Harnessing Energy from the Oceans
Forever moving – our restless oceans have enough energy to power the world. As long as the Earth turns and the moon keeps its appointed cycle, the oceans will absorb and dissipate vast amounts of kinetic energy – a renewable energy resource of enormous potential. But harnessing this resource has proven more difficult than first thought. In this the latest installment of the GLOBE-Net Series on Renewable Energy – we look at how the power of the oceans might eventually find its place among other forms of renewable energy.
Ocean Energy – What is it?
According to the United Nations, 44% of the world’s population lives within 150 km of an ocean coast. In Canada and Australia the number is much higher at 80%. In the United States 53% of the population lives in close proximity to an ocean.
Thus it is only natural that many countries look to the oceans as a source of energy to be harnessed. How they seek to exploit this resource varies according to factors of geography and available technologies.
The two main forms of energy associated with our oceans are tidal power and wave power – born of the same source, but different in how they turn energy into electricity.
Tidal power coverts the energy of tides into electricity utilizing the rise and fall of the ocean tides. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal electricity generation.
Tidal generators act in much the same way as do wind turbines, however the higher density of water (832 times that of air) means that a single generator can provide significant power at velocities
much lower than those associated with the wind power generators.
Tidal stream systems make use of the kinetic energy of moving water to power turbines. This technology simply relies on individual turbines which are placed in the water column; moored to be suspended, floating or anchored to the ocean floor. As the tide flows in or out, electrical energy is produced as water moves through the turbine.
Tidal power boasts several advantages over other types of renewable energy technology, because tides are more predictable and reliable than wind energy or sunny days for solar power. Tidal energy has an efficiency ratio of approximately 80% in terms of converting the potential energy of the water into electricity. Tidal stream system turbines are only a third the diameter of wind rotors of the same power output.
Ocean surface waves are also a considerable source of energy potential, but energy that is not as restricted in terms of location as tidal energy systems. Typically wave energy is captured using buoys which generate mechanical energy as they oscillate vertically from wave motion.
Terminator devices extend perpendicular to the direction of wave travel and capture or reflect the power of the wave. Water enters through a subsurface opening into a chamber with air trapped above it and wave action causes the captured water column to move up and down like a piston to force the air though an opening connected to a turbine.
The Challenges Despite the enormous potential of ocean energy, there remain many pitfalls (if such a word can be used in a watery context) that have proven difficult to overcome, and which explains why ocean energy remains the least developed of all forms renewable energy. Problems still exist regarding cost, maintenance, environmental concerns and our still imperfect understanding of how power from the oceans will impact on the world’s energy infrastructure.
For example, turbines are susceptible to bio-fouling; the growth of aquatic life on or in the turbine. This can severely inhibit the efficiency of energy production and is both costly and difficult to remove. Turbines are also prone to damage from ocean debris.
Turbines may also be hazardous to marine life and the impacts on marine life are still largely unknown, but concern is warranted.
Barrage systems are affected by problems of high initial infrastructure costs associated with construction and the resulting environmental problems. Barrage impacts include a decrease in the average salinity and turbidity within a barrage, significantly altering associated ecosystems.
Wave power systems present their own set of challenges. Most electric generators operate at higher speeds, and most turbines require a constant, steady flow. Unfortunately wave energy is slow and ocean waves oscillate at varying frequencies.
Modern advances in ocean energy technology may eventually see large amounts of power generated from the ocean, especially tidal currents using the tidal stream designs. The technology is still in its infant stage and most projects that exist or are that are in project development stages are mainly pilot projects. But the promise remains.
“It’s not as well-established as solar, thermal, wind and biomass, but it [ocean power] shows a lot of promise,” said Philip Jennings, professor of energy studies at Western Australia’s Murdoch University.
“Water covers more than 70 percent of the Earth’s surface,” said Andy Karsner, assistant secretary for energy efficiency and renewable energy at the DOE. “Using environmentally responsible technologies, we have a tremendous opportunity to harness energy produced from ocean waves, tides or ocean currents, free-flowing water in rivers and other water resources to…provide clean and reliable power.”
According to Jennings ocean power could not match fossil fuels for electricity production but could be competitive with other forms of renewable energy.