Environmental Engineering Reference
In-Depth Information
devices are much less energetic. He estimated that a tidal turbine rotor at a good site
will absorb about 4 kW/m
2
of swept area from the current, whereas typical ship pro-
pellers release over 100 kW/m
2
of swept area into the water column. In addition to
the greater power density, a ship propeller and ship hull generate suction that can pull
objects toward them, increasing the area of influence for strike (Fraenkel, 2006).
Effects of Water Pressure Changes and Cavitation
In addition to direct strike, there is a potential for adverse effects due to sudden water
pressure changes associated with movement of the blade. For example, if the local
water pressures immediately behind the turbine blades drop below the vapor pres-
sure of water, cavitation will occur. Cavitation is the process of forming water vapor
bubbles in areas of extreme low pressure within liquids. As a turbine blade rotates,
cavitation can occur in areas of low pressure (i.e., downstream surface of blades)
causing increased local velocities, abrupt changes in the direction of flow, and
roughness or surface irregularities (USACE, 1995). Once formed, cavitation bubbles
stream from the area of formation and flow to regions of higher pressure where they
collapse. The violent collapse of cavitation bubbles creates shock waves, the inten-
sity of which depends on bubble size, water pressure in the region of collapse, and
dissolved gas content. Within enclosed, conventional hydroelectric turbines, forces
generated by cavitation bubble collapse may reach tens of thousands of kilopascals
at the instant and point of collapse (Hamilton, 1983; Rodrigue, 1986). Cavitation is
an undesirable condition that will reduce the efficiency of the turbine and damage
blades as well as nearby organisms. Properly operating turbines would not cavitate,
and the zone of low pressure that might be injurious to organisms would be relatively
small. The pressure drops associated with the blades of hydrokinetic turbines have
not been measured in field applications, but experimental evidence suggests that
tidal turbines may experience strong and unstable sheet and cloud cavitation, as well
as tip vortices at a shallow depth of submergence (Wang et al., 2007). If this occurs,
aquatic organisms passing near the cavitation zones in the immediate blade area may
be injured. The likelihood of cavitation-related injuries would depend on the extent
of cavitation and the ability of aquatic organisms to avoid the area—the collapse of
cavitation vapor bubbles creates noise which may act as a deterrent.
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MpaCTs
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onversion
An OTEC technology operates a low-temperature heat engine based on the tem-
perature differences between warm surface water and cold deep water (Holdren et
al., 1980). This type of project consists of pumps and ducts for transferring large
volumes of water (several times more flow than is needed for a once-through cool-
ing system of a comparably sized steam electric power plant), large heat exchangers,
and a working fluid that can be vaporized and recondensed (i.e., ammonia, propane,
Freon
®
, or water). Electrical energy could be transported from offshore systems via
subsea cables or alternatively could be converted to chemical energy
in situ
(e.g.,
hydrogen, ammonia, methanol) and transported to shore in tankers (Pelc and Fujita,
2002).
