|Keeping a Wave on the Sand: Generating Clean Energy from the Seas|
|Sunday, 23 June 2013 00:00 | Written by Michelle Wood | Article|
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
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.
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
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
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
Why the Delay in Implementation?
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.
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