Lighter drive pushes the 12+ MW wind turbine limit

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TKI Wind op Zee
Topsector Algemeen

Pioneering SME: Hydrautrans

Hydrautrans has developed an innovative drive train for wind turbines. A modular, mechanical-hydraulic system with unprecedented efficiency is an alternative to the usual direct drives. This innovation could save up to 40 per cent on weight and that is decisive for the next generation of wind turbines. "Now, it is up to a manufacturer who dares," says  Ernst van Zuijlen, together with Bart van Neerbos, both CEO of Hydrautrans.


For which problem did you find a solution?

“The familiar mechanical drivetrains are vulnerable, especially for big offshore wind turbines. That is why the offshore industry increasingly chooses direct drive generators. These generators rotate at the same speed as the rotor. For a rated power of 12 MW or more the dimensions, complexity, desired precision and weight of these slow rotating generators make installation increasingly difficult. A 12 MW generator weighs around 450 tons. With the connected parts, this adds up to around 600 tons. They have to be lifted in one go. The industry considers constructing larger vessels. Vessels that have day rates above several hundred thousand euros. The Hydrautrans Drive Train overcomes these challenges.”

What is the core of your solution?

"The rotor of the turbine rotates at 8 rpm on the main shaft. To produce electricity efficiently this rotational speed has to be increased to more than 1200 rpm. We achieve this by adding a large cylindrical gear wheel that drives 16 planetary gearboxes. Every gearbox drives a hydraulic pump; four pumps drive one hydraulic motor. These four motors drive two 6 MW+ generators that generate electricity. So, our Hydrautrans Drive Train consist of a mechanical power split and a modular hydraulic system that drives high speed - and thus much lighter – generators.”

What is so pioneering about your solution?

“The power split and the efficient hydraulic system. The high power density -  characteristic of hydraulic drivetrains – facilitates limiting the weight of the total drive train to less than 250 tons, including the generators. In addition, the hydraulic system is revolutionary. We apply a new generation of pumps and motors with extremely high efficiency. They work according to the ‘Floating Cup Technology’ which was developed by the Dutch company INNAS. Hydraulic drives are not new but so far they couldn’t be used in transmission systems: metal-to-metal contact causes wear and tear limiting the lifespan and, due to the resistance, limiting the efficiency. In the FCT power transmission takes place through hydraulic forces without any metal-to-metal contact. This enables the efficiency of around 99% per pump and motor. Revolutionary and unique!”

What is the added value of your system?

“First of all, weight saving and the related lower cost price. Because of the high power density, we can suffice with a lighter design. We have calculated that this can save up to 40% on weight, or 200-240 tons. This makes installation with existing, smaller vessels feasible, saving millions of installation costs. As a result, the foundations do not have to be as heavy.

In addition, our drive train is modular. You can split the system into components that can be installed using cranes that carry weights of ‘only’ 250 tons. So an installation or, later, maintenance contractor does not have to lift a nacelle of 600 tons in one go. During maintenance, an even smaller crane can exchange parts. Our partner Mammoet has developed a self-mounting crane system which can lift a load of 250 tons. Then you don't even need a jack-up installation vessel at all.

Our mechanical-hydraulic system is less complex, transparent and reliable. Because there is no wear due to friction from metal to metal, maintenance is also kept to a minimum. And if you have to replace something, you can do that per pump, motor or gearbox.  All that saves on costs. We have calculated that if take everything into account - from installation to maintenance - the costs per kWh can be reduced by 4 per cent.

At the end of our day, our system can push the boundaries of what is technically possible. We can scale up relatively easily up to 15-20 MW. Then the drive train is not the limiting factor but for example the rotor blades."

What are your challenges?

“There is a limited number of potential launching customers. The most important thing is to convince offshore wind turbine manufacturers of our system. They will not simply abandon their own direct drive technology. It is a million-dollar investment. They are reluctant. Even though Bart van Neerbos has earned his spurs by being the first to build a permanent magnet direct drive wind turbine with Lagerweij on the Maasvlakte.

In addition, financing is a challenge. We really have to start building, testing and optimizing pumps and motors. Although we receive project financing from TKI Wind op Zee for optimization, that is not nearly enough."

How far are you now?

“Last year we tested and demonstrated the operation of the hydraulic pumps and motors in a TKI Wind op Zee project. Now that we have completed this phase of Proof of Principle, we want to start producing the pumps and motors. This will be done by our partner VDL ETG. We have ‘interesting’ conversations with wind turbine manufacturers. The progress depends on the outcome of those talks. This is a decisive period. We are in the starting blocks. If we manage to connect at the right time, we will be an important player in the supply chain within two or three years.”


This article has been prepared in cooperation with the Offshore Wind Innovators.

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