Working basics

Flywheels are kinetic energy storage systems. Using a simplified example, Flywheel’s function is the same as batteries, that means, the energy storage, but the technology used for energy storage is different.

Batteries, in general, use electrochemical processes that allow battery recharging and energy storage.

Flywheels transform electrical energy in kinetic energy and the other way round, so that the energy is “stored” as kinetic energy.

Basically, a Flywheel is composed by a shaft that integrates a Flywheel. A rotor of an electrical machine is mounted on that shaft. The Flywheel housing contains the electrical machine stator and other elements needed for the appropriate functioning of the machine, as, for example, the shaft bearings. The Flywheel also includes electronics, which is not explained in this chapter.

When electrical energy has to be transformed into kinetic energy, the electrical machine works as motor, in such a way that absorbs electrical energy accelerating the shaft until working speed is reached.

Once that speed has been reached, the electrical machine is disconnected from the net, but the shaft, due to the inertia of the Flywheel, goes on rotating for a very long period of time. This way, electrical energy has been transformed into kinetic energy, and therefore, energy is “stored” as shaft is rotating.

In order to get the shaft rotating “indefinitely”, mechanical energy losses must be eliminated, such us, bearing friction, aerodynamic losses, and so on. For that purpose different solutions have been adopted. For example, a vacuum atmosphere or helium atmosphere is created inside the housing.

When the stored energy must be extracted from the machine, the kinetic energy is transformed into electrical energy. In this case, the electrical machine works as generator, whose shaft is already in movement. This way, electrical energy is obtained through the generator as shaft speed is reduced.

The electrical parameters of the extracted electrical energy are controlled by the electronics, in order to have the appropriate tension, frequency and power.

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Railways

The installation of Flywheels in Railways, essentially in short distance (metro, trams, commuter trains), is due to the usage of Flywheels as energy management during regenerative braking of rolling units.

The regenerative braking consists of using the traction electrical machine as generator, so that the energy generated by the electrical machine is dissipated in resistors mounted on the trains. Due to the fact that the traction electrical machine is working as generator, a braking moment is transferred to the wheels, reducing the train speed. In case a Flywheel is installed in a substation, instead of dissipating the electrical energy in the train resistors, this energy is transferred through the catenary to the Flywheel, so that the electrical energy is transformed into kinetic energy. Therefore, the energy has been stored. This way, when any train unit accelerates, the Flywheel transforms the stored kinetic energy into electrical energy, and provides the accelerating unit with the necessary electrical energy. Through this application, energy is substantially saved.

The Flywheel can be installed in parallel with substations, inside the substation or in any other place, so that it is always connected to the catenary. As a general rule, the Flywheels must not be installed on the train unit, as it generates tilting effects. This is due to having a rotating mass inside the train unit.

Besides, some other advantages can be obtained.

• Tension stability.

The frequent accelerations of train units reduce substantially the line tension, both in the catenary and in the upstream line of the substation. The Flywheel reduces this effect upstream the substation. This happens because the Flywheel provides the train with the needed energy when it accelerates, and therefore, the substations must not provide this energy.

In case the system efficiency is 100% the substation should only provide the train with energy just once. As some energy losses are present in the process, the Flywheel picks up back up energy from the net to cover those losses. But this energy absorption is done with low power, so that the tension stability is not affected.

• Substations size reduction.

  The load and unload cycles of the Flywheels are electronically controlled in such way that the loading process can be executed fast or slowly, the same as the unloading process (energy provide). That means that the Flywheel power can be adjusted to the power needed in each moment. If the trains brakes with full electrical power, the Flywheel stores the energy with full power. If the train accelerates slowly, not using all the electrical power of the motor, the Flywheel can be adapted to that power.  Further away, the Flywheel can be programmed so that only provides part of the energy and the other part is delivered by the substation. This way, the power needed from the substation is lower. Besides, the backup energy that the Flywheel will absorb from the substation to compensate the energy losses can be absorbed with the desired power, so that the substation size is reduced. Therefore, a substantial economical save is achieved.

• Auxiliary feeding.

  A Flywheel stores electrical energy, therefore, from a functional point of view, acts as a rechargeable battery. This suppose that a Flywheel can absorb electrical energy when general energy demand is low, for example, during the night, and the Flywheel can deliver that energy when the demand is high, for example, when train units are running. Even further, as Flywheels can regulate the power electronically, the substation power from which the Flywheel is loaded can be lower. In other words, the Flywheel loads energy in more time that it can give the stored energy. This working modus can be used for auxiliary feeding, guarantying the auxiliary service and reducing substations installation costs.

• Reactive power source.

  One of the elements composing the Flywheel is a electrical machine. This supposes that the Flywheel can not only absorb or provide active power, but also reactive power. Consequently, reactive and active save with Flywheels is a fact to be considered.

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UPS

One of the applications of Flywheels is the usage of Flywheels as advanced UPS systems.

Actual infrastructures and manufacturing facilities imply in many cases the use of computers or systems based on microprocessors. This kind of equipment is, in general, very sensitive to net voltage changes. On the other hand, the power interruption can generate the stop of productive processes and/or information losses. The typical cases in which power supply must be guaranteed are:

• Centre of calculus.
• Data bases of banks, insurance companies…
• Semiconductor manufacturers.
• Security applications.
• Airports.
• Telecommunications.

The usage of our Flywheels in such applications, not only guaranties the power supply, but supposes the following advantages in comparison with other UPS technologies:

• Voltage change protection. The Flywheels are fast enough to reduce substantially the typical voltage changes in the net and which could permanently damage the electronic equipment.
• High reliability. The mean time between failure of a Flywheel is intrinsically inferior to that of batteries.
• Low Maintenance. The Flywheels life time is between 15 and 20 years, requiring punctual maintenance jobs every 2 years, with small pieces substitution. The cost of this maintenance is much lower than the batteries replacement usually required in standard UPS.
• High energy scalability. The increase of time in which a Flywheel provides the power is easily scalable with Flywheel height increase, in such a way that high powers can be provided for minutes.
• Fast recharge. A Flywheel can recharge it’s energy as fast as it can provide the energy. The charge/discharge time can be controlled electronically. The charge/discharge possibility is not possible with the conventional battery UPS. Even more, conventional UPS are slower in the charge process.

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Wind Energy and Diesel Generators

In some cases, one or several wind generators are installed with one or several diesel generators as back up system. The diesel generators provide electrical energy when there is no wind. Wind interruption is a phenomena that occurs often, making the diesel generators to start and stop frequently.

In other installations where diesel engines are installed as emergency energy suppliers, also frequent start up and stop occur.

In both cases, the installation of a Flywheel in parallel with the diesel generator involves the following advantages:

• Increase of the life cycle of the diesel engine. The energy stored in a Flywheel is such that reduces drastically the starts up of the diesel engine, providing itself the required energy.
• Increase of net quality. The fast response to events of the Flywheel allows the improvement in the net quality supply of wind generators.
• Frequency control. The Flywheels can be used to control the frequency of the voltage supplied by the wind generators.
• Reliability improvement. As the meantime between Failures is much higher in Flywheels than in batteries, the reliability of the system is highly increased.
• Less maintenance work. Flywheels expected life is between 15 and 20 years, with maintenance jobs every two years for small pieces substitution. Therefore, the maintenance costs are reduced.

The table at the right is a comparison between the energy and power densities of different storage systems.

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Others

The quality, reliability and versatility of Flywheels allow its use in several applications.

• Desalinization plants. To minimize the environmental and economical impact of the high energy consumption of desalinization plants, it has been analyzed the possibility of installing a wind park beside the desalinization plant, to generate the necessary electrical energy. The inconvenient of the solution is that the energy demand cycles of the plant are not coincident in time with the wind cycles that generate energy in the wind plant. Flywheel installation will make feasible this solution as they can store the electrical energy generated by the wind park for later supply of such energy to the desalinization plant depending on the demand. This way, desalinized water production is not linked to wind park electrical energy generation.
• Lifts. The actual tender to eliminate the machine room in lift installations leads to the use of synchronous motors regulated with frequency converter. The installation of Flywheel can suppose an energy saving if the energy produced by motor braking is stored at the Flywheel. This stored energy can be used to supply the motor when acting as so, or in emergency situations, in which there is no external power supply and electrical energy is needed for rescue operations (lift movement to the closest floor, door opening,…)
• Cogeneration plants. At the cogeneration plants where energy supply flexibility is needed, diesel engines are installed, because they have less inertia than gas turbines, but less efficiency. The use of Flywheel in parallel with a gas turbine is a perfect combination to obtain a cogeneration system flexible and with high efficiency. When the turbine generates more energy than required, the extra energy would be stored at the Flywheel. Beside, in cases where the energy demand increases, the Flywheel supplies such energy until the gas turbine can change the working regime to  provide the energy required.

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