The ocean is an enormous source of energy. Covering approximayely 70% of the Earth, it is estimated that 0.1% of the energy in ocean waves could be capable of supplying the entire world's energy requirements five times over. The possibility of generating electrical power from this energy has been recognized for many years, with the first patent on wave energy conversion was issued as early as 1799. Unlike other renewable resources, ocean energy is not captured from a single source but is stored in a variety of forms and specific technologies have developed to capture energy from each form. These include tidal and marine energy, wave energy, difference of temperature, and salinity energy, with the two most advanced areas of development being tidal and wave energy.
The current installed capacity of ocean energy systems only supplies 0.01% of the world's power, yet the unused potential of ocean energy systems is enormous.
Tidal energy is captured from the kinetic energy of tides, tides are the result of gravitational forces that the sun and moon exert on the earth. The use of tidal energy to generate power has been widely researched and is often compared to the technology used in hydroelectric power plants. The most significant difference in a traditional hydroelectric power plant is that water only flows in one direction, whereas in the tidal system, energy is captured as the water moves back and forth. This potential generation cycle means that, depending on the site, power can be delivered twice or four times per day on a highly predictable basis.. There are three systems commonly used to capture tidal energy: tidal barrages, tidal stream systems, and tidal lagoons.
Tidal barrages use the potential energy from the difference in height between high and low tides. In this system, water flows in and out of gates and turbines installed across a dam or barrage built across a tidal bay or estuary. Tidal barrages, consisting of a large, dam-like structure built across the mouth of a bay or an estuary in an area with a large tidal range, act to control the flow of water in and out of the bay. As the level of the water changes with the tides, a difference in height develops across the barrage. Water is allowed to flow through the barrage via turbines, which can provide power when the tide recedes, or allow water to fill the reservoir via sluice gates when the tides comes in, or during both tides.
Rather than using turbines in a dam-like structure, tidal stream systems use current devices that are placed directly "in-stream" and generate energy from the ﬂow of the tidal current. The devices used for capturing energy this way use a similar technology to wind power, however the density of water is 850 times that of air, so the power intensity in water currents is higher and the turbines used are smaller and fewer than those needed to generate wind power.
|Tidal stream turbine|
Offshore tidal power generators use an impoundment structure, similar to having a circular dam, built on the seabed. It is completely self-contained and independent of the shoreline. Offshore tidal lagoons put tidal power generation amongst the choices for commercial-scale renewable power generation by being more economical than other options.
Wave power is the capture and use of kinetic energy that is contained in the ocean surface waves. The first wave energy system was implemented in 1909, a harbour lighting system in California. There was a prominent interest in wave energy after the oil crisis of 1973, however, the most recent surge has been in Europe with wave energy conversion and major research in China, Japan, Russia, Canada, and the United States.
Wave energy systems can be divided into 3 groups; shoreline devices, near-shore devices, and off-shore devices.
Shoreline devices are fixed to the shoreline, having the advantage of easier installation and maintenance and do not require deep-water moorings or long lengths of underwater electrical cable. The disadvantage of shoreline devices is that they experience much less powerful waves. The most advanced type of shoreline device currently in use is the oscillating water column (OWC).
Near-shore devices are deployed at moderate water depths (20-25 metres), at distances up to 500 metres from the shore. They have nearly the same advantages as shoreline devices, but are also exposed to higher power levels. Several point absorber systems are an example of near shore devices.
Off-shore devices utilize the more powerful waves available in deep water (more than 25 metres). More recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays. Examples of such systems are the AquaBuOY system (a freely floating heaving point absorber system that reacts against a submersed tube filled with water), and the Wave Dragon (using a wave reflector to focus the wave towards a ramp and fill a higher-level reservoir).
The theoretical potential of ocean energy has been estimated to be between 20,000 and 90,000 terawatts per hour (TWh) per year by the International Energy Agency's Implementing Agreement on Ocean Energy. This amount is far beyond what the world presently requires with its current electricity consumption at approximately 16,000 TWh/year. Tide and marine current resources represent estimated annual global potentials exceeding 300 TWh and 800 TWh, and wave energy has an estimated annual potential of between 8,000 TWh and 80,000 TWh.
As of 2009, the only actively generating systems were tidal barrages, with tidal streams and lagoons still in the research and development stages. Success first came in 1966 with the La Rance Barrage in France with a capacity of 240 megawatts (MW). Then came a 20 MW station in Nova Scotia on the Bay of Fundy, and 0.5 MW station located in Russia on the White Sea. Additional research will no doubt increase the number of tidal power systems over time.
The global wave power resource in deep water (100 metres or more) is estimated to be 110 terawatts (TW). The economically exploitable resource varies from 140 to 750 terwatt-hours per year (TWh/year) for current designs of devices when fully installed, and could rise as high as 2,000 TWh/year if the potential improvements to existing devices are installed. Wave energy systems could potentially supply up to 13% of current world electricity consumption, which is equivalent to approximately 70% of what is currently supplied by hydroelectric schemes. With the potential wave energy market estimated at US$1 trillion U.S. worldwide by the World Energy Council, the possibility arises to bring renewable electricity a significant percentage of the world's population living within 60 miles of a coastline.
The ocean covers approximately 70% of the Earth's surface, making it one of the largest renewable resources in the world. Ocean energy has significant power potential and has relatively little impact on the surrounding environment; its predictable tides and currents enable accurate projections. Tides are now calculated to the minute and oceanographers have mapped all of the major currents in the world. One of the largest currents, the Gulf Stream, is approximately 56 miles wide at its core and travels at average speeds of 4.5 miles per hour. Ocean energy systems also usually sit at the surface of, or below, the water and they run silently.
Despite the numerous positives, there are still a number of concerns regarding ocean energy systems. Reliability can be an issue as strong currents can potentially damage or destroy units that need to be lightly secured in order to extract energy from the movement of the water. Costs can also be high due to the nature of the energy location, connecting wave energy devices to the electricity transmission often require underwater cables. There are also potential environmental concerns by putting man-made structures into a marine habitat. Tidal barrages, for example, can block fish from their normal spawning grounds, as well as prevent silt from flushing.
The past few years have seen some major achievements in the ocean energy sector, with various systems having been deployed at sea in several countries.
Though the industry is still in its infancy, and ocean energy technologies are not yet economically competitive with more developed renewable energy technologies such as wind, in the immediate future these technologies will become significant contributors to markets adjacent to the resource. In the longer term, ocean energy could become a much larger contributor to the world's energy portfolio.
As of 2009, less than 0.03% of global renewable energy was obtained from tidal sources. However, this number could be much higher in the future, with some estimates suggesting potential generation could be as high as 1,000 to 3,500 terawatt-hours. The United Kingdom is a leading country in terms of implementation and support to the ocean power sector. The two are a fitting match because estimates suggest that 6% of the U.K.'s total electricity requirement could be harnessed from tidal energy. Scotland in particular plays an important role, with 25% of Europe's tidal resources and 10% of Europe's wave energy resources are found there.
Top Ten Potential Locations for Tidal Energy
|Canada||Bay of Fundy – Cumberland Basin,shepody,cobequid|
|Russia||Mezan Bay and Tugur,penzhisnk|
|Korea||Siwha & Garolim,,cheonsu|
|Australia||Secure Bay & Cape Keraudren,Walcott inlet|
|Argentina||San Jose / Nuevo,rio Gallegos,santa cruz|
|UK||Severn & Mersey Yy|
|USA||Pasamaquoddy,knik arm,turnagain arm|
Source: WEC, 2001
The potential worldwide wave energy contribution to the electricity market is estimated to be around 1-10 terawatts. This can largely be attributed to the fact that wave energy has the highest density among all renewable energy sources. The most intense wave energy locations are found between 30o and 70o latitude in the northern and southern hemispheres, where the available resource is 30-70 kilowatts per metre (kW/metre) with peaks at 100 kW/metre. The supply potential is estimated to be 7 TWh/year from 200,000 MW installed wave and tidal energy power by 2050 with a load factor of 0.35. In the United States, the Pacific Northwest has the best wave resources. In Europe, Spain, Portugal, the U.K., and Norway are all located near excellent resources. According to the Carbon Trust Future Marine Energy report (2006), in the U.K. alone, the exploitable wave resource has been estimated at 50 TWh/year – enough to meet approximately 14% of their electricity demand. Additionally, an estimated 30% of Portugal's electricity demand can be met from wave power.
Due to the current size of the industry, installation of any ocean energy system can incur high capital costs. As well, maintenance is often costly on these systems, due to limited accessibility and lack of skilled professionals. The capital cost can be broken down into: the cost of the generation device itself (materials, components and labour in manufacturing and fabrication processes); the costs associated with installing it; the costs of keeping it on station (foundations or moorings); and the costs of connecting it to the grid (electrical cables and switchgear). This puts the cost of wave energy between 9 and 16 cents per kilowatt-hour. For tidal energy, which benefits from advancements made in wind technologies, the cost is slightly lower, at about 6 to 9 cents per kilowatt-hour. This high cost is the main barrier to implementation, as coal is only 3 cents per kilowatt-hour, and natural gas is 4.7 cents.
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