ON/OFF - Offshore Center Danmark

The map shows the potential
for wave energy.
Numbers indicate kW / m of
crest length.
SeaGen
SeaFlow, North Devon, UK
Closely related to wave energy is energy
from sea and tidal currents. Examples
of devices within this category include
the SeaFlow and SeaGen from British
company Marine Current Turbines. These
works by blades being rotated by sea/tidal
current. Additionally, Danish company
Modus 2 is currently looking into the
potential of energy from sea currents.
Converting Wave Energy
is a Challenge
By Povl-Otto Nissen, Povlonis Innovation,
Ribe, Denmark, http://www.povlonis.dk
There is plenty of energy in ocean waves, but
of rather low quality. Therefore it is a challenge
to find ways to concentrate and convert
it into more useful forms of energy, such as
electricity.
One of the challenges in producing electricity
from waves is that, in spite of strong
forces in action where waves are hitting,
the movements in the wave crests are rather
slow. Bigger wave heights also
give longer wave lengths and
periods of time between the
energy bursts. Approximately
with a factor of 20, but it is
not quite linear. It means that
the power in watt is rather low
and not much to go for, unless
you build a plant covering a
big area and try to accelerate
the movements by some
sort of gearing or temporary
storing. Also, it is necessary
to find ways to smoothen out
the irregularity of natural waves, and collect
as much as possible to run a turbine and a
generator.
The state of art in wave energy converters
can be summarized to quite a few different
types: Oscillating water Columns (OWC),
overtopping systems, float based plants with
different kinds of power take off and both
vertical and horizontal wave rotors, not to
mention tidal and sea-drift plants.
The principle of the OWCs is to use the water
surface as a piston for making changes in
air pressure in a closed but downward open
chamber, and then put an air turbine in the
chamber wall. This type has not been attractive
to Danish inventors, although some of
them work with air pressure caused by the
waves. Countries with shore lines of cliffs
have a better potential for that type.
Two types of converters using overtopping
to collect the energy have been developed by
Danish inventors, and are now being tested
in open sea. They are the “WavePlane”
(Bølgehøvl) and the “Wave Dragon”. Both
are floating for anchor. Both have power
input from grasping the wave crests and are
converting the energy, when the water by
gravity goes back to sea level. WavePlane is
cutting the waves into layers, which causes a
whirling flow in a horizontal channel in the
plant. Wave Dragon collects the water from
the wave crests over a ramp to a temporary
storing basin on the plant. This water seeks
Testsite at Nissum Bredning
back to sea level through vertical cylinders
with turbines and generators. The Dragon
has parabolic arms to optimise the wave
heights just in front of the ramp.
The float-based types can be categorised in
different types of converting process and
power take off: The first Danish big scale
plant of this type was the Point Absorber, In
the first version the float was driving pumps
and a turbine in a chamber on the sea bed.
The power conversion is now changed to
take place in the float itself with the piston
fastened to the sea bed. AquaBuOY looks
very much like Point Absorber but have apparently
a different converting system.
Other float-based Danish converters can
be seen. One type is pumping water or
hydraulic oil through tubes and cylinders
with valves and with a turbine somewhere
Offshore Center Denmark
Newsletter ON/OFF 10 - May 2007
3