Tenchnology on White Paper

RESOURCE POTENTIAL

Ocean waves represent a form of renewable energy created by wind currents passing over

open water. Capturing the energy of ocean waves in offshore locations has been demonstrated as

technically feasible. Also, basic research to develop improved designs of wave energy

conversion (WEC) devices is being conducted in regions such as near the Oregon coast, which is

a high wave energy resource (Rhinefrank 2005). Compared with other forms of offshore

renewable energy, such as solar photovoltaic (PV), wind, or ocean current, wave energy is

continuous but highly variable, although wave levels at a given location can be confidently

predicted several days in advance.

The common measure of wave power, P, is

ρg 2TH 2

P=

32π watt per meter (W/m) of crest length (distance along an individual crest),

where:

ρ = the density of seawater = 1,025 kg/m3,

g = acceleration due to gravity = 9.8 m/s/s,

T = period of wave (s), and

H = wave height (m).

Because wind is generated by uneven solar heating, wave energy can be considered a

concentrated form of solar energy. Incoming solar radiation levels that are on the order of

100 W/m2 are transferred into waves with power levels that can exceed 1,000 kW/m of wave

crest length. The transfer of solar energy to waves is greatest in areas with the strongest wind

currents (primarily between 30o and 60o latitude), near the equator with persistent trade winds,

and in high altitudes because of polar storms.

Waves are also efficient transporters of solar energy. Storm winds generally create

irregular and complex waves. In deep water, after the storm winds die down, the storm waves

can travel thousands of kilometers in the form of regular smooth waves, or swells, that retain

much of the energy of the original storm waves. The energy in swells or waves dissipates after it

reaches waters that are less than ~200 m deep. At 20-m water depths, the wave's energy typically

drops to about one-third of the level it had in deep water.

2

The total annual average wave energy off the U.S. coastlines (including Alaska and

Hawaii), calculated at a water depth of 60 m has been estimated (Bedard et al. 2005) at

2,100 Terawatt-hours (TWh) (2,100 × 1012 Wh).2

Estimates of the worldwide economically recoverable wave energy resource are in the

range of 140 to 750 TWh/yr for existing wave-capturing technologies that have become fully

mature (ETNWG 2003). With projected long-term technical improvements, this could be

increased by a factor of 2 to 3 (Thorpe 1999). The fraction of the total wave power that is

economically recoverable in U.S. offshore regions has not been estimated, but is significant even

if only a small fraction of the 2,100 TWh/yr available is captured. (Currently, approximately

11,200 TWh/yr of primary energy is required to meet total U.S. electrical demand.) WEC

devices have the greatest potential for applications at islands such as Hawaii because of the

combination of the relatively high ratio of available shoreline per unit energy requirement,

availability of greater unit wave energies due to trade winds, and the relatively high costs of

other local energy sources.

RESOURCE UTILIZATION TECHNOLOGIES

A variety of technologies have been proposed to capture the energy from waves;

however, each is in too early a stage of development to predict which technology or mix of

technologies would be most prevalent in future commercialization. Some of the technologies that

have been the target of recent developmental efforts and are appropriate for the offshore

applications being considered in this assessment are terminators, attenuators, point absorbers,

and overtopping devices.

Terminators

Terminator devices extend perpendicular to the direction of wave travel and capture or

reflect the power of the wave. These devices are typically installed onshore or nearshore;

however, floating versions have been designed for offshore applications. 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 to force the air though an opening connected to a turbine. A full-scale,

500-kW, prototype OWC designed and built by Energetech (2006) (Figure 1) is undergoing

testing offshore at Port Kembla in Australia, and a further project is under development for

Rhode Island.

2 This estimate was made at a specified water depth of 60 m (irrespective of the

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