Wave Energy
Wave energy can be considered as a concentrated form of solar energy because are generated by the earth being heated differently at different points (differential heating). This causes winds which in turn pass over bodies of water transferring some of their energy to create surface gravity waves - the result of the balance between gravity and wind shear on an otherwise undisturbed free surface [1]. The amount of energy transferred depends on wind speeds, the length of time the wind blows and the distance over which it blows - or 'fetch'. Essentially, this 'stores' the solar energy where typical solar power levels are of ~100 W/m2, this can be magnified into waves with power levels of 1000 kW/m of wave crest length. These waves created over long distances - called 'swell waves' - allow the energy to travel thousands of kilometers across the ocean [1].
World ResourceThe global wave power resource in deep water - i.e. 100 m or more - is estimated to be ~ 8000-80,000 TWh compared to a global electricity production of 21,431 TWh in 2010 [2]. the economically exploitable resource varies from 140-750 TWh/year for current designs of devices when fully mature [3] and could rise as high as 2000 TWh/year (Thorpe 1999). These values have been somewhat confirmed more recently with wave power maps which indicate an exploitable resource of 800 GW corresponding to ~ 2000 TWh.
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Some Useful Equations
Below are some useful wave equations which will be used during the project in MATHCAD relating to waves:
Deep water wavesA wave is considered a deep water wave when in is in water that is deep than half its wavelength. This is because the effects of the sea bed when this condition is true are negligible. The wavelength, λ, of deep water waves depends only on its period, T, and gravitational acceleration g :
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Shallow water wavesWave are considered shallow once the depth is less or equal to 1/20th of their wavelength. At this point there is a lot of friction between the wave particles and the sea bed which is slowing the wave. Now the wavelength also depends on depth, d:
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Intermediate water waves are more complicated and fall somewhere between the two but according to linear wave theory, waves at all depths can be described by iteration from:
Where λ0, is the wavelength of deep water waves [5].
Wave number, k
The wave number, k is defined as:
and is the number of complete wave cycles per meter with units of (1/m). This is used to non-dimensionalise the different parameters of the wave as well as the dimensions of the scale model making experiments more easily reproducible and comparable at different scales.