a brief Synopsis
DAEDALUS Informatics
Athens, Greece 01/10/1994*double click on empty space anywhere on page, to access pagetop
3. The AKROKERAMOS breakwater3.4. Evaluation of the area and site specific Energy Spectra
Prior to considering the AKROKERAMOS site specific recorded parameters for wave wind data, it would be advisable for the reader to consider some key parameters on evaluating a real sea's energy potential.
Description of Sea States
Waves are created by the transfer of energy from winds blowing over the sea. The energy transferred depends on the wind speed, the distance over which it interacts with the water and the duration of time for which it blows. Waves once formed will continue to travel in the direction of their formation after the wind dies or turns.
Linear theory is currently used to study ocean waves. Sinusoidal waves can be characterised by their height (crest to trough) H, distance between crests (wavelength)
and time between crests (period) Tor the frequency f = 1/T. The wave speed is
, longer waves move faster.
Real waves are a mixture of heights, periods and directions so wave data analysis requires a statistical approach. The local state of the sea can be described by its spectrum
that represents the energy distribution within each band of frequency and direction
. This spectrum in turn can be summarised by a small number of basic statistics, usually height, period and direction. Significant wave height
(where
is the spectral moment of n-th order) is widely used. For wave energy purposes the most appropriate mean period is the energy period:
Te = m –1/m0
In deep water the mean power or flux of energy per unit wave front in a sea state is given by P = 0.49 Hs² Te kW/m (Hs in metres, Te in seconds). Typical oceanic values of Te are in the range 5-15 seconds; Hs varies from 0 (flat calm) to around 15 metres (severe Atlantic storm), with median values in the Atlantic of up to 2 metres in summer, 4 metres in winter. The third crucial parameter is the principal direction of the power flux. Often an oceanic sea state will include both locally generated wind sea, whose principal direction should be that of the local wind, and swell generated up to several days earlier by distant weather patterns, which may have a quite different principal direction. In this case an adequate summary of the sea state will require separate heights, periods and principal directions of wind sea and (occasionally more than one) swell components. For a more precise description one can add standard deviations of period and direction for each component, or a numerical summary of the complete directional spectrum.Note that for resource estimation the relevant quantity is usually the power flux in a given direction. Even for the optimal direction, this net power flux in deep water will on average be at best about 75% of the gross power flux (Mollison and Pontes 1992); though in shallow water, where wave components line up perpendicular to the depth contours, the two may be virtually equal. The complete directional spectrum, or a good approximation to it (e.g. the frequency spectrum S(f) and mean direction
The Variability of Wave Energy
In order to describe fully the wave climate at a site, that is the long term distribution of waves, we need to consider their variability over the whole range of time scales, from an appropriate sampling interval short compared with the wave period up to year-to-year variability and the even slower scales of climatic change.
The short-term variability of waves, over a few hours and a few tens of kilometers (in the open deep sea), is well described as a Gaussian random process.
Thus the wave to wave and group to group variation, which are crucial for modelling the power take-off of devices, can be calculated with sufficient accuracy from knowledge of basic sea state parameters and the shape of the spectrum. For instance, 'groupiness' is associated with a spectrum which has only a narrow range of periods, such as arises in swell from distant storms.
The duration of a sea state is important for estimating extreme waves within that state. The duration of weather systems is important in determining the limits on forecasting, in particular forecasting calms when no power is available
Year to year variability is important, first of all, in order to confirm that wave data covering a specific number of years (ten or at least five) are representative of the long-term wave climate of the site considered. Moreover, year to year and longer term climatic variability are especially important for estimating the lifetime extremes which
a device will experience.
Nearshore and Coastal Wave Climates
The offshore wave climate is approximately steady over distances of tens of kilometres (Mediterranean, European continental shelf) to a few hundred kilometers (North Atlantic). As the waves travel towards a coast through waters of decreasing depth, interaction with the seabed (and currents) may lead to major changes. Focussing, defocussing and sheltering also occur in indented seafront zones due to wave shoreline interaction. Consequently in the nearshore region (water depth 15-25m) or at the shoreline the wave climate can vary significantly over distances of tens of meters (e.g. Pontes and Pires 1992), the resource generally being lower compared with offshore conditions.
Shallow water phenomena can be usefully classified according to their characteristics of maintaining energy or not. We can so distinguish between conservative (or nondissipative) and dissipative phenomena.Phenomena in the first class have the effect of altering the spatial distribution of wave energy and its distribution between frequency and directional components in a spectrum. The main conservative processes include shoaling, refraction, diffraction and certain types of reflection. The rapid increase in wave height in very shallow waters due to shoaling often causes waves to break. Refraction (due to water-depth variation or wave-current interaction) can be an important positive factor for wave energy utilisation because remarkable concentration of energy occur in specific areas (hot spots). Also
the turning of the crests, which tend to become parallel to the bottom contours in shallow water, is responsible for the decrease of directional spreading of energy in the shoreline. Diffraction is almost always a negative factor for the present purpose because it promotes the smoothing of the spatial distribution of wave energy and spreads wave energy in the shadowed part of obstacles (islands, promontories).Dissipative phenomena involve a reduction of the total amount of wave energy by converting it into water turbulence, heat or motion of the seabed material. Wave breaking, bottom friction, percolation and wave reflection from sloping or rough surfaced structures belong to this category. Wave breaking is generally the most important of the dissipative phenomena and determines to a great extent the power that reaches the coast where the first generation of power plants is being located. For the present purpose, account of wave breaking should be taken under the viewpoint of energy dissipation as well as of the very important forces that breaking or broken waves exert on the power plant structure. The other dissipative phenomena are generally less important but over wide continental platforms, such as off the Hebrides and in the North Sea, energy loss by bottom friction (increasing with the travel distance) can be quite important (Mollison 1983).
The AKROKERAMOS site
The evaluation of the site specific wave power, period and height spectra, are of prime importance for any further design considerations, for both power uptake and structural parameters. There are several steps in evaluating available wave power, starting from collection of long term wave trend data -usually for a period of at least 5 years, as to have a reliable profile. Availability of wind trend records, could also contribute to a more thorough evaluation.
The theoretical basics in evaluating the Power Spectral Density function S(f) from a given set of sea recordings, are provided in (1) (excerpted from "Wave Energy: a Design Challenge" by Ronald Shaw, courtesy Halsted Press).
Section (2) provides a detailed group of monthly charts for wave data isopleths, as per waveheight to wavelength division. A chart for recorded annual wind data is also included.
Section (3) is an integral wave & wind potential presentation, at a national level. It is simply provided for demonstrative purposes, as to indicate the type of work that may result after laborious evaluation of trend data. Such cumulative resource energy mapping is necessary for strategic desing on a national level.
Finally, although the calculated power potential at AKROKERAMOS ranges in the very moderate region of 5-10 KW/m, highly variable seasonally, it was considered adequate for demonstration purposes. Certain aspects of seasonal variation had to be taken into particular account when designing the Wave Energy converter, as it will be recorded in the following chapters.
(1) Characteristics of waves in real seas
- Underlying theoretical evaluation -(2) AKROKERAMOS breakwater
- Annual wave charts -(3) Wave/Wind Resource Distribution
- National Map -
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