Since the mid-1990s, there has been a resurgence in wave energy, led mainly by small engineering companies. As a result, there are a number of projects being built (or about to be built) all over the world. These companies have all learned from the lessons taught by the early experiences in wave energy:
- All the devices are relatively small in size and output (the biggest is 2 MW). This reduces the prototype costs (making funding easier) and the technological challenges. The technological challenges are further reduced for some devices by installing them at the shoreline.
- The devices started to be constructed in the mid to late 1990's. Consequently, they had the benefit of two decades of experience in constructing and operating offshore oil and gas platforms, with the corresponding scope for technology transfer.
- Nearly all of the devices described below are being built by industry (typically small engineering companies), so the focus is on the economics of the technology from the start.
The following describes a few of the more important projects.
Shoreline Devices
Being constructed at the shoreline, these devices are the easiest to fabricate and maintain but they capture much less energy than offshore devices. Most are oscillating water columns (OWCs). These consists of a partially submerged, hollow structure, which is open to the sea below the water line (see figure below). This structure encloses a column of air on top of a column of water. As waves impinge upon the device they cause the water column to rise and fall, which alternatively compresses and depressurises the air column. If this trapped air is allowed to flow to and from the atmosphere via a turbine, energy can be extracted from the system and used to generate electricity. This is usually achieved using a Wells turbine, which has the property of rotating in the same direction regardless of the direction that the air passes the blades.
Fig. Operating principles of am oscillating water column (OWC)A number of OWC devices have been installed world-wide with several commercial schemes currently being built in Europe:
- The European Pilot Plant (Ref. 1) on the island of Pico in the Azores. This plant is already built and operating. It will be used to supply electricity to the local grid and as a test bed for various technologies associated with OWCs.
- The Wavegen Limpet (Ref. 2, 3) is a 500 kW device on the island of Islay (Scotland), which uses a novel construction method to reduce construction costs. The device is constructed in a hollow excavated from a cliff behind a rock bund; the bund is removed at the end to allow ingress of sea water. This device came on stream in November 2000 and is performing satisfactorily.
Offshore Devices
This class of device exploits the greater amount of wave energy available in deeper water (> 40 m depth). They are mainly floating devices held in place by different types of mooring. Because they face more powerful waves than shoreline devices, they have greater technical challenges. In general, they are less developed than the OWCs. There are several designs currently being developed, each with its own pros and cons. Some of the promising ones include,
- The McCabe Wave Pump. Three narrow steel pontoons are hinged together across their beam and point into the incoming waves. The front and back pontoons move in relation to the central pontoon, which is held relatively still by the damper plate, by rotating about the hinges. Energy is extracted from the rotation by hydraulic rams. This energy can be used to provide electricity (~ 400 kW) or else to produce potable water by supplying pressurised sea water to a reverse osmosis plant. A 40 m long prototype of this device was deployed off the coast of Kilbaha, County Clare, Ireland and a commercial demonstration scheme is currently being built (Ref. 3).
- The Pelamis. The Pelamis device is composed of a number hollow, cylindrical sections linked by hinged joints. These sections point into the oncoming waves and move with respect to each other as the waves pass down their length. Again, energy is extracted by hydraulic rams at the joints, which drive electrical generators. Here the emphasis is on building the device with 100 % 'off the shelf' components. A device is being developed for deployment in Scotland, which is rated at 375kW and is 130 m long and 3.5 m in diameter (Ref. 4).
- The Archimedes Wave Swing. This consists of a cylindrical, air filled chamber (the "Floater"), which can move vertically with respect to the cylindrical "Basement", which is fixed to the sea bed. The air within the 10m - 20m diameter Floater ensures buoyancy. However, a wave passing over the top of the device, alternatively pressurises and depressurises the air within the Floater, changing this buoyancy. This causes the Floater to move up and down with respect to the Basement and it is this relative motion that is used to produce energy. This is the most powerful device currently under construction: a 2 MW Pilot scheme is being built for Portugal (Ref. 5).
There are other promising offshore devices (e.g. Ocean Power Technology's Wave Energy Converter) which are undergoing deployment but less is known about them because of commercial confidentiality.
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