Environmental Engineering Reference
In-Depth Information
Bahamas. Because spawning success is important to the viability of populations, the
siting and operation of ocean energy facilities would need to avoid interfering with
these activities.
C ollisions and s Trikes
Submerged structures present a collision risk to aquatic organisms and diving birds,
and the above-water components of floating structures may be a risk to flying ani-
mals. Wilson et al. (2007) defined “collision” as physical contact between a device or
its pressure field and an organism that may result in an injury to the organism. They
noted that collisions can occur between animals and fixed submerged structures,
mooring equipment, surface structures, horizontal- and vertical-axis turbines, and
structures that, by their individual design or in combination, may form traps. Harmful
effects to animal populations could occur directly (e.g., from strike mortality) or indi-
rectly (e.g., if the loss of prey species to strike reduces food for predators). Attraction
of marine mammals and other predators to fish congregations near structures may
also expose them to increased risk of collision or blade strike. In an attempt to define
the risk of collisions from marine renewable energy devices, Wilson et al. (2007)
reviewed information from other industrial and natural activities: power plant cool-
ing intakes, shipping, fishing gear, fish aggregation devices, and wind turbines. They
concluded that, although animals may strike any of the physical structures associated
with marine renewable energy devices (i.e., vertical or horizontal support piles, duct,
nacelles, anchor locks, chains, cables, and floating structures), turbine rotors are the
most intuitive sources of significant collision risks with marine vertebrates.
Effects of Rotor Blade Strike on Aquatic Animals
Many of the hydrokinetic and ocean current technologies extract kinetic energy by
means of moving/rotating blades. A wide variety of swimming and drifting organ-
isms (e.g., fish, sea turtles, driving birds, cetaceans, seals, otters) may be struck by
the blades and suffer injury or mortality (Wilson et al., 2007). Mortality is a function
of the probability of strike and the force of the strike. The seriousness of strike is
related to the swimming ability of the animal (i.e., ability to avoid the blade), water
velocity, number of blades, blade design (i.e., leading edge shape), blade length and
thickness, blade spacing, blade movement (rotation) rate, and the part of the rotor
that the animals strikes. A vertical axis turbine will have the same leading edge
velocity along the entire length of the blade. On the other hand, blade velocity on a
horizontal axis turbine will increase from the hub out to the tip. The rotor blade tip
has a much higher velocity than the hub because of the greater distance that is cov-
ered in each revolution. For example, on a rotor spinning at 20 rpm, the leading edge
of the blade 1 m from the center point will be traveling at about 2 m/s—a speed that
is likely to be avoidable or undamaging to most organisms. However, a 20-m-diam-
eter rotor spinning at 20 rpm would have a tip velocity of nearly 21 m/s. Fraenkel
(2006, 2007b) described a horizontal axis turbine with a maximum rotation speed of
12 to 15 rpm, which results in a maximum blade tip velocity of 12 m/s. Wilson et al.
(2007) suggested that rotor blades tips will likely move at or below 12 m/s because
greater speeds will incur efficiency losses through cavitation.
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