Feasibility of wave- and tidal devices for power generation
Group B3
JOAKIM HIRAMSSON
DAVID JOHANSSON
LARS JOHANSSON
Electrical Engineering
CHALMERS UNIVERSITY OF TECHNOLOGY
10/11-2010
Göteborg, Sweden
Abstract
With oceans of untapped energy, wave- and tidal energy is one of the energy sources that are least
exploited. With modern engineers realising this, many investors start developing devices to capture
the energy of the oceans. The purpose of this report is to evaluate the solutions for capturing waveand tidal energy that exists today and examine if they are feasible for a specified fictional scenario
which includes achieving a total power output of 117 MW by these types of devices. Consequently,
this report includes a presentation of several wave- and tidal power energy converters, along with an
evaluation of the feasibility of using these devices for the specified scenario. The report concludes
that there are presently no available wave converter solutions, as all developers are currently
working on prototypes. Of the tidal energy converters included in this report only SeaGen is a
plausibly cost-efficient alternative for meeting the required power output of the scenario. The
evaluations also conclude that a tidal barrage power plant is a feasible solution for the scenario’s
power production. As tidal barrages implement a larger effect on the local environment than stream
generators, SeaGen is recommended as a final alternative for the power production.
Table of contents
1 Introduction .......................................................................................................................................... 1
1.1 Problem description ...................................................................................................................... 1
1.2 Scope ............................................................................................................................................. 2
1.3 Method .......................................................................................................................................... 2
1.4 Background .................................................................................................................................... 2
1.4.1 Wave power ........................................................................................................................... 2
1.4.2 Tidal power ............................................................................................................................. 3
2 Wave energy devices ............................................................................................................................ 4
2.1 Water action on movable bodies .................................................................................................. 4
2.1.1 Point absorbers ...................................................................................................................... 4
2.1.2 Self reacting point absorbers ................................................................................................. 5
2.1.3 Attenuators ............................................................................................................................ 6
2.1.4 Oscillating wave surge converters .......................................................................................... 7
2.2 Water-to-air interface ................................................................................................................... 9
2.3 Water storage method ................................................................................................................ 10
3 Tidal energy devices ........................................................................................................................... 12
3.1 Tidal stream generators .............................................................................................................. 13
3.1.1 Horizontal-axial turbine ........................................................................................................ 13
3.1.2 Deep Green........................................................................................................................... 13
3.1.3 TidalStream Triton ................................................................................................................ 14
3.2 Tidal barrages .............................................................................................................................. 16
4 Environmental effects ........................................................................................................................ 17
5 Results ................................................................................................................................................ 18
6 Discussion ........................................................................................................................................... 20
7 Conclusions ......................................................................................................................................... 21
8 References .......................................................................................................................................... 22
1 Introduction
1.1 Problem description
With a substantially increasing energy need the ocean is a vast, yet untapped, energy source which
has the potential of supplying a significant part of the world with green energy – if the technology
becomes available.
The technical communication project’s objective is supplying a fictional island with power. The only
given circumstances are the cities’ energy consumption and the map in Figure 1. The main project is
divided between several groups.
The main purpose of our group’s project is to explore and calculate the possibilities of producing 10
% of the island cities’ electricity by wave- and tidal energy converting devices in the top right corner
of the map below.
Figure 1 Island Map
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1.2 Scope
The report will study different solutions for extracting energy from waves and tidal differences by
presenting and evaluating different techniques. An important factor to a solution’s feasibility is the
conditions it requires. Assumptions regarding the local environment, wave energy potentials and
tidal conditions will therefore have to be made to be able to evaluate different devices. Another
factor in the assessments is how the devices affect the local environment and marine life.
The main part of the report consists of a presentation of wave power devices (section 2) and tidal
power devices (section 3) where several examples from existing companies are in attendance. In
section 4 the environmental effects of ocean power installations is examined and further on – in
section 5 – the devices are compared to each other to leave the solution which fits the given
circumstances best.
1.3 Method
The project is carried out by studying reports, articles, scientific books and developer’s internet sites.
1.4 Background
1.4.1 Wave power
The oceans are covering 75% of the earth’s area and are a much more reliable energy source
compared to energy from fossil fuels. Unfortunately, the methods for extracting hydroelectric power
has not been developed much and is far behind other renewable sources like wind- and solar power.
Projects are still in the beginning and there is much to learn about how to extract the energy the
ocean contains (Wave power faces rough seas, 2009).
In 1972 the world was hit by the “oil crisis” and the oil price was quadrupled in just a few months.
This made the whole world panic and countries started to worry about the future. Countries with a
high oil-import were almost forced to find a new way to extract energy and the UK started to expand
the wave power “industry”. The west coast of UK is ideal for wave-energy plants because the wind
blows from the west and all the Atlantic Ocean’s waves hit the coast. The waves are even stronger
during the winter, and that is the period with the highest need of electrical power. So why is the
wave energy industry not much more expanded?
During the recent years, scientists and companies have started to develop new methods and
techniques of using the energy in waves to generate electricity and the UK is one of the leading
countries in the world regarding wave power technology (Wave Power, 2009).
Among the few who have contributed to the development of wave power conversion there are some
notable persons; like Yoshio Masuda, a former naval commander in Japan. He was the first person to
extract energy from waves and he developed the Oscillating Water Column (OWC), which will be
further described later on in the report. Stephen Salter is also one of the foremost and he is the
inventor of Salter’s duck, a technique that uses the motion of waves to make a pendulum shuttle and
generate electricity. Most of the other persons that have contributed to the history of wave power
are students at universities that researched about wave power during the early 70’s (Wikipedia,
2010) (Borgström, 2004)(Ryan, 2009).
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1.4.2 Tidal power
Tidal power plants take advantage of movements of large amounts of water to generate electricity
(Wikipedia, 2010). Using tidal differences as an energy source is not a new idea; it has been used for
hundreds of years, at that time to power mills and similar buildings. The use of tidal differences to
power mills can be traced back to the Roman Empire (Wikipedia, 2010).
The need for environmentally friendly energy sources has increased very much during the late 20th
century and in the beginning of the early 21st century. Therefore engineers had to look into new ways
of generating electricity without polluting our planet. The first attempts of building a tidal power
plant started in 1925, but the project was cancelled due to insufficiency of money. The first tidal
power plant was installed in the French river Rance, at Saint Malobay. It was finished in 1965. The
power plant has a capacity of 240 Megawatts (Engström, 2010). Another power plant of the same
type is under construction in Lake Shiwa, in South Korea. With a capacity of 254 megawatts it will be
the world’s biggest tidal power plant. The South Koreans are already planning another – even bigger
– power plant near the city of Incheon. The construction of this power plant will be finished by 2017.
This will be the biggest power plant in the world. With a capacity of 1320 megawatts it is over three
times bigger than the plant in Shiwa (Sun-Ha, 2009).
The last couple of years, scientists have looked into new ways of building tidal wave power plants.
This because the conventional power plant where the difference in height is used to generate
electricity is only possible to build at a few locations in the world where there is sufficient difference
in height during tidal cycles. The new power plants looks like wind power plants that are submerged
in the water (Nationalencyklopedin, 2010). In 2003 two prototypes were installed, one in
Hammersfest, Norway, and another in Lynmouth, England. The power plant in Norway has a
maximum capacity at 300kW, and was connected to the grid in September 2003 (Penman, 2003). The
power station in England was connected to the grid in spring 2003. This power plant also has a
maximum capacity of 300kW, but it has been about 25 percent more efficient than expected
(Renewable Energy UK, 2007). In April 2008 a 1,2 MW station was installed in Strangford Lough,
Northern Ireland. This plant started to generate full power in December 2008 (Marine Current
Turbines, 2008). South Korea is planning to build a power plant park with 300 turbines, which will
generate about 300 MW. This station is planned to be ready in 2015 (Nationalencyklopedin, 2010).
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2 Wave energy devices
In this report three main types of wave energy converting devices are covered:
2.1 Water action on movable bodies
Water action on movable bodies is the most common type of wave energy converters. There are
numerous companies developing these types of devices and most government and private
investments are made in these.
This technique depends on the wave action moving a solid, a flexible body or linked bodies in
different ways and by that transferring wave energy to a generator (Bhattacharyya and McCormick
2003). These devices can be sorted in the following categories:
2.1.1 Point absorbers
Point absorbers are common among wave energy developers. As seen in Figure 2 and Figure 3, they
consist of a floating buoy and a structure on the sea floor, connected with a wire.
Figure 3 (Wind Power, 2009)
Figure 2 Seabased AB’s point absorber. (Seabased, 2010).
The wave’s motions make the buoys pull the wires which in turn either power a hydraulic pump or
directly convert the motion in a generator (Wind Power, 2009). The name “Point absorber” is derived
from a device which can absorb energy from an area larger than its dimensions, hence absorbing
energy at a point. (Aquaret, 2008).
The Swedish company Seabased AB is currently in the process of deploying a park of wave point
absorbers for evaluation on the Swedish west coast. The absorbers generators are linear meaning
that instead of conventional rotors there are translators which are pulled up and down by the wires.
Several powerful neodymium-iron-boron magnets are placed inside the translators and they induce
4
alternating currents in the stationary stators (Seabased, 2010). A model of Seabased’s absorber can
be seen in Figure 2.
The point absorbers require a voltage substation that rectifies, transforms and inverts the current
before it is transmitted into the land power grid, as the absorbers alone produce an alternating
current which varies in frequency and amplitude and the power grid requires a more constant
current. Depending on the size of the wave park there are one smaller and one bigger type of voltage
substation available at Seabased (Seabased, 2010).
Seabased’s solution is scalable and a wave park would require around 1 km2 per 1000 units. Each unit
is expected to output 25 kW at average. (Seabased, 2010)
2.1.2 Self reacting point absorbers
Instead of using the difference in motion between the
sea floor and the surface, self reacting point absorbers
produce energy without any interaction with the sea
floor. As the waves move vertically the floating absorbers
interact with themselves as the larger and heavier part of
the structure moves differently than the buoy-part which
follows the wave motions more precise. To convert the
motions into electricity either a linear generator or a
hydraulic pump is used. When hydraulic pumps are used
high pressure water is fed to an onboard water turbine
and a generator. In both cases the current is transformed
in a substation – much like the earlier mentioned
Seabased voltage substation – to achieve a steady
current. (Wavebob, 2010) (Aquaret, 2008). An example
of a self reacting point absorber can be seen in Figure 4.
Figure 4 (Wavebob, 2010).
Wavebob Ltd is a developer of self reacting point
absorbers. Their absorber is the one seen in Figure 4 which according to its creator differs from other
point absorbers by having a more adjustable frequency meaning it can generate electricity from both
large and small waves better. One unit is estimated to deliver an average power of 500 kW. The
structure has a mass of 500t, an outer diameter of 20 meters and a depth of 40 meters (Wavebob,
2010) (Farley, 2008).
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Ocean Power Technologies Inc is a producer of self
reacting point absorbers very similar to Wavebob’s
devices. OPT has got a range of differently sized buoys
with varying output power collectively called
PowerBuoys. The company’s latest addition to its
PowerBuoy series is the PB150 which is to have a
maximum power of 150 kW, averaging between 30-50%
which is equivalent to 45-75 kW. The PB150 is seen in
Figure 5. As for most point absorbing systems the PB150
can be arranged in a scalable wave park to generate
hundreds of MW (Ocean Power Technologies, 2010).
Figure 5, Powerbuoy (m) (Ocean Power
Technologies, 2010)
2.1.3 Attenuators
An attenuator is a long, semi-submerged device which operates perpendicular to the wave fronts.
The waves make the attenuator oscillate in resonance with the waves. This generates mechanical
waves in the device. This is possible as the device is separated into two or more pieces whose
connections usually are restrained by hydraulic pumps powering a generator inside the device
(Farley, 2008) (Aquaret, 2008).
Pelamis Wave Power Ltd is a company developing attenuators. Their main device is called Pelamis
and can be seen in Figure 6. It operates both horizontally and vertically as it has got hydraulic pumps
restricting its movement in both ways. Hence the device does not just move up and down as seen in
the figure below, but from a top view it moves sideways in a snakelike way. The most recent version
of Pelamis consists of 4 joints with power conversion modules unlike the model in Figure 6 which has
only 3. Pelamis is slack-moored and can be arranged in arrays to achieve a scalable wave park. A
single power cable is connected to each device and several devices can be connected to a substation
to enable only one cable leading from the park to the land grid. The device is 180 meters long, has a
6
diameter of 4 meters and has a peak output of 750 kW. The average electricity production over a
year is 25-40% of the peak output, equivalent to around 190-300kW. The device’s weight is around
650t and when fully loaded with ballast the weight increases to around 1300t. Pelamis operates
optimally at depths of 50-150 meters (Farley, 2008) (Pelamis Wave Power, 2010).
Figure 6, Pelamis attenuator. (Pelamis Wave Power, 2010)
2.1.4 Oscillating wave surge converters
Fitting in the oscillating wave surge converter class is Aquamarine Power Ltd’s device Oyster (Figure
7). This is a device mounted directly on the sea floor and requires a depth of around 10 meters. As
seen in the figure below, the device consists of a hinged flap which swings back and forth with the
near shore waves. The motion is used to pressurize water in hydraulic pistons. The pressurized water
flows to an on-shore facility where it powers a conventional turbine generator. Three units’ water
tubes are joined at the sea floor to a single tube leading to one on-shore generator. One set of three
units and one generator station is able to deliver an output of 2.5MW at maximum. This implies an
average of around 450kW per unit (Farley, 2008) (Aquamarine Power, 2010) (Aquaret, 2008).
Unlike most point absorbers and attenuators which have to shut down or at least restrict their
movable parts to withstand rough seas, Oyster simply ducks under large near-shore waves and
continues producing electricity (Aquamarine Power, 2010).
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Figure 7 Oyster (Treehugger, 2009)
Table 1 below lists the mentioned different water action on movable bodies devices.
Company
Seabased AB
Wavebob Ltd
Ocean Power
Technologies Inc
Pelamis Wave
Power Ltd
Aquamarine Ltd
Average
power/unit
(kW)
25
500
45-75
Required
number of
units
4640
232
1500-2600
Required area
at sea (km2)
190-300
400-600
12,5
450
250
Weight (t)
4,6
500
650
100
Table 1
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2.2 Water-to-air interface
Water-to-air, or OWC (Oscillating Water
Column), is when waves enter a hollow
structure and in doing so pressing the air
contained by the structure through a turbine.
When the water flows out of the structure
again, air is sucked in backwards through the
turbine which means that the plant is
converting energy both when the waves are
coming in and when they are going out
(Bhattacharyya and McCormick 2003). This can
be seen in Figure 8. OWC devices are most
common as shoreline-based devices but do
sometimes occur as floating devices. When
installed near shore, the turbines of these
devices tend to make loud noises, but these are
reduced by installing noise filters on the
turbines (Farley, 2008).
Oceanlinx Ltd is one of the OWC developers that Figure 8, OWC. (Wind Power, 2009)
construct floating devices. Their OWC consists of
a structure with an underwater opening in which the water oscillates up and down, forcing air in and
out through a turbine. The device is rated to have a maximum power output of more than 2.5MW
with an average output of up to 1MW depending on the specific site where it is to be installed. In the
future, the company plans on building a structure that includes three turbines which will enable
higher output power. Oceanlinx’s OWC can be installed in arrays which will form a wave park
(Oceanlinx, 2010).
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2.3 Water storage method
Devices that use the water storage method relies on the principle of waves overtopping a wall of a
basin in which water is trapped and then released back to the sea as the gravitational force makes it
spin a turbine connected to a generator. The method is illustrated in Figure 9 below. There are
devices that are built at the shoreline which are completely fixed and there are those that float and
are held in place with moorings (Aquaret, 2008) As seen in Figure 10 some devices use a form of
angled collector which forces wide waves into a narrowing structure to overtop into a reservoir when
they have higher amplitude (EMEC Orkney, 2009).
Figure 9 (EMEC Orkney, 2009)
A Danish wave device developer, Wave Dragon ApS, has constructed a device similar to the one in
the diagram in Figure 10 below. The unit uses the principles described in the previous paragraph and
uses its wings to be able to convert energy from a larger area of the sea. The Wave Dragon is a
floating, slack-moored device that can be placed in arrays to make a large wave power plant. It is also
scalable as a machine; the unit can be built in different sizes to fit the specifics of the intended site of
deployment. The only moving parts in the construction are the turbines, which – according to Wave
Dragon (2010) – makes the requirement for maintenance lower compared to other devices that
includes more moving parts (Wave Dragon, 2010).
Figure 10 (Wave Dragon, 2010)
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Below is a table of the Wave Dragon’s specifications. The column labeled “Nissum Bredning
prototype” states the figures statistically proven in an implemented project. The three columns to
the right shows calculated data for feasible future projects.
Nissum Bredning
prototype
24 kW/m
36 kW/m
48 kW/m
237 t
22,000 t
33,000 t
54,000 t
58 x 33 m
260 x 150 m
300 x 170 m
390 x 220 m
Wave reflector length
28 m
126 m
145 m
190 m
Height
3.6 m
16 m
17.5 m
19 m
Reservoir
55 m3
5,000 m
8,000 m3
14,000 m3
7
16
16 - 20
16 - 24
7 x 2.3 kW
16 x 250 kW
16 - 20 x
350 - 440 kW
16 - 24 x
460 - 700 kW
20 kW
4 MW
7 MW
11 MW
-
12 GWh/y
20 GWh/y
35 GWh/y
6m
> 20 m
> 25 m
> 30 m
Wave Dragon key figures:
Weight, a combination of reinforced concrete, ballast and
steel
Total width and length
Number of low-head Kaplan
turbines
Permanent magnet generators
Rated power/unit
Annual power production/unit
Water depth
0.4 kW/m
Table 2 (Wave Dragon, 2010)
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3 Tidal energy devices
There are many tidal energy devices around, and most of them are operating using the same
method. In general there are more or less two variations: tidal barrages and offshore turbines.
A tidal barrage power plant relies on the ability to control water and use its potential energy. This is
achieved by building a barrage across an inlet, creating a dam. Tidal barrages use the differences in
water level between tidal phases, which make water pass though turbines in the barrage which
produce electricity.
Since there are lots of variations and designs of offshore turbines, there is not any specific type that
is the most effective. Regardless of variations the method of producing energy is the same, namely
by converting the energy in underwater currents. An offshore turbine is a turbine that is lowered into
the water. They consist of one or more rotating blades. One model looks a lot like a wind power
turbine built under water and it produces energy when the blades rotate because of the underwater
current. Another model consists of a tube turbine which is placed at the sea floor, which works by
the same principle as the previously mentioned model (Energy Resources, 2010).
Tidal devices are not a very common in the world. That is mostly because few counties have coasts
with sufficient water level difference during tidal cycles. To get an effective and economical
extraction of energy from tides, the tidal range should be around seven meters (Ocean Energy
Council, 2010).
Tides occur because the moon’s position changes relatively to the
Earth’s. At the smallest distance between the moon and the
Earth, the moon’s gravity pulls the water of the Earth’s oceans
towards it and creates an ellipse form of the oceans (Figure 11).
Depending in what position the moon is, the water level increases
and decreases. As the moon rotates in the same direction as the
Figure 11 (NOAA, 2010)
Earth and creates tides, this phenomenon leads to two high tides
and two low tides in approximately 24 hours. This concludes that which time the tide is low or high
can be forecasted, namely every 6 hours (NOAA, 2010).
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3.1 Tidal stream generators
Tidal stream generators are a new
concept, which have not got a clear
“best” technology concept yet. But there
are a few different technologies; some
of them are more developed than
others.
3.1.1 Horizontal-axial turbine
Horizontal-axial turbine power plants
(Figure 12) are very similar to
conventional windmills. The currents in
the ocean drive an axial two bladed
rotor. The rotor is connected to a
generator through a gearbox. The rotor
blades can be pitched to optimize the
power output. It is possible to turn the
rotor 180 degrees, so the power plant
can generate electricity both at high and
low tides. It is possible to raise the whole
rotor up over the surface when it is time
for service or inspection (Marine Current
Turbines, 2010).
Figure 12, SeaGen (Marine Current Turbines, 2010)
The first axial turbine was SeaFlow which was installed in 2003 north-east of Lynmouth, England. It
had a capacity of 300kW. The construction was 50 meters high and the single rotor was 11 meters in
diameter (Fraenkel ,2003).
The successor of SeaFlow – SeaGen (Figure 12) – is about 40 meters high, and the crossbeam is 29
meters wide. At a stream speed of 2.4 meters per second the two 16 meter rotors can generate 1.2
MW (SeaGen, 2010).
3.1.2 Deep Green
Deep Green (Figure 13) consists of a
kite, which can be compared to a normal
wind kite. The kite consists of a 12
meters wide wing, a turbine, a rudder
and a generator, which are attached to
the wing. Deep Green is via a wire
attached to a fixed point on the seabed.
The kite travels through the water in an
eight-shaped circuit. Deep Green’s
design is based on that type of
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Figure 13, Deep Green (Minesto, 2007)
movement. Thanks to the movements through the water the kite does not need a high stream
velocity to generate maximum electricity. Compared to a conventional tidal stream power plant,
which is fixed on the seabed, Deep Green can receive a force on the turbine that is ten times higher
than a traditional power plant. This technique makes it possible to generate much electricity even
though the speed of the stream is not very high. Since the high velocity of the water gives the turbine
a high rpm, the turbine does not need a gearbox between itself and the generator. This makes Deep
Green quite small and very light compared to conventional stream generators. Deep Green weighs
only 7 tonnes, and at a speed of 1-2 meters per second it can produce 500kW.
Deep Green is a new concept developed by the company Minesto – established in Gothenburg.
Minesto is a recently started company, which is owned by the Saab Group, Midroc New Technology,
Verdane Capital and Chalmers owned Encubator (Minesto, 2007).
3.1.3 TidalStream Triton
A device which has been very successful in the United Kingdom is Triton, a tidal stream generator.
The company that develop Triton is TidalStream which was created by Dr. John Armstrong and
Michael Todman in 2005. Both Armstrong and Todman have backgrounds in the wind, marine and
power industries.
Triton is one of the offshore turbines that operate underwater. The latest version has six rotors and
has been developed off its predecessor version. The development of the six rotor device was started
in 2008 and in 2009 it was tested on the coast of France.
In Pentland Firth, in northern UK, a tidal power park consisting of Triton devices has been installed. At
the site Triton is operating at a depth of 60-90 meters and in current speeds of approximately 30km/h.
The construction consists of two spars that floats
vertically, and between them are six rotors (see
Figure 14). The construction is anchored to a solid
foundation on the seabed with a “swinging arm”.
This device is flexible because the swinging arm
allows it to rotate to all horizontal directions which
makes it possible to achieve a higher maximum
power output.
Figure 14, Trition (Tidalstream, 2010)
The six rotors of Triton are specified to deliver up to 10MW of power output. To be able to fulfil this
specification, the water current speed must be relatively high and the device must be placed at the
right depth. According to TidalStream a wind power plant with equal power output as 14 Triton
devices requires roughly a four times larger area.
As for the environment TidalStream states that Triton has no special impacts on marine life, rather
the opposite; it may prevent marine creatures from being swept away by giving them a shelter
(Tidalstream, 2010).
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Table 3 below lists the specifications of the mentioned tidal stream generators.
Type
Maximum
Power Output
SeaFlow
SeaGen
Deep Green
Triton
300kW
1,2 MW
500kW
10 MW
Required
number of
units
390
98
234
12
Weight per MW
14 tonnes/MW
Cost per kWh
(SEK)
1,34-2,68
1,34-2,68
0,54-1,07
Cost
(manufacture &
installation)
(SEK)
53 900 000
41 070 000
Table 3
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3.2 Tidal barrages
Tidal barrages can be compared with those of hydro power plants. A barrier is built to enable
containment of water at one side. To be able to convert energy, generator turbines are installed in
openings in the barrier together with adjustable hatches which control the water flow.
Tidal barrage plants work similarly to hydro power plants. The largest difference is that water can
flow both ways through the turbines in a tidal power barrier while in hydro plants only one way.
When the tide rises, the water flows though the openings in the barrier – making the turbines
generate electricity – into the dam to even out the water levels. When the dam is filled the plant can
be maneuvered to either let the water flow out into the sea again when the sea water level drops or
to seal off the basin so the potential energy of the captured water can be converted at a later time,
depending on the power need at the moment (Ocean Energy
Council, 2010).
A significant negative of tidal barrage plants is that it can only
produce electricity when the tide is flowing out or in, which is
about 10 hours per day (Energy Resources, 2010). But as tides
are predictable, it may be possible to plan for other energy
sources to be available at the times when the tidal plant is
unable to produce energy.
The largest tidal barrage power plant in the world is at the
moment Rance Tidal Power Station (Figure 15), in France,
which and was built during the 1960’s. It built across a 330
meter wide river and contains a reservoir of 22km2. The
Figure 15, Rance Tidal Power Station
barrage holds 24 turbines and has a total capacity of 240 MW
(EDF, 2010)
(EDF, 2010). The cost of Rance Tidal Power Station has been
calculated to approximately 880 million SEK
Table 4, (EDF, 2010)
(REUK.co.uk, 2010).
Rance Tidal Power Station
Table 4 at the right lists some of the specifications of
Rance Tidal Power Station.
Long
High
Wide
Weight
Turbine
750 m
13.5 m
5.3 m
470 ton
24x10 MW
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4 Environmental effects
True for all wave energy converting installations is that they are renewable and that they produce no
atmospheric pollutants – except for when the devices are constructed. Wave energy has the
potential to contribute to the global stride of making the world’s energy sources renewable
(Bhattacharyya & McCormick 2003).
There are as stated many different devices for converting wave energy, each with a different design.
This means that each device causes different effects on its surroundings and every scenario needs a
unique evaluation.
For all floating devices the main issues are that they require a certain area at sea, they require some
sort of power transfer cable on the sea floor, and they need an anchor cable to hold them in position.
These issues are minor to those of other types of wave energy devices.
Devices constructed directly on the sea floor can affect life forms negatively and change sedimentary
flow patterns. On the other hand the installations can serve as artificial reefs over the long term.
Larger structures as those that require a basin to function often involve a significant transformation
of the local environment which on a larger scale may affect the marine life (Bhattacharyya &
McCormick 2003). Installations that include water turbines may result in marine animals being
harmed as they are dragged into the grating of the turbine.
Effects perceptible on land are e.g. noise from turbines and alike, and that visual and acoustic
impacts from land installations might disturb people living in or visiting the area like tourists
(Bhattacharyya & McCormick 2003).
Even though tidal barrage devices produce renewable energy they also have negative impacts on the
environment. Building a barrage across a bay changes the water level on one side. This might give
negative effects on the local ecology (Tidal Energy, Ocean Energy Council).
Marine Current Turbines, who are the developers of SeaGen, is working very hard to minimize the
environmental footprints of SeaGen. Independent consultants have made different studies of the
impacts that SeaGen does to the environment. SeaGen is almost entirely submerged into the ocean,
which makes the visual impact on the landscape very small. The rotor makes no over water noise, but
it makes some under water noise, this to avoid fish swimming into the rotor. The rotor itself is not to
be considered a big risk for fish and other under water animals because of its low rpm – only 10-15
rotations per minute – and because the rotor is stuck in one place. Compare this with a boat
propeller, which rotates at least ten times faster and moves very fast through the water. Fish who
live in high velocity currents is also considered to have a very good observation ability and excellent
manoeuvrability (Marine current Turbines, 2010).
Deep Green is entirely submerged under the water, which makes the visual impact very small. Deep
Green does not make any heavy impacts in the seabed because no heavy installations are needed
under water because Deep Green is only attached to the seabed with a wire. The device also travels
relatively slow through the water and is not considered to be a threat to fish or other under water
animals (Minesto, 2007).
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5 Results
This section evaluates and compares the devices described in the earlier sections. As the wave power
industry is not very expansive, this evaluation is based on several assumptions and focuses more on
the required conditions and environmental effects of the devices than on specific costs.
To be able to evaluate wave energy converters, a reasonable average wave climate is assumed to
give the converters their most important condition.
As mentioned in the introduction, the aim of this project is to supply an island with 10 % of its
electricity need. The following figures represent each city’s energy requirements:
City
A
B
C
D
E
People’s power
consumption (MW)
330
210
55
170
120
Industry’s power
consumption (MW)
100
70
25
60
30
Summa:
Total (MW)
430
280
80
230
150
1170
Table 5, (Projectdescriptions2010_all_100908.pdf, page 3)
The islands total power need is 1170 MW. This results in the 10 % which should be produced by
wave- and tidal power to be 117 MW.
Judging from the specifications of the different wave power converters all of them are feasible –
under optimal conditions. All units except for Aquamarine’s Oyster operate at sites with water
depths of around 50-150 meters. On the other hand point absorbers like Seabased’s device that are
not slack moored floating devices will not stand tidal changes as well as the floating devices. Oyster
requires a shoreline with a depth of around 10 meters and it too does not operate well with
significant tidal changes. It would take – as stated in section 2.1 – around 250 units of Oyster to meet
the necessary power output, meaning a rather long stretch of shoreline with around 10 meters of
depth is required for the solution to work completely by itself. With all facts considered, the slackmoored, floating devices are more flexible to different conditions than the others mentioned. These
devices are the self reacting point absorbers Wavebob and Powerbuoy, the attenuator Pelamis, the
OWC Oceanlinx and the Wave Dragon. The area required at sea by these devices is around 5-15 km2.
Regardless of the technical feasibility of the wave energy converters, all of the in section 2
mentioned companies claim that their solution is scalable to meet the required power output for the
island scenario. On the other hand there is no existing wave park in the required size and there are
no companies that have a finished product that is mass-produced. This concludes that presently,
there are no available solutions that will be able to produce 117 MW of power by wave power to the
island.
Stream generators are not a widely developed concept yet. There are a few plants that operate in
the world today, and some more are being planned in the next few years. Both types of stream
generators operate at deep water. SeaGen and SeaFlow need about 50 meters of depth, and Deep
Green needs at least 20 meters between the surface and the top of its track, plus the depth of the
18
track, which varies from different locations. What separates these two concepts is the velocity of the
currents needed to generate electricity. SeaGen and SeaFlow need higher velocities than Deep
Green. As stated in section 3.1.1 SeaGen needs 2.4 m/s or more to generate maximum power (1.2
MW), but Deep Green only needs a current velocity of 1.6 m/s to generate maximum power
(500kW). This makes Deep Green suitable for more places around the world. Deep Green is also
smaller than SeaGen or SeaFlow; it weighs less than 10% of SeaGen or SeaFlow. This makes Deep
Green much easier to transport, maintain and repair.
Accordingly to Minesto – the developer of Deep Green – Deep Green is said to be more cost effective
than other stream generators which is shown in table 3 in section 3.1.
To reach the islands requirement of 117 MW it would take, according to the maximum power values
described in section 3.1, about 100 SeaGen plants or about 235 Deep Green plants. Currently there is
no existing stream generator park in the world that is large enough to meet these requirements. Of
the three different stream generators described in this report SeaGen is the only one that has been
connected to the grid during a test session and is said to be available for relatively large farms. Deep
Green has not been tested in a full scale yet, which means it needs to be tested and developed more
before it is possible to use it for power generation.
Triton is considered fully developed as it is the result after plenty of research. It has operated in
several areas around the coast of the United Kingdom but it is not fully implemented for
commercialising. According to its manufacturer Triton devices could provide the UK with 16% of the
country’s total electricity requirement, but for that to be realised better cost effectiveness and more
efficient implementation of tidal turbines would need to be achieved. At this moment it would take
about 12 Triton devices to meet the demand of 117 MW. Triton is a technically possible alternative
for the project but as it is not yet cost-efficient according to its developer it would be ruled out by the
investors of the island scenario.
As tidal barrages tend to make a larger impact on the environment than the smaller devices
mentioned in this report, a barrage would be avoided if another solution is available. However, with
the right conditions regarding location and tides, a barrage would be a possible solution for the island
scenario. On the other hand SeaGen is a device ready for manufacture to some extent and would be
a more reasonable choice of device regarding environmental effects if the required under water
currents are available at the specified site. Hence this report concludes that of the evaluated
solutions SeaGen would be the most reasonable alternative to choose if one assumes that optimal
conditions for each device are a certainty.
19
6 Discussion
As the field of wave power is at the very beginning of its advance to becoming an energy source
competitive to other sources such as wind power, there is still no clear winner among the wave
converters. This changed the way the evaluation of different solutions in this report took shape. The
wave energy section became more of an examination of what solutions will be available in the
future. Given a few years it is plausible that a converter has come up with specifications and
efficiency so that it would be suitable to an investor with similar needs as the one in the island
scenario.
In the beginning of the research phase of the project, the group was aiming towards finding specified
costs for the different solutions to be able to calculate which one was the most profitable. This was
quickly abandoned regarding wave power converters as there were almost no figures regarding costs
as there were no finished products available. But we were able to find some production and
installation costs for stream generators. The project however had to be a little more flexible than if a
more established energy source was evaluated. The evaluations of the wave devices mainly
considered technical specifications, effects on the local environment and what conditions they
required.
When reading the different wave energy converter developers’ sites, one gets the idea that the
focus, so far, has been on achieving a prototype which simply has the strength to withstand the
rough conditions at sea and that has a life span that is reasonably high. These goals appear to have
been reached in some cases where companies claim to have a sustainable solution which can deliver
a specified average output power. However, it will take a lot more engineering before a wave energy
converter will be ready for mass production and be competitive to other energy sources.
The research for tidal and stream power started with looking for different solutions that are available
on the market. We found that there were only a few different solutions that really work, but that
there are several prototypes under development. During the research we discovered that tidal wave
power has been in use for quite long, but that scientists are also working on new concepts. Stream
power, though, is somewhat more untested. As described in the report there are a few models which
have been tested, but that they will take some more years of development and testing before they
will be connected to the power grid. If it in the future it would be able to capture the vast amounts of
energy in the major ocean streams – for example the gulfstream – it should imply a huge addition to
the world’s energy production.
20
7 Conclusions
This report concludes that it is possible to meet the objective of generating 10% of the island’s power
requirement by using any one of the tidal stream generators SeaGen and Triton, or a tidal barrage
power plant. Although, this is only possible if the right conditions regarding build site, tides and
underwater currents are optimal. As Triton’s developer states that their device is not yet
commercially valuable it is not recommended for this project.
Because of the larger environmental effects of tidal barrage power plants this report recommends
the SeaGen stream generator for the island scenario.
Wave power devices are ruled out because there are presently no devices ready for mass production.
This also applies to some of the stream generators evaluated in the report, leaving just the above
mentioned SeaGen and tidal barrage constructions left. However, with a few more years of
development and testing all devices examined in this report appear feasible for future projects
similar to this.
21
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