficiency is low, hydrogen production efficiency is only 40%, and the efficiency of synthetic natural gas is less than 35%.

In the five types of electric energy storage technology, comprehensive comparison is made according to the characteristics of various energy storage technologies to select appropriate technologies. The available indicators mainly include: energy density, power density, response time, energy storage efficiency, and equipment life. (year) or number of charge and discharge, technology maturity, economic factors (investment costs, operation and maintenance costs), and safety and environmental considerations, and then select the type of energy storage, installation location, capacity, and each according to the purpose and needs of the application. The cooperation of various technologies. Among these five types of energy storage methods, batteries in which electrochemical energy storage is developed are very rapid.

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