The demand for electricity generated by wind turbines grows each year, with legions of engineers working frantically to implement new technology. In this article we will look at the “bleeding edge” of generator design: the emerging use of superconducting materials to produce direct drive generators that are in theory capable of generating 10 MW of power and beyond.
As we saw in a previous article in this series, engineers like to increase the given output per turbine, in order to reduce the number of towers that need to be installed. However, in the process of increasing that output, we run into the challenge of ever-increasing mass at the top of the tower, and problems with mechanical reliability. Although permanent magnet-based hybrid and direct drive generators can increase the power output with less mass in the nacelle, even these configurations have their limit, as mass inevitable creeps up with increased power rating. The prevailing wisdom in the utility-scale wind energy industry is that for present technology, this practical limit is somewhere around 6-8 MW. So – how do we go beyond this limit?
Enter the superconductors. Scientists have known for decades that below a critical temperature, certain materials exhibit zero resistance to electrical current – a phenomenon known as superconductivity. However, because these material-specific critical temperatures were so low [below 30 K (-405 °F)] these materials had few practical applications. In the late 1980s, a new family of superconducting materials were discovered, including the synthesis of an yttrium-based [YBCO] material with a critical temperature of 93 K (see Figure 1). Since then, materials with even higher critical temperatures have been discovered. This family of so-called high temperature superconducting [HTS] materials was hugely significant, because their critical temperatures were greater than the boiling point of liquid nitrogen. This meant that for the first time, practical applications might be viable since the liquid nitrogen cryogen needed to cool the devices was inexpensive [did you know that the cost of liquid nitrogen is actually less than that of milk?!].
After their discovery, tremendous amounts of effort were spent on producing cost-effective HTS materials that could be manufactured into coils, wires and cables. As a result, a whole family of superconducting electromagnets came on the market, used for anything from SQUID magnetometers to the control systems in particle accelerators. HTS coils are today able to carry more than 100-150 times the current of a conventional copper wire of similar size. It was inevitable, therefore, that attention would eventually turn to the use of HTS materials in motors and generators, particularly in applications where mass and bulk needed to be minimized, such as marine propulsion systems – and wind turbines.
There are a number of companies presently working on the development of HTS generator systems for wind turbines. In 2007, American Superconductor Corp [ASMC] began work in partnership with TECO-Westinghouse, on the design of a 10 MW wind turbine that would utilize a direct drive HTS generator system, as part of a National Institute of Science and Technology grant. ASMC has been working on large-scale HTS-based electrical machines for some time. In early 2009, they completed testing of a 36.5 MW HTS motor for the US Navy, in conjunction with Northrop Grumman (see Figure 2). Using their proprietary 344 YBCO HTS material, ASMC’s initial data indicates that the 10 MW wind turbine generator will weigh around 120 tonnes [264,500 lb], as opposed to an estimated 300 tonnes [661,000 lb] required for a permanent magnet direct drive generator to produce the same output.
Earlier this year, AMSC started work on a project with the Department of Energy’s National Renewable Energy Lab and its National Wind Technology Center [NWTC], to properly evaluate the economics of the 10 MW wind turbine. Interestingly, engineers at the NWTC have been quoted as saying that it may take as long as 10-15 years for us to see 10 MW+ commercially-available wind turbines based on HTS materials.
Advanced Magnet Lab and its subsidiary AML Energy [AMLE] are working to incorporate its proprietary Double-Helix technology into a 10 MW direct drive HTS generator system for wind turbines. AMLE claims that this technology, once proven, will be scalable to 20-30 MW power outputs. They report that the design will be 75% lighter and 50% smaller turbines than the best turbines available today, with greater efficiency and reliability of operation. They hope to have a demonstration unit built by 2011-2012. AMLE is also looking into applying their technology into hydroelectric turbines.
Since 2007, Zenergy Power and its partner Converteam have been working on a UK Department of Trade and Industry [DTI]-sponsored project to develop an 8 MW direct-drive wind turbine, based on Zenergy’s proprietary HTS coils (see Figure 4). This same team was recently in the news for their work on the world’s first HTS hydroelectricity generator. According to Zenergy, a key goal is to ramp up the manufacturing of HTS wires, so that the cost of these materials can be reduced. The first prototype of their turbine is scheduled for production and testing in 2010.
A particularly attractive feature of HTS generators in general, is that once they have been “charged” by passing current into the coil, so long as the coil remains at cryogenic temperatures, the current will not deteriorate. This further reduces the weight in the nacelle since the additional power supply needed to energize conventional induction generators, is eliminated.
After reviewing the current state of HTS-based systems, it would seem then, that although we are some years away from viable, 10 MW+ commercially-available wind turbines based on HTS generators, we are well on our way to achieving this output. If companies like those mentioned above are any indication, we’ll be pushing through to 15 MW and 20 MW in no time at all!
I hope that this article was of some use to you; I welcome feedback and am always looking to improve the content and quality of the Terra Magnetica blog. If you missed the previous article in this series, titled “How Does The Use of Permanent Magnets Make Wind Turbines More Reliable?“, you can find it here.