WAVE ENERGY
A large part of the major influx of energy to this planet, solar energy, is converted by natural processes, i.e. through wind generation, to energy associated with waves. Waves are generated by the wind as it blows across the ocean surface. The energy thus contained is significant, in favoured latitudes with values of around 70 MW/km of wave frontage.
Ocean waves represent a considerable renewable energy resource. They travel great distances without significant losses and so act as an efficient energy transport mechanism across thousands of kilometres. Waves generated by a storm in mid-Atlantic will travel all the way to the coast of Europe without significant loss of energy. All of the energy is concentrated near the water surface with little wave action below 50 metres depth. This makes wave power a highly concentrated energy source with much smaller hourly and day-to-day variations than other renewable resources such as wind or solar.

Since in principle hundreds of kilometres lined with generating stations are conceivable, wave energy could contribute significantly to world energy supply if an economic way of extracting this energy could be found. The highest concentration of wave power can be found in the areas of the strongest winds, i.e. between latitudes 40 deg. and 60 deg. in both the northern and southern hemispheres on the eastern sides of the oceans. Countries like the United Kingdom are thus the world’s most favoured locations for the extraction of wave power.

TECHNOLOGY
Typically ocean wave devices capture the energy of waves and convert their energy to electricity. Wave energy devices include hydro-piezoelectric, oscillating water columns, wave run up (tapered channel) and sea clams. Particularly ‘sea clams’ involve wave action forcing air between blades located on the perimeter of a circular barge structure. The air is then run through air turbines which rotate at a shaft connected to an electrical generator.

Europe, and in particular the United Kingdom, are looking at wave power. A recent review by the UK government has shown that there are now types of wave power devices which can produce electricity at a cost of under USD 0,10/kWh, the point at which production of electricity becomes economically viable. The most efficient of the devices, the “Salter ”Duck can produce electricity for less than USD 0,05/kWh. The “Salter ”Duck was developed in the 1970s by Professor Stephen Salter at the University of Edinburgh in Scotland and generates electricity by bobbing up and down with the waves. Although it can produce energy extremely efficiently it was effectively killed off in the mid 1980s when a European Union report miscalculated the cost of the electricity it produced by a factor of 10. In the last few years, the error has been realised, and interest in the Duck is becoming intense.

The “Clam” is another device which, Like the “Salter ”Duck can make energy from sea swell. The Clam is an arrangement of six airbags mounted around a hollow circular spine. As waves impact on the structure air is forced between the six bags via the hollow spine which is equipped with self-rectifying turbines. Even allowing for cabling to shore, it is calculated that the Clam can produce energy for around USD 0,06 /kWh.

Both the Duck and the Clam generate energy from waves at sea. This is useful for generating energy for offshore structures and low-lying islands. However, where islands offer suitable sites, cliff-mounted oscillating water column (OWC) generators have a number of advantages, not the least of which is the fact that generators and all cabling are shore-based, making maintenance of the former and replacement of the latter much simpler. The OWC works on a simple principle. As a wave pours into the main chamber, air is forced up a funnel which houses a turbine. As the wave retreats, air is sucked down into the main chamber again, spinning the turbine in the opposite direction
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OWC machines have already been tested at a number of sites, including Japan and Norway. The UK is on the verge of deploying Osprey II, a second generation OWC. There is particular interest in OWC systems because of the large amount (7,000 MW) of shoreline wave energy available for exploitation. Costs for OWC machine-generated electricity is likely to start at USD 0,10 /kWh. The first-generation system, based on the island of Islay takes advantage of a natural rock gully to drive a 180 kW turbine attached to an electricity generator. Built by researchers from the Queen’s University of Belfast the system supplies electricity to the local grid, which is connected to the mainland national grid by submarine cable However, both OWC-systems and ocean-wave systems suffer from trying to harness violent forces. The first Norwegian OWC was ripped off a cliff-face during a storm, the Islay station is completely submerged under storm conditions.

PELAMIS
There have been several proposals to harness ocean waves to generate electricity or to make other useful products such as fresh water or hydrogen. To date none of these has been successfully commercialised. Ocean Power Delivery Ltd. is developing a novel offshore wave energy converter called Pelamis. The company has successfully bid for a contract to install a pair of 375kW prototype devices off Islay, Scotland, under the 1999 Scottish Renewables Obligation. The device has an annual capacity factor of 38% at the site chosen. It is approximately 130metres long and 3,5metres in diameter. It is scheduled to be installed early in 2002 and will generate over 2,5 million kWh’s of electricity per year, enough to provide power for 150-200 homes.

The Pelamis device is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints. The wave induced motion of these joints is resisted by hydraulic rams which pump high pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power from all the joints is fed down a single cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single seabed cable. A novel joint configuration is used to induce a tuneable, cross-coupled resonant response which greatly increases power capture in small seas. Control of the restraint applied to the joints allows this resonant response to be ‘turned-up’ in small seas where capture efficiency must be maximised or ‘turned-down’ to limit loads and motions in survival conditions.
The complete device is flexibly moored so as to swing head-on to the incoming waves and derives its ‘reference’ from spanning successive wave crests.
The Pelamis device has a number of important advantages over other existing or proposed Wave Energy Converters, these include:
Tuneable response allows power capture to be maximised in small seas while limiting loads and motions in extreme conditions
The head on aspect to severe waves presents the minimum resistance to the high velocities in extreme wave crests
The finite length of the device is optimised to extract power from shorter wavelengths and is unable to reference against the long waves associated with storm conditions
The small diameter leads to local submergence or emergence in large waves limiting the forces and moments in the structure
The flexible mooring system has a range of motion able to accommodate the largest waves

THE MIGHTY WHALE
The Marine Science & Technology Centre of Japan launched the world’s largest offshore floating wave power device in July 1998, and the full-scale prototype will be tested until the year 2000.
This floating device, called the Mighty Whale, converts wave energy to electricity. The device measures 50 metres long by 30 metres wide, and uses waves in the Pacific Ocean to drive three air turbines (one with a rated output of 50 kW + 10 kW and two of 30 kW) on board the platform, to generate 120 kW of electricity.

After being towed to its mooring about 1.5 km from the mouth of Gokasho Bay, the Mighty Whale was anchored to the bottom of the sea (about 40 m deep) with six mooring lines; four lines on the seaward side and two on the lee side. Mooring lines are designed to withstand typhoon winds, and the unit is designed to handle waves of 8 m. The Mighty Whale converts wave energy to electricity by using oscillating columns of water to drive air turbines. Waves flowing in and out of the air chambers at the ‘mouth’ of the Mighty Whale make  the water level in the chambers rise and fall. The water forces air into and out of the chambers through nozzles on the tops of the chambers. The resulting high-speed air-flows rotate air turbines which drive the generators. The Mighty Whale can be remotely controlled from on-shore. In the demonstration prototype, the energy produced is mostly used by the instruments carried on board; any surplus is used to charge a storage battery or, when this is fully charged, is used by a loading resistor. A safety valve protects the air turbines from stormy weather by shutting off the flow of air if the rotation speed of the turbines exceeds a predetermined level. So that it can be used in the future to improve water quality, the prototype is also equipped with an air compressor to provide aeration.
Because it has absorbed and converted most of the energy in the wave, the Mighty Whale also creates calm sea space behind it, and this feature can be utilised; for example, to make areas suitable for fish farming and water sports. The structure of the Mighty Whale itself can be used as a weather monitoring station, a temporary mooring for small vessels or a recreational fishing platform.

SUMMARY
At the present time both tide and wave energy are suffering from orientation problems, in the sense that neither method is strictly economical on a large scale in comparison with conventional power sources. In addition, neither will produce electricity at a steady rate and thus not necessarily at times of peak demand. Wave power stations suffer even more from these problems, their rate of production being unreliable. In Norway development of wave power was taken a step further, concentrating on small applications on remote islands and the like, and for quite a while a small power plant (500 kW) operated successfully in Toftestallen until it was swept away by the sea.

The disadvantages of wave power stations compared to maybe their closest rival - wind power - are obvious: A wave power unit will probably not have much more than three times the output of a single wind turbine, but the construction costs are likely to be far higher due to mooring problems, the bulkiness and comparative complexity of the whole structure and the water-based location. It will take some time - and far more investment into renewable energy sources - before the only comparative bonus, the fact that they use up and deface less land, will prevail over economic considerations.

And while wave energy is used successfully in very small scale applications, such as powering lighthouses or navigation buoys, its short term prospects as a major contributor to large scale energy production seems to be economically almost ruled out. So until the cost of maintaining the present rate of carbon dioxide emission is taken into account when building new power stations and a policy is adopted that depends less rigorously on market forces, the likelihood of tidal or wave power playing a major part in the energy supply of western industrialised nations even in the medium term future is small.

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