Harvesting energy from the Earth’s oceans can, generally, be classified into three main types: ocean wave energy, ocean thermal energy and tidal energy. Ocean wave energy is, in essence, a form of wind energy, since ocean waves (as opposed to waves formed, for example, by fast flowing rivers entering large basins of water near shore) are a product of wind blowing across the surface of the ocean. Ocean thermal energy is the result of the varying geo-physical temperatures throughout the surface and interior of the Earth, as well as solar heating of the oceans. Ocean tides, on the other hand, are the result of the direct gravitation influences of the Moon and Sun on the Earth’s Oceans.
Tides
Tide energy is readily available due to the fact that Moon mass exerts a gravitational force on the Earth and its oceans. According to the laws of physics, the force due to gravity is proportional to the inverse of the distance between two masses, squared. In other words, the further two masses are from one another, the less attracting force the two masses will feel.
The water on the surface of the Earth closest to the moon at any given time (remember, the Earth is rotating once per day and the Moon is orbiting around the Earth once about every 28 days) feels a stronger attractive force in the direction of the moon than the water on the ‘sides’ and ‘back’ of the Earth, further away from the Moon. As the Earth rotates, ocean waters flow from the ‘sides’ of the Earth, resulting in periodic tidal currents and the lowering and raising of the ocean level (a tide) at different times across the entire surface of the Earth.
The different positions of the Moon are due to the Earth rotating once per day, not the Moon orbiting around the Earth. As seen in the illustration, every area on Earth will experience two high and two low tides each day, making tidal energy a readily available resource to any community or nation with access, or close to, any tidal body of water.
Tidal Power
The generation of energy from tidal forces takes two main forms, categorized by the basic method of harvesting the tidal energy. The first form relies on capturing and harvesting the potential energy from the relative differences in the height of water between tides. This potential energy is converted to kinetic energy (by releasing the water and allowing it to flow down) and passed through turbines, which generates mechanical energy, which is then (most often) converted to electricity. An analogy (with similar environmental impacts) is the traditional hydroelectric dam; however, in the case of tidal power, the flow of water is reversed several times per day.
France and Canada
This form of power generation has its roots in the Middle Ages, when tides were used in the place of water wheels to power a mill. A modern operational example of harvesting tidal energy in this manner is the Rance Estuary tidal power plant, located in Rance, France. Commissioned in 1966 by Electricit de France (now the EDF Group), the Rance tidal power plant project constructed a damn (or barrage) 330 metres long to house turbines, a lock to allow the passage of small boats, a rock-filled dam 165 metres long, and a mobile weir with gates to rapidly empty and fill the tidal reservoir. The Rance tidal power facility has a peak power output of 240 MW, with an annual production of 600 GWh. Future tidal power plants on the size and scale of the Rance facility would have significant impact on the coastal, estuary and river environments. Another example of this type of tidal power generation is Nova Scotia Power’s (NSP) Annapolis tidal power plant. The Annapolis plant is the only tidal power generation facility operating in the western hemisphere. The plant harvests the tidal energy generated in the Bay of Fundy, which has the world’s highest tides.
Tidal Currents
The second method of capturing wave motion harvests the kinetic energy in tidal currents. Tidal currents are the result of the ocean water flowing in or away from the general direction of the Moon, keeping in mind that the Earth is rotating, and the flow path of any specific body of water in the direction of the moon changes throughout the day, ebbing and flowing.
Tidal flow is not uniform due to differences in landmass and ocean depth; the oceans waters must flow up, down and across submerged mountain ranges and valleys; and into bays and seas from and to the oceans. Very much like a wind farm, tidal current power production harvests the energy of the water’s flow, not it’s rise and fall. Water is over 800 times denser than air, thus a properly designed and sited tidal power generator can provide significant power. As with wind power, site selection for placement of the tidal turbines is key; however unlike wind, tidal currents are predictable (just look at your daily newspaper for tomorrow’s tides), and suitable areas with fast currents can be reliably identified.
Generating power from tidal currents is a relatively new approach. Primarily prototypes and small scale generating systems have been demonstrated; however several large-scale commercial facilities are in final development and will soon be fielded. Marine Current Turbines, Ltd., working under the Sea Generation Ltd., project umbrella (SeaGen), has installed and tested a tidal current generation system near Lynmouth, England, since 2003. Although somewhat delayed, SeaGen is scheduled to install a 1.2MW operational tidal current energy converter in Strangford Lough in 2008.
Tidal Power Advantages
The primary advantages of tidal power generation are:
• Tidal energy is renewable, produces no greenhouse gases or other waste. The energy is provided via gravitational forces and the relative motion of the Sun, Moon and Earth.
• There are two energy-harvesting approaches: converting the potential energy of the tides (the rise and fall in water height) and converting the kinetic energy of the tides (harvesting the tidal currents).
• Unlike wind energy, tidal energy has a predictable, daily schedule, resulting in highly reliable generation.
• Once built, tidal power is inexpensive, with no fuel costs and relatively low operations and maintenance cost.
• The higher density of water (over 800 times that of air) means higher densities of energy generated from a given installation footprint.
• Smaller installation footprints have the additional benefit that the impact of offshore and vertical-axis turbines are manageable. Good engineering design, supported by existing research and development, can mitigate the environmental impacts.
Tidal Power Disadvantages
An individual harvesting approach best presents the disadvantages of tidal power generation. In general, the disadvantages of harvesting the potential energy of the tides (water height rise and fall) are:
• Constructing a barrage or sluiced damn across an estuary is expensive to build.
• There is a risk of adverse effects on the environment for many miles upstream and downstream of the barrage facility.
• Birds, sea life, marine and even land animals rely on the tides uncovering the mud flats so that they can feed.
• Many sea animals require unobstructed access to migratory paths for feeding, reproduction and seasonal migration.
• There are only a limited number of suitable sites for tidal barrages.
Harvesting energy from tidal currents also has disadvantages. They include:
• The power generation equipment must be built to withstand operation underwater in relatively harsh environments.
• Effects on marine life during construction phases.
• Operation and control must be provided remotely and maintenance is complicated due to sea-basing of the generation facilities.
• Sea-based moorings and towers to hold the generators must be placed on the sea bottom.
• The generating facilities and mooring infrastructure are potential navigational hazards.
• There are only a limited number of suitable sites with strong tidal currents.
• Only able to provide power for around 10 hours each day, when the tide is actually moving in or out.
Resources
British Columbia Sustainable Energy Association: http://www.bcsea.org/
Clean Current: http://www.cleancurrent.com/
The Engineering and Physical Sciences Research Council (EPSRC), Marine Energy Research (United Kingdom): http://www.epsrc.ac.uk
The European Marine Energy Center, Ltd., Orkney Islands (UK): http://www.emec.org.uk
Marine Current Turbines, Ltd. (UK): http://www.marineturbines.com
Nova Scotia Power (Canada): http://www.nspower.ca
OpenHydro Group, Ltd., Dublin (Ireland): http://www.openhydro.com
Ocean Renewable Energy Coalition: http://www.oceanrenewable.com/
Underwater Electric Kite US: http://www.uekus.com/
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