Tidal or Wave Power

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Tidal or Wave Power

Introduction

Large amounts of energy are involved in the motions of ocean water. These motions include steady ocean currents, the repetitive motions of the tides, and the irregular motions of surface waves. For decades, machines have been in development to harvest energy in the form of electricity from these motions. Although a few projects have been in operation since the 1960s, a commercial market for ocean-power harvesting devices has only opened up in the early 2000s. Ocean power is poised to make a significant contribution to a future portfolio of renewable energy sources that will also include wind and solar power.

Historical Background and Scientific Foundations

Twice a day, over much of the world, sea level rises and falls, anywhere from a few inches to 56 ft (a few centimeters to 17 m). The most extreme variation occurs at the Bay of Fundy in coastal Canada. This rise and fall entails the movement of trillions of tons of water, involving an immense amount of energy. Tidal energy ultimately derives from the Earth-moon system: The energy dissipated as friction by the tides is subtracted from Earth’s rotational kinetic energy, slowing the planet’s spin. Because of the tides, Earth’s day is getting longer at the rate of about 0.0017 seconds per century.

Earth’s seas also circulate in currents and are covered everywhere by the short-lived, small-scale, up-and-down motions of waves. Some of the energy in currents and waves is derived from the tides, but most is from the sun, which drives air and ocean movements around the planet by heating it unevenly.

French engineer Pierre Girard (1765–1836) patented an early device for gathering wave power in 1799. In the centuries since this time, many schemes have been proposed for harvesting tidal and wave power, but until the second half of the twentieth century none produced large amounts of power. Development efforts are underway to find less expensive means of harvesting these energy flows. Challenges remain, but commercial sales of a variety of sea-power systems were ongoing in the early 2000s, and coming decades may see greatly increased output from ocean energy.

Tidal and Current Power

In some locations, bays are connected to the open ocean through narrow openings, forcing tidal waters to flow back and forth like a reversing river. Energy is particularly easy to gather in such locations: all that is needed is to place a windmill-like turbine or other energy-capturing device in the tidal current. A prototype of this kind of device was installed near Hammerfest, Norway, in 2003, generating 300 kilowatts (300,000 watts, 300 kW), with the Norwegian government planning to install a larger facility after gaining experience with the prototype.

In April 2008, a similar but larger system was installed in Strangford Lough, Ireland, a SeaGen dual-turbine device capable of generating a peak power output of 1.5 megawatts (1.5 million watts, five times more than the Hammerfest system). The SeaGen machine consists of an anchored column tall enough to protrude from the sea’s surface with two wing-like underwater supports, one on each side, each supporting a two-bladed turbine resembling a windmill. Each turbine is 50 ft (15 m) in diameter. The system, due to begin producing power later in 2008, was to be monitored by an independent team of government scientists to see what effects it had on seals, fish, and other aspects of the local environment. The blades turn too slowly to be a threat to most marine life.

WORDS TO KNOW

RENEWABLE ENERGY: Energy that can be naturally replenished. In contrast, fossil fuel energy is nonrenewable.

Such systems involve little modification of coastlines and are therefore environmentally relatively low-impact. A larger, but higher-impact, type of system is the tidal dam. A tidal-dam energy project has been operating at La Rance, France, since 1966, generating a round-the-clock average of 65 MW (240 MW peak). At La Rance, the rising tide is allowed to fill a reservoir behind a dam; when the tide begins to fall, floodgates are closed and the water in the reservoir is allowed to run out through turbines, generating electricity. Such systems will probably never be built in large numbers, both because of their great expense and because they involve large-scale modifications of coastlines.

Another low-impact system is the hydrofoil or pulse generator. A hydrofoil is a wing-like object designed to experience an up or down force when water flows over it. Hydrofoil generators being tested in the early 2000s were large structures that sat on the ocean floor with a 33-ft (10-m) hydrofoil blade held out on a horizontal arm. The current flowing over the hydrofoil forced it up and down, like the handle on a car jack or a dolphin’s tail being wagged. This motion operated a water pump, and water pressurized by the pump flowed through a generator, producing electricity. In April 2008, the government of the United Kingdom, a world leader in tidal and wave-power research, gave its go-ahead for the installation of a 0.1-MW pulse generator supplying power to the national grid (network of power lines supplying homes and businesses with electricity). Experience in operating the prototype would, if all went according to plan, lead to the design and installation of units 10 times larger.

Wave Power

Tidal power has the advantage of being completely predictable: The tides occur with perfect regularity every day. In contrast, waves vary in strength, from a mere ripple to large storm waves. Yet sites with deep, concentrated tidal currents are not common, and waves wash all shores. There are at least six different technologies for harvesting wave power:

Attenuator: Long, tubelike floats linked end-to-end by flexible hoses rock as waves pass under them. The motion pressurizes oil, which drives generators. The world’s first commercial wave-power farm, along the coast of Portugal, uses this technology and was being installed as of mid-2008.
Point absorber: A float bobs up down on the surface of the waves. A vertical shaft is worked by the motion, pressurizing generators.
Oscillating wave surge converter: A buoylike arm is tethered to the bottom and waved back and forth like an upside-down pendulum as waves surge past.
Oscillating water column: A sealed column with water in its lower portion and air in its upper portion is connected to the sea. Wave action causes the water part of the column to rise and fall, raising and lowering the air and causing air-driven turbines to rotate.
Overtopping device: Waves slosh over the top of a barrier and the water flows back to the sea through a turbine.
Submerged pressure differential: A drum or tube along which a piston moves is anchored to the ocean floor not far below the surface. Water pressure on the piston varies as waves pass above, causing it to move back and forth in the shaft.

Impacts and Issues

Why not put up windmills instead of tidal and wave power machines? There are two answers. First, windmills produce power irregularly, only when the wind blows. Tidal machines produce it on a reliable schedule, and even though waves are also irregular, they are easier to predict than winds. Second, water is 800 times denser than air (i.e., each cubic foot of water weighs 800 times as much as a cubic foot of air). This means that even at lower velocities, intercepting a relatively small amount of water in motion can yield a large amount of energy. Tidal and wave machines should, therefore, be able to harvest more energy for their size than wind machines, and so ultimately compete with them in cost.

However, wind power will never be displaced by sea power. Winds blow over continental interiors, far from the ocean, and are a very large, well-distributed energy resource. Also, windmills face fewer mechanical design challenges, due in part to the low density of air: They are subject to less extreme forces (for example, during storms). Also, sea-power systems are more difficult to access for maintenance and must survive immersion in a metal-corroding salt solution, namely seawater. It is likely that in the long term, renewable-energy systems harvesting energy from many sources, including geothermal, photovoltaic, solar thermal, passive solar, tidal and wave, wind, and other, will be used to supply some or all of society’s energy needs.

Wave power can also be used to drive oceangoing ships, gathering energy by using horizontal fins that rock up and down as the vessel encounters waves. A vessel demonstrating the technology set out from Hawaii in March 2008, bound for Japan. By mid-May 2008, it had reached the halfway point to its destination.

See Also Tides