Flywheel energy storage-technical principle, features and prospect

Flywheel energy storage refers to the energy storage mode that uses the motor to drive the flywheel to rotate at a high speed, and then uses the flywheel to drive the generator to generate electricity when needed.

Flywheel energy storage technology is one of the new energy storage technologies. It is a new energy storage technology with great prospects in recent years. This article will introduce the technical principle, characteristics and development prospects of flywheel energy storage in detail.

What is flywheel energy storage

Flywheel energy storage technology is a kind of energy storage technology that uses reciprocal bidirectional motor (motor/generator) to realize the mutual conversion between electrical energy and mechanical energy of high-speed rotating flywheel.

Technical principle of flywheel energy storage

The flywheel energy storage power system is mainly composed of the following three parts:

● Flywheel
● Motor
● Bearing

The entire system of flywheel energy storage is placed in a vacuum container. The motor in the flywheel energy storage power system is both a motor and a generator. When charging, the motor accelerates the flywheel as a motor to convert electrical energy into mechanical energy.

When discharging, the motor acts as a generator to convert mechanical energy into electrical energy for external power supply; When the power is supplied externally, the speed of the flywheel decreases continuously. When the flywheel is idle, the running loss of flywheel energy storage power system is very small.

When the flywheel is in the atmosphere, it needs to overcome the air resistance (friction) and bearing friction loss when the flywheel rotates at high speed. By placing the flywheel system in a vacuum vessel and using superconducting magnetic levitation technology, the energy consumption of the flywheel at high speed can be minimized.

Another key issue is the design and selection of bearings. The bearing that can operate at high speed shall have sufficient bearing capacity to ensure the reliability and stability of flywheel operation, in addition to the minimum friction loss of the bearing.

Analysis of key technologies of flywheel energy storage

Selection of flywheel materials

The energy storage density and the strength that the flywheel can withstand will directly affect the selection of flywheel materials. The density of flywheel material is inversely proportional to the allowable stress of flywheel material.

Among several common materials used for flywheel, carbon fiber has low density and high strength, so it is a relatively good choice. At the same time, once the flywheel made of carbon fiber is disassembled, the flywheel itself will become flocculent and fly out, reducing the harm caused by accidents.

Vacuum chamber

At present, the vacuum degree of the vacuum chamber has reached 10-5 Pa, which is used to reduce the friction between the flywheel and the air during its rotation, and also to prevent external forces from affecting the normal operation of the flywheel.

Transparent high-strength glass fiber reinforced plastic can be used in the vacuum chamber, which is convenient to observe the operating condition of the flywheel. At the same pressure, the thermal conductivity of helium is seven times that of air, and the friction loss with flywheel is only about one seventh of that of air.

In addition, the process of filling helium is simpler. Therefore, choosing helium as the medium gas of vacuum chamber has certain advantages.

Bearing technology

Among the numerous losses of flywheel energy storage system, the loss of bearings accounts for a large proportion. With the advent of various advanced bearing technologies, this loss can be greatly reduced. Several bearings for the flywheel energy storage system are described below.

● Mechanical bearing

The more common mechanical bearings are rolling bearings, sliding bearings, squeeze film damping bearings and ceramic bearings. Because the friction losses of rolling bearings and sliding bearings are relatively large, they are generally only used as auxiliary bearings in high-speed flywheel energy storage systems. Squeeze film damping bearings and ceramic bearings are used in flywheel energy storage.

● Passive magnetic bearing

Passive magnetic bearing is a kind of passive magnetic bearing, which uses permanent magnets to suspend two or more magnetic rings in axial or radial direction. With the continuous development of permanent magnet in recent years, its bearing capacity has been greatly improved, and its application is more and more extensive.

● Active magnetic bearing
Active magnetic bearing, also known as electromagnetic bearing, controls the change of magnetic field by changing the on-off and size of the current in the control circuit, and adjusts the control current in a timely manner through real-time feedback position signal and output current signal, so that the bearing stator and rotor can be stably suspended.

● Hybrid bearing
In practical applications, these bearings are usually combined to achieve complementary advantages.

What are the types of flywheel energy storage hybrid bearings

● Combination of mechanical bearing and permanent magnetic bearing

The main disadvantage of mechanical bearings is that the friction loss is large. Permanent magnet bearings can help overcome the pressure between the stator and rotor due to gravity, thus reducing the friction loss.

● Hybrid bearing of superconductor and permanent magnet

Superconductor is used as stator and permanent magnet is used as rotor. The rotor can be suspended in a certain position. At the same time, the magnetic flux captured in the superconductor will not move freely due to the existence of the pinning force, which ensures the axial stability and makes the rotor suspend stably.

● Electromagnetic and permanent magnet hybrid bearing

In order to reduce power consumption, permanent magnet is used to generate bias magnetic field and current generates control magnetic field.

Technical features of flywheel energy storage

Parameter performance

Flywheel energy storage technology, especially high-speed flywheel energy storage system, has the advantages of high power density, long service life, real-time monitoring of the system’s state of charge, insensitivity to ambient temperature, etc., but it also inevitably has serious self discharge phenomenon.

In the energy type application, flywheel energy storage is expensive, which limits its development in the energy type application field to a certain extent.

Technical advantages

At present, large-scale electric energy storage mainly focuses on pumped energy storage. Various new energy storage technologies under development have good application prospects, such as flywheel energy storage, supercapacitor energy storage, superconducting magnetic energy storage, compressed air energy storage, lithium ion battery, liquid flow battery and sodium sulfur battery energy storage.

According to capacity, electric energy storage can be divided into long-term large energy and short-term high power. Long term large capacity pumped storage power plants can provide power supply for several hours on the grid scale; The short-term high power flywheel energy storage can provide high-quality uninterrupted power supply for high-end users.

Among various energy storage technologies, flywheel energy storage has many advantages, such as high efficiency (up to 90%), high instantaneous power (single megawatt level), fast response (several milliseconds), long service life (100000 cycles and more than 15 years), and low environmental impact. It is one of the most promising short-term high-power energy storage technologies.

Key problems of flywheel energy storage

● Low energy density

The energy density of flywheel energy storage is not high enough. The release of flywheel stored energy can only last for a short time, usually only tens of seconds.

● High self discharge rate

If you stop charging, the energy will be exhausted within a few to dozens of hours.

● Low efficiency

For example, the loss of tens of thousands of revolutions of high-speed flywheel system is about 100 watts, and the system of 1 kilowatt hour can only maintain self discharge for 10 hours.

● High technical threshold
High speed flywheel energy storage system has a high technical threshold. Composite material structure technology, magnetic bearing technology and high-speed and efficient motor technology in vacuum still have some problems to be solved.

Therefore, flywheel energy storage is most suitable for high power, short time discharge or frequent charge discharge energy storage requirements. For the data center, whether the second level discharge time of flywheel energy storage is reliable has to be considered.

Traditional UPS vs flywheel energy storage UPS

The magnetic suspension flywheel energy storage UPS has attracted more and more attention. This technology discards the traditional way of UPS using lead-acid battery for energy storage. The battery in the traditional power supply system needs air conditioning cooling and consumes huge energy. 

The magnetic suspension flywheel energy storage UPS system does not need air conditioning, which greatly saves operating costs; Moreover, the space occupied by it is also greatly reduced; Low maintenance cost, no need to replace the battery; The service life is up to 20 years.

In the traditional UPS power supply system, when the power is interrupted, the battery will support the normal operation of the system, at the same time, the diesel engine starts. During this process, the battery type UPS provides “minute level” power supply. The flywheel energy storage UPS can only provide power supply for 30s to 1min.

However, today, the reliability of mains power supply and backup power supply can reach 99.9%. It only takes 10s technically to switch from mains power supply to backup power supply, which is an open standard.

At present, Europe has set the time as 8s. It can be concluded that the flywheel energy storage UPS can provide 30s of power, which can fully meet the requirements of reliable switching from mains power to backup power.

Development status of flywheel energy storage technology in the world

The development of flywheel energy storage technology in the United States, Germany, Japan and other developed countries is in a leading position. Japan has manufactured a variable-frequency adjustable speed flywheel energy storage power generation system with a capacity of 26.5MVA, system output voltage of 1100V and rotational speed of 510690r/min.

The United States has also developed a 24kWh electromagnetic suspension flywheel system for power peak shaving. The flywheel weighs 172.8kg, the operating speed range is 11610~46345rpm, the breaking speed is 48784rpm, the system output constant voltage is 110~240V, and the overall efficiency is 81%. Economic analysis shows that all costs can be recovered after 3 years of operation.

In China, the current research on flywheel energy storage mainly focuses on small capacity series. On the whole, many achievements are still in the research stage.

The prospect of flywheel energy storage technology

Although the energy storage technology of chemical battery (lead acid battery, lithium ion battery, etc.) has been developed very mature, the lead acid battery has the risk of polluting the environment, and the charging and discharging times of lithium iron phosphate battery are limited.

Many industries such as new energy, electric vehicles and UPS power supply urgently need new energy storage technology to meet some special technical requirements. 

Flywheel energy storage technology is a kind of energy storage technology with unlimited charging and discharging times and green environmental protection, which has been increasingly widely used in many industries around the world. Now, the development of large-scale energy storage has become a national strategy, and the flywheel energy storage application will enter a rapid development period.