EcoHearth Environmental Website - Best ecology websites, environmental blogs, news, activism, green jobs, green products

According to Celtic history, the concept and practice of harnessing tidal power has been around for at least 2000 years; the mill at Eling, Ireland, has been operating for 900 of them. But wave energy isn’t just an ancient concept. The past 50 years have seen a rapid expansion of imaginative technologies for tapping the power of the ocean, and the ideas just keep getting better. Three of the most promising are: tidal turbine arrays, wave converters and ocean thermal electric conversion, or OTEC.

Tidal turbine arrays operate like underwater wind farms, while generating both more electricity and less aesthetic aversion. Wave converters make use of the constant movement on the ocean surface. And OTEC is an ingenious and sustainable variation on the refrigerator and the geothermal heat pump. Here’s more about how each works along with their environmental pros and cons.

Tidal Turbine Arrays
Wherever the differential between high and low tides is at least five meters, generating electricity from tides via tidal turbine arrays is practical. Tidal turbines can be envisioned as undersea, out-of-sight, high-efficiency wind farms. They are best positioned in the tidal currents of coastal waters between obstructions, such as islands or headlands, where the ocean is 20-30 meters deep. Because the density of water is so much greater than that of air, tidal turbines yield more energy than those relying on wind moving at the same velocity: a single seawater turbine can generate as much power as a wind turbine four times its size.

Unlike tidal dams, barrages and fences, submerged tidal turbine arrays have a negligible impact on marine ecosystems, including life forms that many people think of as important resources. And unlike sun and wind, tides are always available to continuously generate predictable quantities of electricity.

Wave Converters
The Pelamis wave converter undulates on the ocean surface like the species of serpent for which it is named. Think of it as a harmless, red, giant robotic sea snake that happily generates electricity. Totaling 150 meters in length, a wave converter looks like a very long articulated submarine, anchored at its nose and composed of four cylindrical sections hinged together. When wave swells cause the hinged joints to move, fluid under high pressure is forced through a hydraulic generator.

A wave-converter farm occupying one square kilometer of ocean can continuously generate 30 megawatts of electricity. The Pelamis wave converter has a redundant failsafe system in place to capture hydraulic leaks should they occur, and the snake’s transmission fluid is biodegradable.

Ocean Thermal Electric Conversion
In addition to systems that tap the motion energy in tides and waves, there is another technology that uses the ocean’s natural thermohaline circulation system and simple heat transfer to generate electricity. It is known as ocean thermal electric conversion (OTEC).

The global thermohaline circulation system is like a looped river current that runs through the world’s oceans, driven by the sun’s heat and the Earth’s rotation. In the equatorial Atlantic, surface water is continuously heated by intense daily sun and moved north by trade winds. Near the arctic, that same water becomes very cold and dense, sinks deep and returns to the tropics. In some locations, the ocean is deep enough to maintain a significant temperature differential between that deep arctic water and the warmer water on the surface, and this situation can be exploited to generate electricity.

Two kilometers off the southeast coast of Puerto Rico, the ocean is more than 950 meters deep—enough to maintain a temperature difference between surface water and deep water of approximately 23°C (40°F). An OTEC platform floating there supports a closed system that uses warm surface water to vaporize liquid propane. Warm water is adequate to the task, because propane has a very low boiling point. The vapor turns a turbine, which generates electricity to be sent via underwater cable back to the island. Cold water is then pumped up from the depths to cool the vapor back into a liquid, and the process is repeated. It might be thought of as the greened-up version of offshore oil drilling: offshore boil chilling.

Potential for Replacing Fossil Fuels
All three of the above technologies are promising sources of power. The energy in wave motion holds about 1,000 times that of wind. And OTEC plants capturing 0.1% of the solar heat stored in equatorial waters on a given day could generate enough electricity to power the United States for 20 days.

Conveniently, tidal turbine arrays are most suitable for areas where OTEC systems are not, and vice versa. Wave converters can be anchored in dense formations anywhere offshore, provided they are close enough for underwater cable to carry the power back to land and suitable allowances are made for marine traffic, including setting up navigation markers.

Prototypes and Working Models
Pelamis wave-converter prototypes underwent extensive sea trials from 2004 to 2007. Three such snakes anchored offshore now send 2.25 megawatts of electrical power to the Portuguese grid. Tidal turbines have been sea-tested in several locations over the past ten years, and a few have begun feeding power to local populations. A turbine off Kvalsund, Norway, that generates 300 kilowatts has been connected to the grid since late 2003. The first successful test of an OTEC system was in Hawaii in 1979. The OTEC platform floating off Puerto Rico generates 800,000,000 kilowatts of electricity; a total of 40 linked together would completely satisfy that island’s electrical demands.

Why the Delay in Implementation?
So why haven’t we manufactured hundreds of these brilliant sustainable energy systems for installation in every suitable location?

According to the US Department of Energy, the answer is simple: cheap fossil fuels. The initial construction cost for any of these offshore plants is substantial, and for now, the prices of our dwindling supplies of oil, gas and coal remain artificially low. Until the true and complete costs of their use—including all externalities—is incorporated into the prices of these fossil fuels, or scarcity drives their prices up, or both, the projected return on ocean-power investments will not be enough to justify the risk to venture capitalists.

Additional resources:
US Department of Energy: Ocean Tidal Power
The Pelamis Wave Converter
The Ocean Energy Council
What is Ocean Thermal Energy Conversion?

Help the Earth, Spread the Word: Share this article with family and friends by clicking on the "Email This" or "Share This" links below right. Then see TODAY'S TOP STORIES.

Comments (6)

Show/Hide comments

Write comment