UPDATE: On Tuesday, August 14, 2012, the ocean electric generating turbine was successfully lowered into the Bay of Fundy, just off of Eastport, Maine. Costing $21 million, and benefitting from $10 million in U.S. Dept. Of Energy subsidy, Ocean Renewable Power Company will begin generating some 180 kilowatts, enough to power about 30 homes, according to The New York Times.
The Bay of Fundy has one of the largest tidal changes in the world, as much as 50 feet from low to high tides, and huge quantities of water flow in and out. That’s what makes the Bay attractive for generating electricity.
Eventually, there might be as many as 18 generators generating enough electricity for as many as 1,500 homes. This is an encouraging step. Harnessing the power in the Bay of Fundy’s tides has been discussed and plans put forward since the Franklin Roosevelt administration. The big technological step forward of this new project is that it does not involve building a dam, which would block fish migration and have to fight the huge power of the Bay’s tides. Instead, a turbine something like a wind turbine lying on its side will be placed in the water. It won’t block fish passage and it won’t have to fight the tidal forces in the same way and at the same level as a dam.
How much energy could the world obtain from the oceans? The Bay of Fundy has one of the highest tides in the world, and there are few similar opportunities around the world.
The ocean holds two kinds of energy: one from its moving currents, waves, and tides, which we might informally call mechanical energy, and the other from the temperature difference between surface water and deep water, which we might informally call thermal energy. The World Energy Council writes that the ocean could provide the equivalent of “twice the world’s electricity production,” and that wave energy alone, with current technologies, could provide 15% of the world’s electric energy.
These are rough estimates, to some extent limited to the efficiencies of existing technologies. It’s hard to forecast improvements in efficiency and advances in the kinds of environments where ocean energy could be tapped, and therefore it is difficult to figure out how this energy could be captured and turned into electricity. Engineers generally try to take limitations into account.
For example, another estimate assumes that only 20% of America’s offshore wave energy would be harnessed, and that would be at 50% efficiency (meaning that half of the energy in the waves would end up as usable electricity). Even given these limitations, wave energy could still provide an amount of energy equal to all U.S. hydropower in 2003.
In theory, energy in ocean currents, tides, and waves could provide twice the world’s current energy use. And in theory, wave energy alone, using current technology, could provide 15% of the world’s electricity. If the United States could harness 40% of the nearshore wave energy, it would capture as much energy as is generated by all freshwater hydropower now available in the U.S.
The oldest use of ocean energy is from the tides. During their occupation of Great Britain, the Ancient Romans built a dam that captured tidal water and let it flow out through a waterwheel. In Medieval England, use of tidal power was not uncommon. Currently proposed is the world’s largest tidal power plant, in Great Britain’s Severn River estuary, where the Romans also made use of tidal power. It would have a generating capacity of 2 billion watts.
The most successful modern tidal power plant is La Rance, off the coast of Brittany, France, producing 10 million watts of electricity a year and shown as the featured photo for this post. For years it was the only full-scale tidal power station in the world. It was built in 1967 and produces 10 MW—--enough to provide electricity for 300,000 homes, or for 4% of the population of Brittany—--from 24 turbines, which produce 0.5 billion kilowatt-hours of power a year Wisely, this facility was built along a part of the Brittany coast that has one of the greatest tidal ranges in the world, about 40 feet between high and low tides. This maximizes the energy that can be obtained. Also impressive is that this power plant has never suffered serious damage or mechanical breakdown and, in contrast to many major energy sources that are eyesores, has become a tourist attraction. We can’t expect La Rance imitators to be set up and work everywhere, because only a few places have such favorable topography.
There are also good sites along the northeastern United States. Present conventional technology requires a tidal range of at least 26 feet.
Experimenting with ocean waves and currents
With all the energy potentially available from the ocean, a lot of imagination is going into designing machines to turn the energy in ocean currents and waves into electricity. The idea is to get away from building dams and other fixed structures, which fight the very motion that they try to use and instead seek devices that would be immersed in the ocean and convert the motion of currents into electricity in a more forgiving way. An Australian company has taken a bioengineering approach, using designs that occur in nature. One of their devices looks like a frond of giant kelp, the brown seaweed that forms underwater “forests.” Like the kelp, the new device has holdfasts that anchor it to the bottom of the ocean and a flexible arm that waves back and forth with the motion of the water like the kelp’s frond. This motion turns a generator that makes electricity.
Another device, shaped like a shark’s fin, rolls with the moving water and lets extremely large waves pass by. Still another, called the Pelamis, is being developed by Ocean Power Delivery in Edinburgh, Scotland. The New York Times describes it as “a snakelike wave energy machine the size of a passenger train, which generates energy by absorbing waves as they undulate on the ocean surface.”
Much of this post was excerpted from Powering The Future: A Scientist's Guide to Energy Independence, by Daniel B. Botkin (2010, FT Press.) It is directly from my book and is copyrighted material that can only be reproduced with the author’s permission, but is available here to be read online.