Question of the week: Do we need a 10MW turbine?

As the next generation of 6MW+ turbines pushes into the seabed, the industry is looking to the possibility of creating even larger machines, with Siemens the latest company to indicate it is considering a 10MW turbine.

As with Sway's 10MW turbine - 10MW machines only exist on paper

We spoke to Siemens wind CTO Henrik Stiesdal, DNV GL head of turbine engineering Tim Camp and Hyunjoo Lee, leader of drivetrains at Narec, about the need for such a turbine and the practicalities of its development.

Question: Is a 10MW turbine a necessary and practical objective for the wind industry?

Henrik Stiesdal - CTO Siemens Wind Power

I am convinced that the trend towards larger turbines will continue at least to the 10MW class. Ultimately the physics of the square-cube law will limit the growth in size, but we are not there yet.

The fundamental motivation of larger turbines is the reduction in the cost of energy. For offshore turbines the infrastructure costs do not increase proportionally to the turbine size, so even if the specific cost per megawatt hour for the turbine itself were to be constant we would still see a lower overall cost of energy.

Ten-megawatt turbines can be made with today's technologies, and some of the subsystems actually benefit from the size increase – large generators are cheaper per torque than small ones, and many control and support systems are basically the same as in much smaller machines, thereby reducing in cost per megawatt.

The main challenges relate to the supply chain — at this time the world has no serial production of any industrial equipment of the relevant sizes. Larger machines are made, but only as single units or small batches. But here we will benefit from the industrialization of the 6MW class ongoing now – this will in itself prepare the suppliers for the next big step.

Tim Camp - Head of turbine engineering, DNV GL

There will be a need for a 10MW wind turbine, in time. Fundamentally, however, if you simply keep on scaling up the size of turbines then they become less economically attractive. Power is proportional to the linear dimension squared whereas costs are roughly proportional to the dimension cubed.

What justifies the bigger turbines we see offshore today is the higher balance of plant costs — cabling and foundations and so on — as well as O&M costs. Simply put, if you go to the expense of building an offshore grid connection and turbine support structures, then you need to put large turbines on top to justify the costs.

Our work looking at envisaged North Sea projects to date finds that turbines with a rating of 6-8MW are optimum today and that the lowest cost of energy is around that mark. We need some real experience in the 6-8MW bracket before we can move towards 10MW turbines with confidence.

Increasing the size of turbines is about the amount of energy you are going to get out of it in a year. Any company could increase a turbine's rated power by increasing the rated wind speed, but it is the energy capture that the area of the rotor allows that is more important.

And this offers challenges. Existing turbines are already comparable to or bigger than the largest aircraft. Previous increases in turbine size have relied on technical solutions from other industries; that's not the case with the 10MW turbines. Manufacturers have to find original solutions. This is particularly the case with main bearings and castings, which it would be hard to source from other industries, resulting in supply chain issues.

Current offshore wind turbines have evolved from their onshore cousins, and changes have been modest. There are higher tip speeds and marinisation features, but they are still generally three-bladed, upwind, pitch-controlled turbines. It is easy to see why — in high-investment projects no one wants to come out with anything that's too radical. However, offshore is quite a different design space and perhaps we should be looking for more radical concept changes to facilitate bigger machines offshore.

Hyunjoo Lee - Leader of drivetrains at Offshore Renewable Energy Catapult, National Renewable Energy Centre.

In the 10MW range, not only has AMSC announced the completion of its design around three years ago, but also Sinovel announced it completed the 10MW design last October — hence the possibility is actually nearing reality.

In more technical detail, the required rotor diameter for a 10MW wind turbine is about 160-200 metres, which will exceed the current longest blade developed for Samsung's 7MW turbine. In addition the offshore crane size required for the installation of a 10MW wind turbine is well covered by existing giant cranes, with a lifting capability well over 1,500-8,000 tonnes. The testing facilities for large blades and drivetrains have also been constructed and are ready to accept blades up to 100 metres in length and turbines rated up to 15MW.

Some people might point out more specific barriers, such as material limits for large main bearings, narrow air gap in big size generators or high ratio gearboxes, but we must remember these issues are not new and were raised when we started to develop and manufacture the 3MW and 5MW-rated turbines.

So, with a supply chain already geared up, and with more and more research and development taking place to take the industry to the next level we believe it is a possibility.

However, considering that it took more than ten years for industry to make turbines of 5-7MW commercially available, the development of a 10MW wind turbine is still in the early stages.