COST EFFECTIVE WAVE AND CURRENT MEASUREMENTS FOR OCEAN ENERGY

COST EFFECTIVE WAVE AND CURRENT MEASUREMENTS FOR OCEAN ENERGY Joerg Bendfeld University of Paderborn Fakultät Elektrotechnik, Mathematik und Informat...
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COST EFFECTIVE WAVE AND CURRENT MEASUREMENTS FOR OCEAN ENERGY

Joerg Bendfeld University of Paderborn Fakultät Elektrotechnik, Mathematik und Informatik Lehrstuhl für Elektrische Energietechnik Pohlweg 55 D-33014 Paderborn

ABSTRACT Open water waves are one of the natural phenomena of our environment that have attracted attention for a long time. But the description and the prognosis of waves is a complex issue. In most cases a wave is described mathematically by a wave equation in only two dimensions (namely deflection and propagation direction of the wave). The superimposition is not trivial. It involves nonlinear components (particularly in the case of wind-induced waves), which are predictable only with great difficulty or not at all. It is simpler to assess the wave events according to the probability of their occurrence. In this case measurements of the wave height are of primary interest. The measurement of waves, and in particular their direction, has been one difficult problem in observing the wave climate offshore.

Thus the two measuring methods differ fundamentally in their physical functional principle. Therefore all parameters that are relevant with respect to the measuring instrument must be evaluated separately. Subsequent comparisons and any differences in the respective measuring sequences can thus be interpreted.

1. Introduction The ocean can produce two types of mechanical energy from the current and the waves. Oceans cover more than 70% of Earth's surface. Ocean mechanical energy is a complex issue. Even though the sun affects ocean activity, currents and tides are mainly driven by the gravitational pull of the moon, and waves are driven mainly by the winds. As a result, tides and waves are volatile but predictable sources of energy.

Two different measuring approaches will be discussed: The distance between ground and surface (ast, acoustic surface tracking) or the accelerated motion of the water surface (buoy) is utilized as measured signal.

Ocean wave energy is one of the most concentrated and widely available forms of renewable energy in coastal areas. Despite significant progress in recent years, ocean wave energy conversion technology remains in an early stage of development. Similar to wind power in the

1980’s, device developers are pursuing a large number of very different device concepts at various scales and there is no consensus as to which technology is superior. This is typical for early stage markets where no technology lock-in has occurred. (1) John Kelly Jr.in “Kelly, John M: Surf and Sea; A.S. Barnes and Co. Inc., New York 1965”, proposed two ways of measuring waves - "you can over estimate or you can under estimate". For this reasons it is important to get comprehensive knowledge about waves and currents.

2.

WAVE CHARACTERISTICS

Visual estimates of sea and swell height, period and direction are sometimes included in meteorological reports. But those estimates are influenced by the skills of the observer and by subjective impressions.

Fig. 2: Simulation of the observable sea waves by nonlinear superimposition (2)

It is important to know the desired parameter and their importance: • Swell Direction is the direction that the swells are coming from. Swells are waves not produced by the local wind and come in at a higher period (longer wave length) than waves produced by the local wind. • Swell Height is the estimated average height of the highest one-third of the swells. It is estimated from determining how the wave energy is distributed among various periods (frequencies), determining if a separate swell energy peak exists, and then, picking a frequency to separate swell and wind-waves. The swell height is calculated from the wave energies below the separation frequency. • Swell Period is the peak period in seconds of the swells. If more than one swell is present, this is the period of the swell containing the maximum energy.

Fig. 1: Definition of wave parameters • Wind-Wave Direction is the direction that the wind-waves are coming from. Wind-waves are produced (or were recently produced) by the local wind. If a swell is present, these waves arrive at a lower period (more frequently) than do the swells. • Wind-Wave Height is the average height of the highest one-third of the wind-waves. Again, it is

estimated by the process mentioned under "Swell Height", except that it is the calculated from the energies above the separation frequency. • Wind-Wave Period is the peak period in seconds of the wind-waves. • Significant Wave Height (SWH) and Period is the significant wave height and dominant wave period that has been traditionally available. Significant wave height is the average height of the highest third of the waves. If both swell and wind-waves are present, it should equal the square root of the sum of the squares of the swell and wind-wave heights. Dominant period is the period with maximum energy and is always either the swell period or the wind-wave period. • Steepness: For a given wave height, steep waves represent a more serious threat to capsizing vessels or damaging marine structures than broad swell. It is determined by examining the significant wave height and the dominant wave period when compared to climatology. (3)

Fig.3: Attenuation with depth

Ocean waves are generated under the influence of wind on the ocean surface. Once ripples are created on the surface, there is a steep side available against which the wind can push and waves begin to grow. In deep water, waves can travel for thousands of miles without losing much power until their energy gets dissipated on a distant shore. Representing an integration of all the winds on an ocean surface, ocean waves are very consistent. Ocean waves are an oscillatory system in which water particles travel in orbits. As the water depth decreases, the oscillation becomes smaller. Close to shore, in shallow water, the ocean waves are influenced by the ocean floor, which results in a loss of energy because of the friction of water particles on the ocean floor. (1)

3.

Currents

Currents are the coherent horizontal movement of water. Density currents are driven by gravity. Density differences in a fluid in a gravitational field lead to pressure differences that drive flows. Examples of density currents are turbidity currents or the thermohaline circulation. Geostrophic currents are controlled by a balance between a pressure gradient force and the Coriolis deflection. Geostrophic currents flow along isobars, in contrast, to our everyday experience of fluids flowing from high pressure to low pressure. Large-scale mid-latitude ocean (and atmospheric) flow are in approximate geostrophic balance. The other significant component of large-scale ocean circulation flow is winddriven and is known as Ekman flow. (4)

4.

Measuring devices

Waves influence so many processes and operations at sea, hence many techniques have been invented for measuring waves. Here are a two of the more commonly used shown.

conventional current sensors

Because of the resulting physical effects which may influence the wave measurement, it is important to know how the applied device works.

ADP/ADCP Seabed

Fig.5: Current measurements

Wave measurement

Fig. 4: Difficult real wave situation

5.

ADP/ADCP

Current measurements An acoustic Doppler current profiler or ADP is used to measure how fast water is moving through the whole water column. They can be fixed on the seabed to measure the current across the water. ADPs work by sending out a pulse of sonar and “listening” for a return. This return is generated from particles in the water, if the particles are moving it is subject to Doppler shift. The amount of the Doppler shift can be used to calculate the speed and direction that the water is moving. This is detailed information about the current for this very location.

Measurements of directional waves are often required. In the past, specially designed equipment was often used to provide wave data, which required complex installations and significant costs. Earlier pressure-velocity systems lacked the accuracy especially when to water gets deeper. Recently introduced ADP-based beam systems are reported to be capable. There are some improvements like the AST-Method. The Results of this Acoustic Surface Tracking (AST) are very comparable to the standard method of wave measurements like buoys.

Direct measurements with movable devices such as a buoy are affected by unwanted acceleration signals as a result of the movement of the water surface. Buoys provide accurate wave information at a specific location. However, the buoys must be moored and can be lost due to bad weather and collisions by other ships.

Fig.6: Wave measurements

6.

Buoys:

Fig.8: Measurement of different wave parameters

Modern Buoys can be equipped with a current sensor. This current sensor works like a normal ADP/ADCP (Acoustic Doppler Current Profiler).

Fig.8: Buoy hull with integrated ADCP/ADP (5)

Fig.7: Directional Wave Buoy with currents

Wave Buoy 8.

ADP/ADCP

conventional current sensors

Seabed

Fig.9: Buoy hull with integrated ADCP/ADP

7.

Conclusion

All the different measurement devices and techniques do have their pro’s and con’s especially under different weather conditions. An ADCP with AST is a promising solution to measure both, waves and current. As an all-inone solution it saves money, it is robust because of no movable parts and very accurate in measuring the direct water elevation. But for its deployment and maintenance divers are often needed. A Buoy with currents is also capable to measure the majority of the needed parameters and the maintenance effort is less.

References

(1) Previsic, Mirko. 2006. California Ocean Wave Energy Assessment. California Energy Commission, PIER Renewable Energy Technologies Program Area. CEC-500-2006119 (2) ADCP and Waverider Measurements for O&M at offshore windfarm locations, Bendfeld, Ditscherlein, Splett, Voss Ewec 2008] (3) http://www.ndbc.noaa.gov/waveobs.shtml (4) http://oceancurrents.rsmas.miami.edu/glossary.h tml (5) http://www.axystechnologies.com

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