Direct Drive Wind Turbines

A direct drive wind turbine converts rotor rotation to electrical power directly, without the use of a gear box. Traditional wind turbines use gearboxes to step up the rotational speed (about 100x) from the rotor to the generator, which makes electrical power. This article discusses direct drive wind turbine generators, including pros and cons.

Background

Modern wind turbine rotors spin around 8-16 revolutions per minute (RPM). This speed is far too slow for a typical generator, which needs over 1000 RPM. For this reason, a gearbox is used to step up the rotation speed roughly 100x from the rotor to the generator.

Wind turbine gearboxes are exposed to large and variable torque, for example when a wind gust hits the rotor. This wears on the gearbox and shortens its lifespan to about 7-10 years, making it the highest maintenance item in most wind farms. For more information about gearbox failures, see "Zeroing In on the No. 1 Cause of Gearbox Failures".

The high maintenance cost for gearboxes is one of the main motivations for direct drive turbines. Direct drive machines have no need for a gearbox; a special generator creates electrical power directly from the (low speed) rotor rotation.

Pros and cons

Advantages of direct drive turbines include

• less maintenance,
• simpler overall design, and
• increased efficiency (no power lost in the gearbox, better efficiency at lower wind speeds).

Disadvantages are

• they require rare earth materials that can be hard to access,
• mining the above materials can be very pollutive, and
• the upfront cost can be much higher.

The pros of direct drive include the avoidance of gearbox maintenance and improved efficiency. Traditional wind turbine generators becomes less efficient at lower wind speeds—the electrical power output is a smaller portion of the wind power absorbed. A direct drive essentially maintains its efficiency over the full range of wind speeds / power output. In addition, a traditional wind turbine suffers gearbox inefficiencies proportional to the number of stages. As turbines get larger, the number of stages required in the gearbox will increase, so gearbox losses will likely get worse.

There are three often-cited cons to direct drive technology. All are associated with the rare earth materials used in the direct drive generator. The materials are expensive, subject to being unavailable, and are often mined in an environmentally unfriendly way.

Two primary materials are neodymium and dysprosium. These materials are expensive, pushing the upfront price tag well above an equivalent turbine without direct drive. Of course, this is often more than made up for in decreased maintenance cost, depending on the time value of money among other things.

These materials are mined in only a few locations, mostly in China. This means manufacturers face political risk accessing these materials. For example, a deal to purchase the materials from China may get canceled due to politics, after a company has already designed a product around them.

The last problem with these materials is mining. Mining and processing neodymium in China has led to extensive environmental pollution, tarnishing the clean image of wind turbines.

A note on scale

As wind turbines reach the 8MW range, traditional direct drive generators must be very large and heavy, becoming less attractive relative to traditional geared machines. However, geared machines become less energy efficient at these larger sizes, as more stages are required in the gearbox (each subtracting a bit of energy). Most experts think that direct drive generators have more potential in future, larger, turbines. Many researchers are working on technologies to make direct drive generators cheaper, smaller and without reliance on problematic rare earth material.

More on gearbox costs

The cost of replacing a gearbox far exceeds the cost of the gearbox itself. First, when a gearbox fails, the turbine stops producing energy (money). The turbine can be down for longer than you think, because replacement requires a number of things to come together on a day with benign weather.

Gearboxes are very heavy and need to be transported to the (often remote) turbine location. Once the gearbox has arrived, it must wait for the proper crew, equipment and weather before being installed. A large (and expensive) crane must be transported to the turbine to remove the old gearbox and place the new one. To make matters worse, the installation must be abandoned and restarted later in some weather conditions (excess wind, lightning, …). A video of a wind turbine gearbox being replaced.

As you might imagine, all these things add up to a very costly process, typically over $300,000, plus the lost money when the wind turbine was inoperative.