Ocean-Based Renewable Energy
Fossil fuels like petroleum or other hydrocarbon resources are considered nonrenewable, since it takes millions of years to convert organic matter into these energy resources. As oil and gas becomes increasingly difficult to recover, as geopolitics complicate the access to both the resources and the markets for such energy products, and as existing supplies diminish and the world approaches peak oil production, interest in renewables has increased. Additionally, recent attention on carbon emissions from hydrocarbon use and their role in global warming and other climate changes means that renewable energy resources are fast becoming a preferred alternative. However, renewable energy resources are still at this point in time generally more expensive than conventional non-renewables, and production tends to be small scale.
There are four major classes of renewable energy available at sea: 1) kinetic energy unique to the ocean, such as energy provided by surface waves and tides, 2) renewable energy not unique to the ocean, such as wind and solar energy, 3) thermal energy, such as that produced by the temperature differential of surface and deep ocean waters, and 4) marine biofuels, such as those derived from algae.
Kinetic
Attempts have been made to harness the enormous energy potential of moving ocean water for decades. As far back as the middle of the 11th century, people were making the logical extension from exploiting energy in running rivers, streams, and canals (an ancient technology that probably predates even waterwheels for grinding flour) to trying to harness that same mechanical energy contained in waves and tides. The first commercial scale wave energy plant was commissioned for the Isle of Islay (Scotland) in 2000; at about the same time, the Japan Marine Science and Technology Center created a large scale experimental wave energy platform.
Early attempts to harness the kinetic energy contained in moving seawater were focused on estuaries, where both river hydrology and tides influence the movement of seawater or brackish water. But wave energy can be harnessed, in theory, anywhere where there are predictable waves, including in offshore areas. The first commercial scale wave power station was established in Scotland at the beginning of the century.
Oceanlinx, a company formerly known as Energetech, has developed wave energy projects in three areas of Australia (New South Wales, Victoria, and Tasmania), in two sites in the U.S. (Rhode Island and Hawaii), in South Africa, in Mexico, and in the United Kingdom. The wave energy devices follow the simple principle of the model below:
Water inside a chamber which is open on the bottom rises and falls as waves pass through, compressing and displacing the air inside which then drives a turbine. Wave energy is converted to mechanical energy that drives an electric generator. In some units, the turbine is driven on both the upward and the downward movement of water in the chamber. This is the case with the Oceanlinx turbine, which has variable pitch blades that produce maximum energy efficiency – each unit can produce enough energy to power 1,500 homes and can save thousands of tonnes of CO2 and SO 2 annually3. Other units rely on what is known as heaving buoy technology, which captures the kinetic energy in the orbital motion of surface waves. And tapering channels funnel waves into natural or artificial channels, filling an elevated reservoir which then allows water to flow back to the sea past turbines that capture and convert the energy.
Tidal energy also has great potential as a renewable energy source, and has the advantage over waves of high predictability. Tides are caused by the gravitational pull of the moon and sun and their effects on the rotating Earth. In near shore areas, the differential between low and high tide (both of which occur twice a day) can approach 15 meters. However, there are a limited number of areas around the world where tidal range is high and topographic conditions would allow the utilization of tidal energy. According to the U.S. Department of Energy, only about 20 locations have good inlets and a large enough tidal range - (at least 3 meters) - to produce energy economically4.
Most tidal energy plants use a dam, known as a barrage, which spans a narrow bay or inlet. Sluice gates on the barrage allow the tidal basin to fill on the incoming high tide and empty through the turbine system on the outgoing, or ebb tide. As in wave energy systems, there are units that generate electricity on both the incoming and outgoing tides5. The La Rance Station in France began making electricity by harnessing kinetic energy of tides in 1966 and now produces enough energy to power 240,000 homes (240MW/year – or about 1/5 the power generated by a nuclear power plant). According to an article in the Financial Times, total wave and tidal power could eventually exceed 2TWh/year (2 trillion watts) of electrical energy generation.
Open ocean currents also contain potential power for energy generation. The government of Taiwan is looking into harnessing the strong Kuroshio Current to generate an expected 1.68TWh per year6.
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