In order to illustrate how the different wave energy devices work and the differences between them a classification based on function has been made.

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The purpose of a wave energy device is to harvest the energy in the waves. Since wave energy is a renewable resource, the environmental impact is limited to the resources used for production, set-up and dismantling. The evaluation of wave energy devices should be based on the relation between realistic energy production and monetary investment – a simple case of kilowatts per dollar.

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Traditionally wave energy devices have been classified by placement (on shore, near shore, off shore), rather than by principle of operation or how much energy the device can produce. The following classification system is based on the principle of operation, where each group has inherent limits on energy potential. This system is therefore useful for evaluating and comparing the efficiency and energy potential of different wave energy devices.

 

This system is based on the several hundred wave energy devices which have already been designed and patented. The classification groups have been kept deliberately simple so even non-professionals can easily understand them.

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Sizing and energy considerations

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The energy in the wave is both kinetic and potential.

Ekin = 1/2 m v2

Epot = m h g

Wave energy devices will always be limited by which part of the wave energy they are designed to harness and by the capacity a given area has.

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Movers- the wave is used to move a body (float, fluid or air)

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Most wave energy devices belong to the group: "Up and down". These use only the potential energy. They can only extract an amount of energy equal to the mass of displaced water multiplied by the height and acceleration due to gravity. In this group the focus has been aimed at utilizing the difference in potential energy between wave crest and wave trough.

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Many “up and down” devices use a float, whose movement up and down with the wave is used to create a pushing/pulling force in relation to a fixed point. This fixed point can be an anchoring block or a damping plate. The possible amount of extracted energy in these devices can never be greater than the potential energy component of the wave. Floats also have size limitations: If the horizontal length of the float perpendicular to the wave front is larger than 1/4 of the wave length, the efficiency drops, as the float starts to roll about its centre of gravity. A float larger than the wavelength will ride on several wave crests without "dropping down" in the wave trough. Also, since wave amplitude decreases the deeper you are, the maximal potential energy of a wave exists only at the wave surface. As the float gets larger vertically, its centre of gravity will be deeper and the potential energy available will be less.

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Smaller floats can transform some of the potential energy to kinetic energy with latch, which stops the float at its high or low extreme, and releases is a certain delay later. This causes the float to overshoot the crest or trough of the wave, giving more energy.

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The limitations for floats mentioned above also apply to oscillating water column type devices. In these devices the water oscillates up and down in a fixed pipe. The pressure variation of the water surface underneath the pipe forces the air above the surface in the pipe to oscillate.  This oscillating airflow can then be passed through a turbine. The horizontal length of the pipe has to be smaller than 1/4 of the wavelength, or the efficiency drops markedly, reaching zero when the length is one wavelength. Another limitations on the amount of energy that can be extracted is that these devices rely on the compression/expansion of a body of air, causing heat losses.

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Devices of the roll type can extract both kinetic and potential energy. The limitation in energy extraction of these devices is largely governed by the individual design. Although they can extract all the energy from a wave, they rely on oscillating with exactly the optimal phase and amplitude in relation to the incoming waves. Their horizontal length perpendicular to the wave front must also be smaller than the wavelength. Rollers are made up of two parts, which moves relative to each other. One of the parts can be a fixed body, firmly mounted or kept in place by a gyro, and the other part can be the housing of the device or its float. Most of the energy loss in these devices is due to friction.

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The impact devices are generally constructed to use a very narrow spectra of energy frequencies. In the energy transformation of the device a part of both the kinetic and potential energy will be lost due to the uneven pulsating character of the energy delivery whether the uptake takes place on a firm body or a flexible body.

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Eaters– The water is flushed into the device.

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Eaters use both potential and kinetic energy. The limitations on the possible energy extraction are very dependable on the design. A wash up device converts some of the kinetic energy to potential by making the wave run up an inclined plate. It will use only the potential energy of the water that has been lifted into the reservoir – any kinetic energy it has when it reaches the reservoir is lost. Both devices for wash up and flush in will lose some energy to energy as friction. Principally eaters can be scaled to fit any size of wave, which does not apply to the movers.

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