When you think of flywheels, what is the first thing that comes to your mind? You may think of magnificent steam engines with huge flywheels; the ones you find in museums nowadays. Beautiful machines that powered the industrial age. But do you know that flywheels still play a vital role in powering today’s modern data centers? And that they keep the factories going that produce chips for the Internet of Things (IoT)? Flywheels today may be less visible, but they are at the heart of our rotary UPS systems.
This post touches on flywheels for storing energy. It describes how flywheels store energy and explains why we prefer flywheels over batteries in our rotary UPS systems.
What Is a Flywheel?
A flywheel is “A heavy revolving wheel in a machine which is used to increase the machine’s momentum and thereby provide (…) a reserve of available power,” according to the Oxford Dictionary.
This definition contains some interesting elements. It mentions that the purpose of a flywheel is to store energy and make it available for later use. This power reserve is available in the form of momentum. This means that it takes power to start rotating the flywheel. The heavier it is, the more force is required to increase the speed. The opposite is also true. To stop the object from rotating, an opposite force must be applied. Otherwise, it keeps spinning forever, assuming there is no friction. This is how a flywheel stores kinetic energy.
The definition mentions that flywheels are heavy. That, of course, is relative. The flywheel of a diesel engine is heavy if you need to lift it by hand, but compared to the total weight of the engine, it is not that much. The mass of a flywheel is one of the factors that determines the amount of energy that it can hold. So, one way to store a lot of energy is to make the flywheel heavy. But that is not the only factor that counts.
The formula that describes the amount of kinetic energy in a flywheel is:
E is the kinetic energy in Joule.
k is a constant that depends on the shape of the flywheel. For example, k = 0.606 for a solid disk.
m is the mass in kilogram.
r is the radius in meter.
\omega is the angular velocity in radians per second (simply said, the rotation speed).
According to this formula, the radius and the speed have more influence on the energy than the mass. So, if you want to store a lot of energy in a flywheel, it is more effective to make it big and to spin it fast.
As always, constraints limit the possibilities. The tensile strength of the flywheel material determines the speed that it can run. And the bearings must be able to support the weight and the speed.
Uninterruptible power supplies need an energy reserve to be able to bridge interruptions in the mains supply. The amount of energy depends on the power rating of the UPS and the desired bridging time.
Rotary UPS systems use flywheels to convert electrical energy into kinetic energy and vice versa. This principle is well known, and there are good reasons for that:
- Flywheels are simple devices. In essence, a flywheel is a round piece of steel that is sturdy and reliable.
- There is no limit to the number of charges and discharges.
- It is an efficient way to store and retrieve energy.
- The stored energy is instantly available when it is needed.
- A flywheel has a long lifetime. The recycling and reuse of the materials is well possible after the usable lifetime.
The bridging time that in practice can be achieved is measured in seconds. Many applications, though, need longer bridging times. In those cases, the UPS uses a second source of energy to get the desired autonomy time. This can be, for example, a diesel engine. In such a case, the main purpose of the flywheel is to bridge the startup time of the diesel engine.
Manufacturers of rotary UPS systems have produced various designs to meet the requirements of applications. In a later post, we will explore some popular designs.
Flywheels Versus Batteries
Most UPS systems today use batteries for energy storage. Most of these systems still use lead-acid batteries. The price of Li-Ion batteries has dropped over recent years with the rise of large-scale production facilities. Yet, lead-acid batteries are still cheaper. Li-Ion has advantages, such as higher power density. But in most UPS applications, this does not outweigh the higher cost.
UPS systems based on batteries are solid-state, all-electrical systems. The bridging time is measured in minutes and, sometimes, hours. For most UPS systems, those rated below 100 kVA, this is the technology of choice.
Batteries have some drawbacks compared to flywheels:
- Batteries, in particular lead-acid batteries, are sensitive to temperature. The available capacity drops significantly at low temperatures. At high temperatures, battery performance is irreversibly degraded.
- Batteries can perform a limited number of charge-discharge cycles. The capability of the battery degrades with every cycle. After 500 full cycles, the battery usually has reached the end of its useful lifetime.
- The environmental impact of batteries is high. The lead compounds used in batteries are extremely toxic. The recycling of battery lead is possible in recycling plants. Unfortunately, many batteries still end up in landfills after their useful lifetime.
These may all be good reasons to favour flywheels. But the main reason UPS systems utilize flywheels is that they are very suited to high-power applications. Think of UPS installations with ratings of 1 MW and above used in, for example, large data centers or factories.
In these applications, another difference between flywheels and batteries is a decisive factor. Lead-acid batteries can deliver around 180 Watt per kilogram (the specific power). For flywheels, this figure is much higher. This is also true for the power density, the power per volume. If you store the same amount of energy in a flywheel and in a battery of equal weight or volume, you will be able to retrieve it faster from the flywheel. In other words, if you need a lot of energy for a short time, a flywheel is smaller and weighs less than a battery. This gives flywheels the edge in today’s high-power UPS applications.
This post is the first in the UPS Fundamentals series, which explores key concepts and technologies of uninterruptible power supplies (UPS). In the next post, I will investigate the technologies UPS manufacturers use to get the most out of the flywheel.
A Spanish translation of this article appeared on https://upsdinamico.com.
Courtesy to Angel Garcia Torreiro from IEM Power Systems.