Note: Descriptions are shown in the official language in which they were submitted.
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System for the exploitation of tidal- and river current energy
This application concerns a system for generating energy from water currents.
The invention is particularly suited for use in tidal currents, river currents
or ocean
currents, and particularly where there is a limited cross section that water
moves
through, for instance rivers or narrow passages or sounds. Compared with other
sources of energy, the use of the tidal energy is one of the most
environmentally
friendly and predictable of all known sources of energy.
In sounds that become narrower, the current will converge towards the
narrowest
cross section. In these places, not only the velocity will be the greatest,
but also
most evenly distributed and such localization will therefore normally be the
ideal
site for a tidal plant. In passages the local bathymetry is absolutely
decisive, and
placing the tidal plant in places there it is exposed for strong currents with
differ-
~s ent direction simultaneously should be avoided. In rivers, similar
evaluations
should be made.
Plants for collecting flowing current energy has seen limited use, because the
costs will become to prominent as very large plants are required to collect
even
zo fairly modest amounts of energy. In the past it has been focused on the use
of
differences in level, and the use of the energy from the pressure height of
the
water, as in a low-pressure hydroelectric power plant. In such plants, it is
re-
quired that the water is dammed up, is exploited through turbines, and will
result
in interference of the environment similar to those related to the building of
tradi-
Zs tional hydro electric power plants. The size of the plants has done that
they rep-
resent environmentally unfavourable changes and at the same time have been
obstructing boat traffic etc.
Several technical solutions have been suggested to reduce the above-mentioned
3o disadvantages, and the solutions have concerned various turbine wheels or
im-
pellers with blades, and turbine/blade combinations that as a whole are placed
below the surface of the water.
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US 3 922 012 shows a patent where a multitude of vertical turbines are placed
on a frame that it is to be lower down to be placed on the sea bed. The frame
has floating elements enabling the plant to be buoyant and to be towed to a
suit-
able location. The floating elements are then been ballasted, such that the
plant
sinks and becomes located on the seabed.
Patent US 3 912 937, describes a system for the production of electrical
energy
from ocean currents based on a two-part turbine blade construction. The plant
is
completely placed under water and comprises a horizontally placed turbine.
Patent US 5 440 176 is based on a movable turbine, that is, it can be adjusted
up
and down in relation to the platformlframe depending of the operating
conditions.
In connection with the design of the turbine, the previous patent US 2 250 772
~s shows an impeller.
However, with these designs, it is a problem that they are covering a relative
small cross section of the flowing water current, or the units becomes so
large
that installation and maintenance becomes difficult.
Accordingly the present invention concerns a plant that can cover a large
cross
section of a flowing mass of water, but nevertheless with components at a more
manageable size, where the maintenance is simplified, and where the compo-
nents or the entire plant more easily can be raised to the surface for
maintenance
2s and repair.
This is achieved with a plant for the production of electric energy from ocean
or
river currents that comprises turbines with blades, shafts and side
units,.genera-
tors for the production of electric current; frame assembly equipped with
buoyant
so vessels, assembled from one or several sub frames, where the plant as a
whole
is placed below the surface of the water. The shafts of the turbines are
oriented
substantially perpendicular to the direction of the velocity of the water, and
the
turbine shafts are supported in the frame assembly. The plant has positive
buoyancy adjusted by the buoyant vessel and a'backstay system is anchored
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3
below surface of the water, such that the plant is kept below the surface of
the'
water by the backstay system. The blades are shaped as wings such that the
turbines are rotating in the same direction regardless of the direction of the
flow
of the water.
s
An embodiment of the plant according to the invention will now be described
closer with reference to the enclosed figures where:
Figure 1 shows the plant according to the invention as it will be seen from
the sea
bed looking towards the system in the direction of the flowing current.
Figure 2 shows the plant seen from 2-2 on figure 1.
Figure 3 shows the plant seen from 3-3 on figure 2.
Figure 4 and 5 shows turbines in detail.
Figure 6 shows an elevated view corresponding to figure 2 where the
localization
~s of the system is shown.
The turbines (A), comprises wing shaped blades (G), formed such that the tur-
bines (A) will rotate in the same direction, regardless of the direction of
the water
flow. The blades (G) are supported on each side. Generators (not shown) are
ao connected to the shaft of the turbines (A), and will produce electric
energy, that it
is transferred through a cable (not shown). The generator is protected against
ingress of water by means of a water resistant enclosure and possibly an over-
pressure in the enclosure.
as The turbines (A) are supported in frames (C) that comprise a number of
vertical
and horizontal frames or sub frames that are assembled as modules. The mod-
ules are assembled to create. a plane with several turbines. This plane can
thereby be adjusted to a desired cross section of current in a passage or a
river.
It is also possible to build the most critical components in such a way that
they
3o are reasonably easy to substitute in a running phase. This is particularly
relevant
in terms of exchanging components having an expected high frequency of repair
and maintenance. The framework design gives in general a favourable distribu-
tion of the design forces, particularly those that acts on the turbine
bearings. The
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buoyant vessels (B) are also installed on top of each end, and in the middle,
to
allow easy adjustment of the buoyancy of the plant.
Figure 4 and 5 shows a cross section of the impeller (A) where the shape of
the
wing shaped blades (G) is better shown. The shape ensures that the impellers
rotate in the same direction regardless of the direction of the water flow.
The
blades (G) can furthermore be made with adjustable pitch to better utilize the
en-
ergy of the current.
The blades (G) are secured to circular plates at each end to form the turbines
(A). The shafts of the turbines are connected at the centre of each of these
circu-
lar plates. The shafts of the turbines can be secured on each circular end
plate
to avoid that the shafts extend through the turbine. This will improve the
flow
conditions in the turbine.
~s
It is shown that the turbine wheels (A), 8 in total, and the 3 buoyant vessels
(B)
are assembled in the frame (C) that is anchored to the ocean/river bed (D) and
is
furthermore anchored through anchor wires, anchor lines or backstays (E) to an-
choring block (F). From figure 6, the direction of the ocean/river currents
(H) in
zo relation to the system (I) is shown.
The backstay, or anchoring, comprises tight anchor lines (E) keeps the plant
un-
der water and acts against the buoyancy. The anchoring is chosen from a re-
quest of the smallest possible play of the plant. Steel ropes or wires of the
type
zs "spiral strand" are chosen to achieve a long expected lifespan. The
backstays
(E) should furthermore be treated to avoid corrosion.
The anchor lines are secured to five submerged winches (not shown) installed
on
the structure. The power to the winches is supplied through hydraulic hoses
with
3o quick release couplings placed on top of the plant.
The total forces on the plant are relatively high. It is therefore used 10
anchor
lines (E) for reducing the load on the anchor lines (E) and to reduce the
effect in
the event of breaking a line.
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The anchor lines (E) are secured to the five vertical frames (C) in the
carrying
assembly. Two anchor lines (E) are secured to each end of the plant. These
anchor or backstay lines (E) are loaded with vertical and horizontal forces
both in
the length axis and transversally of the plant. For the three intermediate,
inner
s frames, the backstays are secured to the bottom or lower part of the frames.
All
the backstays (E) have an angle 45° in relation to the seabed.
Below is an example of a passage with a tidal current as the plant according
to
the example can be placed in.
Water depth: 50 m
Width of the passage on the place of installation: 500 m
Maximum current velocity: vs = 2.5 mls (5 knot)
Significant wave height : Hs = 0.5 m
~s Maximum wave height : Hm = 1.0 m
Wave period range : Tz = 6.0 - 15 s
Below are design parameters for the plant according to the described embodi-
ment:
Coefficient of drag for the turbine, Cd: 1.2.
Drag area is set At = 12 x 12 m2 x 0.75 = 103 m2,
where 0.75 is the factor of the permeability for the turbine to accommodate
for
the flow through the turbine.
2s The carrying assembly is designed as a steal frame. It comprises five
vertical
frames or sub frames with a height of 14 m and a centre distance 16 m intercon-
nected by means of a horizontal frame. Total centre distance between the outer
frames is 64 m. Because of the requirement for 10 m sailing height above the
plant, the steel frame (C) will be situated 23 m below the water surFace, that
is,
so 27 m above the seabed. The distance to the seabed is thereby 20 m. The
frame
(C) comprises 500 mm tubular profiles with 20 mm wall thickness. This gives a
net dry steel weight of 171 t for the frame (C) and a total buoyancy of 126 t.
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The buoyant vessels (B) are placed on the top of the vertical frames (C) in
the
carrying assembly, one in each end and one on top of the mid-frame (C). Total
buoyancy for the installation is chosen such that the anchor lines will not
lose
tension in the existing weather conditions. Each of the five vessels are 3.5 m
in
s diameter and 20 m long. This gives 197 t gross buoyancy per vessel. It is as-
sumed steel tanks with a buoyancy to weight ratio of 3, that is, weight for
each
vessel becomes 66 t.
The turbines (A) are arranged in two horizontal levels with four turbines in
each
level and horizontal shaft support. Thereby the centre distance of the
turbines
(A) becomes 14 m in vertical direction. The turbine consists of five blades
that
spans 12 m unsupported between two end plates with diameter of 12 m. The
blades are of NACA 0016 profile meaning that the greatest blade thickness is
16% of the length of the blade. The length of the blades is set to 3.2 m such
that
Is that the greatest thickness of the impeller blade is 512 mm. Hollow profile
blades
are chosen to save weight. The end plates comprise circular plates with corru-
gated core to reduce the weight. Total thickness is 120 mm.
Total dry weight per turbine is estimated to 64 t. Similarly total buoyancy
per tur-
bine is 81 t. Thereby each turbine will have a net buoyancy of 17 t, which is
suf-
zo ficient to carry the weight of the generator and gear system in connection
with the
turbine.
The greatest spring tide velocity of 2 m/s gives a maximum effect per turbine
of
208 kW. It is then assumed a turbine with exposed area of 144 m2 and an effi-
zs ciency of 22%. The plant will, with 8 turbines, have an installed effect of
ca. 1,66
MW.
The duration curve for the measured flow velocity flats out at ca. 1.75 m/s
and it
should be inquired if it is profitable to dimension for greater velocities
than this.
3o This gives a turbine effect of 139 kW. The contribution from velocities
above
1,75 m/s to an annual production is ca. 10%.
The plant comprises 8 turbines where each generator shaft is connected to two
turbines. This gives a maximum shaft effect of 416 kW per generator.
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The power cables to the surface can be connected in the atmosphere or under
water. When coupled in the atmosphere, the cable is led through the enclosure
with a water resistant cable gland or passage, which is a standard product on
the
market, and is connected to the generator terminals inside the enclosure. The
s enclosure must then be coupled to the generator before it is installed under
wa-
ter. This requires then that the generator with gear and enclosure is
installed and
secured to the carrying assembly after the carrying assembly has been
installed
and anchored or moored. In the invent of a breakdown of the generator, gear or
cable, the cable will be pulled up along with the generator/gear module and is
disconnected onboard a vessel. This requires that the cable is sufficiently
long
to, or the cable has sufficient play, so that the module can be elevated. If
con-
necting under water, an electric underwater coupler or connector is used. One
part of the connector is then secured on the outside of the enclosure. The
other
part is then secured to the cable and is connected to the enclosure when the
ca-
~s ble is laid. The generator/gear module can then be mounted on the carrying
as-
sembly onshore and be installed as a part of the total assembly. in the event
of
breakdown, the cable can be released from the generator/gear module under
water and the generator/gear module or the cable can be hoisted up for repair.
The electric under water connectors are expensive units, but this method can
ao prove to reduce installation and maintenance costs more than the added cost
for
the electric system as a whole.
Another critical factor will be the cables. Because of the tidal currents, the
cable
will be exposed for mechanical loads. It is therefore required that this is
secured
as properly to avoid wear and fracture of the cable. Sea cables are normally
flushed
down or anchor to the seabed. The critical part becomes the length from the
seabed and up to the generators. If it is chosen not to use underwater connec-
tors to terminate the cables to the generators, an additional extra length of
ap-
proximately 40 meters per cable is necessary to allow for extra play for
hoisting
so up the generator/gear module to disconnect the cable. This extra cable
length
must be secured to avoid fracture of the cable, but must be simple and quick
to
release in the event of breakdown of the generator/gear module.
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When installing the anchoring system, the seabed can be surveyed by video
where the plant is to be anchored. Seismic examinations of the seabed should
also be performed.
In the example it is assumed that a rigid anchoring by means of poles cemented
s securely in boreholes is used. The poles will typically have a dimension
between
30" to 60", and are installed in 10-15 m deep boreholes.
The plant is positioned on the place of installation with the short side
towards the
direction of the current flow, and five anchor lines on one side are secured
by
means of a smaller craft. Then, forerunners are secured to the five anchor
lines
on the other side to five under water winches that are installed on the
structure.
Power supply for the winches comes from hydraulic hoses with quick release
couplings on the upper part of the carrying structure.
~s In the change of tide, tugs turn the entire plant 90 degrees such that the
long side
is perpendicular to the direction of the current. When the forces from the
current
are tightening the five anchor lines that are secured to the structure, the
under
water winches are run until the plant is in position. In this position the
next five
anchor lines are locked, and divers or a ROV releases the hydraulic hoses
during
ao the change of tide. How the anchor lines are to be locked in the structure
has not
been evaluated in detail, but will be important during the design of the
details.
The method requires somewhat longer anchor lines than those that are antici
pated for the construction of the carrying structure. Furthermore the use of
fibre
Zs ropes with a protective layer (against fouling and sand intrusion) is
recommended
to ease the installation work. Any increase of costs with this change, is in
this
connection assumed to be of a smaller magnitude.
The tidal plant will be submerged 10 meters below the ocean surface and will
3o therefore not be visible. Where the cable is to be landed on shore, houses
for
frequency converters and transformers will be installed and will be visible,
as be-
fore the installation. The plant will reduce the tidal current with
approximately
20% in the area the plant will be placed.
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The water mills will rotate the same direction regardless of the direction of
the
water flow.
A tidal power plant will have two opposite effects on the tidal current: A
reduction
s of the velocity due to the increased resistance of the flow given by the
plant, and
an increase of the velocity because the plant blocks some of the cross section
and chokes or throttles the current. If the water mills are distributed in the
pas-
sage as windmills are distributed in large parks, the mills will only increase
the
resistance and reduce the current. But if the watermills are placed tightly
together
~o in the one and same cross-section, the velocity of the current in this
cross section
will increase even though it is reduced elsewhere in the passage. Thereby the
watermills can drain more energy when they are placed in the same cross sec-
tion and block the passage than they would if they were distributed.
~s The given example is only described to help understanding the invention,
and the
invention is only defined by the appended claims.
