WPP A/S - Wave Energy
Classification of wave energy devices based on function
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
Movers - the wave is used to move a body (float, fluid or air)
– The water is flushed into the device.
Sizing and energy considerations
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.
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.
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.
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
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.
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.
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
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 most
energy to energy as friction. Principally eaters
can be scaled to fit any size of wave, which does not apply to the movers.