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Superconductivity Promises More Efficient Generators

March 10, 1998

Superconducting energy-storage systems could provide more power with less waste. (Photo: Chubu Electric Power Co., Inc.)

Research into ways of saving electricity through superconductivity, in which resistance drops to zero at extremely low temperatures, has become a hot field. If superconductivity can be exploited in power generation, transmission, and storage, losses of electricity can be slashed, leading not only to cuts in energy use but also in carbon dioxide emissions. Over the past 10 years, researchers have made great advances in superconductor materials and cooling technology, and are abuzz with expectations of practical applications.

Leaner, Cleaner Power Plant
Superconductivity is the phenomenon whereby electrical resistance in solid materials disappears when they are cooled below a certain temperature. If a superconductor could be turned into an electrical circuit, the current would flow for ever because there would be no energy loss due to electrical resistance. Further, superconductors have a strong aversion to external magnetic fields. So if a magnet is placed on superconductors, forces of repulsion lift it so that it floats above.

Researchers have long been studying how these properties could be useful in energy conservation. In the last 10 years, improvements in superconductor material and cooling technology have finally brought practical applications in the generation, transmission, and storage of electricity onto the horizon. Because it does not use thermal energy, superconductivity holds out the promise of reduced carbon dioxide emissions. It is regarded as a potential energy-saving technology.

A municipal research team set up by electrical companies and manufacturers in Osaka is working on superconducting materials and related apparatus at an experimental center. The team hopes to develop a superconductive generator for practical use in around 2010. Roughly 2% of the electricity provided by a regular generator using powerful electromagnets is lost during generation in the form of heat caused by resistance and other factors. But if superconducting magnets are used, no power at all is lost to resistance. With this technology introduced at large power plants, the loss of electricity can be pushed down to as little as 1%, even after taking into account power consumed in the production of the liquid helium needed as coolant.

An efficiency improvement of just 1% makes a huge difference when it is achieved across an entire industry. In autumn 1997, a trial superconductor power generator developed at the Osaka center managed an output of 70,000 kilowatts, the highest ever for any superconductor generator in the world. Assuming that the technology were used in all thermal power plants in Japan, it is calculated that 5.5 million barrels of petroleum could be saved a year.

Cost-Effective Storage, Transmission
Chubu Electric Power Co. and Shikoku Electric Power Co., which supply electricity in those two regions of Japan, are developing ways of taking advantage of the ability of superconductors to repel magnetic fields as a solution to the problem of uneven demand for electricity. In Japan, consumption peaks in the daytime in the hot summer. Electrical companies have made massive capital investments to be able to meet this peak demand, but they could be much more efficient if it were possible to store power generated at night for daytime use. At the moment the main technology for this is pumping-up power plants, which raise water up to a reservoir using night electricity and generate power hydroelectrically during the day by releasing it. The trouble with this method is the need to use nearly a third of the initially generated electricity in pumping and other processes.

An alternative superconductor technology currently being researched is flywheel energy storage. A wheel containing magnets is floated above a superconductor material; by making it spin, electricity can be stored in the form of rotational energy. If electrical current is sent through coils wrapped around the wheel, the wheel will begin to revolve in accordance with motor principles. If air friction is removed by creating a vacuum around the wheel, it will continue to revolve without losing energy. The wheel can later be used to generate electricity as is needed. A far smaller site would suffice for this system than that needed by conventional generation facilities.

Researchers are also looking into ways of using superconductivity not only for generation and storage of electricity but also for its transmission. Sumitomo Electric Industries, Ltd. and Tokyo Electric Power Co. are jointly developing electrical cable in which the copper tubing is covered with insulator formed by twined tape made from superconductive wire material; as coolant, liquid nitrogen is made to flow outside and inside the wrapped copper tubing. In regular supply cables with copper wires, 50 kilowatts are lost to resistance each kilometer. But with superconductive materials, the loss is thought to be reducible to 35 kilowatts. And at 13 tons per kilometer, superconductor cable is one-tenth the weight of conventional cable, making it easier to handle. In addition, installation costs for underground cable could be cut by an estimated 10-20%. The goal is to have cables ready for practical use that are capable of handling output from 1-million-kilowatt class nuclear plants by round 2010.

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