Note: Descriptions are shown in the official language in which they were submitted.
Steerable Underwater Frame for Submerged Turbines
This invention relates to an underwater turbine support that can be steered
while submerged
to increase energy generated from the turbines supported thereupon.
Many systems have been proposed for the mounting of turbines for the
generation of power
from water currents associated with river, estuarine, thermal or tidal flows.
The more practical
of these incorporate the means for installation and maintenance access as well
as for
positioning the turbines in the best part of the current. This is usually near
the surface where
the flow tends to be stronger, but deep enough that the rotors do not break
the water surface
in even the strongest sea states.
Whilst many proposed supports use tethering fore and aft to keep the turbines
aligned in the
same direction, others allow free swinging of the support around a single
seabed anchorage
so that the turbines can be self-aligning with the current flow. This
maximises energy capture
and reduces loads from off-axis operation. In the case of generation from
tidal flows, this
single-anchor approach means that the support will swing through a half-circle
at each tide
change to align first with the flood flow and then with the ebb.
Some systems, for instance that described in G82348249, propose that no
special steering is
necessary for turbines operating downstream of a single anchorage: the
turbines on their
support frame will naturally find their own alignment with the water current.
"Aligned"
underwater turbines, therefore, only change their orientation if the water
current changes
direction. Another example of an "aligned" underwater turbine is disclosed in
GB 2 450 624.
In this example, thrust of the turbines is varied in order to maintain the
turbines aligned with
the prevailing water current flow. In other words, the variation of thrust
maintains the
orientation of the turbines in alignment with the flow.
In all the above-described systems, an essential feature is that the turbines
remain "aligned"
with the flow at all times, so that energy is not lost (and loads do not rise)
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because of misalignment of the turbine rotors. However, it is also the case
that a rotor
fixed so as to be stationary in a water current slows the water down as the
flow
spreads out to go around the rotor rather than through it: this is the
inevitable
consequence of the diffusing effect of the turbine operating in a fluid flow.
This limits
the theoretical energy of an "aligned" turbine to the so-called Betz limit,
namely 59%
of the energy available in the flow. In practice the highest efficiency of
"aligned"
turbines is lower than this limit because of unavoidable hydrodynamic and
mechanical
losses in the rotor and drive train.
One way to boost the energy capture of a turbine in a flow is to mount it on a
tethered
wing, the wing being constrained to travel across the flow in a circular or
closed path
pattern, with energy being gathered from the turbine moving along the
direction of
travel of the wing rather than normal to the flow. Such a system is described
in US
2015/0316931. In effect, the wing acts as a means of scanning across the water
current
flow, the wing being driven by the current flow, much like a kite in the
wind.. As
such, the winged underwater turbine moves rapidly through the flow, driving
the
turbines. As a result of the speed at which the winged frame travels through
the water
the turbine thereby gathers many times more energy. However the wing itself
constitutes a large structure attracting considerable loadings to itself, and
by moving
rapidly through the flow - at several times the velocity of the flow - does
constitute
both a potential hazard and a vulnerable additional component.
The purpose of the present invention is to achieve the benefits of increased
energy
capture in a similar way but without the complication of the additional
feature of the
wing described above.
According to a first aspect of the present invention there is provided an
underwater
frame supporting a plurality of tidal turbines, the frame being tethered to a
seabed
fixing such that the frame and tidal turbines are positioned downstream of
tidal flow,
.. wherein the frame is steerable under the influence of differential thrust
from the
turbines, wherein the underwater frame is configured to be driven continually
into
fresh flow, so as to increase energy capture from said turbines by exposing
them to
undisturbed tidal flow.
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In this specification, the terms "fresh flow" and "undisturbed tidal flow",
refer to
underwater current flow that has not yet been slowed down by the diffusing
effect of a
turbine operating in the flow. It will be understood that undisturbed tidal
flow is only
obtainable for a very limited amount of time, until the flow starts being
diffused by the
presence of the underwater turbines. As such, the invention suggests to
continuously
steer the frame and turbines slightly out of alignment and into fresh, that
is, non-
diffused flow. This differs from the disclosure of US 2015/0316931 in that
these
common winged frames expose the wings to fresh flow, not the turbines.
The frame may be steerable laterally and movable in a reciprocating side to
side
motion.
The differential thrust may be achieved by varying the speed of the turbine
rotors. In
particular, the speed of the turbine rotors is typically characterised by the
so-called tip-
speed ratio. The tip-speed ratio (or TSR) for turbines is the ratio between
the
tangential speed of the tip of a blade and the speed of the tidal stream at
the plane of
the rotor. It is known that the thrust created by a turbine is dependent on
the tip-speed
ratio of the turbine and generally increases as the turbine blades rotate
faster. As such,
varying the tip-speed ratio on at least one of the turbines will inevitably
change the
thrust of said turbine and therefore introduce a differential thrust that can
be used to
steer the frame with respect to the tidal stream. Introducing a differential
thrust by
varying the tip-speed ratio of at least one turbine can be achieved by either
increasing
or reducing the speed of said turbine.
The tip-speed ratio of the turbines is further directly linked to its power
coefficient,
which determines its output power. The maximum power output, occurring at the
power coefficient peak value of any turbine, is dependent on several
parameters but is
typically achieved at tip-speed ratios of between 4 and 6. As will be
described in
more detail below, the optimum output power is achieved at the peak of a bell
curve,
which decreases if the tip-speed ratio is above or below the tip-speed ratio
that
provides the maximum power output.
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In an embodiment of the present invention, the differential thrust is achieved
by
varying the tip-speed ratio of at least one of the turbines so that the power
coefficient
of said turbine falls below its peak value. The skilled person will appreciate
that,
depending on various factors, such as the size and form of the turbines, the
shape of
the frame, etc., every turbine has a peak power coefficient that will be
obtained at a
specific tip-speed ratio. The peak value is described in more detail herein
below and
can be determined by calculation or experiment. . The tip-speed ratio may be
varied in
such a way that the power coefficient of said at least one turbine remains
above 90%,
preferably above 95%, of the peak value. This embodiment is based on the fact
that,
for most turbines, the thrust coefficient of a turbine changes monotonically
and more
significantly as a function of the tip-speed ratio around its maximum power
coefficient
value than does the power output coefficient. As such, it was found that it is
generally
sufficient to sacrifice no more than 10% of the power coefficient peak value
of the at
least one turbine in order to achieve the desired differential thrust.
Since the thrust of a turbine reaches a level of saturation at high tip-speed
ratios, the
change in thrust may be more significant when the tip-speed ratio of the
respective
turbine is decreased. In other words, the change of thrust per incremental
change in
tip-speed ratio may be higher at lower tip-speed ratios, resulting in more
substantial
differential thrust at lower tip-speeds. However, it is of course also
feasible to increase
the tip-speed ratio of the respective turbine. In another embodiment, the tip-
speed ratio
of at least one turbine might be decreased, while the tip-speed ration of at
least one
other turbine might be increased in order to maximise the differential thrust.
In
particular, turbines on opposite sides of the underwater frame may be adjusted
in
opposite ways such that one or more turbines on one side of the frame are set
at an
increased tip-speed ratio, whereas one or more turbines on the opposite side
of the
frame are set at a decreased tip-speed ratio. The term "opposite sides" refers
to
opposite sides of a vertical or horizontal axis that intersects a centre of
gravity of the
underwater frame.
The preferred method of variation of the tip-speed ratio of at least one
turbine is by
electronic variation of the electrical loading on the turbine. It may also be
achieved in
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other ways, for instance by application of a brake or by adjusting the blade
pitch of the
rotor blades.
In yet an alternative embodiment the frame may be provided with lateral
thrusters and
5 is then steerable by the forces generated by said thrusters. In a further
alternative
embodiment the frame may be provided with controllable lift surfaces and is
steerable
by manipulation of said lift surfaces.
In addition to the aforementioned lateral motion, in the frame may
additionally be
steerable vertically and movable in a reciprocating up and down movement. The
lateral and vertical movement components may be combined such that the frame
follows a circular, elliptical or figure of eight path.
In order to facilitate the aforementioned movement, the frame is preferably
tethered to
a single point on the seabed fixing. The tether may be either a flexible or a
rigid
tether. Where the tether is rigid, it will be understood that appropriate
articulation
means are provided at opposing ends of the tether between, respectively, the
frame
and tether, and tether and sea-bed fixing.
The torque of the turbine rotors may also controllable so as to provide roll
stabilisation
and/or underwater positioning of the frame, in use. Alternatively, or in
addition to,
such torque control, the buoyancy of the frame may be controlled so as to
provide roll
stabilisation and/or underwater positioning of the frame. The frame may be
provided
with one or more chambers to which water can be admitted to, or removed from,
in
order to alter the buoyancy of the frame.
The frame is preferably movable between a substantially vertical operating
position
and a substantially horizontal maintenance position. In the maintenance
position the
frame lies at or near the water surface and thereby allows access to the
turbines.
Preferably a portion of the frame extends above the water surface when the
frame is in
the vertical operating position. This provides a variable buoyancy reaction to
oppose
horizontal thrust forces, and also serves to indicate the position of the
frame to vessels
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in the vicinity. In the absence of a part of the frame above the water
surface, ballast
water may be pumped in or out of one or more chambers to provide reaction to
horizontal thrust forces and stabilise the vertical position of the frame in
the water.
According to a further aspect of the present invention there is provided a
method of
operating an underwater frame supporting a plurality of tidal turbines, the
frame being
tethered to a seabed fixing such that the frame and tidal turbines are
positioned
downstream of tidal flow, wherein the frame is continuously steered into fresh
flow,
so as to increase energy capture from said turbines by exposing them to
undisturbed
tidal flow.
The purpose of the present invention is to arrive at the same goal of enhanced
energy
capture described in the prior art by amplifying the area of flow intercepted,
but by a
different method. Rather than using differential thrust to "align" multiple
turbines on a
tethered frame with the flow, in one embodiment, the present invention
proposes to
scan the turbines in an arc across the flow first in one direction and then in
the other,
thereby continuously steering the turbines into undisturbed flow. A further
beneficial
side-effect of this side-to-side motion of the frame and turbines is that the
speed of
frame and turbines through the water is increased, thereby enhancing energy
capture
from the turbines.
If water depth allows, it may also be possible to allow an up-and-down
oscillation of
the frame, such that the turbines move in an ellipse rather than a straight
line, further
enhancing the intersected area. It will be appreciated that vertical and
horizontal
movement components may be utilised in order for the frame to follow different
path
configurations. In addition to an elliptical path, the frame may, for example,
follow a
circular or figure of eight path. It will be appreciated that other path
shapes are
possible. From the point of view of the turbines, rather than being subject to
flow
velocities reduced by diffusion, they are instead subject to fresh or
partially fresh
water flows, less affected by diffusion, with higher velocities giving rise to
greater
energy capture. Since the diffusion loss in velocity - known as 'induction' -
can be
typically one third of the upstream velocity, and since energy is proportional
to the
cube of velocity, the energy capture from totally fresh flow would be (3/2)^3
or 3.4
times that from flow with an induction factor of one third. Of course, such
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enhancements could not be practically obtained, since some degree of induction
would
still accompany laterally moving turbines, but nevertheless a useful degree of
enhancement could in theory be obtained.
This invention is quite distinct from that described in US 2015/0316931. In
the first
place, no 'wing' is involved; just turbines supported on a tethered frame.
Secondly, it is
not the speed of movement through the water that leads to the enhancement of
energy;
rather it is the displacement of the turbines to a zone of fresh flow that
produces the
enhancement. The speed of movement of the frame does not in itself contribute
to the
enhancement, however if the speed is too slow the induction velocity loss
associated
with diffusion will have time to build up and will to that extent limit the
enhancement.
Too fast a motion of the frame will on the other hand lead to drag losses that
offset the
energy enhancement, so that the speed of displacement is in fact critical, but
for
different reasons from the wing speed of US 2015/0316931. It is likely that
different
frame speeds will be required to maximise the enhancement at different levels
of flow
velocity.
Motions of the frame can be induced by a number of means, for instance by
differential thrust induced by varying the rotor speed or blade pitch of the
turbines
individually or in groups. Alternatively; sideways-acting thrusters can
physically
propel the frame in its arc of motion, or lift surfaces can be exposed or have
their
angle of attack altered to generate sideways force to provide the motion. With
sufficient turbines, motions can also be induced in a vertical direction or
combined to
give an elliptical or circular path of motion. It may also be desirable to
include in the
control system means for also varying the differential torque between turbines
such
that the frame remains stable in roll in the desired configuration.
Other features of the invention will be apparent from the following
description of a
preferred embodiment shown by way of example only in the accompanying drawings
in which:
Figure 1 shows a perspective view of an underwater frame supporting a
plurality of turbines;
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Figure 2 shows a plan schematic view of the lateral motion of an underwater
frame according to an aspect of the present invention; and
Figure 3 shows a perspective view of an underwater frame supporting a
plurality of turbines and thrusters.
Figure 4 shows a schematic diagram of the output power coefficient and thrust
coefficient as a function of the tip-speed ratio of a typical turbine.
Referring to figure 1 there is shown a frame 10 having a plurality of tidal
turbines 12.
In the embodiment shown, the frame 10 includes a body 14 having a pair of legs
16
extending downwardly therefrom. Each leg 16 is provided with a respective
further
body 18 towards its distal end 20. The frame 10 further includes a plurality
of lateral
extensions 22 to which the turbines 12 are mounted. The bodies 14, 18 may be
provided with internal compartments to which sea water may be admitted to and
removed from. Admission and/or removal of sea water from the compartments
.. permits the buoyancy of the frame and turbine combination to be altered.
Alteration of
the buoyancy in this manner may be used to, for example, trim the structure
while
submerged and move the structure between a submerged operating position and a
maintenance position at or near the water surface. Additionally or
alternatively,
floodable compartments may be provided in the legs 16. The form of the frame
10 is
.. provided for the purpose of illustration and is not intended to be
limiting.
The frame 10 is tethered to a seabed fixing 24 by a flexible tether 26. A
rigid tether
with appropriate articulations at opposing ends may also be used.
.. Referring now to Figure 2, the thrust of the turbines 12 is varied
differentially (by
rotor speed or blade pitch) so as to provide rotation of the frame 10 anti-
clockwise
from position 1 to position 2. The net rotor thrust will then cause the whole
frame 10
with its tether 26 to rotate anti-clockwise to position 3 and then to position
4.
Once at the extreme position of position 4 is reached, the thrust differential
is reversed
so that the frame 10 rotates the other way, and hence pulls its tether 26
clockwise up
to position 8. In this way the turbines 12 are driven continually into fresh
flow and as
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a result capture more energy. The motion of the frame 10 is reciprocated in a
side to
side motion.
Motion of the frame 10 in the manner described above may be achieved by
methods
other than turbine thrust variation. For example, movement of the frame 10 may
be
achieved by the provision of lateral thrusters on the frame 10 and/or the
provision of
controllable lift surfaces.
In Figure 3, two of the turbines at the extremities of the frame have been
replaced by
thrusters 27 which may be powered- in either direction - differentially to
generate the
moment required to displace the frame and followed the desired oscillatory or
cyclic
motion.
Figure 4 shows a schematic diagram of the power output coefficient Cp and
thrust
coefficient CT as functions of the tip-speed ratio TSR for an exemplary
turbine. The
behaviour of the output coefficient Cp as a function of the tip-speed ratio
TSR is
shown by line 30: a generally bell curved shape. As discussed hereinbefore,
the peak
value or maximum power output coefficient Cpmax will be reached at medium tip-
speed ratios, typically between 4 and 6. The peak value of the bell curve
described by
line 30 is labelled in Figure 4 as point b. In a normal operating condition,
all of the
turbines of the present invention are typically set to a tip-speed ratio
(TSR1) that
results in the maximum power output coefficient Cp., at point b. As will be
understood, this tip-speed ratio TSR1 may change dependent on several
characteristics
of the turbine, however it will generally be easy for the skilled practitioner
to
determine TSR1 via calculation or experiment.
As can be derived from Figure 4, if the tip-speed ratio is reduced, the power
output
coefficient Cp decreases. For example, if the tip-speed ratio is set to value
TSR2,
which is lower than TSR1, the power output coefficient Cp decreases to value
Cpmm
which will be described in more detail below. Similarly, if the tip speed
ratio is higher
than TSR1, such as the tip-speed ratio TSR3 shown in Figure 4, the power
output
coefficient Cp will also decrease. For example, if the tip-speed ratio is set
to value
TSR3, the power output coefficient yet again drops to Cp min.
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It is further derivable from Figure 4 that changing the tip-speed ratio not
only affects
the power output coefficient Cp but also changes the thrust coefficient CT of
the
exemplary turbine. This behaviour is depicted by line 40, which shows the
thrust
5 coefficient CT as a function of tip-speed ratio TSR. As will be
appreciated, during
normal use, that is when the exemplary turbine rotates at tip-speed ratio
TSR1, the
coefficient of thrust has a value of CT1, represented by point e along line
40. If the tip-
speed ratio of the exemplary turbine is reduced from TSR1 to TSR2, the
coefficient of
thrust decreases from value CT1 to CT min along the path between points e and
d of
10 line 40. If, on the other hand, the tip-speed ratio is increased from
value TSR1 to
value TSR3, the thrust coefficient increases from CT1 to CTmax between points
e and f
along line 40. This change of thrust in at least one turbine can be used to
create a
differential thrust within the frame that can be applied to steer the frame
laterally
and/or vertically. To this end, the underwater turbine of the present
invention
comprises a control unit (not shown) configured to continuously steer the
frame into
undisturbed flow by creating differential thrust, e.g. through variation of
turbine
speeds and/or use of thrusters and/or controllable lift surfaces. The tip-
speed ratio of
the turbine is preferably adjusted electronically by control of the electrical
loading on
the generators and hence of their power output. However, it is also feasible
to adjust
the pitch of the turbine blades or implement mechanical brakes.
It is preferred to vary the tip-speed ratio TSR within certain limits so that
power loss is
minimised. In particular, the tip-speed ratio TSR should not be changed so as
to cause
the power output to drop below Cpmin as this will result in unacceptable
losses. In one
embodiment of the present invention, this threshold is set to at least 90% of
the peak
value. i.e. CPmm = 0.9 x Cumax In other words, the tip-speed ratio of the
exemplary
turbine will only ever be varied between tip-speed ratios TSR2 and TSR3, such
that
the power output coefficient Cp never drops below Cpinin, which is at least
90% of
C Pmax =
Figure 4 further shows that adjusting the tip-speed ratio TSR between values
TSR2
and TSR3 will cause a proportionally bigger change in the coefficient of
thrust than
the change in the power output. For example, it may be that changing the tip-
speed
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ratio such that the coefficient of power Cp reduces by about 10% will reduce
the
coefficient of thrust of that same turbine by about 20%. In other words, Cpim,
will be
around 90% of Cpmax, whereas CTmul will be around 80% of CT' As such, a small
decrease in the coefficient of power will cause a proportionally higher change
in
.. thrust, which is sufficient to steer the frame in the desired direction.
The above is also true for an increase of the tip-speed ratio between TSR1 and
TSR3.
That is, the increase of coefficient of thrust will again be proportionally
higher than
the decrease in the coefficient of power. However, as line 40 reaches
saturation at
high tip-speed ratios, the proportional increase in coefficient of thrust
between TSR1
and TSR3 is smaller than the proportional difference in thrust between TSR2
and
TSR1. In other words, CTmax may be around 115% of CT1 whereas CImm may be
about
80% of CT1 Accordingly, it may be found that obtaining a differential thrust
by
reducing the tip-speed ratio may be more effective than by increasing the tip-
speed
.. ratio.
