Flywheel Technology | Energy Storage

Steel flywheel for energy storage
A recent addition to the DBS tech portfolio, this energy storage product comes as part of an innovation at City University. Innovative changes to the geometry of the rotor have led to a dramatic reduction in rotor size and weight for a given amount of energy storage, leading to significant reductions in cost.

General Flywheel Characteristics

  1. Very rapid response time to full rated power (< 25ms).
  2. Excellent life expectancy (> 20 years) and no capacity degradation.
  3. Power rating and energy storage are independent: can be bespoke designed for specific applications.

Innovation

Due to a number of innovations in its geometry, the DBS flywheel has several advantages over competing flywheel technologies.
  1. By moving the central hole where the support shaft is located to several holes at the laminate periphery, the stress in the flywheel was halved
  2. The periphery holes are elliptical, further reducing stress in the steel.
  3. Tri-axial stresses are very low as they cannot propagate through the flywheel due to the laminations.

Advantages

High Safety

Use of laminations of steel instead of a solid forged piece dramatically improved the safety of the system because a catastrophic failure would only release a fraction of the energy contained rather than 100%.

High Energy Density

The geometry improvements and intrinsic safety combine to produce a significant energy density improvement as compared to all other high-speed steel flywheels and enables the de-rating of the motor as less torque is required.

Suitable for Mass Manufacture

​Using laminates rather than a solid piece has a number of manufacturing benefits:
  1. Laminates have better material properties as they are cold rolled rather than forged
  2. They are cheaper and easier to manufacture than alternative steel rotors and are already being manufactured in high volumes.

Low Cost

The above characteristics all contribute to the technology being lower cost than its competitors.

Alternative technologies

Composite Flywheels

Composite materials are much stronger in tension than steel, leading to higher possible rotational speeds and therefore greater energy stored due to the square relationship between energy stored and rotational speed. However composites are much more expensive than steel which is why composite flywheels are only superior in situations where low weight is a driver. Steel is a better option where low cost is a driver.

Due to the innovations in the geometry of the rotor, our flywheel has an energy density that is 4 times a conventional steel flywheel, meaning that it takes up a similar volume for a given amount of energy stored.

Chemical Batteries

Batteries have very high energy densities and are a mature technology which means they are low in cost. However they have significant capacity degradation issues, exacerbated by a number of common operating conditions, such as high temperature, high charge/discharge rate and normal cycling. This means that the life expectancy of a battery is very low (3-4 years) and hence their operating cost (due to replacement) is very high.
Lithium-ion batteries tend to have longer life expectancy the lead-acid and are not as affected by changes in temperature. However Li-ions are also considerably more expensive (3-4 times) and this means that the long term cost is similar.
The following diagrams show that the flywheel’s main commercial advantage lies in its long life expectancy which allows it to continue operating many years longer than a battery and this is what leads to its exceptional price per kWh cycle over its lifetime despite a greater initial investment.

It should also be noted that although a composite flywheel has a greater specific energy (ie weighs less for a given unit of energy capacity), the steel flywheel has an equal or greater volumetric energy density (ie takes up s similar amount of space for a given unit of energy capacity). This partly because there must be a hollow section in the centre of a composite flywheel and partly because there containment must be much stronger and larger as a catastrophic failure of a composite flywheel is less predictable and can release 100% of its energy.

DBS’s laminated flywheel’s characteristics therefore pitch favourably against batteries and composite flywheel for applications where power density, long life, charge/discharge rate and cost are drivers.

Opportunity

Energy storage is seen as an enabling technology for renewable energy and technology. This is because of the inherent ​unpredictability of renewable power sources that can cause high power during periods of low demand and visa versa. Energy storage allows you to balance these inconsistent loads and flywheels have particular properties that make them suitable to certain applications in this sector.

Ground power

The pressure to reduce carbon emissions worldwide has dramatically increased the penetration of renewable power sources in the last few years. This has caused increasing second-by-second instability of the frequency of electricity grids which must remain within certain bounds (±1% of 50Hz in the UK) to avoid rolling blackouts. This very rapid response times required in this application mean the flywheels are well suited.

Regenerative braking

The automotive industry is also under a lot of pressure to reduce emissions and one promising idea is ‘regenerative braking’. This is where energy is recovered during braking and then re-used during acceleration. Currently this is partly achieved using chemical batteries, but due to the limited power capacity of batteries (high power charges/draws cause capacity degradation) they are not able to regenerate the majority of the braking energy and deteriorate over time. Flywheels are ideally suited to this application.

Current developments

Steel rotors for laminated flywheel prototype
We are currently involved in two development projects looking into this application. One with a Tier 1 manufacturer, developing a system for the automotive industry and the other looking into modernising the British rail network. The following images show our current prototypes and models.

In line with these projects, we have run the flywheel up to speed and completed burst tests demonstrating the systems inherent safety and technical feasibility.

Collaboration

We are still at an early stage of development and are always looking for partners to collaborate with to develop this technology. If you are interested, please contact us at enquiries@dynamicboost.com