EPIC Material Lets Flywheels Compete with Lithium Batteries!

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But that historically means lithium-ion batteries, which requires exotic materials and we can’t build them fast enough. But, what if we could store energy without mining and building batteries, but instead, by storing it in a rotating flywheel?

Let’s talk about flywheel energy storage and some game-changing breakthroughs in materials that could really take flywheels to the next level. I’m Ricky, and this is Two Bit da Vinci.

I’m all about replacing fossil fuels with clean and renewable energy, but unlike coal and gas that you can modulate to produce electricity predictably, the sun shines differently throughout the day and the wind speeds vary too. So renewable energy is equal parts energy production and energy storage, to help level out the variance in output.

A recent study published in the international journal of Renewable Energy suggests that we’ll need to store at least 5% of the total renewable energy production capacity in order to replace fossil fuels completely [1]. That means A LOT OF BATTERIES.

To give you an idea, in 2019 alone, we consumed roughly 173,000 terawatt-hours of energy, 85% of which came from burning fossil fuels [2].

Image source: [2]

So, we would need approximately 7,350 terawatt-hours of storage capacity, to run a stable grid, assuming, for the sake of the argument, that all that energy was consumed as electricity.

That’s an insane amount of storage capacity. That’s enough energy to power 1 million average American homes for almost 700 years [3]! or to charge 1 million Model 3 Performance Teslas from 0 to 100% almost 90,000 times each[4]!

Considering that the worldwide energy storage capacity is approximately 9 terawatt-hours [17], which is only 0.12% of what we would need, we have an uphill battle in front of us, and we need to take advantage of every storage technology available, and one promising solution is flywheel energy storage.

Additionally, given that over 90% of our storage capacity comes from pumped hydro [17], and there are many places where this simply isn’t an option (like in Delaware and Mississippi, for example), we need alternative ways to store energy.

In places where pumped hydro isn’t an option, thermal storage and batteries are the main competitors, followed by compressed air and flywheels coming in fourth with a total deployed capacity of only a little over 97 megawatt-hours [18]. This would only be enough to power 3,300 average US homes for a day!

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Yet flywheels have amazing potential! and storage capacity is growing exponentially. Only a couple of years ago, Beacon Power built one of the world’s largest flywheel energy storage systems in New York that can pump out 20 Megawatts for 15 minutes for a storage capacity of 5 Megawatt-hours.

Another even bigger storage facility that’ll hold 80 megawatt-hours with a power rating of 20 megawatts, is set to be built by a company called Amber Kinetics in Fresno County, California.

Now, these are mostly bulky and heavy flywheels made for grid storage applications, but, as we’ll see later in the video, newer technology can make them lighter and more energy dense, making them ideal for mobile applications like EVs. In fact, flywheels are already used in formula-1 cars to store excess mechanical energy.

But, what exactly are flywheel batteries?

Flywheel batteries, or, more accurately, flywheel energy storage systems, FESSs for short, are a type of electromechanical energy storage system. These systems transform one form of energy, like electrical energy, into rotational kinetic energy.

The physics behind flywheels

At a fundamental level, a flywheel is pretty easy to understand. An electric motor uses energy to spin a massive rotor or flywheel, storing that electrical energy as kinetic energy in the spinning mass.

These things can spin really fast! A modern flywheel is designed to spin at speeds upwards of 50,000 rpm, while the engine on a standard sedan will rarely go over 3,000 rpm while cruising down a highway [10].

That rotational kinetic energy remains in the flywheel as long as it spins at a constant speed. Then when we want to discharge the flywheel, we use a generator to slow it down and convert the kinetic energy back into electrical energy, just like an EV’s regenerative braking system does by slowing down the car.

It’s as simple as that.

But how good are flywheels at storing energy? One way to know is by looking at its energy density, so let’s see how that works for flywheels.

Thankfully, this is also quite easy to understand and it’s only based on a couple of simple equations.

To get a feel for this, let’s put on our engineering hats and set to work on designing the world’s greatest flywheel energy storage system by looking at how to maximize its energy density.

As I just said, in a flywheel, energy is stored as rotational kinetic energy, which is given by the equation you see here:

KRot=1/2 I2

Where I is the moment of inertia and (omega) is the angular speed, or how fast the flywheel is spinning, as in Hertz or RPMs.

This little formula basically tells us that we can increase the flywheel’s energy by increasing the inertia or the angular speed, but the latter is more effective because it’s squared. That means that if we double the speed, we’ll multiply the kinetic energy by four.

The moment of inertia, on the other hand, depends basically on the flywheel’s design or shape, its dimensions, and what it’s made of, and it’s usually a fixed quantity.

So, to store more energy, we want to increase speed as much as possible because kinetic energy increases with the square of the angular speed, and
We also want to increase the radius of the flywheel as much as possible, because the moment of inertia also increases with the square of the radius.

But, there’s a problem. You see, spinning faster and increasing the radius, generates a great deal of tensile stress in the material that tries to rip it apart. If that stress becomes too high, the material will fail and the flywheel will really fly… into pieces, that is.

This is a typical optimization problem. We want to increase a good thing, like maximum speed and radius for more energy, but as we do, something bad also increases, like tensile stress, in this case.

Engineers already found the optimal solution for this problem, and it’s summarized in a very simple equation that determines the maximum energy per unit mass of a rotating flywheel, or its specific energy density:

Energy Density=K*max/

In this equation, K is a constant that depends exclusively on the rotor’s shape.

Its highest possible value is 1 for a special theoretical shape called a Laval disc, but we can get real values in the range of 0.8-0.95 if we trim the laval disc and add a rim [8].

Here are some examples of K for other rotor shapes:

Images Source [8]

Rotor material

The rest of the equation is the ratio of the material’s maximum tensile resistance or yield strength (sigma-max) to its density (rho). So, the stronger and lighter the material, the more energy a flywheel can store, while spinning at its maximum velocity, without failing.

We should note here that the real limiting factor in flywheel energy density isn’t so much the flywheel’s design, which, in the best of cases, would double the energy density, but rather its construction material.

So, what should we make our flywheel of if we want to maximize energy density?

Table Source [8]

This table here ranks flywheel construction materials based on their expected energy density calculated with the previous equation.

Notice how a strong, lightweight metal like titanium offers an energy density of just 32.3 Watt-hours per kilogram, which is about one-tenth the energy density of Tesla’s new 4680 cells.

On the other hand, a cylindrical rotor made of T1000 carbon fiber composite can reach theoretical energy densities of 254 watt-hours per kilogram, making them as good as the best Li-Ion battery packs. So, in principle, we could use flywheels to power EVs.

But it doesn’t stop there. Notice how I blurred out a row at the top of the table? That’s because I don’t want to give away the number one spot just yet. But we’ll get back to that in just a bit.

Advantages of Flywheel Energy Storage Systems

First, let’s talk pros, in other words, how can flywheels benefit you.

Well, first of all, flywheels are encased in an inert, frictionless vacuum, meaning they require very little maintenance and can easily withstand 20 or more years of constant use. This makes them an almost set-it-and-forget-it energy storage system.
Secondly, we can charge them with a high round-trip efficiency of 90% and more in a matter of seconds to minutes, depending on the total capacity, without generating almost any heat. You can’t do that with Li-ion batteries, for example.
Thirdly, flywheels can withstand 100,000 to 1 million charge-discharge cycles, including both shallow and deep cycles, without a hiccup. Try that on your mobile phone’s battery and see what happens [12].
But one of the best things about flywheels, though, is their power density, which is second only to supercapacitors. You can draw almost as much power as you want from a flywheel without overheating it, making them great for covering peak electricity demand in power grids and other applications.

Disadvantages of Flywheel Energy Storage Systems

But of course, flywheels aren’t perfect.

Most common flywheels have low energy densities compared with the best chemical battery systems.
They require precision milling and construction techniques to ensure the rotor is adequately balanced and the vacuum chamber doesn’t have any leaks.
This makes this technology rather expensive upfront, especially considering the cost of high-tensile strength carbon fiber and other novel materials.
But, of all the issues pertaining to flywheel energy storage, the worse by far is energy loss in standby mode, which ranges from 3% to 20% per hour! [13]

This is the limiting factor that prevents using flywheels for long-term energy storage and it’s also the #1 reason why we don’t see flywheels absolutely everywhere. Even in the best of cases, losing 3% of the charge per hour means losing all of the charge in a little over a day.

However, this still works for some mobile applications like EVs and the 1940s Swiss Gyro-Bus, which charged for a couple of minutes in every station. I made a video about the Gyrobus, link’s in the description.

In spite of these limitations, there’s a new material called carbon nanotubes that is helping make flywheel batteries better than ever before.

This is a form of carbon arranged into a hexagonal honeycomb structure rolled up into a tiny tube 1 to 2 nanometers thick [14]. That’s about one ten-thousandths of the thickness of the thinnest human hair.

It’s also super light and incredibly strong. That said, it’s time to unblur the table I showed you earlier.

As you can see in the table, these little guys (labeled CNTs) have the highest yield strength known to man, reaching a staggering 30,000 megapascals along their axis. That’s over 20 times stronger than the strongest steel.

This unique combination of tensile strength and low density is just what the doctor ordered to maximize a flywheel’s energy density, which can reach almost 27 hundred Watt-hour per kilogram. That’s ten times more than the best chemical battery to date.

Increasing a full order of magnitude in capacity will totally change the game in energy storage and possibly shift a large chunk of funding towards improving other components of the system even further.

There are still some challenges to address, like making these nanotubes long enough and in large enough quantities, but researchers were already able to produce forests of carbon nanotubes 14 cm long [16]. These could be enough to build these awesome flywheels. If we manage to make them at a commercial scale, we’ll have hit the flywheel of fortune jackpot.

But what do you think? Is there anything I missed about the mechanical flywheel battery? Do you think we’ll ever see EVs powering up their flywheels in a couple of minutes in the morning before we commute to work? Sound off in the comments below, as you always do. I love reading what you have to say!