Back to the future – high capacity mechanical flywheels

Posted on June 5th, 2019 in News

“Meaningful innovation does not need to be based on outright invention. Rather, there is an exhilarating shortcut. It is based on bold, new combinations of already existing components that simultaneously unlock heightened levels of consumer value and reduce costs.” –  Gabor George Burt

Mechanical flywheels have been a part of human history for many centuries. From the potter’s flywheel, to more recent alterations and combinations, we are still exploring how new combinations of this simple mechanism are able to help us improve our lives and power our future with 100% renewable and clean energy.

Firstly, what is a mechanical flywheel? “A flywheel is a simple form of mechanical (kinetic) energy storage. Energy is stored by causing a disk or rotor to spin on its axis. Stored energy is proportional to the flywheel’s mass (more accurately, its mass moment of inertia) and the square of its rotational speed.” – Brown and Chvala, 2003. If the concept is so simple, and the technology has been available for decades, why isn’t it more popular as a storage technology?

Traditionally, the flywheel has been designed with high power and short duration capacity, and so has been used as an Uninterruptible Power Supply (UPS) system. And in the current market, this is where flywheels still see most use today and dominate over batteries. Examples of such technology are easily found and promoted, meaning that flywheels have been dismissed for medium-term storage applications.

However, our emerging technology – a high capacity, medium duration flywheel energy storage system – is different, as it combines several new components to give a high capacity, medium duration storage solution, designed for supporting intermittent solar generation. Each modular unit provides at least four hours of storage at peak discharge of 30kW, with 120 kWh rated storage capacity. Self-discharge losses are able to be reduced to the region of 1% loss within the first few hours, thanks to a combination of magnetic bearings, vacuum containment and other proprietary technologies and design features. These combine to make a storage technology that has some of the lowest cost per discharged kWh in the industry today, lasts longer at up to 30 years design life, with no material degradation in efficiency level over the project life.

What is different about this technology?

Some of the key highlights for a flywheel energy system is:

FeatureCommercial Benefit
Very high cycle life of 22,000 cycles. This is equivalent to around 2 full cycles per day, every day for 30 years (Note some manufacturers claim unlimited cycle life as the core mechanism degrades incredibly slowly, however we prefer to put a practical limit as ancillary elements are subject to degradation and replacement)Lowers the investment and price risk – Matching your storage to your panels and project life of 25-30 years
High capacity – 120 kWh per unit regardless of use Known performance means lower risk and

Lower complexity and O&M cost – with less units required to reach MWh scale, project installation is simpler, and there are less units that have a risk of failure leading to simpler and cheaper O&M

Minimal degradation of capacity and efficiency from ongoing use – maintain an 84.9% (peak) to 90% average efficiency level for the duration of the asset lifeImproved economies of scale – The asset can be treated like a normal industrial machine and utilised with minimal downtime or consideration of opportunity cost. This completely changes the economies of scale and encourages use rather than restricting it
Micro-cycling friendly – The flywheel is able to withstand rapid changes in direction of electrical flow, which allows it to trickle charge from solar and more effectively smooth production.Greater PPA revenues – Improves the predictability for power plants’ output and will likely allow greater PPA revenues from existing generation capacity
Greater temperature tolerance – the flywheel unit itself is all steel in construction and can therefore withstand much greater ambient temperature ranges, reducing the need for HVAC on the batteriesLower cost on a delivered kWh basis – For the same amount of usable storage capacity, there is less parasitic load as the units do not need to be air conditioned. (Note that power electronics would need to be, as with any storage system)

Greater reliability – in times of extreme heat, the storage will work when you most need it to

Fully recyclable – the flywheel, as an all steel construction is therefore recyclable at the end of its useful life with existing recycling methodsGreater salvage cost and lower remediation costs at end of project life
No chemical fire risk – as the construction is steel and does not include chemicalsExpanded areas of operability – such as remote locations or high fire risk areas.
Full depth of discharge friendly – the flywheel is able to do a full depth of discharge, whereas doing so with many other technologies will damage the battery and void warrantiesLower cost per delivered kWh – each unit is able to provide the full amount of stated capacity on discharge, reducing capacity over-sizing requirements and paying for unusable capacity
Low O&M – as the core elements do not degrade, reducing the need for a replace or repair scenario except for occasional balance of system replacementLower O&M cost and greater applicability for remote locations.


It can’t all be good, what are the drawbacks?

No technology is a silver bullet for all use cases, and all of them have some drawbacks. It would be unethical and unfair to cast this emerging technology as completely positive. The weaknesses of this system are as follows:

  • Slightly larger footprint compared to some battery technologies. A 20 foot container can hold 60kW/240 kWh storage, and generally this is placed in a concrete vault for safety
  • Risk of mechanical failure – though insignificant, there is a risk of mechanical failure. Our technology uses all-steel construction using appropriate alloys to ensure that the components do not disintegrate under use and stress, unlike carbon-fibre alternatives
  • Lower peak power rating – the technology is currently standard with a peak 30 kW power rating, reducing its effectiveness as a short-term peak power support case
  • Relatively higher self-discharge – although the charge can be stored for hours, not minutes like other mechanical solutions, it is not a medium or long term storage solution for weeks without use.
  • Heavy – these units are heavy at over 10 tonnes each and therefore are only suited for stationary applications

What are the best applications and use cases to unlock value at lower cost?

Let’s now focus on some of the best applications, as storage is an industry where the old adage “horses for courses” is never truer.

With these features, a scenario with twice daily energy arbitrage and time shifting, every day for 30 years, is unlocked and makes the best use of the technology’s unique properties. Energy arbitrage or time shifting is simply moving the generated electricity (or load, if an energy user) to more economic times of the day to make use of different time of use electricity rates. A generator may want to do this to take advantage of the afternoon and evening peak demand (and price) periods, or increase self-use if they are a so-called solar “prosumer”.

Smoothing of intermittent generation – with almost no impact from micro-cycling (rapid and frequent changes of direction of the battery current), these units support a medium to large-scale solar installation where maintaining the generator output will reduce FCAS penalties,

Matching long asset life and long project life requirements – this technology is perfectly suited for projects (or load profiles) set to exist for over 10-30 years, with requirements for twice daily to continuous use of the storage system.

Specific use cases

Although the range of example use cases is endless, some possible specific use case examples are for the flywheel to support:

  • Utility-scale solar generation projects
  • Cold storage and logistics warehouses with desire to use solar PV to reduce costs
  • Factories with night-time operations and demand profile and large (>30kW) solar installation
  • Other water pumping stations or flow control machinery
  • Agribusinesses that want to install solar water pumping stations for irrigation and high-pressure watering as well as diesel offset
  • Micro-grid support to reduce diesel generator use as a daily baseload or smoothing element

How do I find out more?

Key Energy is bringing this technology, among others, to Australia and is looking for interested parties to collaborate with on pilot demonstration projects. We will be at stand 314 in the emerging technologies section of the Australian Energy Storage Conference at ICC Sydney 13-14 June 2019. Come and enquire with us further to see whether our emerging technology could be a solution for your situation!

For more information please contact Howard Leong at