Page 187 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 175

hence the main focus of this section, is via surface following wave buoys.
However, alternatives such as acoustic devices and pressure transducers are
also available, and these are also discussed in this section. In Section 7.3,
remote-sensing methods that are suitable for measuring wave properties are also
introduced.

7.2.1 Wave Buoys
The most common way of characterizing waves in situ is via a surface following
wave buoy. As with measurements of tidal elevations (Section 7.1.1), various
networks of such wave buoys exist around the world, such as UK WaveNet
(Fig. 7.13), or the Integrated Ocean Observing System (IOOS) in the US.
However, wave networks are driven by factors other than quantifying the wave
energy resource, such as coastal flood risk, and so there are many energetic
regions that are not covered by wave buoys. For example, the west coast of
Scotland, despite having the most energetic wave conditions around the United
Kingdom, is sparsely populated by wave buoys. Therefore, within many wave
resource assessment studies, it is necessary to deploy a project specific wave
buoy.

General Principals
Wave buoys measure their vertical acceleration using an on-board accelerom-
eter, supplemented by an artificial horizon that accurately defines the vertical
[15]. Modern wave buoys both obtain their position and transmit their data via
satellite communications. GPS (i.e. Differential Global Positioning System) is
so accurate that wave buoys have now been developed which do not need an on-
board accelerometer or other motion sensors [16]. This has several advantages
over accelerometer-based buoys, including lower cost and lack of long-term
calibration of the instrument. However, GPS-based wave measuring systems
have significantly higher energy consumption, and require a constant connection
with GPS satellites. This latter requirement can be challenging under rough
sea conditions, including whitecapping waves and spray, which affects the
reliability of the connection.
In the case of a wave buoy that measures vertical acceleration of the device,
the time series of water elevation can be calculated by integrating the vertical
acceleration twice, that is, vertical displacement of the sea surface η is

dw
η = (7.6)
dt
where w is the vertical velocity.
Wave directional information can be obtained by measuring either the slope
3
of the sea surface or the horizontal motion (surge and sway )ofthewave
buoy. The sea surface slope can be determined by including additional sensors
(inclinometers) to detect the tilt of the buoy in two orthogonal directions, and a

3. Vertical motion is defined as heave.