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The World’s Largest Flywheel

The World’s Largest Flywheel
July 16, 2024 Jacob Cunningham

The world’s largest flywheel is installed at the site of a re-purposed coal power plant, in County Glare, Ireland. But why go to all the trouble?

The flywheel is a solution to the somewhat lesser-known issue countries with high and growing wind power resources are facing; grid stability. A country’s grid must maintain a very specific alternating current (AC) frequency in order to operate, in the UK this is 50Hz. A deviation from this frequency by even 1Hz could cause widespread blackouts. As such, if a wind turbine is to be connected directly to the grid, its output must match the grid frequency.

Wind Turbine Technology Challenges

Considering the inconsistency of wind day to day and hour to hour, matching a turbine frequency to the grid is no simple task, and wind turbine technology has gone through several iterations to address this problem. Earlier turbines simply use a series of gears to increase the output rotation speed (and therefore frequency) and would only operate when the wind speed would allow the output to match the grid frequency; standing still if the wind speed was too low, and using breaks if the wind speed was too high. The problem with this system is that the gears in the turbine were experiencing a very high torque and were prone to cracking and failing, increasing maintenance costs. Additionally, slowing the turbine’s blades in strong winds caused them to flex and induced stresses on the blades, further increasing maintenance costs.

largest flywheel

Figure 1:  Wind Turbine.

Next, turbines began to implement rotatable blades, called variable speed turbines. Rotating the blades allowed for further control of the output frequency in the face of varying wind speeds, whilst reducing the stress on the turbine blades. The rotating blades can now also be used to spin the turbine further, getting more energy out of favourable weather conditions, however the issues with the gears remained.

The next step was to reduce maintenance cost of the gears. The solution? Remove the gears entirely. This is called direct drive, and instead of using gears to spin a small generator fast, it skips the gears and spins a large generator slowly. Direct drive is used in tandem with variable speed turbines.

Although both the variable speed turbines and direct drive turbines are improvements on their predecessor, they introduced a new problem. Because the variable speed wind turbine can be used to get more power out of the same wind speed by rotating the blades faster, the output power is no longer in sync with the grid’s required frequency. The solution to this, while quite simple, removes one of the benefits wind turbines have over other renewable sources of energy like solar panels. The solution is to convert the output power from the wind turbine into direct current (DC), and then convert it back into AC at the right frequency, this is a rather simple process which results in minimal power loss, it is the same process used to connect solar panels to a grid. The downside is that the wind turbines are no longer directly connected to the grid, and so power grid operators lose access to what is called the “inertia” of the wind turbines. This inertia is an important part of a power grid, other sources of power also have inertia such as gas and coal power plants. Inertia is essentially an inbuilt battery which allows very quick provision of energy at exactly the right frequency, for example, when everyone in the UK goes to turn their kettle on at the exact same time during an ad break in a popular TV show. Batteries cannot provide the same sort of quick release energy to the requirement for the energy to be converted to AC, the same goes for the turbines utilising variable speed blades.

Flywheel Solution

And now we come to the giant flywheel installed in a re-purposed coal power plant in Ireland. At 177 tonnes [1], it is the largest flywheel ever built and serves one purpose; providing inertia to Ireland’s power grid. It will be housed in a large vacuum chamber to reduce air resistance and excess power from renewable energy sources will spin the flywheel up, taking 20 minutes to reach its maximum speed. From there, a small trickle of energy will be needed to maintain its inertia. The official name for this system is a synchronous condenser, it is essentially a power storage facility, much like a battery storage facility or pumped storage hydropower, except in this case, the flywheel stores a small amount of energy but can release it extremely quickly to counteract small, fast changes in the power grid.

It is estimated that Ireland will need 5 more of these synchronous condensers to match its current wind energy goals. If this new system is proved successful in the long run, it is likely that we will see flywheel synchronous converters being implemented in other power grids which are focusing heavily on wind power, such as the UK and Texas.

References

[1] “Siemens Energy: World’s largest flywheel shipped”, PowerProgress, 14 April 2022.
(Online) Available: https://www.powerprogress.com/news/siemens-energy-world-s-largest-flywheel-shipped/8019928.article#:~:text=Siemens%20Energy%20said%20the%20world’s,developing%20at%20ESB’s%20Moneypoint%20site. [Accessed 15 July 2024].

Figure 1: Wind Turbine, unsplash.com, 14 September 2020. (Online) Available: https://unsplash.com/photos/white-wind-turbines-on-green-grass-field-during-sunset-9zjHwQRIJ7A  [Accessed 15 July 2024]

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