Do you know that the subway you usually take can also store energy?

Metro energy storage is to recover energy when the subway brakes at the station, and store this energy on a high-speed rotating flywheel device. When the subway train is about to start at the station, the flywheel energy storage system will output energy to the subway.

In urban rail transit, such as subway, the distance between stations is not long, and the number of train braking and starting is relatively frequent. The energy generated by train braking is often wasted. If this part of energy can be recycled, the energy generated by rail transit train braking can reach 20% - 40% of the energy consumption of traction system, reducing the operation energy consumption of rail transit. The first megawatt flywheel energy storage device in China was put into use in Qingdao Metro Line 3, which is a set of energy storage system with independent intellectual property rights.

What is flywheel energy storage?

Flywheel energy storage is a mechanical energy storage method, which uses the motor to drive the flywheel to rotate at a high speed to convert electric energy into kinetic energy for storage, and then uses the flywheel to drive the generator to generate electricity when needed to convert kinetic energy into electric energy.

The metro train provides DC power through the DC bus, which is equivalent to the power supply of the metro, and the flywheel system is related to the DC bus. When the train arrives at the station, the voltage of the bus bar rises, which provides power for the flywheel system. The flywheel system absorbs energy, converts electrical energy into kinetic energy, and stores it. The speed is up to 20000 revolutions per minute. When the train starts, the bus voltage drops, and the flywheel system releases energy to convert kinetic energy into electric energy, which is fed back to the bus for use by the train.

Since the flywheel energy storage system has a certain weight, in order to reduce the burden of the train, the flywheel energy storage system will be made into modular products and installed at each subway station. Generally, a subway station needs to be equipped with 1-2 flywheel energy storage systems.

When the subway enters the station, it will generate energy loss. The "flywheel" is like an energy saving system, which recovers the previously lost energy and feeds it back to the train again to achieve energy conservation. If the subway runs every three to five minutes, each subway can save 3-4 kilowatt hours of electricity, and 500-600 kilowatt hours of electricity can be saved in a day. It is conservatively estimated that the flywheel energy storage system can help a subway station save about 500000 kilowatt hours of electricity a year.

Flywheel device relies on mechanical device for energy conversion and has a long service life. Theoretically, the flywheel itself can be used indefinitely, but its electronic bearings and power electronic devices need regular maintenance and replacement. Power electronic devices need to be maintained after 10 years of use, while electronic bearings can operate normally for 20 to 30 years under good conditions.

| The global flywheel energy storage accounts for only 0.22% of the installed energy storage capacity, and there is huge room for future development

Flywheel energy storage is a kind of energy storage technology with high power density, high reliability, long life and environmental friendliness. In addition to being used in the field of rail transit, it can also be used in electric energy, petrochemical, shipbuilding and other industries, power grid frequency modulation, UPS and other fields. At present, the market penetration rate is still low, and there is huge room for future development. Flywheel energy storage technology was first used in the aerospace field. As the integration of satellite attitude control and energy storage, its key technology has gradually been transformed into the civil field. According to the data of the Energy Storage Special Committee of China Energy Research Association, by the end of 2021, the global flywheel energy storage accounts for only 0.22% of the installed energy storage capacity, and there is huge room for future development.

Flywheel energy storage is used for UPS. In the power system, there are a large number of users with high requirements for power quality, such as semiconductor manufacturing industry, bank computer system, communication system, hospital precision medical equipment, etc. In case of interruption of external power grid or abnormal power supply quality, flywheel energy storage UPS can be equipped to ensure continuous and reliable power supply for these users. At present, the flywheel energy storage UPS products produced by Active Power of the United States have been widely used by major data centers, telecom operators, etc. in the world. The domestic flywheel energy storage UPS product market is in the initial stage, and the 200kW flywheel energy storage device produced by the National Machinery Corporation has been put into use in Deyang Branch of Sichuan Mobile.

In the operation cost of urban rail transit such as subway, the electric charge accounts for about 40%, and the train traction power accounts for 45% - 60% of the total electric power cost of subway operation, of which 17% is recyclable regenerative braking energy. In the future, more cities in China are expected to use flywheel energy storage system as energy-saving equipment, with a market space of about 20.4 billion yuan.