Wave Energy
The idea of extracting energy from ocean waves is not new. In Paris in 1799, during the Napoleonic era, a Monsieur Girard obtained a patent for a machine he and his son had designed to mechanically capture the energy in ocean waves.
The energy potential in waves is apparent on any beach where they can be seen and felt repeatedly crashing onto the shore. These steep and breaking waves result from the effect of the shallow water depth on the waves shape and speed. In these so called shallow water waves the energy release is both dramatic and explosive and for obvious reasons it is very difficult to directly capture this energy in a controlled manner.
It is the effect of the shallow water depth close to the shore which has this significant effect on wave. Whilst some on shore wave energy devices aim to capture the surges and wave effect on the shore line they typically do so indirectly by allowing the waves to overtop a physical structure or dam and fill a lagoon, or alternatively by allowing them to translate that energy into air compression within a chamber and subsequent pass that air through an air turbine turning a generator.
In deeper water where the bottom effects are less significant or negligible the waves travel in a more predictable manner dictated by their speed or wavelength. These are deep water waves and this is where most of the other wave energy capture devices aim to extract energy and do so directly.
Courtesy: Queens University Belfast – Energy 2100 Dated 2006.
Where do Waves come from?
Waves can be formed by seismic activity, landslides, or by tides but generally speaking most waves are the direct result of the wind blowing across the ocean surface. Wave energy can therefore generally be considered as converted wind energy (and of course wind energy can be considered as converted solar energy.
When wind blows across the surface of the water it pushes the surface with it and develops waves. The stronger the wind, the longer it blows, and the greater the surface distance it has to work on (the fetch), the bigger the waves will be and the more energy available.
Since there is such a strong link between wind energy and wave energy it is not surprising that as well as having the best wind resource in Europe when combined with the long Atlantic fetch the west coasts of Ireland and Scotland also have amongst the best wave resource. Likewise in the southern hemisphere it is perhaps no surprise that the southern tips of South America, South Africa and Australasia which jut out into the Southern Ocean also have both excellent wind and wave resource.
Global Wave Resource - Average Annual Wave Resource kW/m Challenges in extracting the Energy
The energy resource potential of ocean waves has been recognised for a considerable time and various devices have been proposed and tank tested in an effort to extract this energy efficiently and economically. However, despite the level of resource available and the significant work undertaken to date in trying to extract it, it is only now after some 30 years or more of research and experimentation we are beginning to see commercial exploitation at any scale. This is an indication of just how big an engineering challenge wave energy capture is.
Efficiency: Generally speaking, wave energy is available as intermittent low-speed events acting with high forces. Also, unlike wind which essentially blows horizontally and perpendicular to the rotor, these forces do not act in a single direction (illustrated by the wave particle motion figure above). This makes energy conversion difficult since most readily available electric generators operate at high speeds, and at relatively constant speed. This difficulty is further compounded because waves do not have a constant height nor do they have a constant wavelength and consequently a wave capture device must operate efficiently across a wide range of conditions. Efficiently converting wave motion into electricity is a challenge.
Survival: The tracks of the low pressure systems or depressions which cross the Atlantic (and which largely drive our winds) tend track off the north west coast of Ireland and Scotland before heading further north. When these depressions are deep they can generate strong, sometimes Hurricane Force winds (up to Force 12) and huge seas. In its shipping forecast the Met Office describes rather poetically a sea state with wave heights of 14+ meters as ‘Phenomenal’. In these extreme heavy weather conditions, even deep water waves can break resulting in massive energy releases. This is clearly no place for a device which has not been designed to withstand the rigours of such conditions, and certainly no place for a service boat trying to maintain them.
Constructing devices that can survive storm damage without being so heavily over engineered and therefore prohibitively expensive to construct and maintain is a major issue.
Economics: The resource is large but of relatively low density when compared with that of fossil fuels. This combined with the high total cost of any form of activity offshore makes electricity production (at least in the short term) expensive. It is anticipated that this will change over time as equipment manufacture costs come down with higher device numbers installed but offshore maintenance will always be expensive and getting the energy costs down to those of other land based technologies is a challenge.
Devices
At the University of Edinburgh, Professor Stephen Salter’s 1974 invention of Edinburgh Duck also known as Salter's Duck or Nodding Duck marked an important milestone in the modern scientific evaluation of wave energy resource. Its development was prompted by the 1973 Oil Crisis. The Duck consists of a curved cam-like body moored to the sea bed which is allowed to oscillate or nod with the wave motion as a wave passes.
In laboratory tests the Duck reportedly was capable of extracting 90% of wave motion and could convert 90% of that into electricity. However, a full-sized version of the Duck has never been deployed at sea.
Most of the modern wave designs being tested and evaluated absorb far less of the available wave power than that theoretically demonstrated by Salter’s Duck. These designs are vaery varied and have largely concentrated on minimising some of the complexity inherent in the Duck design and as a result their Mass to Power Ratios remain far away from the theoretical maximum.
There are many different types of wave device and they can be grouped in a number of ways – for example based on deployment location (on shore, near shore or off shore), on their operation (flapper, oscillating buoy, air column) or based on their working fluid (air, water, hydraulic oil) but for simplicity they are listed below by location.
The following is not intended to be an exhaustive list of devices or manufacturers or an endorsement or otherwise of their technology but simply to provide a sample of devices to illustrate their general principles and a flavour of the technology and what these devices might look like.
Shore Based Devices
Generally speaking on shore devices should be cheaper to install and easier to maintain since the device requires no sub sea grid connection and is accessible by road. Many of them rely on natural site features (such as a deep narrowing gulley) to enhance the wave effect. Whilst having some obvious advantages shoreline devices do, however, capture less energy than their deeper water counterparts since seabed friction significantly reduces power levels closer to shore.
Although there are a number of on shore devices perhaps the best known is the Oscillating Water Column (OWC) where the rise and fall of a trapped water column acts directly on an air volume which moves in and out under pressure driving through a Wells air turbine.
Oscillation Water Column (OWC)
The 150kW LIMPET demonstrator installed on the island of Islay off the west coast of Scotland by Wavegen (www.wavegen.co.uk) is an OWC device, and has been in operation since 2000. Several larger schemes are now being proposed, in Scotland, Spain and Portugal.
Wavegen’s LIMPET Islay, Scotland
The OWC principle can be adapted to open deeper water by creation of an ‘artificial shore’ onto which the wave acts exactly as if it were onto land. For this offshore environment Wavegen developed a multi air column version of the Limpet named OSPREY, which was designed to be anchored in around 15m of water.
In Japan JAMSTEC (www.jamstec.go.jp) also developed a moored OWC device the Mighty Whale, which was designed to be operated in 40m of water. The prototype contains three air chambers that convert wave energy into pneumatic energy.
Mighty Whale OWC device
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Other shore based devices utilise the wave’s energy to fill a reservoir whose level is above that of the sea thereby creating a hydrostatic head. Energy is then extracted from the returning water using a conventional low head turbine. One such example of this is a tapered channel (TAPCHAN) device which is usually constructed of concrete in a suitable inlet which feeds and accelerates waves into a reservoir several meters above sea level. An early example of this is the Wave Power Plant at Toftestallen in Norway built in 1985.
Toftestallen Wave Power Plant
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Just as the OWC concept can be adapted to offshore, the overtopping concept has also been adapted to open deeper water by use of a simulated inlet with a tapering channel fabricated from steel or concrete, moored some distance off shore. The 4-7MW Wavedragon (www.wavedragon.net) employs such a strategy with a pre-commercial demonstrator project off the Pembrokeshire coast of South Wales currently awaiting planning consent.
Wave Dragon
Near shore devices
Near shore devices operating in shallower waters include those which utilise a flapper type arrangement either boxed or free mounted on the sea bed. An example of this the Oyster device being developed by Aquamarine (www.aquamarinepower.com) which extracts energy by utilising the forward and backward movement of the flapper to actuate a ram filled a sea water. This water can then be piped ashore and fed through a simple Pelton wheel turbine to generate electricity.
Aquamarines Oyster Device
Off Shore Devices
One consequence of moving further offshore to capture the higher energy resource of the deeper ocean waves is that water depths become significant (from 30 to over 100metres) and it is usually not possible to solidly mount the energy devices to the sea bed. Relative movement generated by the ocean waves must thus be created either between separate elements of the device itself or by slack or tight mooring of some element of the device to the sea bed. There are a number of general categories of the offshore wave device or OWEC (Offshore Wave Energy Converter) and some examples are given below.
Some float based point absorber devices rely on the harmonic motion of the floating part against the stationary anchor on the sea bed. One such device developed by Finavera Ltd (www.finavera.com) uses a specially reinforced elastomeric hose which through stretching and relaxing, pressurises water in the column which is then fed to a hydraulic pump and generator.
Finavera’s Aquabuoy Point Absorber
Other point absorbers are designed to be non surface piercing utilising a submersed but essentially buoyant element moving against a lower fixed element. The Archimedes Wave Swing (AWS) developed by AWS Ocean Energy Ltd (www.awsocean.com) is an example of such a device. With the AWS an air-filled upper casing cylinder acts against a lower fixed cylinder, moored to the seabed. As a wave approaches, the increasing water pressure compresses the gas within the cylinder whilst the reverse happens as the wave trough passes and the cylinder expands. The relative movement between elements is converted to electricity by means of a hydraulic system and motor-generator set. The concept has been proven at full-scale in 2004 via a pilot plant off the coast of Portugal and engineering for a pre-commercial demonstrator is now ongoing.
The Archimedes Wave Swing
An alternative method of generating power is through so called moving body devices which rely solely on the relative movement of different parts of the device with the waves to generate pressure in a working fluid. The working fluid might be sea water or hydraulic oil held in a sealed tank which is then passed through a turbine to generate electricity.
The McCabe Wave Pump, conceived in 1980 was one of the early exponents of this principle. It consists of three tubular sections or pontoons which face into the wave and pivot as wave fronts move across. The McCabe Wave Pump currently under development by Hydam Technology Ltd was installed in the Shannon Estuary off the west coast of Ireland with trials completed in 2004.
McCabe Wave Pump
A number of other technology suppliers have adopted the moving body device principle. The PELAMIS device developed by Ocean Power Delivery Ltd (www.oceanpd.com) is one such device. Currently 3 x 750kW devices have been installed off the coast of Portugal at Aguçadoura with a further 20MW likely to be installed in the near future.
OPD Pelamis
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