Wind turbines are already the size of skyscrapers – are there any limits to their size?

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In 2023, about 100 miles off the coast of North East England, the world’s largest wind turbines will start generating electricity. This first phase of the Dogger Bank offshore wind farm development uses General Electric’s Haliade X, a turbine that sits more than a quarter of a mile from the sea surface to the highest point of the blade tip.

If placed in London, it would be the third tallest building in the city, taller than One Canada Square in Canary Wharf and only 50 meters lower than the Shard. Each of its three blades would be longer than the height of Big Ben’s clock tower. And Dogger Bank will eventually have almost 300 such giants.

Comparison of a large wind turbine and famous buildings

Comparison of a large wind turbine and famous buildings

Just two decades have passed since the UK’s first offshore wind farm was built off the coast of North Wales. Each of his turbines was capable of producing 2 megawatts (MW) of electricity under ideal conditions – considered huge at the time. Haliade X, on the other hand, is capable of producing 13 MW of electricity, and the 15 MW turbines will only be available in a year or two.

So why are turbines growing so fast, and is there a limit to their size? In short, the first answer is to reduce energy costs, and the second is that there must be a limit – but no one has yet specified that.

Big turbines, cheap electricity

Just five years ago, the offshore wind industry hoped to bring energy prices down to under £100 per megawatt hour by 2020 with new projects in UK waters. Even at this level, projects would still rely on government subsidies to be economically viable compared to other types of electricity generation.

But in fact, costs quickly dropped to the point where offshore wind farm developers soon pledged to sell their electricity at much lower prices. Today, developers are building wind farms such as Dogger Bank, where they have committed to a price of less than £50 per megawatt hour. This makes offshore wind competitive with other forms of power generation, effectively eliminating the need for subsidies.

The main factor reducing these costs was the size of the turbine. Increasingly larger turbines were arriving on the market faster than anyone in the industry expected.

Dogger Bank offshore wind map

Dogger Bank offshore wind map

The blades must not rotate too fast

Theoretically, turbines can get bigger. After all, a larger blade draws energy from the wind over a larger area as it rotates, which generates more electricity.

However, there are some engineering limitations. One concerns blade erosion caused by collisions with raindrops and sea spray. For current designs, blade tip speed must be limited to 90 meters per second (which operates at nearly 200 miles per hour) to avoid erosion. Therefore, as turbines get bigger and their blades get longer, their rotors have to spin more slowly.

A consequence of having to slow down the rotor is that the blades must deflect the wind more to produce the same power. This results in significantly increased forces acting on the entire turbine. We can deal with these high forces, but only by increasing the mass and cost of the turbine. This means that the point where the turbine becomes unprofitable – the point where the extra cost is no longer worth the extra electricity generated – is reached much sooner than if the blade tips could move faster.

Also, as the blades get longer, they become more flexible. This makes it harder to fully control the aerodynamics of the airflow around them, and harder to ensure that the blades don’t hit the turbine tower in extreme wind conditions.

Logistics constraints

However, such engineering challenges can be solved in the long term. This means that wind turbines will be constrained more by manufacturing, installation and operation issues than by physical constraints in the design of the turbine.

Just transporting the blades and towers from the factory to the construction site and installing the turbine on site is a huge challenge. Each of these Big Ben-sized blades must be shipped in one piece. This requires huge ports, giant ships and cranes that can operate safely and reliably far offshore. This is where the limit most likely comes from.

Wind farm under construction

Wind farm under construction

These limitations are seen in practice in the UK, which is surrounded by windy and shallow seas, ideal for power generation. Even so, the UK is unlikely to meet its ambitious target of more than tripling its offshore wind capacity by 2030.

This is not because of technology or lack of offshore places. It is unlikely that the industry will be able to produce the turbines quickly enough, and the port infrastructure and the number of installation vessels, suitable cranes and workers with the required qualifications are unlikely to be sufficient.

So if the UK is to maximize the benefits to its economy from what has been a fantastic success so far, the focus now needs to be on pure cost-cutting on developing workforce skills and the offshore wind supply chain.

I’m sure turbines will get bigger, but I suspect at a slower rate than in recent years. And if the turbines are placed 100 miles offshore, will anyone care? After all, the audience will not see them.

Read more: Why a sunken island is the perfect place for the world’s largest wind farm

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This article has been republished from The Conversation under a Creative Commons license. Read the original article.

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Simon Hogg is Ørsted Professor of Renewable Energy at Durham University. Ørsted is a leading developer of offshore wind farms. Professor Hogg is the chairman of the Energi Coast Innovation Group (https://energicoast.co.uk/). It has received research funding for projects from partners in the offshore wind industry and from the UK Government Research Council.

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