Rezachek & Associates'
Energy & Environmental Resources

WAVE ENERGY

WAVE ENERGY - What Is It?

Waves are generated by the force of the wind acting on the surface of the ocean. The wind imparts a portion of its energy to these waves. The energy of these waves is then converted, through various devices, to a rotary or oscillating motion or to movement of a working fluid (such as air, seawater or hydraulic fluid) which can be used to drive an electrical generator.

Advantages

Wave power plants can be based on land, on caissons in relatively shallow water (5-15 m depth), or in deeper, offshare waters.
Wave energy systems can be incorporated into breakwaters, thus reducing the cost of such systems, and providing for dual use.
Use of wave energy as a renewable resource of electricity will help reduce Hawaii's almost complete dependence on imported fossil fuels.
Hawaii has a large wave energy resource.

Disadvantages

Wave energy systems, if not properly sited, could interfere with other uses of coastal areas such as fishing, surfing, and beach use.
Land-based systems involve significant shoreline modifications and associated environmental impacts.
Offshore floating systems might have unacceptible visual impacts.
Wave energy is an intermittent resource.
Wave energy conversion devices are in an early stage of development and are not yet commercially viable. System costs and performance are uncertain.
Such devices are subjected to a very harsh environment and have not been tested on a long-term basis.

The Basic Process

There are several basic types of wave energy conversion devices. Land-based systems include the tapered channel and a variety of fixed oscillating water column (OWC) devices. Caisson-based systems include fixed OWC devices, pivoting flaps, and confined, heaving floats. Offshore devices include floating OWC devices, heaving buoys and other devices.

The tapered channel consists of a collector which funnels waves into an ever-narrowing channel which increases their height. They then spill into a reservoir. Water drains back into the sea through a conventional hydroelectric turbine which generates electricity.

Fixed oscillating water column devices use a vertical circular cylinder, open at both ends, whose bottom end is submerged. As a wave crest passes, the column of water entrained in the submerged portion of the cylinder rises, pushing air out the top. Likewise, as a wave trough passes, the water column falls, drawing air in. If the cylinder is capped, valves and ducts can be arranged to allow electrical energy to be generated through a pneumatic turbine.

Heaving buoys and pivoting flaps use the up and down motion imparted by passing waves to pump fluids, create oscillatory movements which are translated to rotary movements, or use oscillatory movements directly in a variety of devices to generate electricity.

A number of these modular devices can be connected together to create a larger wave energy conversion system. The electricity generated can then be transmitted to the utility grid.

Use of Wave Energy in Hawaii

There are no wave energy conversion systems in Hawaii. However, a preliminary assessment of the wave energy resource and the economics of such systems has been conducted for Hawaii.

The summary for Wave Energy was prepared by David Rezachek and is based, in part, on information contained in a report prepared by SEASUN Power Systems of Alexandria, Virginia, USA, entitled "Wave Energy Resource and Economic Assessment for the State of Hawaii." This information does not necessarily reflect the views of the State of Hawaii, the United States Government, or any agency thereof.