Power ring: This three-megawatt wind turbine uses permanent magnets and a design that makes it significantly lighter than a conventional geared turbine.
Siemens
Both Siemens and GE bet on direct-drive generators.
Wind turbine manufacturers are turning away from the industry-standard gearboxes and generators in a bid to boost the reliability and reduce the cost of wind power.
Siemens has begun selling a three-megawatt turbine using a so-called direct-drive system that replaces the conventional high-speed generator with a low-speed generator that eliminates the need for a gearbox. And last month, General Electric announced an investment of 340 million euros in manufacturing facilities to build its own four-megawatt direct-drive turbines for offshore wind farms.
Most observers say the industry's shift to direct-drive is a response to highly publicized gearbox failures. But Henrik Stiesdal, chief technology officer of Siemens's wind power unit, says that gearbox problems are overblown. He says Siemens is adopting direct-drive as a means of generating more energy at lower cost. "Turbines can be made more competitive through direct-drive," says Stiesdal.
Siemens's plans hinge on a new design that reduces the weight of the system's generator. In conventional wind turbines, the gearbox increases the speed of the wind-driven rotor several hundred fold, which radically reduces the size of the generator required. Direct-drive generators operate at the same speed as the turbine's blades and must therefore be much bigger--over four meters in diameter for Siemens's three-megawatt turbine. Yet Siemens claims that the turbine's entire nacelle weighs just 73 metric tons--12 tons less than that on its less powerful, gear-driven 2.3-megawatt turbines.
Much of the weight reduction comes from the use of permanent magnets in the generators' rotor--a trick that GE is also using. Conventional turbine generators use electromagnets--copper coils fed with electricity from the generator itself. Henk Polinder, an expert in permanent-magnet generators at Holland's Delft University of Technology, says that a 15-millimeter-thick segment of permanent magnets can generate the same magnetic field as a 10- to 15-centimeter section of copper coils.
Stiesdal says Siemens reduced weight further by inverting its generator's design. Rather than a steel rotor covered with permanent magnets spinning inside a stationary doughnut-shaped stator (the design GE is using in its four-megawatt direct-drive turbine) Siemens's rotor is a steel cylinder with permanent magnets on the inside, and this rotor spins around a column-like stator.
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Actually, on a TV documentary- can't remember the name, there is a gentleman working on a system of 6 250k generators that connect to an axis using tires. If one tire blows, the other 5 can keep going AND they can change out the tire while the turbine keeps producing. From the description, because they have two means of controlling the generator - variable field and the number of tires in contact with the shaft, the turbine would be able to operate at lower speeds.
would modulation work out if the inner generator element were a male cone and the outer element a female cone? To modulate, adjust one element axially to vary the air gap?
You bet there is concern about the rare earth elements needed to make permanent magnets. China has the monopoly of REEs.
I suggest you read China’s Rare Earth Elements Industry: What Can the West Learn, a report by the Institute for the Analysis of Global Security (IAGS) www.iags.org/rareeartIAGSh0310hurst.pdf.
China announced that by 2012 it no longer will export neodymium and several other REEs, rather, they will sell the finished products. Expect manufacturing to shift to China for the generators.
small mine owners, who already smuggle half of their REE output out of the country due to restrictions on some REEs already in place.
They don't have a monopoly. They have the lowest cost production. Other locations would have to invest heavily to produce REEs but they are in other places around the globe. The chinese also are lucky to have them not mixed with radioactive elements. In other places they largely are.
Another alternative is superconducting generator which generates large field and reduce total weight of the system
Permanent Magnets Don't Need REEs
There was also a recent article about a compound producing record strength permanent magnets using no REEs. IIRC it was CrN or FeN - a really surprising combo of elements producing the permanent Mag Field.
Going further, again IIRC, the research had gotten to the point of proving a stable magnetic field, a stable material, and a well-measured value for the field strength that was confirmed to break the max previously measured field strength for a permanent magnet.
Where they had NOT gotten was to the point of detailing a method of mass production...and thus they were even a step removed from being able to stay that a method of mass production would result in cost-effective magnets.
However - the more China charges for their REE magnets, the easier it is to create a cost-effective manufacturing method for these new magnets. Me personally, I'm pretty excited about this.
I'm also excited about the possibility of using Halbach arrays to limit the necessary material even more. Currently, these arrays aren't much used...but if they become commonly used, then we can use fewer REEs per generator *and* we can also use less of any expensive synthetic material such as the one I vaguely remember. Although this is neutral in terms of the benefits for REEs v. the benefits for CrN/FeN, either way it causes turbines to weigh less and cost less.
What I'd love to see is research on the effectiveness of Halbach intensification in curved magnets with given radii.
Re: Permanent Magnets Don't Need REEs
Please: tell me what exactly REEs means.
Re: Permanent Magnets Don't Need REEs
I was responding to RD who discussed REEs above, defining them as Rare Earth Elements - neodymium, etc (mostly the lanthanides). These metals include those like Nd that are crucial to the most powerful permanent magnets available. While other elements are needed for magnets such as NdFeB, the rare earths, or REEs, are the much rarer component and thus it is the amount of REEs available that ultimately limits production of these magnets. Therefore finding an alternative to REE magnets would dramatically reduce the cost of producing permanent-magnet generators and other vital tech components.
It also, btw, would contribute to evening out the trade balance between the US and China: a non-trivial contribution.
Why not try to use the hydraulic system for speed transmission? I donot think the direct-drive design is a good idea, the speed of blade is really too low, without certain transmission device to improve the rotation speed, the efficiency of generator is too low.
Lot of the failures are due to bearings dumping material into the gearbox (sealing bearing distortion problems?). Using hydraulics to separate the drive from the gearbox and generator helps get over this and has a big influence on the nacelle weight and hence structure as generator can be located in base. However you still need good cleanliness and condition monitoring of the hydraulic fluid, you need to cool it due to fact high power transmission needs high flows and pressure and you do lose some efficiencies. Alway wondered whether the micropitting evidenced in the bearing contact was due to a frequency of fretting vibration being transmitted from the vibration of the blades through the shaft structure.
And You Thought Peanut Butter Was Good
This has to be the best news since the creation of peanut butter for a lot of people. No more gears, no more gear oil to change and no more failed bearings - yippee! Any engineer will also tell you that eliminating even a small portion of the complexity of a wind turbine is also a good thing. We should have and could have done this years ago.
Now if we could just move most of the other electrical stuff to the ground level, we would be well on our way to meeting our goals of more cost effective energy.
Gearboxes are terribly wasteful especially step up ones, easily wasting 30% of power input. The AIR 400 for example has used Rare Earth Permanent Magnet Alternators since inception, but it runs at up to 400 prm so thats relatively easy. The trick with the large alternators is to have several hundred poles in the armature, with a thin PM rotor creating high frequency AC which is more efficiently transformed. We were building these at Inperial College London in the early 70's and I still have some parts in storage! Just imagine turning a conventional alternator inside out and using Permanent Magnet instead of coils. Not only do you save weight, but additional advantages are - up to 30% power gain (from not having to energise the magnets) AND electronic braking. If you short some of the the coils (tricky with a big machine, I know) the rotor will 'cog' and quickly stop, with no brake pads to wear out etc.
Great for serviceing.
Hydraulics are great , but a pain in mid-air and very expensive. They also wear considerably.
This is a real breakthrough.
Go PM at last!!
HaroldusO,
Gearboxes are not as inefficient as you claim. A typical 3 stage 1.5MW gearbox would have an input-to-output efficiency above 95%.
As for direct drive PM generator systems being more reliable than a gearbox and synchronous AC generator, this is also a fallacy. Statistically, the most common source of failure in a typical gearbox/DFIG wind turbine drivetrain are the power electronics. The PE's fail at a rate of 3 or 4 times that of a gearbox. However, a gearbox failure tends to result in more downtime overall since it is much more difficult to replace.
But to claim that replacing a gearbox/synchronous AC generator with a statistically less reliable direct drive PM generator and 100% power conversion PE system will improve reliability is untrue. The fact of the matter is that such a system will be statistically less reliable.
We need to also consider the root cause of gearbox failures. They are due to the large out-of-plane rotor moments that get transferred back to the gearbox causing edge loads/stress concentrations in the gear meshes and bearings. A direct drive generator's rotor shaft bearings will be subject to the same loads, and would likely have the same issues.
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ryuuguu
44 Comments
high speed turbines
Another way to get high speed without gear boxes is too put multiple smaller turbines on a single axis.
http://selsam.com/
Reply
carlhage
84 Comments
Re: high speed turbines
The reason (fewer) large turbines are used is the power is proportional to the square of the rotor size. So it would take 100 10m turbines to equal one 100m turbine. The 4GW turbine is only twice the size of a 1GW machine. Also the larger ones are usually taller and get better wind higher up (power grows with the cube of wind speed).
Enercon has been making gearless turbines for many years. I wondered why the other companies were still building gearboxes. Enercon uses a large ring rotor with lots of poles in lieu of a gearbox with the usual 3.
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Aviator
1 Comment
Re: high speed turbines
Good comment! Believe one of the reason was patents owned by Enercon / GE. Glad to see competition emerging towards what I see as the obvious future of wind power. Weight advantage is really important for land base installation.
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