WAVE TECHNOLOGIES
This section of the report will investigate a number of wave energy devices, looking at
the technology, and the potential to contribute to the UK’s energy requirements. The
devices investigated here range from being fully working commercial devices such as the
LIMPET through to devices in demonstration schemes such as the Wave Dragon. Due the
difference in how advanced the technology is the range of information available is very
large. This makes direct comparisons of the devices extremely hard. Therefore this
section of the report only aims to narrate the current status of the devises. And make
general high level suggestions or prediction of how wave energy devices can and should
be used in the future to contribute to the UK’s energy requirements.
Oscillating Water Columns
An OWC comprises of a partly submerged structure called the collector, which has an
opening below the water level. The column collector contains a column of water that
oscillates up and down with the waves. Above the water within the column is some
trapped air. The column of water acts like a piston displacing a trapped air as it oscillates.
The movement of the air causes a turbine to rotate. The turbine is coupled to a generator
to produce electricity. A schematic of an OWC is shown bellow.
Schematic of an OWC device. [1]
There are two main variations of the OWC today, the LIMPET and the OSPREY both
will be discussed in the following sections.
Limpet
The LIMPET, shown in the figures below is a shoreline OWC device located on the
island of Islay, off the west coast of Scotland. The Limpet was installed in the 2000, by
Wavegen and is connected to the national grid. The acronym LIMPET stands for Land
Installed Marine Powered Energy Transformer.
The LIMPET. [2]
Technology
The LIMPET comprises of three distinct components:
 a shoreline oscillating water column collector
 a turbo generation unit
 and a control and monitoring station
The LIMPET has an inclined shoreline oscillating water column collector, inclined at an
angle of 40 degrees from the horizontal. An inclined collector has two main advantages:
 It offers an easier path for water ingress resulting in less turbulence and lower
energy loss.
 It also increases the water plane area of the column for a given chamber cross
section. This permits the primary water column resonance, which is influenced by
the ratio of the water plane area to the entry area, to be better coupled to the
predominant period of the incoming wave [3].
The collector has a width of 21m, this width has been divided up into three columns
because:


As the width of a column increases there is an increased risk of transverse wave
excitation within the column. Which would reduce the energy capture
performance of the devise.
The design of the roof of the collector capable of spanning 21m without
additional supports would have been too expensive.
The collector is made from BISTEEL a sandwich of steel-concrete-steel. And has been
described as containing a higher density of steel than a nuclear bunker [4]. This is all to
cope with the hostile environment of the shoreline. To construct the collector they used a
technique called protective excavation. This involves excavating an area for the collector
just behind the cliff edge, but leaving a protective bund between the mouth of the
collector and the sea. This bund is removed once the construction is complete. The figure
below illustrates this technique.
Protective excavation technique. [5]
The air exits the collector and enters the turbo generation unit through two 2.6m diameter
openings in the back wall of the collector. The turbo generation unit consist of two
500kW counter rotating Wells turbines. Each turbine has a flywheel to smooth out the
energy supply, as well as a sluice gate offer protection in stormy seas.
Current Situation
The LIMPET was estimated to have an average net electrical output of 202kW, however
only 21kW where being outputted in 2002. This is due to a number of factors being lower
than expected as illustrated in the table below [6]. The lower than expected output is not
due to any fundamental problems with the concept, but instead with the modelling used
to predict the output.
Wave Power (kW/m)
Pneumatic Efficiency (%)
Turbine Efficiency (%)
Initially expected
20
80
60
Actually recorded
12
64
40
Despite the lower than initially expected power output the LIMPET has been haled as a
success. It is important to remember that this is still a developing technology and the
lesions learnt here will help improve the next generation of LIMPETs. And it is still
believed that this technology has the capability to output approximately 200kW.
Osprey
The OSPREY is nearshore OWC device, designed by Wavegen. The acronym OSPREY
stands for Ocean Swell Powered Renewable EnergY.
History
In 1995 a prototype device called
OSPREY1 was launched, towed and
installed near Dounreay in Scotland. The
steel structure comprised of a 20m wide
collector, located between two ballast
tanks. The tanks focused the waves
towards the collector. Unfortunately
during the installation phase when the
ballast tanks where being filled with sand,
a three-meter swell developed. Due to the
ballast tanks not being filled the structure
failed, and OSPREY1 never became
operational.
Fig Sketch of the OSPREY
Current Situation
OSPREY2 is now under development, but its structure differs greatly from OSPREY1.
OSPREY2 is made from concrete and the ballast tanks are built into the walls of the
collector. Above the collector there will be two stacks each containing a 500kW counter
rotating Wells turbine.
Fig Schematic of OSPREY2
OWC UK Potential
In the UK shoreline resources have been estimated to be approximately 2TWh/yeat, and
nearshore resources to be between 100-140TMh/year [9].
Below is a table showing the key data regarding both the LIMPET and the OSPREY
[10]:
LIMPET
Wave Power (kW/m)
32
Annual Power Output 2300
(MWh)
Capital Cost (£k)
1400
Annual Operating Cost 29
(£k)
OSPREY
30
4955
275
19
For the LIMPET the right geographic factors, i.e. the right combination of shoreline
topography and geography, together with low tidal ranges and closeness to the grid, have
been identified in 72 sites around the UK.
Overtopping
An overtopping device has three stages. The first stage is the absorption stage, this is
where the wave energy is focused and the wave is allowed topple over the structure. The
second stage is the storage stage, once the wave has toppled over the structure the water
is stored in a reservoir above sea level. The third stage is the power take off stage the
water is allowed to leave the reservoir via a hydro turbine. An example of an overtopping
device is the Wave Dragon.
Schematic of an overtopping device. [11]
Wave Dragon
Technology
The Wave Dragon, a floating
overtopping device, can
generate up to 7MW. A
Wave Dragon power plant
would
be
capable
of
producing 77MW, electricity
for 60000 homes would
consist of 11 individual
devices [12]. Covering an
area of 5.5km2. The mooring
would be either by a concrete
gravity base or a pile secured
to the seabed. A Wave
Dragon Prototype has been
Wave Dragon Prototype. [13]
tested in Danish sea, a photo of which is shown in the figure bellow.
Current Situation
A Wave Dragon demonstration is due to be conducted off the Pembrokeshire Coast.. The
demonstration will consist of one full size device. The Wave Dragon demonstration
project will be in place for 3-5 years before the site is completely decommissioned.
Wave Dragon demonstration device. [14]
UK Potential
This technology is very much in the early stages of development. But pending the results
of the Wave Dragon demonstration project, an overtopping device has the potential to
make a contribution to the UK energy requirements.
Point Absorbers
A point absorber is a device where one section moves with the waves, relative to a fixed
sectioned, often secured by mooring. Point absorbers are often uni directional in their
ability to pick up wave power, and they will only absorb energy in one plane of motion.
Salter Duck
Introduction
The Edinburgh Duck, or Salter Duck as it is commonly known, is one of the forerunners
of wave power devices. It has been developed over many years by Stephen Salter and the
Wave Energy Group at Edinburgh University. Falling under the category of a point
absorber, the Duck faces into the direction of the waves and the “beak” moves up and
down with the wave motion, while the “bottom” section remains stationary, anchored to
the sea bed by a mooring device. An array of around 30 ducks would sit interconnected
along a spine, all bobbing in differing phases. Since its conception in the 1970s the Duck
has undergone various alterations in design and this report will look at some of the
technical alterations between the 1982 Duck and the current day Duck. Although the
Duck is one of the most famous offshore devices, as well as being the most highly
developed, it has never seen any commercial success, although in the 80s it did come
close to government funding.
The research on the Duck has so far been performed under lab conditions, with Stephen
Salter saying he wants to solve all the problems before putting the device to sea. Some
people are critical of this approach and believe it to be counter productive for the
development of the Duck commercially.
The tests on the Duck have required the development of highly complex wave tank
systems, designed by the Edinburgh Wave Energy Group. Many test tanks in use for
other wave energy devices are developed by the Edinburgh Wave Energy Group, or using
many of their revolutionary ideas.
There was one high profile use of a duck outside the laboratory environment, but this was
infact a fake duck created by XXXX University [4]. This was tested on Loch Ness and
showed good performance, until it sank. Steven Salter claims these tests to have damaged
the Duck’s reputation, however many believe it highlighted the feasibility of wave power
to the general population.
The main idea behind the Duck was for a device to gain the most energy available from
the waves [4]. To achieve the highest power the Duck was optimised for use in depths of
around 80m, far offshore. This approach meant that many inherent problems of the ocean
environment would need to be tackled and it was understood from early stages that it
would be a time intensive product development. Because the design was so innovative
many new technologies have been developed by the Edinburgh wave research team for
testing wave devices and for the functioning of them. If the Duck never manages to
become commercially realised, no one can deny the invaluable contribution its
development has brought to wave power technologies.
Figure xx: Outline of the 1983 Duck [7]
Technology
Gyroscope - The original idea for the Duck met problems in the power take of, because
motion was slow and unstable. For the 1983 Duck design Salter had a breakthrough on
the power take of in his idea for use of gyroscopes [7].
The main problem with the gyroscopes was one of cost. It was a very complex system, to
complex to explain in this report, and the parts were not only expensive, but difficult to
protect from the elements, which drove the costs up very high. Also the gyroscopes
weighed a great deal, which lowered the efficiency of power conversion.
Ring-cam pump - When Salter heard about a new material called Ceremax, which was a
ceramic coating for offshore devices, he discarded the gyroscopes for a simpler Ring-cam
pump design [7]. The ceremax prolongs the life of parts before corrosion sets in. With
the ring-cam pump, the motion of the duck bobbing on the waves, relative to the
stationary spine of the duck, forces a ring to move across a hydraulic fluid filled
container, pumping the fluid into a digital hydraulic motor, known as the “wedding
cake”. I believe this motor to be an early stage of the hydraulic motors developed for
some modern wave technologies. This pump set-up allowed transfer of twice the torque
of the gyroscope design, with only half the mass [4].
Current Situation
It is hard to asses the current position of the Duck technology, because there is little
information about any recent development. As mentioned previously, there are many
aspects of the Duck’s development that have contributed to other technologies. Perhaps
the most the Duck it will ever contribute to a UK renewable energy solution is to be a
source of ideas, but never more than a concept.
Attenuators
An attenuator works by the motion of one part relative to another moving part. Unlike in
a point absorber there is no fixed part, also there is often no restriction to the direction of
motion for power take off, although the amount of energy taken from the waves tends to
be reduced.
Pelamis
Introduction
The Pelamis could be considered the most commercially developed of the UK’s offshore
wave power devices, with a test device contributing up to 750 kW to the national grid. It
looks much like a large snake floating on the sea, consisting of 4 sections held by 3
hinged joints, totalling 150m in length and 3.5m diameter. Power is generated through
the motion of each section relative to the others as the wave crests roll by. The motion of
the sections is resisted by hydraulic rams, which pump high pressure oil through a
hydraulic motor, which in turn drives an electrical generator.
The company Ocean Power
Development was set up in 1998
with the specific goal of
developing the Pelamis wave
power device. The concept of the
Pelamis can perhaps be attributed
back to Sir Christopher Cockerell,
the inventor of the hovercraft. In
the 1970s he came up with the idea
of interconnected rafts, bucking in
respect to one another due to the
wave’s motion. Sir Cockerell has a
Figure xx: Pelamis on sea trials in Orkney [10]
long history working with ships
and has spent a great deal of time
working to stop boats buckling in the waves, which led to the ideas of designing the
opposite. Cockerell’s design was the first device falling in the category of a hinged
contour device and the design of the Pelamis is clearly along the same lines, although
much of the technology and concepts are significantly moved on from Sir Cockerell’s
original ideas.
Over the past 2 years Ocean Power Delivery have worked closely with Atkins, who
verified their prototype against DNV offshore codes and used their extensive knowledge
of offshore systems to support the development of Pelamis.
The Pelamis has been developed over many years, with several tests being performed on
scaled prototypes, to assess, validate and optimize the device for power capture,
survivability and mooring requirements. A total of 14-wave tank test programs were
carried out at 1:80, 1:35, 1:33, and 1:20 scale.
Technology
In designing wave power devices, one main concern is survivability, as waves greatly
vary in their power. Ocean Power Delivery saw survivability as the main design criterion
and this can be seen in many features of the design. The low cross section of the Pelamis
to the oncoming waves serves to reduce loads facing the waves, and control features,
which stiffen the joints in powerful seas, serve to reduce loads normal to heavy waves.
The stiffening of the joints in strong seas also works in reverse in small seas. The Pelamis
is designed to respond resonantly in small seas and joint stiffness can be altered to utilise
this resonant response; this allows more power to be generated in smaller seas, which is
important for wave energy to be a viable energy solution.
The concept behind the self referencing arrangement of the device is that it gives a very
large dereferencing effect in long wavelength. This, coupled with 3 separate joints,
further increases the energy available in small seas.
The device was developed using proven existing technologies from the offshore oil and
gas industry. This eliminated the need for such extensive testing as was required for
devices like the Salter Duck, for which most components were newly designed.
Each Pelamis consists of 4 sections and is 150m long and 3.5m diameter, weighing 700
tonnes in total, including ballasts. In the device there are 3 independent power conversion
units, each of modular construction and can be removed individually for repair. The
power converters each contain 4 hydraulic rams, 2 heave and 2 sway, which drive 2
variable displacement motors, via smoothing accumulators. The motors each drive a
125kW generator, which totals to 750kW across the whole Pelamis device.
The hydraulic rams are set up with 2 driving each variable displacement motor. They are
set up one in the heave direction, which takes power from vertical flexing of the Pelamis,
and one in the sway direction, which takes power from horizontal flexing. There are 2
motors in each power module, therefore 4 rams, so that as one ram in being compressed,
the coupled ram is being expanded.
The power from the pistons is
converted into rotational motion
by hydraulic motors, developed
by Artemis Intelligent Power.
For electricity generation a
constant rotational speed and
torque
is
required.
The
hydraulic motor converts an
irregular and slow moving input
into this required motion.
Assisting the smoothness of the
hydraulic
motor
is
an
accumulator, which stores the
Figure xx: Instantaneous pressure and smoothed pressure by
accumulators[2]
unevenly input fluid and releases a smooth output flow. Figure xx
The energy from the hydraulic motor is converted into electricity by 125kW ABB
electrical generators [5]. The hydraulic motors are developed by Artemis technologies
and allow slow irregular input from the waves to be converted into a high speed stable
rotation, utilising a constantly varying transmission ratio. This varying transmission is
controlled by Digital DisplacementTM technology, developed by Artemis. Using a
hydraulic motor also serves to limit impact of high loads, inherent in waves, by sharing
the load across many pumping modules, rather than single contact points in gear box
transmissions. [6]
The power from all 3 generators will be transmitted down one umbilical cable, most
likely to a sea bed junction, as will
be the case for off the coast of
Cornwall in the Wave Hub
initiative mentioned later in this
report.
Deployment of the Pelamis is by a
rapid release deployment system,
which allows for the device to be
quickly towed from its deployment
position to a sheltered quayside
location for any maintenance work.
All internal aspects of the device
are modular, and removable by a
Figure xx: Pelamis being towed into position [11], the tow bar
standard 5T mobile crane, ensuring
detaches when on site.
easy maintenance when the device
is back onshore. The downside of
this near shore maintenance is that the device is out of service for the whole maintenance
period, although in a wave power farm one device out of service would have minimal
impact, and additional spare devices could be on standby at limited overheads.
Optimum Conditions
Like all wave power devices the efficiency
of the Pelamis is dependant on the wave
conditions. The optimum positioning for
the device is around 5-10km offshore in
depths of 50-70m, although Ocean Power
Delivery claim it also performs in depths
up to 100m. At these depths the device
can maximise the potential of the larger
wave whilst still remaining close enough
to shore to reduce the costs of submarine
cables. The optimum wave height is 6-7m,
Figure xx: Limiting power at wave height of 6m[2]
at which height the maximum output of 750 kW will be obtained and any extra power
will be shed to protect the device. Wave heights below 1m will generate no power, and
there is a fairly linear rise from the lowest to the highest power rating. Figure.
The power rating is not solely influenced by the wave height, but also the wave period.
The power matrix in figure shows wave heights around 6-7m with periods around 7-10s
to give highly effective power outputs, although for these conditions to be consistent is
very optimistic. Positioning of the Pelamis will need to be in an area where the optimum
wave conditions can be successfully achieved through much of the year.
The power matrix provided by Pelamis can be used to predict annual power production
by looking at site specific data over a few years, as shown in figure.
Figure xx: Power matrix for Pelamis[2]
Efficiency
The predicted efficiency of the Pelamis is 25-40% of the rated power across the year.
This will clearly vary across the year, depending on the quality of the waves. From figure
xx it can be seen that in the winter months output is closer to 50-60% of rate power,
whereas in the summer
months it is much less. A
convenient aspect of
wave power is that in
winter the UK sees
stronger seas, which is
the time of year that a
greater energy demand is
placed on the grid.
Looking at the actual
efficiency of the device
in converting wave power
to electrical power the
Figure xx: Wave power across the year and Pelamis power conversion [2]
Pelamis appears less effective. Because the primary design objective was survivability its
efficiency is high power waves is greatly reduced. Although I have found no data to
support this I am inclined to believe that the power conversion efficiency is at its highest
at low to medium wave sizes, trailing of rapidly in large waves. I wouldn’t consider the
loss of conversion efficiency as a big problem with the Pelamis; the main thing is the
amount of its rated power it achieves. As Stephen Salter said, “efficiency itself is of no
concern when the gods pay for the waves”[4].
Current Situation
Currently the Pelamis device is undergoing a trial connection to the grid at the European
Marine Energy Centre in Scotland. It is the first deep water grid connected terminal in the
UK, and tests have been successful enough for interest to be shown by many companies.
Enersis in Portugal acquired 3 Pelamis devices for a 2.25MW in the Port of Peniche, with
potential plans for a further 28 devices to expand to 22.5MW. E-on, in conjunction with
Ocean Prospect have laid out plans for acquiring 7 Pelamis for use off the coast of
Cornwall and are one of the 4 companies agreed to make use of the Wave Hub, which
was given planning permission in September 2007. ScottishPower have secured
investment to go ahead with plans for a large scale instalment of 4 devices at the EMEC
site in Orkney, which will provide 3MW to the grid, enough for 2000 homes.
Cost
It is difficult to fully assess the costs of the Pelamis, especially as it is not yet on full scale
production, however to gain an insight, the 4 Pelamis at the EMEC site in Orkney are
requiring an investment of around £4 million[2], so roughly £1 million per device. These
costs will obviously decrease in time and for larger scale projects, but at this cost the cost
for electricity from the Pelamis can be roughly calculated. If a 30MW plant requires 40
Pelamis, this would cost £40 million, and operating at 40% efficiency it would generate
0.1TWh in 1 year. If the plant were 100% available through the year there would be a
positive return in 1 year if the electricity were sold at 0.5p/kWh. This seems really low,
and is probably due to an error in the initial cost of each Pelamis, however it does show
that if the Pelamis only ran at 25% rated power on average through the year the increased
cost for positive return would only increase by about 25%, so if costs we an order of
magnitude higher electricity could still cost as little as 5-7.5 p/kWh. This is obviously
very much a ballpark figure and the cost will need to be increased to incorporate lower
availability of the plant, maintenance costs and other factors.
Potential Sites
To assess the potential sites for the wave devices the average wave heights need to be
taken into account, as do the extreme weather conditions. As the Pelamis also works best
on long wavelengths its optimum positioning will need to take this into account.
As discussed in an earlier section the west coast of England sees the highest power
waves through the year, with the West of Ireland and Outer Hebrides gaining most and
Southwest Wales and western Cornwall second highest. With the highest power also
comes the highest extreme conditions, with Shetland have a 100 times more powerful 100
year design wave than Cornwall, at around 25m.
The Pelamis was designed for
survivability and as such can withstand
some large waves. Its 100 year design
wave is……. This makes it suitable for
all areas off the UK coast. To get
maximum potential it should be placed
further from shore, to maximise
exposure to the larger waves and more
importantly the longer wavelengths it is
designed for. It would also be more
effective placed along the Irish coast and
northern England, although the Wales
and Cornwall areas would still be
effective, especially in the winter
months.
More specific decisions regarding the
sighting of a Pelamis wave farm would
be determined with use of various
assessments of the area, including
Figure xx: Wave power across Europe
finding the area with the most consistent
mean wave direction and determining
the best sea bed type to accommodate the devices mooring.
Production Scalability
With regards to scalability the Pelamis can work at a 30MW maximum capacity from a
site of 1 square kilometre, which would be enough to power 20,000 homes. The UK
energy usage is roughly 350TWh per year; therefore a kilometre square Pelamis site
would provide 0.25TWh per year, which is a percentage contribution to the UK grid of
roughly 7.5x104%. There is a predicted convertible wave capacity of at least 50TWh per
year [1], about 15% of demand, which is roughly what the government requires from
renewable sources by 2015. For Pelamis to meet this demand it would require 200km2
running at full capacity; however the predicted yearly output of a Pelamis in the best
locations around the UK is 25-40%, which would increase the area covered by Pelamis
devices to 500-800km2.
Powerbuoy
Introduction
The Powerbuoy has been developed in USA by Dr George
W Taylor and Dr Joseph R Burns in Ocean Power
Technologies. It is a large floating buoy which bobs up and
down with the waves. The Powerbuoy is a classical example
of a buoyant moored device, where the central section is
held rigid by mooring on the sea bed, and power is
generated by motion of the buoy relative to this fixed
section. There are 3 main Powerbuoy models, with different
power ratings each, the largest providing 40kW per buoy.
The Powerbuoy began its development in 1997 with
prototype tests off the coast of New Jersey. Since then it has
seen major interest and financial backing, now having
several future development plans, including a 1.39MW
build planned with Iberdrola S.A of Spain [8].
Figure xx: Powerbuoy [8]
Technology
The main advantage of the Powerbuoy is the use of an existing device, the buoy. The
technology for power take off was designed to fit into existing structures, simplifying the
testing process as the structure is already tried and tested. Because buoys have been in
use for decades, techniques for the mooring and installation of the Powerbuoy need very
little work and no specialist technology is required.
Simplicity is key for the Powerbuoy and the device is designed to have a simple, modular
construction. The only parts with any real complexity would be the power take off and
control systems, which are all patented new technologies.
As the waves roll past the Powerbuoy its buoyancy causes the main body to move up and
down, causing mechanical stroking of a piston like structure, fixed by mooring to the sea
bed. The power from this is converted into electricity via a High Temp Superconductor
Linear Generator, developed by Converteam Ltd.
The HTS linear generator has improved efficiency, reliability and maintainability over
conventional linear permanent magnet based systems, this is also combined with lower
cost and lower weight, making OPTs working partnership with Converteam highly
advantageous for the Powerbuoy.
Converteam have done extensive work developing power take of for renewable
technologies, focussing on HTS rotational generators and have applied much of this
technology to their linear generator, particularly developments in higher operational
temperatures, allowing use of cheaper coolants [9].
A series of sensors are used to control the bobbing motion of the device, for 2 reasons.
Firstly the system will lock up when the severity of the waves is too great and could
damage the device. The second control is more complex and is used to control the
frequency of the bobbing buoy to best match the waves. If it were left to bob on its own it
would do so at a greater frequency than the waves which were driving it [ref], the result
of this being analogous to pushing a child on a swing. As you push the child, more
energy is transferred if you push when they have finished the backswing. If you push
when they are still swinging back they will end up pushing you and losing energy. If the
buoy were to be moving downwards when the wave was trying to push it up, a great deal
of energy would be lost in trying to change the direction of motion.
I have found no information on the specific control systems use by Ocean Power
Technologies, but the suggested technique by thingy would be to combine a method of
predicting wave behaviour and altering motion to meet the prediction, with a mechanical
means of self rectifying the devices motion to suit the waves[4]. A method of achieving
the same motion from the buoy and waves would be to hold the buoy at the end of each
stroke until the motion of the wave has caught up, measured perhaps by the force of the
wave on the buoy being above a certain threshold level [4].
The electricity generated in the Powerbuoy will be transmitted to shore via an underwater
cable, and like Pelamis, Powerbuoys will be tested on the Wave Hub off the coast of
Cornwall.
The Powerbuoy technology is developed for use in depth of 30-50m with minimum wave
power of 20kW/m, but development is still going on, so more detailed data on optimum
wave heights is not currently available.
Efficiency
Again, detailed data on efficiency is not currently available, but it can be estimated to be
around a 30-40% yearly average of the rated power. This is because they will have been
designed to withstand a certain sized design wave, so can take large forces, but operating
close to these large forces will greatly increase the costs of the device, so for safety and
reduction of maintenance it is best to operate at lower levels.
Current Situation
At present the Powerbuoy has been on trial since 1997 of the shore of New Jersey. Future
plans include a 1.39MW installation off the coast of Northern Spain, for which the first
stages of installation have been completed.
Ocean Power Technologies are also one of the 4 companies which will utilise the Wave
Hub off the coast of Cornwall, with plans for an installation of 5MW capacity.
Cost
I have been unable to find accurate costing of the Powerbuoy, and like the Pelamis have
had to make calculations based on the funding received by the company. The Scottish
Executive awarded roughly £1 million for the construction and testing of a 150kW
Powerbuoy. On the same criteria as the Pelamis, to generate 0.1 TWh in a year, assuming
40% average of rated power, would require 200 Powerbuoys, costing £200 million, so to
make a positive return electricity would need to be sold at around 2p/kWh. This cost
would again vary as the device is further developed and with additional costs factored in.
UK Potential
The potential sites for the Powerbuoy would be very similar to that of the Pelamis,
although the devices survivability is not quite as high and the size of waves it is offline in
are lower than that of the Pelamis, so it is perhaps better suited to the Wales and Cornwall
regions.
The Powerbuoy can be moved around easily, with commonly available technologies used
to move standard buoys. This manoeuvrability could be utilised to position the
Powerbuoy in differing places to take advantage of seasonal wave sizes, for instance the
Cornwall Wave Hub in winter, then Northern regions in summer when Cornwall suffers
from very low wave heights.
Production Scalability
The Powerbuoy is claimed to scale up to 10MW on a site which would take up 0.125
square kilometres, so 1 square kilometre could potentially produce 80MW, enough for
around 70,000 homes. This would be a percentage contribution of 2x103% to the grid.
Using the same data as for the Pelamis it would require roughly 70km2 for Powerbuoy to
meet the governmental legislation of 15% renewable contribution by 2015. As with the
Pelamis, the Powerbuoy doesn’t run at maximum capacity at all times, with a yearly
rating of…..
Figure xx: Array of multiple Powerbuoys [8]