How This Mechanical Battery is Making a Comeback

This is the Dinglun Flywheel Energy Storage Power Station. At 30 MW, this is likely the biggest Flywheel Energy Storage System on the planet. Don’t let that spin you around though. While its sheer size is unrivaled, It’s not alone. More and more people are turning to mechanical energy storage systems, like flywheels, as the solution to large-scale energy woes. Why the sudden uptick of interest in this otherwise niche mechanical energy storage device? And can a spinning wheel really compete with lithium batteries and all the other energy storage systems available to us at the moment?

We took a spin at this topic almost four years ago now, and there’s been a lot of movement with flywheels since then. California-based Amber Kinetics has been rolling out installations, and as I just mentioned, China has brought one of the largest systems online recently.

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But in case you aren’t familiar, what is a flywheel? Or, more accurately, what is a Flywheel Energy Storage System (or FESS)? It’s essentially a mechanical battery made by combining a dual-action electric motor and a big, heavy … well … wheel.¹ You can charge a FESS by using excess electricity to spin up the flywheel. Once it’s spinning, it wants to keep spinning, meaning only a little electricity is needed to maintain its speed. Then, when electrical energy is needed, the flywheel’s inertia is used to turn a generator. The wheel will spin the generator’s rotor, and voila electricity, sorta like regenerative braking in an electric vehicle.²³ This makes for a very efficient mechanical battery.⁴

Flywheel Pros

Flywheels have a unique edge in energy storage—and that advantage becomes even clearer as we dive into their perks. But before we get into the latest breakthroughs, here’s the big question: why choose a flywheel over a chemical battery? Understanding this is key to seeing why flywheels are really picking up speed.

Flywheels are especially valuable for pairing with renewables like wind and solar, which have variable output due to shifting conditions. For example, wind turbines produce more power in high winds and none when still, leading to inconsistent power output (amplitude).

A major advantage of flywheels is their rotational inertia. As heavy, fast-spinning devices, they can store and release energy quickly, stabilizing the grid during short-term power fluctuations. This inertia is crucial for grid stability, especially with renewables that don’t provide constant output. While most power generation, including renewables with inverters, needs to carefully control frequency to deliver the standard 60 Hz AC in the U.S. (or 50 Hz elsewhere), flywheels help with this by operating at steady speeds, minimizing conversion losses.

Though flywheels can spin at extremely high speeds (up to 50,000 rpm), their mechanical inertia primarily addresses amplitude fluctuations, making them a steady partner for renewables.⁵ Their rotational speed can be adjusted during spin-up and generation modes, resulting in less energy loss to heat. Plus, their ability to charge and discharge at supercapacitor speeds makes them incredibly responsive.¹ This quick responsiveness is invaluable for patching over grid dips and absorbing extra power during spikes. It’s also handy in emergencies.⁶

Another FESS perk is its high round-trip efficiency (RTE)—the percentage of stored electricity that can be retrieved. No system has perfect RTE; energy is always lost as it transfers between mediums, and flywheels do lose more energy over time than chemical batteries. But as long as you’re not storing that energy for extended periods, a flywheel can achieve an RTE of 90-95%.⁷ That’s better than pumped hydro or industrial-scale batteries, which reach about 80%.⁸⁹ This efficiency makes FESSs an excellent fit with renewables.

Flywheels offer other advantages, too. Their simplicity means they often last 30 years or more, and, unlike batteries, they don’t rely on chemical storage, so they can handle tens of thousands of full cycles without degradation.¹⁰ They also don’t require rare earth metals; the most advanced flywheels are made from carbon fiber,⁴ which, while not cheap at around $7 per pound, has dropped significantly from $15 a few years ago and is expected to continue falling as production becomes more widespread.¹¹

At this point, you might be wondering, ‘Why aren’t flywheels everywhere?’

Flywheel Drawbacks

If you’ve been wondering how a spinning wheel can store energy without eventually slowing to a stop, wonder no more—because it can’t. There’s still friction between the wheel and the generator, air resistance on the wheel, and many other points where energy converts to heat.

Some flywheels are built in a vacuum chamber to reduce air resistance, and others use magnetic bearings to ‘float’ the wheel instead of letting it drag on ball bearings. Even so, Newton’s first law is unforgiving: even a perfectly lubricated, magnetically levitated flywheel, with literal tons of inertia in a low-friction vacuum, will eventually slow down. Magnetic losses from eddy currents also slow the wheel, and these losses grow with rotational speed.¹² Without maintenance, flywheels tend to last about 15 minutes, limiting them to short-term applications.¹³

This also means flywheels have fast self-discharge rates compared to chemical batteries, losing about 5 to 20% of their energy per hour.¹⁴ At that rate, stored power could be gone in under a day, so using flywheels for long-term energy storage is a no-go.

Then there’s cost. Flywheels are still quite expensive, despite their overall cost-efficiency. Sure, you can have a functional FESS without extras like magnetic bearings, a vacuum chamber, or a high-tensile rotor. Just look at their use in the Swiss Gyro-Bus—often cited in flywheel discussions, including my own. But for grid-scale systems, optimization is crucial, requiring specialized materials, precision machining, and price tags that reflect these demands.⁶

And surprisingly, the lowest-tech part of a flywheel system is often the most expensive: the well. Flywheels are typically buried for a good reason—they’re massive, heavy components spinning up to 50,000 rpm, practically trying to tear themselves apart.¹ If anything goes wrong, you don’t want chunks of flywheel—or the entire machine—flying around. With that kind of weight and speed, it’s a recipe for disaster.⁶ Though rare, it’s still a risk that requires careful design. And while flywheels usually take up less real estate than chemical storage, the excavation alone is pricey.¹²

So, flywheels are highly effective in short bursts and overall a tidy energy storage device, but they come with their fair share of limitations. Cool-but-niche tech, so throw it back into the steampunk curiosities pile, right? Not so fast. Despite these challenges, flywheels really are gaining popularity.

Dinglun

Let’s start with the record-breaking flywheel facility currently online: the 30 MW Dinglun Flywheel Energy Storage Power Station (or Dinglun, for short) in Changzhi, China. For context, that’s much smaller than the average coal power plant’s capacity,¹⁵ but a full 10 MW above the previous record holder in Stephentown, New York.¹⁶

The Dinglun project broke ground in July 2023 and was connected to the grid by September 2024—a remarkably quick turnaround.¹⁷ It’s equipped with 120 high-speed magnetic levitation flywheels, grouped into sets of 10, with a total of 12 energy storage and frequency regulation units forming an array that connects to the power grid at a voltage level of 110 kV. That’s a lot of power.¹⁸

Interestingly, this system is pioneering a sort of half-buried style. Most of the tech is housed underground in a setup reminiscent of wells with windows, allowing Dinglun’s operators to monitor the wheels. This setup provides a safer, clearer view, so workers can easily assess operations and spot maintenance needs.¹⁶

The point of the project is two-fold. First, China is rapidly growing its green energy portfolio and, again, FESSs pair really well with wind and solar energy. It just patches over the energy shortfalls and intermittent zones of those technologies with ease. It acts like a buffer. That leads into Dinglun’s second point: it’s a pilot project. It’s still early, but if all goes well then we could see a lot for Dinglun-scale FESSs popping up around China to help all those other renewable energy facilities.¹⁷

Moneypoint

Let’s look at another record breaking flywheel. German company, Siemens, provided the largest single flywheel for Moneypoint station in County Clare, Ireland.¹⁹ Moneypoint is Ireland’s only coal plant, and with an output of 915 MW at its peak, it was the single largest power producer in Ireland and capable of providing 25% of the entire country’s energy.²⁰ Of course, these features also make it the single largest greenhouse gas emitter in Ireland. That’s why the country is enacting a multi-phase plan to transition the station into a green energy hub, dubbed Green Atlantic @ Moneypoint. Yes, the “@” is part of the official title.

Phase one of this plan is this massive flywheel. The rotor alone weighs over 130 tons! It’s a big boy. When combined with a smaller, 66 ton synchronous rotor condenser (a close cousin of a FESS) they produce around 160 megawatt-hours in the rotating mass, which can be tapped into on demand.²¹²²

With the flywheel up and rolling just days before 2023, Ireland’s ESB has turned to phase 2: the construction of a 1,400 MW offshore wind farm which will be connected to the FESS.²³ The following phases will revolve around turning the area into a wind turbine manufacturing hub as well as the production and storage of green hydrogen. This might give you pause considering hydrogen’s perennial status as a “any day now it’s going to solve all of our problems” kind of technology. But hey, at least they’ve already installed five wind turbines capable of producing 17MW, and the flywheel is up and running. Or should I say up and spinning?²³²⁴

Torus + Gardner Group

Let’s get back to the FESS-tivities but on a smaller scale. U.S.-based company Torus just signed a deal to supply the Gardner Group, a commercial real estate firm, with nearly 26 MWh of energy storage using Torus’s hybrid flywheel and battery energy storage systems (BESS).²⁵ FESS and BESS pair well because they have complementary strengths: FESS is great for short-term, reactive storage, while chemical batteries handle longer-term stuff.²⁶

Torus has also developed a proprietary software platform, enabling smart energy management, demand response, and integration with renewable energy sources. Apparently, it plays well with EV charging, too. Standard “smart monitoring” stuff perhaps, but still nice to see as we move to smarter grids, and putting flywheels in more places.²⁷

Torus and Gardner Group plan to begin installations of the FESS/BESS systems in Q4 2024, aiming for completion by Q1 2026. Once fully operational, these systems will be able to store and dispatch nearly 26 MWh of energy—approaching Dinglun-scale capacity. According to Torus, that’s enough to power nearly 1,000 homes for a full day.²⁵

The exact facilities Torus’s hybrid systems will support aren’t public yet. Historically, flywheels have been ideal for emergency power, not just for the grid but for energy-intensive spaces like data centers or hospitals. Gardner Group manages various commercial properties, from malls and offices to tech centers and even some residential communities, by the look of it.²⁸ Maybe flywheels are widening their niche a little bit. We’ll have to wait and see.

Key Energy + Amber Kinetics

Let’s end with a familiar face: California-based Amber Kinetics. We last checked in with them a few years ago when they debuted their all-steel M32 FESS.²⁹ Back then, the company had recently deployed some FESS units to the West Boylston Municipal Lighting Plant as part of a solar array. As of 2023, the plant still mentions its flywheel assets, so I’m guessing they’re still rolling—and in this case, no news is probably good news.³⁰ At the time, Amber Kinetics was also teaming up with Chinese company Yungao Renewables to support a 60 MW solar installation.³¹ As of 2024 that solar plant is still in operation, though I can find no mention of the flywheel system. That could be a language barrier issue, though.

Since then, Amber Kinetics has expanded further, installing flywheels in Hawaii, Taiwan, the Philippines, and beyond.³² These aren’t massive FESS installations like Moneypoint or large arrays like Dinglun, but it’s impressive to see just how many places have adopted Amber Kinetics’ flywheel tech.

Amber Kinetics is also partnering with Key Energy of Sydney, Australia, to install a system at a residence in Sawyers Valley, Perth. With wildfires being common, the owners wanted a system that posed no fire risk yet offered energy security, given frequent blackouts in the area.³³ Sounds like a job for a flywheel! Key Energy has developed a way to install Amber Kinetics’ flywheels above ground, avoiding the costly excavation. If they’ve truly found a cost-effective and reliable approach, this could make FESS a lot more affordable.

Amber Kinetics has also expressed interest in further expansion in the Philippines. Infrastructure is a challenge on this tropical island chain—though honestly, it’s tough to maintain anywhere. In rural parts of Mindanao, the second-largest island, high electricity costs and grid unreliability are common. Installing flywheels to stabilize microgrids could make a real difference for these communities.³⁴ As promising as this sounds, it’s just a stated goal for now, so we’ll have to wait and see if it comes to fruition.

Are flywheels the ultimate solution to all our energy problems? You’ve watched this far, so you know the answer is ‘no.’ But they’re incredibly effective for smoothing out grid fluctuations—a role that’s becoming more essential as we add more renewables like wind and solar to the grid. So, while flywheels may never be the star of the renewable energy storage world, they’re a pragmatic supporting cast member. And in the near future, I expect to see many facilities following in the footsteps of Dinglun, Moneypoint, and others.