Like all machines, large wind turbines involve several engineering tradeoffs. Perhaps the most significant is the compromise between fast rotational speeds required by the generator and slow rotational speeds needed to prevent rotor and blade damage. It’s possible that superconducting field coils will make that tradeoff a thing of the past. If that's not enough of an enticement, the superconductors could cut the cost of a large turbine by an order of magnitude.
Rotational Speed vs Magnetic Field Strength
Utility-scale wind turbines spin slowly because large blades would be damaged by high rotational speeds. An AC induction generator - the kind typically found in large wind turbines - uses a field coil in to generate a relatively weak rotating magnetic field. The power generated is proportional to the strength of the magnetic field and the speed of rotation. To produce significant power with a weak magnetic field, the shaft must rotate very quickly. To resolve this, designers typically add a gearbox to convert the slow rotor speed into a fast generator speed. But gearboxes are heavy - up to 38% of a turbine’s weight - and expensive. Gearboxes require regular maintenance and are common source of turbine failure.
To generate more power with slower rotational speeds - thus eliminating the need for a gearbox - a stronger magnetic field is required. Smaller turbines - and even some utility-scale turbines - accomplish this by increasing the size of the generator itself and using rare-earth permanent magnets instead of field coils. This reduces turbine cost and eliminates the complexity and unreliability that’s inherent to gearboxes, but doesn’t decrease the weight since you’re trading gears for generator size. It also relies on rare-earth magnets whose long-term availability is questionable.
Strong Magnetic Field through Superconductors
The best of both worlds is to create a strong magnetic field using a small electromagnet. Since the strength of the magnetic field around a coil is proportional to the current flowing through the coil, better conductors produce stronger magnetic fields. Conventional field coils made of copper wire lose almost 10% of their energy to wire resistance. Not only does this waste energy, it also creates heat, which further increases resistance. Superconductors have virtually no resistance, eliminating the power loss and helping the coils to stay cool.
A team of engineers at University of Wollongong (Australia), led by Dr Md Shahriar Hossain, thinks that inexpensive superconductors may provide the key to lighter, cheaper turbines that don’t require rare-earth magnets.
Other turbine designs have used low-temperature superconductors, but those require liquid helium to cool the material down to superconducting temperatures. Dr Hossain’s proposed design uses superconductors made of magnesium diboride (MgB2), a material that achieves superconductivity at temperatures below 39K (-389F, -234C). Yeah, that’s still quite chilly, but they’re able to achieve the superconducting temperatures using an inexpensive cryocooler. I asked Dr. Hossain about the cooling apparatus and he elaborated:
“Cooling the superconductors in a device is, no doubt, the most challenging part. Now a days, off-the-shelf cryocooler is available in the market. Two stage cryocoolers will be used for cooling the rotating components. The cryocooler will be operated at the first stage at 55 K and the second stage at 20 K. At 20 K we have got very high critical density compared to the well-established Nb-based superconductors. These cryocoolers will operate with ambient temperature gas helium supplied from compressors located on the stationary side. The gas helium goes into the rotor and returns through rotary coupling in a closed loop. The cost of these cryogenic arrangement will still be cheaper than using the high temperature superconductor ($25/meter compared to $1/meter MgB2). Niobium based low temperature superconductor can be good option but only the downside is, it can’t operate without the liquid helium which is going to be unavailable very soon and becoming costly (now $25/liter). Many companies like China Techo-westing house, Convertim, GE, Siemens are exploring the 10MW generator using the high and low temperature superconductors but these will be very expensive design options. We believe that simple, lightweight, low-cost and high critical current density at 20 K properties make this superconductor very attractive for the industry. We have done our work on material testing and cable coil fabrication for the rotor and stator. We will start the engineering work very soon with US company.”
Dr. Hossain and his colleagues have developed a small-scale superconducting field coil, but they’re still working to optimize the material. They expect that to continue through 2015, when they plan to have a design that’s ready to scale up.
Here he is explaining the concept:
Same Power for One-Tenth of the Cost
The National Renewable Energy Laboratory (NREL) estimates that a conventional geared wind turbine costs about $1.5M per megawatt. Hossain believes that a superconducting turbine can be produced for about $150k per MW, about one-tenth the cost of a conventional turbine.
Depending on where you live, electricity from wind power may already be at or below the cost of electricity from fossil fuels; reducing the cost of a turbine by a factor of ten will have a dramatic impact on the overall cost of wind power. Maybe Bob Dylan was right!
Images and video courtesy of University of Wollongong