Energy Articles

Harnessing Wave Energy

Scientific opinion supports the idea that we will have to go to the ocean for food if we are to support the newly 7 billion estimate of the world’s population. As well, many believe that both building materials and minerals that our ever growing people’s needs will also come from our sea and our oceans. It will probably not be a surprise to you that our energy needs are beginning to be met by various forms of ocean generated power including but not limited to: wave generated power, current generated power, tidal generated power and offshore solar power.

Wave Generated Power

Naturally, wave power varies dramatically across the world, and wave energy can not be harnessed effectively everywhere. Wave-power rich areas of the world include the western coasts of Scotland, southern Africa, northern Canada, Australia, the northwestern coasts of the United States and some parts of northern Europe and especially Scandinavia.

Waves are caused by the wind blowing over the surface of the ocean. In many places in the world’s oceans, the wind blows with the reliability and force to provide continuous waves that can provide prodigious energy in the ocean waves. Mankind has invented and continues to invent new devices to extract energy directly from the surface motion of ocean waves and even tapping into the pressure variations that occur below the surface.

Ocean Wave Energy Technologies

A variety of technologies are being perfected, some even at commercially viable levels. They are quite promising for those hoping to capture energy from the movement of the ocean waves.

Some technologies are currently being designed to be far-offshore as well as in nearshore and offshore installations. Offshore systems are situated in deep water, usually more than 40 metres deep.

While most wave energy systems are installed at or near the water's surface, they differ in their orientation to the waves and in the method by which they convert the wave energy into other energy forms, predominantly electricity. The examples that follow represent some of those wave energy technologies that are currently in both research and development stages.

A point absorber is a floating structure with components that move relative to each other due to wave action (e.g., a floating buoy inside a fixed cylinder) These devices aren't oriented a particular way toward the waves, but rather can "absorb" the energy from waves that come from every which way. One such device is called the Aquabuoy and was developed by Finavera. In a vertical tube below the water, waves rush in and drive a piston, a buoyant disk connected to hose pumps, up and down to pressurize seawater inside. The pressurized water then drives a built-in turbine connected to an electrical generator. The relative motion is used to drive electromechanical or hydraulic energy converters. (http://ocsenergy.anl.gov)

One such technology that is installed at right angles to the direction that the waves travel is called a terminator device. These capture or reflect the magnitude of the waves and are usually onshore or nearshore. This is because one part of the device must remain fixed and unmoving while another part, attenuators, use long multi-segment floating structures oriented parallel to the direction of the waves that move in interaction with the waves. Currently, floating versions have been designed for offshore applications and usually these must be attached at some point to the sea floor.

The oscillating water column (OWC) is a form of terminator in which water enters through a subsurface opening into a chamber with air trapped above it. The wave action causes the captured water column to move up and down like a piston in an automobile to force the air though an opening connected to a turbine. The differing heights of waves along the length of the device causes flexing where the segments connect, and this flexing is connected to hydraulic pumps or other converters.

The most famous kind of terminator, however, is called Salter's Duck and includes a bobbing tear-shaped head that drives the turbine. Though not fully developed, theoretically, this device would be the most efficient wave energy collector (WEC). The Salter's duck, also called the bobbing duck and officially Edinburgh’s duck, was invented by Stephen Salter in response to the oil shortage in the 1970s and was one of the earliest generator designs proposed to the Wave Energy program in the U.K. (http://www.mech.ed.ac.uk/research/wavepower/).

Overtopping devices are another type of terminator and have reservoirs that are filled by incoming waves to levels above that of the surrounding ocean. The water is then released, and gravity causes it to fall back toward the ocean surface. The energy of the falling water is used to turn hydro turbines. Specially built seagoing vessels can also capture the energy of offshore waves. These floating platforms create electricity by funneling waves through internal turbines and then back into the sea. Several of these devices strung together make up what the industry refers to as a wave farm (http://www.wavedragon.net/).

Environmental Considerations and Obstacles to WEC’S

Potential environmental considerations for the development of wave energy include the following:

• Positive or negative impacts on marine habitat;
• Toxic releases from leaks or accidental spills of liquids used;
• Visual and noise impacts above and below the water surface;
• Conflict with other sea space users, such as commercial shipping and recreational boating;
• Cost in comparison to oil, natural gas, wind and solar;
• Durability of the component parts;
• Estimated as being able to produce only 10% of the world’s require energy;

Despite all the obstacles and the fact that many of the devices are in their R&D infancies, there is much energy promise from the ocean’s waves.