Analysis of flywheel energy storage thermal management

1. Development of flywheel energy storage
2. Overview of key technologies for flywheel energy storage
3. Heat production analysis of flywheel energy storage system
4. Status of flywheel energy storage thermal management
   1. Motor cooling
   2. Magnetic bearing cooling
5. Summary

Flywheel energy storage can be widely used in uninterruptible power supply, grid-connected renewable energy, power peak and frequency regulation, rail transit, aerospace and other fields. In a vacuum environment, the heat dissipation of the motor and the electromagnetic bearing is related to whether the flywheel energy storage system can operate safely.

This is a key scientific and technological problem that needs to be solved in the development of flywheel energy storage technology. The heating of the flywheel energy storage system is mainly caused by the motor and the electromagnetic bearing.

The heating of the motor mainly comes from the iron loss of the stator core, the copper loss of the winding and the eddy current loss of the rotor. The heating of the electromagnetic bearing is mainly composed of iron loss and copper loss.

Development of flywheel energy storage

In recent years, renewable energy technology has developed rapidly, but at the same time, the intermittent and unstable nature of renewable energy has also become one of the main factors hindering its development. Through the introduction of energy storage technology, large-scale renewable energy can be connected to the grid, which is of great significance in improving the efficiency and security of the power system.

In principle, the existing energy storage technologies are mainly divided into mechanical energy storage and electrochemical energy storage. Mechanical energy storage mainly includes compressed air energy storage, flywheel energy storage and pumped hydro energy storage, etc.

Electrochemical energy storage mainly includes lead-acid energy storage. Battery energy storage, lithium battery energy storage, sodium-sulfur battery energy storage, flow battery energy storage, superconducting energy storage and supercapacitor energy storage, etc. And here is a top 10 energy storage battery manufacturers in China to help know about the industry trends. 

The working method of flywheel energy storage is the mutual conversion between electric energy and flywheel kinetic energy. At the same time, the single flywheel can form a 10-100MW energy storage system through array assembly, and the charging and discharging time can reach the hour level, which can be widely used in power quality control, uninterruptible power supply (UPS) and power grid frequency regulation and other fields. At present, flywheel energy storage is developing in the direction of high energy density and high speed.

High-speed flywheel energy storage systems face high technical difficulties, including flywheel composite materials and structural technology, magnetic bearing technology, and vacuum motor technology.

Overview of key technologies for flywheel energy storage

The flywheel energy storage system is mainly composed of flywheel, motor, bearing, vacuum cavity and so on. Its application areas mainly include UPS, grid-connected renewable energy, power peaking and frequency regulation, rail transit and aerospace.

The flywheel has high requirements on the strength and performance of the material in the process of high-speed rotation. At present, metal materials and composite materials are mainly used. Among them, the research on metal flywheels mainly focuses on the performance and structural optimization design, and the development is relatively mature.

The research on composite materials focuses on the complex properties of materials and their failure mechanisms, and has become a current research hotspot. For composite flywheels, multi-ring suits, hybrid materials, gradient materials, fiber preloading, etc. are usually used to increase the energy storage density of the flywheel.

At present, two-dimensional or three-dimensional strengthening has become a new way of flywheel design, which makes the ultimate experimental speed of flywheel rotor up to 867m/s. In the flywheel energy storage system, the shafting support bearing is mainly composed of several combinations of rolling bearings, hydrodynamic bearings, permanent magnet bearings, electromagnetic bearings and high temperature superconducting magnetic bearings.

Due to the large loss of rolling bearings and hydrodynamic bearings and insufficient high-speed bearing capacity, they are usually used in conjunction with permanent magnet bearings, and are not suitable for flywheel energy storage systems with a speed higher than 10,000 r/min. Electromagnetic bearings have the advantages of low loss and mature technology.

When the speed of the flywheel is between 10,000 and 60,000 r/min, electromagnetic bearings should be used. High temperature superconducting magnetic bearing has the advantages of relatively small loss, high reliability, and no need for complex control systems, and has become a hot research topic at present, but its bearing capacity and stiffness are relatively low.

The world’s largest 300kW flywheel energy storage system prototype was built in Mt. Komekura in 2015. It uses high-temperature superconducting magnetic suspension bearings and has been tested for 1500h. It can be seen that electromagnetic bearings and high-temperature superconducting magnetic bearings have been widely used in high-speed, high-efficiency flywheel energy storage systems.

As a key component of the flywheel energy storage system, the integrated motor/generator has the characteristics of bidirectional variable speed operation, and its loss is not only related to the efficiency of the flywheel energy storage system, but also has an important impact on the thermal safety of the system.

Experts proposed a brushless DC motor drive method based on current planning, which can significantly reduce rotor eddy current loss and winding copper loss. Compared with the traditional square wave method, the rotor eddy current loss can be reduced by 90%, and the motor copper loss can be reduced by 10%. 

It can be seen that by using a multi-phase motor or a new type of motor drive method, the eddy current loss of the motor rotor can be effectively reduced, and the problem of difficult rotor heat dissipation of the flywheel energy storage system can be fundamentally solved.

Heat production analysis of flywheel energy storage system

The loss of flywheel energy storage system mainly includes system loss, heat dissipation loss, and system loss includes electrical loss, wind loss and bearing loss. In motors and electromagnetic bearings, stator copper losses, iron losses and rotor eddy current losses are usually converted into system heat sources, generating heat and increasing the system temperature, especially the temperature of the flywheel rotor.

Therefore, the accurate estimation of the loss of the stator and rotor of the motor and the electromagnetic bearing is an important prerequisite for the analysis of the heat source of the motor.

In order to accurately analyze the heat generation of the flywheel energy storage system, scientists pointed out that different stator structures will generate phase harmonics of different sizes, and it is necessary to analyze the eddy current, magnetic field and cooling scheme simultaneously. The results show that the key to controlling the heat induced by eddy current losses is to keep the induced eddy currents caused by the phase harmonics to a minimum.

It can be seen that the heat caused by the eddy current loss of the flywheel energy storage system under vacuum conditions is mainly caused by the system phase harmonics. Through the multi-field synchronous simulation method, the system eddy current loss can be accurately simulated, which is of great significance for the accurate analysis of the heat generation of the flywheel energy storage system.

The rotational speed of the high-speed flywheel energy storage system can reach (4～5)×104 r/min. In order to reduce the wind resistance of the flywheel energy storage system at high rotational speed, the rotor usually operates under low pressure or vacuum conditions. At the same time, in order to reduce the friction loss of the system bearings, electromagnetic bearings are often used to suspend the flywheel rotor in a vacuum, and the efficiency of the flywheel energy storage system can reach more than 95%.

However, in a vacuum environment, the heat generated by the rotor of the flywheel energy storage system can only be transferred outwards by means of radiation heat transfer, and the radiation heat transfer intensity is very weak, making it difficult to dissipate the heat generated by the rotor of the system, resulting in the constant temperature of the rotor. rise.

In the current high-power flywheel energy storage dynamic UPS in industrial applications, the average motor power during high-speed standby is usually lower than 1% of the rated rapid discharge power, and its heat generation is very small, which will not cause thermal runaway of the system. In the case of high-power discharge under working conditions, more heat is generated, and a new thermal balance can only be achieved by dissipating heat for a long time.

Therefore, in the application of flywheel energy storage dynamic UPS, it is usually stipulated that the frequency of charging and discharging within 1h is not more than 10 times or less to prevent the thermal runaway of the system caused by the accumulation of motor heat.

However, the heating of the electromagnetic bearing is still an important issue related to the safety of the flywheel energy storage system. At present, the main methods for solving bearing heating in the world are:

• high resistivity magnetic permeability material; 
• conical lamination;
• groove.

Status of flywheel energy storage thermal management

Motor cooling

Motor stator cooling methods mainly include air cooling, water cooling, oil cooling, heat pipe cooling and phase change cooling. The heat of the flywheel energy storage motor mainly comes from the iron loss of the stator core, the copper loss of the winding and the eddy current loss of the rotor. A large number of literatures have studied the heat dissipation of the motor stator.

The cooling methods of the motor rotor mainly include oil cooling in the shaft hole, filling with inert low molecular mass cooling gas, and strengthening radiation heat transfer. The experts studied the single-loop thermosiphon cooling structure of the rotor of the flywheel energy storage system by means of numerical simulation.

The heat from the heat-generating part is quickly transferred to the cold end of the shaft for cooling. The results show that the single thermosyphon circuit is more suitable for motor rotor cooling. It is worth noting that the volatilization, sealing and corrosion of the cooling fluid in the cooling system under vacuum conditions need to be considered.

At the same time, it is necessary to consider the heat generation and kinetic energy loss of the fluid during the rotation of the rotor. In the case of ensuring a certain degree of vacuum, a mixture of helium and air can be filled into the vacuum chamber to enhance the flow heat transfer capacity of the rotor.

The relevant experimental results show that compared with pure air, filling the vacuum chamber with a mixture of helium and air can effectively reduce the wind resistance, increase the breakdown voltage, and enhance the heat dissipation capability of the rotor.

It is still difficult to dissipate heat from the rotor of the flywheel energy storage system, which leads to the thermal management of the maglev flywheel energy storage system under high vacuum conditions, which has become a key factor restricting the development of the flywheel energy storage system.

Magnetic bearing cooling

At present, the bearings used in the high-speed flywheel energy storage system mainly include electromagnetic bearings and high-temperature superconducting magnetic suspension bearings. Among them, the heating method of the electromagnetic bearing is similar to the heating method of the motor, which is mainly composed of iron loss and copper loss.

Compared with copper loss, the iron loss composed of eddy current and hysteresis loss is relatively small, and its cooling method is similar to that of motor cooling. In the flywheel energy storage system, the cooling system is usually shared with the motor. In the structural design, in order to block the eddy current and reduce the heat generation, the stacked structure is often used.

High-temperature superconducting magnetic bearing has become a current research hotspot because of its low loss, self-stabilization, and simple control. Unlike conventional electromagnetic bearings, HTS magnetic bearings are usually equipped with a cryogenic refrigerator to cool the superconducting blocks and superconducting coils. High temperature superconducting magnetic bearing coils are usually cooled by heat transfer at the low temperature cold end.

For the cooling of the superconducting block of the high-temperature superconducting magnetic suspension bearing, the method of filling the low-pressure helium gas is adopted, and the helium gas is used as the heat transfer medium to directly exchange heat with the low-temperature cold end. However, this is unfavorable for maintaining the vacuum degree of the high-speed flywheel energy storage system.

It can be seen that the main cooling methods used for the magnetic bearing of the flywheel energy storage system are water cooling (traditional electromagnetic bearing) and filling with low pressure helium gas (high temperature superconducting magnetic suspension bearing).

The difficulty is basically the same as that of motor cooling.  Since the electromagnetic bearing loss is smaller than the motor loss, and through the system low-loss design and control strategy research, good thermal management can be achieved.

Summary

The heat generated by the flywheel energy storage system mainly comes from the motor and the electromagnetic bearing, and the heat generated by the motor is dominant. The loss of motor and electromagnetic bearing mainly includes copper loss and iron loss. The analysis of rotor eddy current loss is the basis for accurately predicting the heating of flywheel energy storage system.

In a vacuum environment, the heat dissipation of the flywheel energy storage system includes the heat dissipation of the motor stator, the heat dissipation of the motor rotor, and the heat dissipation of the magnetic suspension bearing. The existing motor stator cooling methods mainly include water cooling, oil cooling, heat pipe cooling and phase change cooling.

Motor rotor cooling methods mainly include filling inert gas to enhance convection heat transfer, oil cooling in the rotor shaft hole, and expanding surface to enhance radiation heat transfer. Magnetic bearing cooling mainly includes electromagnetic bearing cooling (can share cooling system with motor) and high temperature superconducting magnetic bearing cooling (low-pressure helium direct contact cooling).

The flywheel energy storage system operates under special conditions of high vacuum, and the heat dissipation of the system rotor is the difficulty of thermal management of the flywheel.

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