Japan has upwards of 40 pumped storage plants in operation and new river sites are becoming scarce, but peak demand is still rising. A plant under construction in Okinawa offers a solution: make use of the head between the coastal cliffs and sea level, using seawater as the medium. Janet Wood reports.

Using seawater to generate power is not a new idea for Japan’s Ministry of International Trade & Industry (MITI): the concept has been under investigation since the 1980s. The idea is attractive for several reasons, not the least of which is Japan’s long coastline, much of which is hilly and which should offer plenty of sites. Coastal sites would also be close to the many large cities and other high-load areas which are situated near the sea. Finally, using seawater means that there is no need to construct a lower pond for the station.

The first seawater pumped storage plant is now under construction in Japan’s southern island of Okinawa, under the auspices of MITI and the Electric Power Development Co (EPDC). Work on this project began in 1991, and it is now planned for completion in March 1999. This is some months later than the original 1998 completion date, but this is a demonstration project, not a commercial one, and research takes as high a priority as completion.

The plant is sited on the northeast of the island, facing the Pacific Ocean. Its upper pond is on a plateau, some 150m above sea level and 600m inland, in sedimentary rock made up mainly of phyllite. The pond, which is 25m deep and 252m across, is a simple octagonal shape, chosen partly because it required the minimum of earthworks and partly because the pond is to be lined with a rubber membrane, so a simple shape was preferred. To retain the natural look of the area the penstock, powerhouse and tailrace tunnels are all underground. The sea level outlet is sited where the surrounding coral reef is least developed, and surrounded by a breakwater to mitigate the effects of the water intake and discharge.

New technologies

In the pond, waterways and powerhouse new technologies are being employed to ensure both that this development project is as efficient as possible and to minimise its effect on the environment.

A major environmental concern is diffusion of the seawater into the area at the top of, and through, the cliff. It is here that the role of the pond liner is important. The material to be used – an ethylene propylene dien monomer (EPDM) – was chosen following tests on several types of rubber. EPDM had high marks for durability, waterholding, ease of installation and mechanical structure, and its suitability was confirmed in tests on:

•Ozone and UV deterioration.

•Watertightness across joins.

•Deterioration caused by marine organisms such as barnacles.

•Stability in the face of winds up to typhoon speed.

In the construction, a 2mm-thick sheet of EPDM is placed on a drainage layer 50cm thick, made up of gravel, with a polyester cushioning layer between the two. Pipes in the drainage layer detect the seawater ingress and pressure changes that would be caused by damage to the EPDM sheet. In this event the seawater is pumped from the drainage layer back into the pond so that it does not leak into the surroundings, and an alarm is raised.

The penstock and tailrace tunnels have to be designed to resist corrosion by seawater and damage from marine organisms. To meet this requirement the 300m-long straight section of the penstock is made from fibre reinforced plastic – the first time FRP has been used in such an application. The pipes are made of four layers of fibres, orientated alternately in a circumferential and longitudinal direction, topped by layers that protect against wear and against both acid and alkali corrosion. Sleeve joints minimise water head loss and prevent the adhesion of marine organisms. The remainder of the penstock, the curved section, is made of steel and has electrolytic protection, while the tailrace is concrete-lined with steel bars coated in epoxy resin. The filling material around the penstock is a mixture of fly ash, produced by a coal-fired power station, and concrete. The pump turbine and the generator are also required to show a very high corrosion resistance. EPDC specified that the pump-turbine runner and guide vanes must resist cavitation, wear and corrosion, and tested three types of stainless steel – two phase, austenite and mertensite. Austenite, which contains more molybdenum than conventional stainless steel, was found to offer the best resistance. As regards operation, EPDC was won over by the benefits of using a variable speed system, which allows both pumping and generating operations to be adjusted to meet the needs of the grid. The variable system is based on a gate-turnoff thyristor converter-inverter AC excitation system.

As IWP&DC went to press in late February, the pump turbine and generator were about to be put in place.

A question of water

Employing seawater in pumped storage offers an exciting opportunity, but also presents some interesting challenges. To a great extent the effects of seawater on the plant materials and on the surroundings are unknown, and when the Okinawa plant starts up in 1999 its main function will be to try to answer some of these questions. The plant is due to carry out test operations for around five years, investigating such issues as:

• Assessing the measures taken to prevent seawater permeating from the pond into the ground and groundwater.

•Investigating whether, and by how much, efficiency is reduced due to marine organisms adhering to the waterways and the turbine.

•Corrosion of metals by seawater passing at high pressure and speed.

•Maintaining a steady water discharge or input in conditions of different wave heights.

•Examining the impact of the project on marine organisms living near the output.

•Examining the effect on the region of seawater dispersed from the upper pond by the action of wind.

All these issues are still to be examined, but EPDC is excited about the prospect for its new plant. If all goes according to plan, by 2004 Okinawa should be on call to offer 30MW to bolster the overstretched local grid. And in the longer term, EPDC believes it will be the forerunner of a series of plants that will improve the efficiency of the grid and meet peak demands, right across Japan.