Page 173 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

162 Fundamentals of Ocean Renewable Energy

FIG. 7.2 A mechanical flow meter.

Current meters either can be of the mechanical rotor type described above
or can be based on either acoustic or electromagnetic techniques. An acoustic
current meter measures the travel time in both directions of an acoustic pulse
between a pair of transducers [5]. The travel times will be unequal, because there
is a component of the current directed from one sensor to the other. A second
pair of sensors allows the complete horizontal current vector to be resolved.
One problem with acoustic sensors is the accuracy required to measure very
small time differences. For example, for a transducer pair separated by 0.1 m,
the timing must be accurate to within 10 −9 s for a speed resolution of 0.01 m/s.
However, acoustic current meters have an advantage, compared with mechanical
rotor current meters, in that they are less prone to fouling from seaweed and
fishing lines, etc.
Magnetic flow meters are based on Faraday’s Law of Electromagnetic Induc-
tion. If we consider water particles as a conductor, the motion of water through a
magnetic field induces a voltage. This voltage, in general, is proportional to the
flow velocity, the magnitude of the magnetic field, and the length of conductor
(i.e. the path between electrodes). Magnetic flow meters are widely used for pipe
flow measurements as they do not have any moving parts that could interfere
with the flow. They measure just one component of velocity.
Tilt current meters are inexpensive and simple mechanical devices, suitable
for current measurements near the bed, or at the surface. A subsurface buoy is
either anchored to the bed (Fig. 7.3) or hangs from a support that is mounted
above the sea surface. The drag force, which is proportional to square of