Japanese pumped storage embraces the ocean waves

In March 1999 construction of the world’s first seawater pumped storage power plant was completed in Japan. Called the Okinawa Yambaru station, the plant has a maximum output of 30MW, maximum operating head of 152m and maximum discharge of 26m3/sec.

Prior to construction a six-year study of the plant was started in 1981. Analytical studies, experiments and computer simulations addressed potential problems within a saltwater environment and how they may affect civil structures, electrical equipment and environmental consid-erations. Work on the seawater pumped storage plant began in 1987, with con-struction getting under way in 1991.

The unique feature of this scheme is that it is actually a demonstration project which, since commissioning, has entered into a five-year period of testing. So far, the plant has been operating successfully with over 3500hr of generation and pumping in the first year. The main areas under examination are:

•Infiltration and dispersion of land-stored seawater.

•Seawater corrosion of power plant materials.

•Fouling by marine creatures.

•Operation of a pumped storage plant in various sea conditions.

The above subjects were categorised into those which can be dealt with by existing engineering methods, and those for which solutions will have to be provided by new technology. The object of the testing is to verify the application of the new technological solutions to a commercially sized plant through the design, construction and operation of a demonstration plant.

An example of the innovative solutions applied to the demonstration plant included addressing the problem of storing seawater in a land environment. Remedies included lining the entire surface of the reservoir with a synthetic rubber sheeting to prevent seawater infiltrating the surrounding land strata. In addition fibre reinforced plastic (FRP-M) pipes were used for the penstock which is subjected to a high pressure, high speed flow of seawater, while improved austenitic stainless steel was used for the pump turbine runners and guide vanes to prevent corrosion.

The plant has been in operation almost every day since March 1999 and provides electricity for the power system on Okinawa main island. Operation, inspection and monitoring of the plant during the first year (April 1999 to March 2000) were carried out as follows:

•Operation:

Demonstration operations and monitoring of the plant were carried out throughout the year.

•Maintenance and inspections:

Periodic inspection and maintenance were carried out on 14-30 June 1999, and on 19 January-14 February 2000. Routine inspections of electrical equipment took place once a week, and civil inspections twice a week.

•Environmental monitoring:

Land environment surveys were carried out twice a year. There was observation of plants (June, November-December), animals (June and September), birds (November) and noise levels (June and October).

•Seawater dispersion/infiltration survey: Water quality surveys were carried out regularly throughout the year while marine environment surveys were done in August and January.

•Meteorology/hydrograph monitoring:

Meteorology and hydro-graph observations were done throughout the year.

Testing during the five-year period will be carried out through a variety of means:

•Routine inspections during normal operations consist of equipment performance checks, performance checks of sensors and monitoring equipment, and analysis of the data collected by automated sensors and recorders.

•Periodical plant inspections will be performed twice a year during normal operating conditions. In addition detailed inspections will be performed in the second and fifth years of the demonstration plant’s life.

•Monitoring will be performed using various types of measuring instru-mentation. Environmental monitoring will be carried out annually to determine any environmental impacts of the plant.

•Operating results during typhoons will also be considered.

Infiltration and dispersion

Seawater infiltration and dispersion under normal operations are observed by daily monitoring and inspection. To prevent seawater infiltrating into the surrounding land strata, the entire reservoir surface is lined with rubber sheeting. Assessment of the impervious nature of the sheeting started when reservoir inundation began in August 1998. No water leakage has been detected within the upper reservoir for nearly two years, including the test inundation period. However, if seawater leakage does occur it will be detected and collected in the inspection gallery.

To observe the effect of ozone, ultraviolet rays and seawater on the rubber sheeting, test samples attached to the sheets are collected regularly and their physical changes observed.

No substantial change has been observed during routine inspections and at reservoir draining.

Seawater dispersion is regularly checked by measuring the salt content in the surrounding atmosphere, rainwater and soil. As Okinawa main island is surrounded by the sea the salt content is usually high. So far, there is no sign of an increase caused by the operation of the upper reservoir. To confirm the effect of seawater infiltration and dispersion the flora and fauna in the surrounding environment, and the water quality in nearby streams and ponds, are also being monitored. No substantial changes have been observed so far.

Periodic inspections were performed in June 1999 and January 2000. The pump-turbine was pulled out for the inspection. The level of the upper reservoir was reduced virtually to the low water level. The intake, penstock, discharge tunnel and tailrace were left full of water and could be inspected by divers.

Condition of civil structures

Inspections of civil structures mainly consisted of visual inspections. The condition of the water systems, from the intake at the upper reservoir, through the penstock down to the tailrace opening to the ocean, were observed by video cameras carried by divers and by monitors on land.

The penstock interior was examined to a depth of 50m, the maximum depth for the divers. These observations indicated no marine creature adhesion inside the penstock. However, colonies of calico barnacles that favour water movement were found at some locations on the tailrace. The water-impervious sheet of the upper reservoir had no adhesion within the range where the water level normally changes. However, silt had accumulated below the low water level, where algae grew and some shell adhesion was observed.

A maximum of 20 species of fish and crustaceans were found in this newly created environment, although the variety changes with the seasons. Echinoids with sharp teeth were found among these creatures. Careful observation is required to find these as they may bite into the rubber sheets.

FRP(M) piping and stainless steel were the main materials used at this plant. Where this is not possible and conventional steel has been used, this is protected by coatings and electrolytic anti-corrosion measures.

The penstock, where FRP-M has been used, has not experienced any problems such as excessive erosion, cracks or deformation. Even visual inspections of the intake liner tubes and lower penstock piping, which were protected by coatings and electrolyses, reveal no anomalies. However, crevice corrosion has been found on the nuts and bolts of the inspection doors of the intake screen in the upper reservoir. The cause of this is now being observed.

Inspection requirements applicable to a seawater pumped storage plant have been added to those necessary for a plainwater pumped storage plant. Inspection data were compared to those obtained at the time of construction.

The inspection items and equipment conditions are discussed below. No problems which could impede plant operation have occurred so far, and the plant is operating successfully.

Pump-turbine assembly

Corrosion checks were made on the stainless steel parts of the pump-turbine. Visual inspections and tissue examinations were conducted on the stainless steel runner and guide vanes. These inspections indicated that material integrity was maintained and there was no deterioration in strength.

The coatings of conventional steel parts were also examined. External visual inspections revealed no fracture, flaking or swelling and conditions were generally normal. Some coat flaking had occurred on parts of the speed ring and lower draft tube near the pump screen for the seawater supply.

However, measurements of the coating strength were within an acceptable range. Corrosion was observed near the speed ring where the coat flaking had occurred, but this had not progressed into pitting.

Seawater supply system

The seawater supply system was monitored in detail. Corrosion was seen on the valve sheet surface and disc surface of the manually operated butterfly valve at the heat exchanger’s seawater inlet in the seawater supply system. The valve was made of bronze and corrosion was also developing on the disc surface of the seawater strainer inlet valve, formed from a similar material. The valve materials are being re-examined, as are the butterfly valves of other seawater systems.

Gap corrosion and heterogeneous metal corrosion, which had been a subject of concern, were confirmed in the seawater strainer system (although the scale of the corrosion was minor).

No scar, corrosion or rusting had occurred on the pipe linings, including the flange surfaces.

Pipe clogging due to marine creature fouling had been a matter of concern for small diameter piping in the seawater supply system. Inspection up to the present time shows no marine creature fouling inside equipment or piping.

Operating in typhoon conditions

The meteorological features of this region are such that the Okinawa island lies in a typhoon passage, where an average of eight typhoons occur each year. Due to the geographical location of the island, typhoons assault the island during their development stage and change their path nearer land.

As Okinawa experiences severe typhoons the following areas were a concern and verification tests were necessary:

•Seawater dispersion from the upper reservoir due to strong wind.

•Stability of water-impervious sheets under strong wind.

•Pumped storage and power generation operations under high waves during typhoons.

Large typhoons approached and passed Okinawa main island twice in 1999 (August and September).

Typhoon no7 passed through Okinawa main island on 1 August. Although this typhoon was the first to affect the island after the start of operation,

the plant operated normally before, during and after the passage of this typhoon.

The waves had little effect on plant operation. Stable generation and pumped storage were continued according to the direction of the automatic load regulator. Vibration and shaft displacement did not differ from normal conditions.

The bearing temperature also did not change.

During power generation in the typhoon the effective head of the plant fluctuated by approximately 50cm (a one-minute average), according to variations in the tailrace water level, and the guide vanes acted to maintain constant output level. The fluctuation of the output level varied by 1-2%.

This was a little larger than under normal operating conditions, but the hydraulic piston position that actuated the guide vanes was not required to move excessively to maintain a relatively constant output.

During pumped storage operation in the typhoon, the tailrace level was relatively stable and plant conditions were similar to those in normal plant operation.

The phenomenon of the upper reservoir’s water-impervious sheets inflating due to the negative pressure of the typhoon was again observed. The sheets were designed to be problem-free, even when they are inflated to an 8.5m radius (50% of the distance between the fixtures under a maximum wind speed of 43m/sec). In this typhoon, with a maximum wind force of 35m/sec, the swelling due to inflation was only 1m or so.

Typhoon 18 approached Okinawa main island on 22 September and caused substantial damage to the island. Power supplies to 25% of the island’s households failed and the water level of the upper reservoir was maintained at the full water level. The maximum wind speed of this typhoon was 45m/sec. The inflation of rubber sheets was similar to the previous typhoon and salt was not dispersed to the neighbouring areas.

Serious equipment faults were not observed during the first year of operation at the demonstration plant. And despite two typhoons hitting the island the plant operated normally. Periodic inspections have not highlighted any major problems, and the first year of the demonstration test is considered to be a success.

The test plant is now operating continuously like a commercial facility. Demonstration testing will continue for the next four years and then engineering tests will be undertaken to commercialise the pumped storage plant system.