Conversion of wave energy to electricity
Today more than 80 per cent of the worlds electric power
production comes from fossil-fuelled plants. As the demand for electricity
is forecasted to increase, there is an urgent need to find new methods
to extract electric energy from renewable sources. Renewable electric
energy supply is today one of the highest priorities in many parts
of the world.
The Kyoto declaration 1997 and the last agreement at Marrakech
2002 are significant proof of this. Both the EU and the US have
set their targets on future greenhouse emissions. Ocean waves represent
a vast unexplored source of renewable energy. The wave energy potential
in the EU has been estimated conservatively as 120190 TWh/year
offshore and an additional 3446 TWh/year at near shore locations.
However, these estimations depend on assumptions of technology
and energy cost. The actual resource could be a magnitude larger.
In any case, it will be a challenging task to convert the vast energies
in the ocean waves into electric energy. When approaching sustainable
electric power production for the future, attention must be paid
to the economical constraints.
The social, ecological and environmental impacts also needs to
be adressed. The need for research and investigations in this area
must not be underestimated.
The Swedish waters have been estimated to contain too little wave
energy and the general opinion has been that it could not be motivated
to do research on small 550 kW conversion devices. From the
mid eighties the area has been considered difficult and uneconomical.
Despite this, one of the more tested technologies has been developed
in Sweden, the so-called IPS OWEC Buoy with a power of 100 kW or
more. It is now further developed in the USA and UK. The device
is pumping water up and down, thereby driving a traditional generator.
The ocean waves behaviour have been the objectives for many investigations.
However, apart from some tests, mechanical solutions with a traditional
rotating generator (1,500 r.p.m.) have been predominant for the
conversion. Most of the projects remain in the research stage, but
a substantial number of plants have been deployed in the sea as
Several ways of classifying wave energy devices have been proposed,
based on the energy extraction method, the size of the device and
so on. A group of devices, classified as Point Absorbers,
appears to have the approach a performance where commercial exploitation
Point absorber driven linear generator
We work with a concept that combines Faradays law of induction,
Newtons laws of motion, the even older principle of Archimedes
with relative recent advancement in materials technology. In the
spirit of minimizing mechanics by adapting generator to wave motion
a design with a buoy absorbing ocean wave energy at the surface
driving a linear generator at the sea floor is studied, see Figure
Wave energy is directly converted into electricity by a linear
generator consisting of insulated conductors; NdFeB permanent magnet
and steel of different quality like electroplate and construction
steel. Detailed modelling and simulations, as opposed to the traditional
rule of thumb estimates, with a full account of design in full physics
simulation gives detailed data on performance, as illustrated in
The buoy, which drives the linear generator, can be built from
different materials and in different forms.
However, a cylindrical shape is preferred as a uni-directional
point absorber is desired. Buoy dynamics and its behaviours during
ocean wave exposure have been described elsewhere. A buoy connected
with a stiff rope will drive the generator piston as the wave is
rising. When the wave subsides a spring that has stored energy mechanically
will drive the generator. Thus allowing for generation of electricity
during both up and down travel.
When the flux from the piston circumvents its coils induction will
occur in the generators stator, as the piston ideally moves up and
down. Dependent on several parameters, generator design, wave shape,
buoy size, weight, load and springs etc., different voltages with
varying frequencies will be induced in the stator windings.
For open circuit conditions, the generator AC-voltage starts at
zero, when the buoy is momentarily at rest in its lowest position,
increases as the buoy accelerates towards the top of the wave, where
it again reaches zero as the buoy stops.
For a relatively small wave energy converter (WEC) in the regime
of 1020 kW the buoy will have a diameter of three to five
meters depending on wave climate and power rating. The buoy will
have a weight in the regime of a few hundred kg to one metric ton
depending on size and material. The buoy is connected to the generator
with a modern synthetic rope (possible of stretched polyethylene)
trade names such as Dyneema and Spectra, with an optional cover
for handling of fouling. A housing encloses the generator, as indicted
in Figure 1. This could be made of concrete or steel with and integrated
bottom concrete slab.
The total weight of the generator is in the range of a few tons
whereas the bottom slab must have a weight surpassing the floatation
of the buoy, in the range of 10 to 30 metric tons. The slab can
be positioned directly at the bottom and kept in place by gravity.
Only active power is converted
Using rectifier with an externally applied DC voltage makes the
dynamics even harder. Current passes the diodes when the induced
voltages have higher potential than the externally applied voltage.
The retarding force is zero when the current is zero in the windings.
Moreover, as induced and rectified voltage excides the applied DC
voltage, a retarding force will abruptly be introduced, momentarily
reducing the acceleration.
However, only active power transmitted is converted in the rectifier.
Hence, the design has to be render the generator insensitive to
wave and load variations. This can be accomplished by designing
for a load angle close to unity. In practice, the current has to
be relatively low at full load securing small variations in load
angle versus open circuit.
This strategy has advantages of and widen the range of components
used for conversion from stator windings to the grid connection.
Simulations show that a working efficiency of around 85 per cent
can be obtained.
The WEC units will be connected in larger arrays ranging from tenths
up to thousands of individual converters.
For an ocean with moderate wave climate, like the Baltic, four
hundred 10 KW WEC could be interconnected to form a 4 MW plant .
The grid connected can be implemented in various ways:
Furthermore, it can have a relatively large utilisation time as
the power flux variations are attenuated when the waves are induced
by winds which in turn originates from solar power.
The main challenges
However, electrical overloads can and are routinely handled at
moderate cost. Wave energy, absorbed by buoys and transferred via
a rope to the linear generators at the ocean floor, can be converted
into electricity with varying frequency and amplitude. An array
grid connection can be obtained with the use of AC/DC converters
together with transformers in a number of topologies. Selection
of diodes and power transistors are based on cost, losses and maintenance.
Technology for conversion and transmission from offshore to land
has been used in other applications during the last 50 years.
Thus we argue, if raw unsynchronised power can be extracted offshore
it can also be converted into practical form and connected to the
AC grid. Longitudinal flux three phase generators have previously
been regarded difficult or impossible to use for wave power.
However, the use of other types of linear generators have been
suggested. A simple conversion scheme, with moderate investment
cost and acceptable survivability, is anticipated to give wave energy
a competitive potential. n
The div. of Electricity, dept. of Engineering Science,Uppsala University
Latest update 18-10-2006 8:49