Compact electrical generators of stunning power, wind turbine head weights halved, super-efficient power transmission with negligible line losses; it's a tantalising vision for the renewables sector. Making it a reality could transform the economics of wind energy, and that is a key aim of a number of forward-looking companies now bringing a new technology to market.
These firms, along with academic partners, have tackled the issue of electrical resistance, the phenomenon that accounts for massive aggregate power losses – mainly in the form of waste heat – in humankind's technological infrastructure. If resistance could be eliminated, or nearly so, more current would flow in a given wire or machine and more work would be done by a set amount of energy. The technology that could make it happen is superconductivity.
Ever since Dutch physicist Heike Kamlerlingh Onnes discovered, almost a century ago, that the metal mercury could, under certain conditions, lose its resistance to direct current and become a near-perfect conductor, there has been excitement about superconductivity. The sting in the tail was that mercury must be cooled to 4.2 degrees Kelvin – that is within a few degrees of −273 deg C, the absolute zero of temperature – before it will exhibit its quirky behaviour. Achieving this, for mercury and similar low-temperature superconductors (LTS), is an expensive high-tech undertaking that has held back the application of superconductivity ever since.
However, efforts to develop materials able to superconduct at higher, more achievable, temperatures have latterly borne fruit. The 1986 discovery by two IBM scientists that barium-doped lanthanum copper oxide becomes a superconductor at 36 K, some 12 K above the previous highest superconducting temperature, was considered a breakthrough. Other cuprates have since demonstrated transition temperatures of up to 130 K, and several of these can be sufficiently cooled by liquid nitrogen, which liquefies at 77 K, rather than by liquid helium and the expensive cryogenic coolers previously required. Liquid nitrogen is a widely accessible industrial cooling medium and can be used with these materials, dubbed high-temperature superconductors (HTS).
Today, development emphasis is on rare-earth cuprates, in particular yttrium-barium copper oxide (YBCO), though difficulties in producing this in continuous lengths suitable for wire and tape have until recently obliged engineers to rely on an earlier bismuth-strontium-calcium copper oxide (BSCCO) formulation that consequently became the first-generation superconductor workhorse.
Because this complex metal oxide has a ceramic-like brittleness and is difficult to bend, producers like the American Superconductor Corporation (AMSC) surround filaments of BSCCO with pliable silver when making the thick tape that they then wind into final cable. A pipe for nitrogen coolant also has to be incorporated. Even so, the resulting cable carries three to five times as much power as a copper cable the same size. It has proved possible to manufacture BSCCO conductor in kilometre-plus lengths.
Second-generation wires and tapes, based on the rare-earth cuprate YBCO, can be produced less expensively than the first generation by chemically coating the active material onto a nickel wire or other conductor substrate. Silver is not needed in the final product. Companies such as AMSC and SuperPower Inc have developed, with research input from research bodies like the USA's Oak Ridge National Laboratory and Sandia National Laboratory, continuous-feed coating processes suitable for producing 2G wire, which has now begun to supersede the first-generation product in live applications. The latest 2G power cables can conduct up to 10 times the amount of power comparable copper cables manage.
Superconductivity has begun to yield real benefits in pioneer applications. About 7 years ago, some 8 tonnes of copper cable in a main feed to Detroit, USA, was replaced with 110kg of first-generation superconducting cable. Three 120m lengths of cable take up just three of 9 previously-occupied underground ducts, leaving ample room for anticipated demand expansion. Since then there have been many more applications, mainly of 1G cable, but with a growing number of 2G applications now becoming evident.
Superconducting cables from companies like AMSC, SuperPower Inc, the Southwire Company, Ultera – a partnership between Southwire and Denmark's NKT Cables – Zenergy Power, Nexans SuperConductors, Sumitomo Electric Industries and others are contributing to high-power underground distribution networks in urban centres ranging from New York and Columbus in the USA, to Amsterdam in the Netherlands, Copenhagen in Denmark and Seoul in Korea.
AMSC recently shipped 17 km of HTS cable, manufactured by Ultera using AMSC 2G wire, for use in Consolidated Edison's Manhattan grid. This product, which will deliver 10 times more power than a copper equivalent, is called Secure Super Grids (SSG) cable because it will also suppress fault currents. This is by virtue of a characteristic of a superconductor that once its current carrying capacity reaches a natural limit, determined by magnetic and other factors rather than resistance, it ceases to conduct and becomes resistive, thereby blocking fault currents.
Other promising applications for superconductors include powerful electromagnets and the to-date elusive magnetic levitation (maglev) train; compact transformers, generators and motors; and power storage devices. Superconducting wire is in facilities ranging from mobile phone base stations to the Large Hadron Collider at the European Organisation for Nuclear Research (CERN) in Switzerland.
A few years ago AMSC heralded a likely revolution in marine propulsion by manufacturing a 5000hp motor a fifth the size of an equivalent copper-wired motor, and it has recently produced and tested, with Northrop Grumman, a 36.5 MW (49,000 hp) motor that is about half the size and weight of a conventional equivalent. A motor operated in reverse i.e. converting mechanical power into electrical power rather than vice-versa, is a generator and superconducting wire was key to a 100 MW super-generator developed by the General Electric Company under the US Department of Energy's Superconductivity Partnership Initiative.
Benefiting wind turbines
If HTS superconductor cables can live up to their promise of cutting grid transmission losses at acceptable expense, this will help the viability of wind farms that must transmit their power over long distances to established distribution networks. For example, AMSC ‘superconductor electricity pipelines’ are being considered for the proposed US grid that will link wind and other renewable resources in the inland states to the largest centres of population which, in the main, are near the coast.
However they can do more than this, becoming integral to wind turbines themselves. According to Zenergy Power PLC, a UK-headquartered manufacturer and developer of commercial applications for superconductive materials, achieving WT generators a third the size and a quarter the weight of their conventional equivalents will greatly facilitate the construction and deployment of large wind turbines, particularly future offshore units of up to 10 MW. It will also, claims a spokesperson, cut electricity generation cost by up to a quarter.
The company, partnering French electrical systems specialist Converteam Group SAS, is two years into a five-year agreement under which the partners are jointly developing HTS generators for the wind and small hydro power markets. In particular, Converteam is leading a UK BERR - formerly UK Department of Trade and Industry (DTI) - funded project to design an 8 MW direct-drive superconducting wind generator based on Zenergy's HTS wire. The partners, who regard offshore wind as a large and commercially viable market for HTS technology, are preparing to test the first HTS wind turbine this year.
As Michael Fitzgerald, chairman of Zenergy, explained, “wind power generation represents the most mature source of renewable energy production. Converteam shares this belief [with us] and has stated its intention to be at the forefront of this industry. We are excited to be working together on developing cutting-edge technologies based around our patented materials and products.”
Pierre Bastide, president and ceo of Converteam adds, “we believe that the extraordinary electrical efficiency and power density enjoyed by HTS wind turbines represent the most viable solution for overcoming technical and economic challenges facing the renewable power generation industry.”
Direct drive is favoured for this and other projects because it eliminates the gearbox and reduces the number of bearings and other failure-prone components, thereby reducing WT maintenance needs and operating costs. Use of HTS-based superconducting magnets enhances the viability of such machines, not least by transforming their power-to-weight ratio. One industry pundit suggests that if the cost of HTS materials like YBCO decreases as anticipated, superconductive wind turbines with rated MW capacities into double figures could be seen within the next five years.
AMSC, which entered the wind energy business as a logical extension of its original focus on electrical power distribution, is working with its wholly owned subsidiary AMSC Windtec of Austria to analyse the costs of a 10 MW-class wind turbine incorporating a direct drive superconductor generator. The results will be used by the US National Wind Technology Center (NWTC) to benchmark and evaluate the turbine's economic impact, in terms of both its initial cost and its overall cost of energy.
The NWTC is part of the US Department of Energy's National Renewable Energy Laboratory (NREL), the director of which, Dan Arvizu, said after a cooperative research and development agreement had been concluded with the DoE early this year, “high-temperature superconductors hold promise for helping to lower the overall cost of wind energy. We are pleased to be teaming with AMSC to move this technology forward.”
General manager of AMSC's superconductor business Dan McGahn commented, “superconductors are today proving their tremendous power density and efficiency advantages to electric utilities and large power users. This program brings those same benefits to the rapidly growing wind power market.”
AMSC continues to work with the TECO Westinghouse Motor Company to develop HTS and related technologies for a 10 MW-class offshore wind turbine. A US$6.8 million, 30-month design project, on-going since 2007, is 50% funded by the National Institute of Science and Technology's Advanced Technology Program. The partners say that superconductor technology will make it much easier to break the 10 MW barrier for wind turbine power, a new turbine being a fraction of the size and half the weight of a conventional direct drive machine of equal power. A 10 MW HTS-based machine is expected to weigh around 120 tonnes rather than the 300 tonnes likely for a conventional direct drive 10 MW turbine.
According to Jason Fredette, director of investor and media relations at AMSC, the UK is seen as a major target market. He argues, “the UK is talking about tens of Gigawatts of new capacity, up to 33 GW; you can either put up 7,000 to 8,000 smaller wind turbines or 3,000 to 4,000 turbines in the 10 MW class. Our objective is to have a turbine ready for when offshore wind really takes of in the middle of the next decade. That gives us time to commercialise the system.”
AMSC is already known in the UK energy sector for its voltage control systems, including its D-VAR dynamic control system that allows wind farms to be connected to the grid in accordance with UK grid codes. The 10 MW turbine project will also benefit from the company's involvement in a US$100m programme for the US Navy, under which it has developed a 36.5 MW ship propulsion motor using coils of HTS wire rather than conventional copper wire.
Fredette points out that AMSC has been working on power dense machines for 17 years so that the technology is proven. He says that AMSC would not build the turbine itself, but would supply superconductor components to a UK or northern European partner who would construct and supply the final product.
Nevertheless, AMSC is more deeply involved in the wind industry than this suggests. Its acquisition of Austria's Windtec GmbH allows it to design turbines and licence these designs to customers. A number of clients in Europe and the Far East are now producing, or preparing to do so, Windtec-designed turbines. For instance, the company has licensed its WT1650 model (1.65 MW) to Turkish company Model Enerji Ltd, and this is likely to be followed by proprietary 2 and 2.5 MW designs.
In China it is providing the XJ Group Corporation with designs for its WT2000 double-fed induction wind turbine, while additionally supplying core WT components for the Chinese company's own designs. It is doing a similar thing for the Shenyang Blower Works Group and gets to supply full electrical systems for all SBW's wind turbines. CSR Zhuzhou Electric Locomotive Research Institute Company has ordered core WT components for 1.65 MW machines.
Beijing-based Sinovel Wind Corporation Limited has ordered US$18m worth of AMSC systems and components to be deployed in 3 MW machines being developed by AMSC Windtec. Machines of 5 MW are expected to follow. Another Chinese customer, wind turbine producer the Dongfang Steam Turbine Works, is building 2.5MW turbines to a Windtec design.
Korea's Hyundai Heavy Industries has acquired AMSC designs for 1.65 and 2 MW models it intends to start producing this year. Canada's AAER Inc has ordered core electrical components for twenty 1.5 MW machines, a follow-up to previous orders, and is due to start producing a Windtec-designed 2 MW machine. Another licensee is Ghodawat Industries (India) Pty Ltd, starting with Windtec's WT1650 technology.
While none of these involvements result in sales of superconductive elements directly, the overall activity increases AMSC's engagement with wind energy, provides a revenue stream that helps fund development of WT applications for superconductors and positions the company to inject superconductor solutions into key wind energy markets as the technology develops.
Superconductivity can also play its part in enabling low-wind sites to be productive. Developers of a ‘magnetic levitation’ wind turbine generator - unveiled at the 2006 Wind Power Asia Exhibition in Beijing - said it could create new opportunities in low wind areas worldwide, helping to harness previously untapable resources. China's Academy of Sciences and Guangzhou Energy Research Institute added that their maglev generator could boost generating capacity to a fifth more than traditional turbines, while halving wind farm operating expenses.
Superconductivity's time as little more than a tantalising dream may now be past, with the wind energy sector being a likely leader in its adoption and commercialisation. The technology's ability to practically double the power available from a turbine of given size and weight is compounded by its potential to lower the cost of transmitting and distributing the power generated at wind farms. Benefits in other renewable sectors, such as hydro, current and wave power, could follow. Investors will be taking careful note of further developments as the technology continues to transition from dream to reality. For renewables, and wind in particular, it is a potential game changer.
|About the author |
|George Marsh is a technology correspondent for Renewable Energy Focus magazine. |