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Ocean Energy | Technology

Ocean energy systems can be categorised into five different types - Wave; Tidal; Salinity; Marine Current and Thermal.  Click on the links above for more information about the status of each technology, work currently underway, and the challenges that remain.

Wave Energy Technology.
The first patent certificate on wave energy conversion was issued as early as 1799, the intensive research and development study of wave energy conversion began after the dramatic increase in oil prices in 1973.   In the last 5 years, nascent wave energy companies have been highly involved in the development of new wave energy schemes such as the Wave Dragon, the Seawave Slot-Cone Converter and the AquaBuOY.

Though wave energy conversion is being investigated in a number of countries, particularly in the member States of the EU, Canada, China, India, Japan, Russia, the USA and others, Europe still remains the world leader in wave energy technology. With some European countries investing in R&D or demonstration projects, the EU should be well placed to compete when a commercial market for the technology evolves.

Wave Energy Potential
The global wave power resource in deep water (i.e. 100 m or more) is estimated to be ~ 1­10 TW (Panicker, 1976). The economically exploitable resource varies from 140-750 TWh/y for current designs of devices when fully mature (Wavenet, 2003) and could rise as high as 2,000 TWh/y (Thorpe, 1999), if the potential improvements to existing devices are realised. Global electricity consumption is about 15,400 TWh/y (BP, IEA), hence wave could supply up to 13% of current world electricity consumption which is equivalent to about 70% of what is currently supplies by hydroelectric schemes.

The predicted electricity generating costs from wave energy converters have shown a significant improvement in the last 20 years, which has reached an average price below 10 c/kWh. Compared to e.g. the average electricity price in the EU, which is approx. 4 c/kWh, the electricity price produced from wave is still high, but it is forecasted to decrease further with the development of the technologies.

The most important objective for the wave energy sector is to deploy full size prototypes to prove performance at sea and to bring the technology to a point where it becomes comparible with other renewable energy technologies such as wind energy. This step is crucial in order to gain greater confidence in ocean energy as a reliable energy source. This requires suitable funding.

Wave energy systems can be divided into 3 groups :

Shoreline devices: are fixed to the or embedded in the shoreline, having the advantage of easier installation and maintenance. In addition shoreline devices do not require deep-water moorings or long lengths of underwater electrical cable. The disavantage shoreline devices experience is that they experience a much less powerful wave regime. The most advanced type of shoreline device is the oscillating water column (OWC).

One example is the Pico plant, a 400 kW rated shoreline OWC equiped with a Wells turbine that was constructed between 1995 and 1999. Due to malfunction problems the testing programme was delayed. In 2003, the Wave Energy Centre, a Portuguese Association dedicated to the development and promotion of wave energy, refurbished the plant and restarted testing, resulting in real sea testing in September 2005. Based on the experience a 'wave energy breakwater' project is being developed at the Douro estuary in Oporto, Portugal mainly financed by the EDP-group.

Another wave energy system that can be integrated into a breakwater is the Seawave Slot-Cone converter (SSG). The SSG concept will then give the breakwater an added value in therms of income through sale of electricity. The SSG will provide the breakwater with infrastructure, including electricity and may be combined with fresh water production.

Near shore devices: are deployed at moderate water depths (~20-25), at distances up to ~500 m from the shore. They have nearly the same advantages as shoreline devices, being at the same time exposed to higher power levels. Several point absorber systems are near shore devices.

Offshore devices: exploit the more powerful wave regimes available in deep water (> 25 m depth). More recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays. The AquaBuOY system is an example of an offshore wave energy device. The AquaBuOY system is a freely floating heaving point absorber system that reacts against a submersed tube, filled with water. Another example based on the overtopping principle is the Wave Dragon. The Wave Dragon used a wave reflector design to focus the wave towards a ramp and fill a higher-level reservoir.
Seawave Slot-Cone Converter
(Wave Energy AS)

(Finavera Renewables Ltd.)

Tidal energy conversion techniques exploit the natural rise and fall of the level of the oceans caused principally by the interaction of the gravitational fields in the planetary system of the earth, the sun and the moon. The vertical movements associated with the rise and fall of the tides are accompagnied by roughly horizontal water motions termed tidal currents. It has therefore to be distinguished between tidal range energy, the potential energy of a tide, and tidal current energy, the kinetic energy of the water particles in a tide.
Tidal Energy Potential
The global tidal range energy potential is estimated to be about 200 TWh/y, about 1 TW being available at comparable shallow waters. Within the European Union, France and the UK have sufficiently high tidal ranges of over 10 metres. Beyond the EU, Canada, the CIS, Argentina, Western Australia and Korea have potentially interesting sites. At present 3 tidal barrages operate as commercial power plants, amounting to a worldwide total of 260 MW of installed capacity.
Tidal range energy projects require normally higher capital investment at the outset, having relatively long construction periods and long payback periods. Consequently, the electricity cost is highly sensitive to the discount rate used. This problem could be solved by government funding or large organisations getting involved with tidal power.

In terms of long term costs, once the construction of the barrage is complete, there are very small maintenance and running costs and the turbines only need replacing once around every 30 years. The life of the plant is indefinite and for its entire life it will receive free fuel from the tide.

The economics of a tidal barrage are very complicated. The optimum design would be the one that produced the most power but also had the smallest barrage possible.

The technology required to convert tidal range energy into electricity is very similar to the technology used in traditional hydroelectric power plants. Tidal range energy conversion technology is considered mature, but, as with all large civil engineering projects, there would be a series of technical and environmental risks to address.

Tidal Range Energy Projects
At present, three tidal barrages operate as commercial power plants. One of them is the tidal plant that was built on the Rance estuary in France during the 1960's and has now completed over 40 years of successful operation. Because of the high generation costs and the long payback times and their environmental impact on local ecosystems it is unlikely that tidal range energy will be commercially developed.

La Rance
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Salinity Gradient Technology.

Significant research took place from 1975 to 1985 and gave various results regarding the economy of PRO and RED plants. It is important to note that small-scale investigations into salinity power production take place in other countries like Japan, Israel, and the United States. The principle of salinity gradient energy is the exploitation of the entropy of mixing freshwater with saltwater. This energy source is not easy to understand, as it is not directly sensed in nature in the form of heat, waterfalls, wind, waves, or radiation.

Salinity Energy Potential
Salinity power is one of the largest sources of renewable energy that is still not exploited. The potential energy is large, corresponding to 2.6 MW m3/sec freshwater when mixed with seawater. The exploitable potential world-wide is estimated to be 2000 TWh/y. The potential cost of energy from this source is higher than most traditional hydropower, but is comparable to other forms of renewable energy that are already produced in full-scale plants.

Several methods have been proposed to extract this power. Among them are the difference in vapor pressure above freshwater and saline water and the difference in swelling between fresh and saline waters by organic polymers. However, the most promising method is the use of semi-permeable membranes. The energy can then be extracted as pressurized brackish water by pressure retarded osmosis (PRO) or direct electrical current by reverse electrodialysis (RED).

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Marine Current Technology.
At present, marine current energy is at an early stage of development, different pilot plants are in operation or about to be installed. Most devices rely on the horizontal or vertical axis turbine concept. . There are no commercial grid-connected turbines currently operating.
Marine Current Energy Potential
 The global marine current energy resource is very large. Countries with an exceptionally high resource include the UK (E&PDC, 1993), Ireland, Italy, the Philippines, Japan and parts of the United States. Few studies have been carried out to determine the total global marine current resource, although it is estimated to exceed 450 GW (Blue Energy, 2000). The potential for marine current turbines in Europe is estimated to exceed 12 000 MW of installed capacity. Locations with especially intense currents are found around the British Islands and Ireland, between the Channel Islands and France, in the Straits of Messina between Italy and Sicily and in various channels between the Greek Islands in the Aegean. Other large marine current resources can be found in regions such as South East Asia, both the east and west coasts of Canada and certainly in many other places around the globe that require further investigation. The UK has the major component of the EU resource at approximately 4.3 GW.

Marine current energy is one of the most promising new renewable energy sources. The know-how is available to combine existing technologies. Marine currents have the potential to supply significant quantities of energy into the grid systems of many countries. As interest grows, marine current energy is likely to play an increasing role in complementing other energy technologies and contributing to the future global energy supply mix.

Recent technologies open up prospects for commercial deployment of some projects in the near future. The economical viability is yet to be proven but it is a anticipated that the production costs will decrease as the technology advances. Most devices rely on the horizontal or vertical axis turbine concepts. Turbines may be suspended from a floating structure or fixed to the sea bed. In large areas with high currents, it will be possible to install water turbines in groups or clusters to make up a marine current farm. Variants of these two types have been investigated, including turbines using concentrators or shrouds, and tidal fences.

Horizontal axis turbines: (axial flow turbine). This is similar in concept to the widespread horizontal axis wind turbine. Prototype turbines of up to 10 kW have been built and tested using this concept. One example is the Rotech Tidal Turbine (RTT). The RTT is a bi-directional horizontal axis turbine housed in a symmetrical venturi duct. The venturi draws the existing ocean currents into the RTT in order to capture and convert energy into electricity.

Vertical axis turbines: (cross flow turbine). Both drag and lift turbines have been investigated, although the lift devices offer more potential. Some stand-alone prototypes have been tested, including a 5 kW Darrieus turbine in the Kurushima Straits, Japan. The concept of installing a number of vertical axis turbines in a tidal fence is being pursued in Canada, with plans to install a 30 MW demonstration system in the Philippines (Blue Energy, 2000).

Rotech tidal turbine
(Lunar Energy)
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Thermal Energy Technology.
The principle of ocean thermal energy conversion (OTEC) originated with a French physicist, Jacques D'Arsonval, in 1881. His pupil, Georges Claude, built the first plant at Matanzas Bay, Cuba in 1930, with a gross output of up to 22 kilowatts.

The United States became involved in OTEC research in 1974, when the Natural Energy Laboratory of Hawaiii Authority was established. The Laboratory has become one of the world's leading test facilities for OTEC technology. Japan also continues to fund research and development in OTEC technology. Ocean thermal energy conversion generates electricity by using the temperature difference of 20C (36F) or more that exists between warm tropical waters at the sun-warmed surface, and colder waters drawn from depths of about 1000 m. To convert this thermal gradient into electrical energy, the warm water can be used to heat and vaporize a liquid (known as a working fluid). The working fluid develops pressure as it is caused to evaporate. This expanding vapor runs through a turbine generator and is then condensed back into a liquid by cold water brought up from depth, and the cycle is repeated.

Thermal Energy Potential
The world's largest solar collector absorbs a tremendous amount of the sun's energy, averaging about 65 million gigawatts (a gigawatt is one million kilowatts), or 570 quadrillion kWh/y - more than 5,000 times the amount of energy used in all forms by humans on the planet. A typical square mile of that collector - otherwise known as the surface waters of the Earth's vast oceans - absorbs an average of about 500 MW, or annually more energy than the equivalent of 2.6 million barrels of oil. The estimated global resource is 10 000 TWh/y.

OTEC power plants require substantial capital investment upfront.

There are potentially three basic types of OTEC power plants: closed-cycle, open-cycle, and various blendings of the two. All three types can be built on land, on offshore platforms fixed to the seafloor, on floating platforms anchored to the seafloor, or on ships that move from place to place.

Offshore OTEC is technically difficult because of the need to pipe large volumes of water from the seabed to a floating system, the huge areas of heat exchanger needed, and the difficulty of transmitting power from a device floating in deep water to the shore. The latest thinking is that OTEC needs to be applied as a multipurpose technology: for example, the nutrient-rich cold water drawn from the deep ocean has been found to be valuable for fish farming. In addition, the cold water can be used directly for cooling applications in the tropics such as air conditioning. If OTEC takes off, it is likely to be with energy as a by-product.

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Copyright 2008 European Ocean Energy Association