Environmental Engineering Reference
In-Depth Information
Tidal power is appealing because seawater's greater density, compared with wind, means fewer
turbines are needed to create the same amount of power (USDOI 2011a). Water is more than 800
times denser than air, so for the same surface area, water moving twelve miles per hour exerts
about the same amount of force as a constant 110 mph wind (USDOI 2006a). And tides, unlike
the wind, are predictable. But perhaps the greatest advantage is that the underwater equipment is
hidden, unlike wind turbines, so there are no complaints from the public about aesthetics. Accord-
ing to Paul Jacobson, water power manager for the Electric Power Research Institute, the largest
obstacle to development of tidal power is lack of funding for further development of technology
and for permitting and licensing of demonstration projects (Sharp 2010).
Waves are caused by friction between air and water from wind blowing over the surface of the
ocean. In many areas of the world, the wind blows with enough consistency and force to provide
continuous waves. There is tremendous energy in ocean waves. Wave power devices extract
energy directly from the surface motion of ocean waves or from pressure fluctuations below the
surface (USDOI 2011b).
Wave power varies considerably in different parts of the world, and wave energy cannot be
harnessed effectively everywhere. Wave-power-rich areas of the world include coasts of the
northwestern United States, western Scotland, northern Canada, southern Africa, and Australia.
Wave energy resources in the United States are quite limited. In the Pacific Northwest, it is pos-
sible that wave energy could produce forty to seventy kilowatts per meter (3.3 feet) of western
coastline (USDOI 2011b).
A variety of technologies have been proposed to capture the energy from waves, with some
demonstration testing at commercial scales. Wave technologies have been designed for installation
in nearshore, offshore, and far offshore locations. Offshore systems are situated in deep water,
typically more than forty meters (131 feet) (USDOI 2011b). While all wave energy technologies
are intended to be installed at or near the water's surface, they differ in their orientation to the
waves with which they are interacting and in the manner in which they convert wave energy into
other energy forms, usually electricity. Wave energy conversion technology is not commercially
available in the United States.
Terminator devices extend perpendicular to the direction of wave travel and capture or reflect
the power of waves. These devices are typically onshore or nearshore, but floating versions have
been designed for offshore applications. The Oscillating Water Column is a terminator device in
which water enters through a subsurface opening into a chamber with air trapped above it. Wave
action causes the captured water column to move up and down like a piston and forces air though
an opening connected to a turbine, such that the air turns the turbine (USDOI 2011b).
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). The relative motion is used to drive
electromechanical or hydraulic energy converters that generate electricity (USDOI 2011b).
Attenuators are long multisegmented floating structures oriented parallel to the direction of
the waves. Differing heights of waves along the length of the device cause flexing where the seg-
ments connect, and this flexing is connected to hydraulic pumps or other converters that generate
electricity (USDOI 2011b).
Overtopping devices have reservoirs filled by incoming waves to levels above the surrounding
ocean. When waves recede, water is released and gravity causes it to fall back toward the ocean
surface. The energy of falling water is used to turn hydroelectric 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 (USDOI 2011b).
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