WIND turbines may soon get a supercharge. Turbines wound with superconducting wire instead of regular copper could turn today's 2 to 3-megawatt generators into 10-megawatt powerhouses, say teams in Europe and the US that are racing to produce the machines.
At heart, a wind turbine is simple - a series of wire coils attached to the rotor blade spin in the presence of strong magnetic fields, provided by stationary magnets. This generates a current, but the resistance in copper wire limits the amount of current that can flow through the coils. Making the coils from a resistance-free superconductor would cut down on weight and boost power generation.
Using superconductors will not be easy, though, partly due to the ultra-low temperatures they require. Developing a coil that can be cooled while simultaneously rotating with the turbine blades is a big challenge. A research project dubbed Suprapower, funded by the European Union, kicked off in December to address this problem.
Holger Neumann at the Karlsruhe Institute of Technology in Germany and other members of the Suprapower consortium are betting on a new "high temperature" superconductor, magnesium diboride, which works at 20 kelvin. "It's light, easily made into wires and is really cheap compared with the old niobium-titanium superconductors, which needed cooling way down to 4 kelvin," Neumann says. That temperature difference might not sound much but it means, crucially, that cooling the magnesium diboride superconductor requires just one-seventh of the power.
The team will also have to build a casing, called a cryostat, in which the superconducting coil will be kept chilled by gaseous helium. This is tricky as its supporting structure will act as a "heat bridge" to the warmer world outside. Neumann thinks they have cracked the problem with a novel arrangement of an outer vacuum vessel and insulating inner layers of plastic and titanium.
But however good their technology, they have to contend with an unusual property of superconductors - when the wires sweep through a magnetic field, their ability to generate current is reduced. That means more coil turns would be needed to make up for the current loss, which would negate some of the weight savings and make the turbines more expensive to construct.
"Magnetic flux lines interfere with the wires' ability to transport electricity, lowering its performance," says Venkat Selvamanickam at the University of Houston, Texas, where the US government is funding work via its Advanced Research Projects Agency - Energy. Selvamanickam's team thinks they have found a way to solve this problem - adding 5-nanometre-wide particles of barium zirconate to the wire. The team found that this "pins" the magnetic flux lines in place as the wires sweep through the field, preventing the formation of swirling magnetic vortices that reduce current flow. So far they have eliminated 65 per cent of this current-limiting problem.
The US team claims to be within a few years of building their own 10-megawatt wind turbine, and says that their techniques could make superconducting wires attractive for distributing electricity as well as generation.
"If we can demonstrate this superconducting-wire technology in a wind turbine, we think it's more likely that it will make its way into the power cables of the electricity grid," says Selvamanickam.
Merrily spins as laser looks on
Lasers could slash wind-turbine power outages, say engineers at Chonbuk National University in South Korea. If the bolts securing turbine blades to a rotor begin to loosen, or blade mass is lost due to a lightning strike, a blade can strike the turbine tower and fall off. But monitoring for when a blade starts to go out of alignment is expensive as it involves peppering each blade with strain sensors.
A cheaper answer is to place a laser on the tower and instead measure the reflection time from every blade as it passes by. This way, deviation of all the blades is measured using just one low-cost sensor (Smart Materials and Structures, doi.org/j62).
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Wind turbines supercharged with superconductors
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