Due to the characteristics of safety, environmental protection, energy saving and less land occupation, rail transit has gradually become the main mode of transportation for people’s travel, and has developed rapidly in recent years. Energy storage technology also has a place in the development of the rail transit industry. Trains can store electrical energy through energy storage technology and release electrical energy in the event of no contact or emergency to ensure normal running. Subway and intercity high-speed rail are widely used.
Subway energy recovery is a high-power and high-frequency application scenario. Currently, the most widely used is regenerative braking energy absorption and utilization. When the braking energy cannot be absorbed by the vehicle, the traction network voltage rises. After rising to a certain level, the regenerative braking energy absorption device in the traction substation is put into operation to absorb the regenerative current and stabilize the regenerative current of the vehicle. At present, regenerative energy absorption devices can be divided into resistance consumption type, capacitor energy storage type, flywheel energy storage type, and Inverter feedback type. The comparison of each type of device is shown in the table. At present, capacitor energy storage type and flywheel energy storage type are more commonly used.
Comparison of various types of regenerative energy absorption devices
Flywheel energy storage
The electric energy recovered by the subway train entering the station is consumed by resistance heat release, which is a waste of resources. The flywheel energy storage has the advantages of fast response, high frequency, high reliability and long life, which can solve these problems well.
In 2019, the domestic GTR flywheel energy storage device was officially put into commercial use at Guangyangcheng Station on the Fangshan Line of the Beijing Subway. The device consists of a single flywheel with a power of 333 kW and one group of three to form a 1 MW flywheel. When the subway train enters the station and brakes, the flywheel energy storage system can be used to store the energy in the braking process.
Several subway stations in the United States have demonstrated flywheel energy storage, which can achieve an energy saving effect of 20% to 30%. The Los Angeles Metro installed a flywheel-based wayside energy storage substation (WESS) in August 2014. WESS deployed a 2 MW system with a charge/discharge time of 15 s and a capacity of 8.33 kW h, consisting of 4 flywheel modules. Each module consists of 4 independent flywheel units. After the application of the energy storage system, 10% to 18% of the traction power energy can be saved every day.
Supercapacitor energy storage
Supercapacitor energy storage has the characteristics of high power and long life, and it can also recover braking energy and realize the reuse of braking energy. At present, this technology has just started in China and is still in the experimental stage. For example, Guangzhou Metro Line 6 is equipped with a supercapacitor energy storage device and is officially running on the grid. The rated power of the system reaches 1.4 MW, which has the function of stabilizing the DC grid. Alleviate DC voltage fluctuations when high-frequency trains start or brake. The supercapacitor energy storage system consists of connecting units and converter supercapacitors, and has been applied in subways in many cities around the world, such as the ground-mounted energy storage system in Madrid, Spain. Beijing Metro Line 5 is also equipped with a super capacitor energy storage system with a total capacity of 69.64 F and a maximum voltage of 515.2 V, which has the characteristics of reducing traction power and improving subway comfort.
Intercity high-speed rail
The main functions of the intercity high-speed rail energy storage system are to reduce energy consumption, to draw trains, to recover braking energy, and to reduce peak power. The energy storage system in the railway industry is mainly divided into ground energy storage system and on-board energy storage system. The on-board energy storage system is mainly installed in the train to store the recovered energy inside the train, and the required power is smaller than that of the ground energy storage system.
Countries have conducted in-depth research on railway energy storage systems. The MITRAC Energy Saver Unit supercapacitor energy storage system can be loaded on the train. After a period of experiments, it has been confirmed that it can save 30% energy than ordinary trains. The supercapacitor energy storage system used on diesel EMUs in Germany reduces CO2 emissions and emission reduction costs to a certain extent. In terms of ground energy storage, the supercapacitor energy storage systems in Japan and Italy can not only adjust peaks and reduce costs, but also nearly 70% of the regenerative energy can be recovered and reused.
Flywheels are used in energy storage systems earlier than batteries and supercapacitors. The high-speed energy storage systems that have been developed use flywheels as components, which can well control DC voltage and suppress fluctuations. They are used in New York, Lyon and other places.
The battery energy storage voltage fluctuation is smaller, the self-discharge rate is lower, and it can provide larger capacity. East Japan Railway Company and Bombardier Railway Transportation Equipment Co., Ltd. are the main companies developing power units. Both East Japan Railway Company and West Japan Railway Company have studied battery energy storage devices, and the energy storage system developed by them is applied to the Huxi Line. On the train to the Hokuriku Main Line, the power supply of the train is ensured, and the operating conditions and comfort of the train are improved. As a giant in the electric vehicle industry, Tesla has also studied energy storage technology. The picture shows the energy storage system configured on the Kintetsu train line in Osaka, Japan. As a backup power source, the system allows the train to run to a safe location in the event of a train electrical system failure or grid outage.
The Russian railway department plans to deploy a 10 MW·h battery energy storage system to help the Russian railway network run faster and more smoothly.
my country’s subway lines are also equipped with energy storage systems. CRRC Changchun Railway Passenger Vehicle Co., Ltd. installed a lithium iron phosphate battery system on the light rail, which can increase the cruising range of the train, shorten the charging time, and have broad application prospects.