Offshore Technology

SPECIAL | OMAE 2008
Wave ernergy utilisation
Rexroth rises to the Power Take-Off
UTILIZING OCEAN ENERGY One of the main challenges at the concept of ocean power stations
which extract power from ocean energy lies in devising a suitable “power-take off“ system for
converting the kinetic energy of the water into electrical power
Nik Scharmann
Experts tell us that within a few decades,
innovative systems for the
sustainable use of ocean energy may
generate as much electricity as 150–200
nuclear power stations.
On the one hand the important advantages
of utilizing ocean energy here is that around
two third of the world‘s inhabitants live in
coastal regions. The close proximity of ocean
power stations to consumers simplifi es the
infrastructure and minimizes power losses.
On the other hand ocean energy is always
available: tides, currents, and – to a certain
extent – sea swells are ever present, thus enabling
more long-term planning. As a result,
ocean power stations are a much better option
for providing the base load of the electricity
networks.
A number of companies are currently working
on different concepts for large-scale facilities
that will extract power from this renewable
energy source. The main challenge lies
in devising a suitable “power-take off“ system
for converting the kinetic energy of the
water to electrical power while ensuring that
generation costs remain competitive. The
development of the plants and the necessary
PTOs is only in its infancy, but Rexroth is
already supporting numerous projects with
tailored solutions, just as it did for wind
energy a few decades ago. These solutions
Special 12 Schiff & Hafen | June 2008 | No. 6
are based on Rexroth hydraulic components
and cross-technology systems, which have
already proven extremely robust and reliable
in a range of maritime applications.
The ocean energy industry currently consists
of two main sectors: tidal energy and
wave energy.
Tidal energy
Tidal power stations use the energy of currents
to power rotors – be they tidal or natural
sea currents. Water has a density one
thousand times greater than air and can thus
generate signifi cant power even from low
fl ow velocities. This requires specifi cally tailored
solutions.
The diameter of underwater rotors does not
have to be large for them to be able to con-
Mechanic Power-Take-Off
duct energy effectively. Even at low speeds,
the forces acting on the entire system are
signifi cant. Rexroth is currently pursuing a
development concept that adopts the generator
gearbox technology employed in the
wind energy sector, an area in which the
company has established itself as a worldleading
supplier for renewable energies.
Research is also being conducted into the
use of hydraulic converters. This simple yet
highly robust concept transforms rotary
motion into hydraulic fl ow, which powers
an adjustable hydraulic motor for the generator
with great effi ciency. A slow-running
radial piston motor with constant displacement
is employed on the pump side. It
generates a volumetric current based on the
rotor speed. A fast-running axial piston dis-