August 13th, 2020
Between 1978 and 1986, in an international experiment initiated by Japan, an OWC system was tested for the first time on a large scale: The ship Kamai’ carried out three series of tests in the Japan Sea in which turbines with up to a megawatt of power output were installed. Only in 1998 was the idea of a floating OWC again taken up in Japan. The new Japanese prototype ’Mighty Whale’ however has a power output of only 110 kW (Figure 2). An Irish group is currently testing a reduced-scale floating construction at sea.
It has taken a similarly long time for the first continuation of coastal projects to be pursued, with an OWC of relatively high power output. From 1985 to 1988, the Kv№rn – er company in Toftestallen, Norway, tested an OWC built on the rocky coastline with a power output of 0.5 MW; it was constructed mainly of steel. Only since the end of the year 2000, on the island of Islay in Scotland, has a coastal OWC with a similar power output once again been in operation: LIMPET (Locally Installed Marine Power Energy Transformer, Figure 7). It was planned for an output power of 500 kW, but achieved only 250 kW in practice. It is constructed for the most part of concrete, as is a very similar project on the island of Pico, in the Azores, with 400 kW output power. The concrete design has been tested beginning already in 1983 in Sanze (Japan), from 1990 on the island of Dawanshan (China), and from 1988 on the island of Islay with considerably smaller prototype plants. Whether concrete will prove to be more enduring than steel is still under debate by the experts.
Breakwater structures usually consists of many concrete cubes of the size of a single or multiple-family house. For breakwater wave-energy power plants, one or more such cubes are modified in such a way that they can be employed as OWC chambers. The construction of these OWC chambers directly at the locations where they are to be used has proved to be the main problem for all of the test projects mentioned. At a location where waves are breaking onto the shore, people have to work continuously for several months – a dangerous, difficult and therefore expensive undertaking. For this reason, those construction engineers who built OWCs near Trivandrum (India) in 1990 and Sakata (Japan) in 1988 took a different route: Both were built as concrete caissons. These caissons were produced using established methods in a drydock. The firm ART (Applied Research and Technology) also built its steel wave-power plant OSREY (Ocean Swell powered Renewable EnergY) with 500 kW of output power at a shipyard. ART-OSREY demonstrated that even the installation of a previously-constructed wave breaker has its uncertainties: during the installation of the power plant in water 20 m deep off the Scottish coast, a severe storm came up. The structure was not designed to withstand such stresses during its installation phase, and it was destroyed.
All of the OWCs so far built have to be classed as test installations, with which construction techniques can be tried out and turbine technologies developed – although the
Indian OWC has already fed power into the local grid. ART-OSREY was supposed to be the first commercially-operating OWC prototype. A current demonstration project has just been built in the Spanish harbor of Mutriku, as mentioned above (Fig. 1). The first completely commercial breakwater – OWC power plant, with an output of 4 MW, the Sidar Wave Energy Project (SWEP), is planned for the near future on the Scottish Hebrides island of Lewis .