Ocean wave energy

A number of designs for harnessing wave energy have been proposed, and some are in use on various scales, but the vast potential of this power source has not been tapped because of the uncertainties and expense involved.

Background

Waves crashing against a beach are a vast, almost mystical, display of mechanical power. For centuries people have sought ways of tapping it. In 1799, a father and son named Girard applied to the French government for a patent on a wave-power device. They noted that waves easily lifted even mighty ships. Hence, a lever from a ship to shore could power all manner of mills. (There are records of Girard mills on rivers, but the wave machine was probably never built.) In 2024, Eco Wave Power, an onshore wave technology company, received approval to build a wave energy project in the Port of Los Angeles. Eco Wave already had a pilot station in Israel.

The Nature of Waves

Waves are the product of wind blowing on the ocean surface. The energy available comes from the wind speed and the distance (or “fetch”) that the wind blows: A breeze blowing on a small bay produces ripples, whereas a hurricane blowing across several hundred meters builds hill-sized waves. Waves hitting a beach can be the result of a storm on the opposite side of an ocean. From that standpoint, waves are a collecting and concentrating mechanism for wind power. However, there is some loss of wave energy over great distances, so the best places to take advantage of wave potential are along high-wind coasts of the temperate and subpolar latitudes. Specific regions of the world with strong wave actions include the western coasts of Scotland, northern Canada, southern Africa, Australia, and the northwestern coasts of the United States.

Water waves mostly consist of a circular motion of the water molecules as the wave energy continues until it meets a barrier, such as a shoreline. Then the energy hurls water and pieces of the shore until gravity pulls them back. Ultimately, the energy is transformed into heat, hardly noticed in the water. Along the way, the energy is vast. The North Pacific is estimated to have a of 5 to 50 megawatts of mechanical energy per kilometer.

One limitation of wave energy is that timing and power are variable (although not as much as with winds). The crests and troughs of one storm may be out of phase with another, in which case they largely cancel each other out. Winds may be low, or they may be directly against waves approaching the power plant. Any of these factors can limit power production at unpredictable times. Conversely, waves from two or more storms may be in phase and stack, creating monster waves that have been observed as high as 34 meters in the open ocean. Extraordinary waves have been the destruction of countless ships and of more than one wave power station. They are probably the greatest obstacle to widespread use of wave energy.

Methods for Harnessing Ocean Waves

Electrical power generation from waves requires three things: mooring the power stations to the ocean floor or building along the coast, generating power, and transmitting the power to customers inland. As with wind energy, a useful fourth item would be storage to deal with low-wave days.

Building and power transmission are straightforward operations because most wave-harvesting designs are on or near shore. Even though these installations must be reinforced against especially strong waves, they do not have the cost and complexity of deepwater structures.

Proposed energy-harvesting techniques have great variation because many researchers have been attempting to harness wave potential. The researchers face three major problems. First, generators face the previously mentioned fluctuations in awesome power. Second, wave power is large but moves at a slow pace, and the machinery to obtain high speed (needed for an electric generator) is expensive. Third, complex hinges, pistons, and other moving parts need frequent replacement in the salty ocean environment.

The simplest approach is a ramp and dam facility that traps water splashing above sea level. Draining water goes down a pipe (penstock) to turbines, just as in a hydroelectric dam. The “Russel rectifier” is sort of a dam with chambers and flaps so that both rising and falling waves cause water in a turbine to flow continuously in the same direction. The various dam schemes are familiar and can be built on land. The disadvantage is that power dams must be large and capable of surviving the surf; thus, they are expensive.

The “dam atoll” is an open-ocean variant of the ramp. A half-submerged dome in the ocean bends waves around it so that waves come in from all sides, just as with coral atolls. The water sloshes to a central drain at the top and drains back through a penstock. The central collection increases efficiency, and being a floating structure allows submerging below the waves during major storms. However, increased distance from shore increases power transmission costs.

Air pressure can translate slow wave motion into a fast spin. In many schemes, waves rise and fall either in a series of open rooms at the bottom of a floating structure or in cylinders at the end of funnel-shaped passages facing the waves. In both cases, the waves alternately push air out and suck it in. Both processes run turbines at high speed. Extensive work on air-pressure designs has been done by the British, the Norwegians, and the Japanese, who tested the Kaimei, an 80-meter ship with a number of chambers for testing various turbine designs, in the 1970s. In the 1980s and 1990s, the Japanese worked on the Mighty Whale project, which consists of near-shore floating structures with three large air chambers that convert wave energy into pneumatic energy.

Directly harnessing wave motion has some advantages to offset the slow motion and exposure of moving parts. The necessary equipment can be much smaller (thus cheaper) per unit of electricity generated than the other schemes. For many years, Japanese buoys have used pendulums and pulling units to power lights and horns. Scaling these units to larger sizes is difficult and expensive. Experimental units have used hinges between rafts (Cockerell’s design), “nodding duck” cam-shaped floats to activate rotary hydraulic pumps to turn a generator (Salter’s design), paddles on rollers, and many other techniques.

In 2008, the world’s first commercial-scale wave-power station went live off the coast of Portugal. This British-designed and Portugese-financed station is about 5 kilometers off the northern coast of Portugal and consists of several semisubmerged 142-meter-long, 3.5-meter-diameter “snakes” of carbon steel, each with four articulated sections. The wave action drives hydraulic rams in the snake’s hinges, creating energy that generators convert to electricity, which is relayed to a substation in Portugal via seabed cables. At peak output, three machines can generate 2.25 megawatts, which is enough to serve fifteen hundred family homes for a year. Once produced in quantity, ocean wave power may have economics similar to hydroelectric plants—expensive to build but inexpensive overall because of low operating costs.

Bibliography

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