Page 192 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea
P. 192

180 Fundamentals of Ocean Renewable Energy


            spectra. As mentioned earlier, such directional information is useful for wave
            energy devices (e.g. attenuators) that must be aligned with the direction of wave
            propagation.

            7.2.2 Pressure Transducers

            Wave buoys, as described in the previous section, are relatively expensive, for
            example, in the region of $60,000 for a directional wave buoy, in addition to
            operation and maintenance costs. Therefore, under many circumstances, it may
            be desirable to seek a more economical solution, particularly at the earlier
            stages of project development. One such alternative is to install a sea-bed
            pressure sensor. These instruments measure the total pressure, and analysis of
            instantaneous pressure at high frequencies (e.g. > 1 Hz) provides a measure
            of wave height and period. However, the accuracy of wave measurements by
            pressure sensor is strongly influenced by attenuation effects, and so their use is
            limited to water depths of less than 20 m [20]. Such a pressure sensor can also
            be used to concurrently determine tidal elevations, but care should be taken to
            account for changes in the atmospheric pressure when attempting to distinguish
            the astronomical component [18]. Note that by installing an array of sea-bed
            pressure sensors, information on the directional characteristics of waves can
            also be obtained.

            7.2.3 Acoustic Waves and Currents
            Some ADCPs include an additional vertical-orientated transducer that can mea-
            sure the vertical velocity (Fig. 7.5), and can measure wave properties or estimate
            turbulence. Such an instrument is known as an Acoustic Waves and Currents
            (AWAC). Waves are traditionally measured by wave buoys (Section 7.2.1), but
            by using an AWAC, currents and waves can be measured concurrently at a
            location of interest. Because this device is deployed on the sea bed, it does not
            suffer from constraints such as severe weather impacts or the influence of marine
            traffic. Such an instrument is particularly beneficial for studies of wave-current
            interaction. Some researchers have assessed the accuracy of wave measurements
            by ADCPs compared with wave buoys. Bouferrouk et al. [21] compared the
            wave data collected by a five-beam ADCP and four wave buoys at the WaveHub
            (UK)—a site that is dedicated to the testing of wave energy devices. Their results
            show, in general, good agreement between ADCP and wave buoys. However,
            unlike wave buoys, the five-beam ADCP could also detect long-period swell
            waves (0.07–0.08 Hz), for small wave heights, and can only operate in relatively
            shallow waters.

            7.3 REMOTE SENSING
            Although, in some ways, an ADCP could be considered a remote-sensing
            platform, remote sensing really refers to observations made at some distance
            from the medium that is being observed. Remote sensing most often refers to
   187   188   189   190   191   192   193   194   195   196   197