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Using Sea Water and Gravity to Generate Power

October 15, 1997

The experimental facility in Okinawa could be a sign of power plants to come. (Photo: Electric Power Development Co.)

The world's first experimental power station that uses sea water pumped to and released from a height is being built in the north of the island of Okinawa. The brine is raised during the night, when electricity-consumption rates are low, and released from a storage reservoir on high land to generate power during the day, when consumption rises. If the trial plant is successful, it is hoped that Japan, with its long coastline, will be able to more easily solve its problems of finding sites for power generation.

The Inexhaustible Sea
This project at the village of Kunigami is being jointly undertaken by the Ministry of International Trade and Industry and a company engaged in power-resource development. It is a globally unprecedented attempt to find out whether it is feasible to use sea water in a power station using the pumped-water principle. Trial operation is slated to begin in the spring of 1999.

At Kunigami, water is lifted by pump to a reservoir 150 meters above sea level and released en masse to generate electricity at an underground facility before being returned to the sea. Power can be supplied when needed at any time after the preliminary pumping work is complete. Maximum capacity is 30,000 kilowatts for six hours, enough to supply households accounting for 30,000 people.

If it turns out to be feasible to use the inexhaustible resources of the sea in this way to generate power, it will be possible to consider far more locations for power stations in Japan. But there are many technical hurdles, including impregnation of turbines and other generation equipment against sea water corrosion, and stopping saline damage to the surrounding environment. Designers have come up with solutions such as using strong fiberglass-reinforced plastic for the conduits that direct water to the turbines and making the turbines themselves of special stainless steel.

Environmental Concerns
Kunigami is in an area that is home to the Okinawa rail, designated as a Japanese natural monument, and other important flora and fauna including the Ryukyu yamagame (land turtle). This has sparked environmental concerns. The power-resource development company has replaced the normally U-shaped gutters along the access roads with channels featuring sloping walls so that animals that fall in can get out. And workers are made to carry a guide for recognition of important fauna, so that they can report on numbers and species encountered on the site. In this way, the project is working to keep tabs on the natural environment for its preservation.

The Need for New Power Sources
Electricity consumption in Japan peaks in August, the month of intensively televised high-school baseball and heavy air-conditioner demand. At this time, it is 37%higher than in April, at the other end of the usage scale. And daytime demand for electricity can be four times as great as night-time use. Electricity companies are supposed to be able to meet peak demand, but nuclear power needs consistency of output; it cannot respond to variations in demand. For this reason, pumping up sea water is seen as a means of complementing nuclear power at peak times. In recent decades, demand for power has risen rapidly. Total electrical output in 1975 was 7.1 million kilowatts. This had doubled by 1982, and in fiscal 1990 output was 18 million kilowatts. Major electricity companies are already proceeding with plans to meet demand with some of the world's biggest power-generating facilities: One will begin turning out 2.16 million kilowatts in fiscal 2002 and a 2.7-million-kilowatt plant will go online in fiscal 2003.

The coming age of information technology is expected to place an even greater burden on power resources and further stretch the bounds of fluctuation in demand. Thus the importance of this new power-generation technology will gradually grow. The greater the distance between the reservoir and the underground generator, the greater the amount of power that can be generated. For this reason, the best locations for this kind of facility are those that take advantage of cliffs and other rugged coastal terrains. If this trial is successful, places near Tokyo, such as Izu, could become candidate sites for power generation. This would raise the prospect of reduced power-distribution costs, since facilities would be close to an area of concentrated demand.

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