Environmental Engineering Reference
In-Depth Information
The force of the strike is expected to be proportional to the strike velocity; conse-
quently, the potential for injury from a strike would be greatest at the outer periphery
of the rotor. Unfortunately, little is known about the magnitude of impact forces that
cause injuries to most marine and freshwater organisms (Cada et al., 2005, 2006) or
the swimming behavior (e.g., burst speeds) that organisms may use to avoid strikes.
Although the blade tip will be moving at the highest velocity and exhibit the greatest
strike force, animals may be able to avoid the tip of an unducted rotor. Relatively safe
areas of passage through the rotor would be nearest the hub (because of low veloci-
ties) and potentially nearest the tip (because of the opportunity for the animal to
move outward to avoid strike). The central zone of relatively high blade velocity and
relatively less opportunity to avoid strike may be the most dangerous area (Coutant
and Cada, 2005). For rotors contained in housings, there would be no opportunity
for an organism entrained in the intake flow to escape strike by moving outward
from the periphery; safe passage would depend on sensing and evading the intake
flow or passing through the rotor between the blades. This suggestion of relatively
high- and low-risk passage zones has not been tested and remains speculative until
the phenomenon is investigated in field applications.
There have been several studies to estimate the potential of fish strike by rotating
blades (e.g., Deng et al., 2005), but all involve conventional hydroelectric turbines
that are enclosed in turbine housings and afford little opportunity for flow-entrained
organisms to avoid strike. It is likely that both the probability and consequences
of organisms striking the rotor blade are greater for a conventional turbine than
for an unducted current energy turbine, due to the greater opportunities for organ-
isms to avoid approaching the turbine rotor or moving outward from the periphery.
However, passage through a conventional turbine poses only a single exposure to the
rotor, whereas passage through a project consisting of large numbers of hydrokinetic
energy turbines represents a larger risk of strike that has not been investigated.
Wilson et al. (2007) described a simple model to estimate the probability of aquatic
animals entering the path of a marine turbine. The mode is based on the density of the
animals and the water volume swept by the rotor. The volume swept by the turbine can
be estimated from the radius of the rotor and the velocity of the animals and the turbine
blades. They emphasized that their model predicts the probability of an animal enter-
ing the region swept by a rotor, not collisions. Entry into the path toward the rotor may
lead to a collision but only if the animal does not take evasive action or has not already
sensed the presence of the turbine and avoided the encounter. Applying this simplified
model (no avoidance or evasive action) to a hypothetical field of 100 turbines, each with
a two-bladed rotor 16 m in diameter, they predicted that 2% of the herring population
and 3.6 to 10.7% of the porpoise population near the Scottish coast would encounter a
rotating blade. At this time, there is no information about the degree to which marine
animals may sense the presence of turbines, take appropriate evasive maneuvers, or
suffer injury in response to a collision. Wilson et al. (2007) suggested that marine ver-
tebrates may see or hear the device at some distance and avoid the area, or they may
evade the structure by dodging or swerving when in closer range.
The potential injurious effects of turbine rotors have been compared to those of ship
propellers, which are common in the aquatic environment. Fraenkel (2007a) pointed
out that in contrast to ship propellers the rotors of hydrokinetic and current energy
