In Depth: GE's monster wind turbine plans to leapfrog the rest

US giant GE Energy is squaring up to leapfrog the 5-8MW fleet of wind turbines being unveiled for the coming wave of offshore developments with a “transformational” 15MW machine that uses advances in technologies such as magnetic resonance imaging (MRI) and ­nanocomposites.

Under a $3m cost-share deal with the US Department of Energy, the power technology company’s research and development arm, GE Global Research (GEGR), aims to build a direct-drive turbine based on an innovative transmission design that uses ­superconducting technologies shown to have the potential to greatly reduce the ­total weight of a generator, while super-charging its production output.

Adapting superconducting electromagnet (SEM) technology created by GE Healthcare, GEGR reckons it can create a 15MW machine that has up to five times the torque density of its 4.1MW turbine.

The beauty of this capacity boost is that it would come via a compact ­drivetrain featuring a generator “in the same weight range” as the smaller model’s 85-tonne unit.

“The role we have is to anticipate the direction the wind industry is going in and what technologies will be needed to make this possible,” GEGR wind technology platform leader Keith Longtin tells Recharge.

“There have been quite a few studies showing that the 8MW range will have the best dollar-per-kW rating — our feeling here is that this [size] is still expensive power.”

With about 60% of the capital cost of offshore wind power lying with the balance of plant (BoP) and 40% with the turbine proper, “making the turbine bigger and more impactful without driving up the BoP [will mean] you’ll have a real improvement [in the cost of energy]”, he adds.

“To do this, you do need transformational technology — you can’t just scale up from existing industry products — and that is the power that superconducting technology offers.”

SEMs could eventually eclipse the permanent magnets (PMs) being used in large offshore turbines.

PMs, once magnetised, have the benefit of being able to produce “reactive power” — background energy movement — even when not being charged.

An SEM goes a step further, carrying a magnetic-field-generating ­current that has no electrical resistance, so energy “does not dissipate” and the generator can be ramped up to have a much higher power density than any PM generator on the market.

“The energy density of a ­superconducting electromagnet compared to the best permanent magnets out there is more than an order of magnitude higher [within the electromagnetic subsystem of coil, cryostat and steel housing],” says Kiruba Haran, manager of the electric machines lab at GEGR.

After accounting for “structure and other features” needed to integrate an SEM generator into the turbine, the net impact will be a “four-to five-times” improvement on the 4.1MW machine.

“In our healthcare business, we know how to generate very large magnetic fields for our MRI machines to increase signal-to-noise ratio using superconducting electromagnets — five times higher than in conventional electrical machines — [and although] the applications are different [the risks and cost structure are different] we would expect to get close to that five-times entitlement [for a superconducting magnet generator].”

The ultra-high-power magnetic field created by an SEM can also be “pushed” through air rather than traditional magnetic materials such as iron, which is part of the reason that SEM generators would be much lighter than comparable PM models. SEMs also do not need expensive rare-earth materials required in PM manufacturing.

Among the hurdles facing development of SEMs is that they must operate within a few ­degrees of absolute zero (-273ºC). To address this issue, GE is fashioning a ­novel rotating armature concept that would allow the generators to be equipped with cutting-edge cryogenic cooling technology.

“This is a central challenge,” says Haran. “What we need is [a nacelle architecture] that ­allows us to minimise the heat load from the superconductor and transmit torque occurring at very low temperatures along the shaft to the rotor, while not compromising the reliability of the turbine.”

GE’s 15MW monster will not be built on superconducting technology alone. Longer, stronger blades and advanced control systems are also under the ­microscope.

“Today’s rotors are generally in the order of 110-120 metres [in diameter] but to go to 160-180 ­metres, you can’t just keep adding more carbon to the blades [for strength], you need entirely new materials,” says Longtin.

“You need to change the technology curve so that when you do go bigger you are not adding more cost for the power that you are able to produce.”

GE is investigating the development of new ­nanocomposite materials that would allow blades to be made with “30-40% more strength from the strategic placing of these materials during the manufacturing process”.

It is also looking at advancing “high-lift, low-drag” aerofoil designs that capture more energy from the blade’s sweep while curbing the wind-wake that trails behind the turbine rotor.

“Controls are another key area that will help in growing turbines bigger, because a lot of the cost involved is associated with mana­ging extreme loads — ­lower [these] by 20-30% and all the other relevant component costs come down as well,” says Longtin.

The two-year project now under way is organised in two stages. From phase one, which is slated to run for six months, GE expects to produce the first concept drawings of the 15MW turbine. Phase two will see designs for the machine’s components firmed up.

Longtin says a floating foundation is likely to be the eventual home for GE’s mega-scale machine, although he notes “there is a lot of less-than-deep offshore that we can go after first, as well. We are exploring multiple approaches — as GE’s technology arm, that is our charter.”

Although a separate project started this year, GEGR is fleshing out the potential of developing nanocomposite technology to create a PM that uses 80% less rare-earth material than the conventional neodymium-iron-boron versions, while being a third more powerful.

Darius Snieckus, Bristol

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