Note: Descriptions are shown in the official language in which they were submitted.
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WIND TURBINE AND METHOD FOR ICE REMOVAL IN WIND TURBINES
Description
OBJECT OF THE INVENTION
The present invention belongs to the field of energy generation by means
of wind turbines. In particular, the present invention relates to a wind
turbine
having the capacity of removing ice deposited on its blades and a removing ice
deposited in the wind turbine blades method.
BACKGROUND OF THE INVENTION
The deposition of ice or other type of unwanted material on the wind
turbine blades causes inadequate operation in terms of production and loads.
This is due to the variation in the aerodynamic and mass characteristics of
the
blades, partly due to the modification of the geometry of the aerodynamic
profiles. This variation in aerodynamic profiles can even cause aerodynamic
loss in certain parts of the blade. Said variation can result in an increase
in wind
turbine loads and vibrations.
The prior art proposes different solutions to try to solve the previously
described problem.
Therefore, there are systems for removing and/or preventing ice
formation based on the heating of at least part of the outer surface of the
blades. In the first case, the system is brought into operation upon detecting
ice
formation with the aim of removing it as quickly as possible; said systems are
known as de-icing systems. In the second case, the heating system is brought
into operation prior to ice formation when conditions given to ice formation
are
detected. Energy consumption in both types of systems can be considerable
and it is important to reduce required heating time to a maximum.
However, other active systems are based on ice removal in an
exclusively mechanical manner. Thus, there are systems that pursue the
removal of ice by deforming the outer blade surface, such as that disclosed in
GB2481416. This document proposes adding elements that deform at least the
outer blade surface, such as an element that produces a vibration in the
interior
thereof. The main drawback of said system is that it requires the inclusion of
technically complex elements and an adaptation of the structural blade design
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(or at least verification of its adequacy) for integration thereof.
Others only use systems usually present in wind turbines, such as the
blade pitch or the rotor speed control system. Thus, in the method disclosed
in
US829257962, vibrations are induced in the blades by controlling wind turbine
torque. However, said control method for ice removal is only effective with
low-
density ice.
DESCRIPTION OF THE INVENTION
The present invention discloses a wind turbine that solves the problems
of the prior art allowing the removal of ice from the blades with less energy
consumption than the devices of the prior art. Also, it does not require
substantial modifications in the structural design of the blades.
Specifically, the present invention discloses a wind turbine of the type
comprising:
- a rotor having at least two blades;
- a control system;
- means for detecting the presence of ice on the blades;
- a heating system having at least one heating element configured to
carry out a heating stage of at least one of the blades.
Preferably, the heating system is connected to the control system and is
configured to carry out a heating stage of at least one of the blades by
activating the power supply of at least one heating element disposed in each
of
the blades. To this purpose, the control system comprises at least one ice
removal routine that comprises the following stages:
- a heating stage of at least one of the blades;
- an induction movement stage on at least one of the blades, also
called mechanical ice removal stage.
Preferably, the heating stage, wherein at least one of the blades is
heated, is executed first. The mechanical ice removal stage on at least said
blade is carried out when the heating stage has been carried out at least
partially.
In addition, the mechanical ice removal stage can also be carried out, at
least partially, simultaneously with the heating stage (having heated the
blade
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for a certain time, after which the mechanical ice removal stage is carried
out
while continuing to heat the blade).
Executing firstly the heating stage for a sufficient time facilitates faster
ice
removal because the heating stage reduces the adherence of the ice to the
blade (although it does not eliminate it fully) since it contributes to the
thawing of
part of the ice surface. After the heating stage has been carried out for at
least a
certain time, the mechanical ice removal stage is carried out, which
accelerates
the removal of the ice that could remain adhered to the outer blade surface
after
the heating stage.
In one embodiment, the ice removal routine includes a heating stage
having a predetermined duration, after which the mechanical ice removal stage
is carried out.
Likewise, the routine can comprise sequences wherein the blade heating
stages are alternated with mechanical ice removal stages on said blades,
wherein the duration of each of said stages can be predetermined.
The mechanical ice removal stage requires the execution of a movement
(or sequence of movements) in at least one blade. Thus, the mechanical ice
removal stage comprises at least one of the following substages:
- execution of a blade movement by adjusting blade pitch angle,
- execution of a blade movement by adjusting rotor rotation speed.
To this purpose, the wind turbine control system sends the
corresponding rotor rotation or blade pitch adjustment orders.
In one embodiment, the blade pitch angle is modified by means of
controlled acceleration and deceleration.
In one embodiment, the mechanical ice removal stage is carried out with
the wind turbine stopped and the adjustment in rotor rotation speed comprises
performing a wind turbine rotor starting sequence such that the rotor starts
rotating.
The heating stage of at least one of the blades comprises the activation
of the heating system. In a particular embodiment described below the heating
system is aerothermal. In other embodiments, the heating system can be based
on thermo-resistant fabrics embedded in the blade material and near the outer
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blade surface or disposed on the outer surface thereof.
In both cases, it energetically compensates to include the mechanical ice
removal stage in the ice removal routine. However, when used in aerothermal
systems it is more advantageous for the ice removal routine to include a
mechanical ice removal stage, as said heating systems are less efficient than
systems based on thermo-resistant materials (due to the fact that the heat
must
flow outwards through the blade shells, which are usually made of a compound
material and which in certain zones are very thick). The invention therefore
makes it possible to reduce the time required to remove the ice with respect
to
the use of a single heating stage.
Thus, the wind turbine preferably comprises an aerothermal heating
system that comprises, in the zone next to the blade root, a fan for impelling
air
through a duct that comprises the heating element, comprising, in one
embodiment, electrical resistors that transfer heat to the air in the interior
of the
blade. Said air is conducted through the interior of the blade towards the
blade
zone where the ice must be removed, typically located between half the blade
length and the tip of the blade in the leading edge zone.
In one embodiment, the interior of the wind turbine blade comprises a
first chamber formed from a part of the blade shell that includes the blade
leading edge and at least one spar thereof. In said first chamber, a duct is
disposed for conducting the hot air that extends from the root to a point
disposed at a distance from the root between 1/3 and 2/3 of the length of the
blade wherein the air is released inside the first chamber and will flow
towards
the tip zone. When the air reaches the tip it returns towards the tip zone
through
a second chamber disposed between the spar and the trailing edge of the blade
or between two spars.
In one embodiment of the invention, a temperature sensor is disposed at
the exit of the duct that comprises the electrical resistors for monitoring
the air
temperature at the exit of the aerothermal system. According to this
embodiment, the control system controls the feeding of the resistors in
accordance with the magnitude of the difference between the temperature
measurement at the exit of the aerothermal system and a reference
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temperature.
Said reference temperature, in one embodiment, is calculated in
accordance with the maximum temperature that can be supported by the blade
material, thereby preventing any type of damage.
5 In one
embodiment, the control of the feeding of the resistors is of the
on-off type. In one embodiment, the control system has implemented a
hysteretic control loop for controlling the aerothermal system output air
temperature such as to ensure that its temperature does not exceed the
reference temperature by more than a predetermined margin. Once said
temperature is exceeded, at least part of the resistors of the heating system
are
disconnected so that the air does not exceed said temperature. Furthermore,
when the aerothermal output temperature falls below the reference temperature
within a certain margin (which may be different to the foregoing), at least
part of
the resistors are reconnected to the power supply. Depending on the switching
system of the resistors, the control can be more or less continuous and
implement a P1-type control loop or similar.
The reference temperature may vary slightly depending on the weather
conditions. Thus, under certain conditions, there is margin for increasing the
reference air temperature while always maintaining the maximum temperature
that can be supported by the blade material, or reduce it so as not to exceed
said maximum temperature in order to increase the effectiveness of the heating
system.
The wind turbine comprises means for detecting the presence of ice on
the blades. Said means may comprise, inter alia:
- a comparing algorithm for comparing between a signal indicative of
the power generated and a signal indicative of the power expected at
the wind speed measured (or incoherence between the wind speed
and power signals measured);
- equipment for estimating the natural frequency to each blade based
on accelerometers and comparison measurements with respect to the
inherent frequency without ice;
- ultrasound or capacitive ice detection sensors disposed in the interior
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of the blade;
- an algorithm for estimating the presence of ice and thickness thereof
based on the measurement of temperatures in the interior of the
blade.
In order to implement said algorithm, the blade is equipped with
temperature sensors in at least one zone where the ice must be removed. In
one embodiment, the temperature sensors are disposed in a blade control
station, disposed at a point situated at a distance from the blade root
between
60% and 90% of the blade length. This is where the greatest quantity of ice is
deposited and whereon action must be done. Preferably, at a point placed
between 75% and 90% of the blade length.
Preferably, at said control station at least one temperature sensor is
disposed on the inner wall of the blade in a zone of the leading edge, i.e.
disposed in contact with the wall of the leading edge. This sensor makes it
possible to determine a temperature value T1 in the interior of the blade on
the
leading edge. Additionally two more sensors are disposed in said control
station: an internal air temperature sensor, based on which a temperature
value
in the interior of the blade Tia is obtained, and a temperature sensor of the
blade
wall, in a zone where ice is not usually deposited, wherewith a temperature
value of the blade wall in an ice-free zone T2 is obtained. This sensor
arrangement makes it possible to perform the corresponding calculations to
determine the presence of ice and, where applicable, the thickness of the ice
layer.
In order to calculate the thickness of the ice layer and the temperature on
the blade surface, the three previously described sensors (which provide the
values T1, T2 and Tia), which are disposed in the control station in the
interior of
the blade, are used.
An algorithm for estimating the presence of ice on the blades and
quantifying the thickness thereof, and which is included in the wind turbine
control
routine, is used. Said algorithm uses a mathematical programming based on the
electrical similarity of the thermal problem of heat transmission that uses at
least
the following parameters:
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- Conduction resistance (Rõnd) through the blade walls. At least the
blade control station is characterised to determine the thermal
resistance values Rcond in each zone of said station, as there can be
different values of Rcond in each zone, as both the materials used (and
therefore their thermal conductivity) and thickness vary from one zone
to another. In one embodiment, the thermal resistance values Rcond are
determined in points 1 and 2, RcondK1 and Rc0ndK2, where sensors for
measuring inner blade surface temperature are disposed.
- Internal convection resistance (Rconvi). This value is considered
known
(characterised on the basis of a finite element calculation model, such
as for example a CFD (computational fluid dynamics) model having
identical value in points 1 and 2, where the inner blade surface
temperatures T1 and T2 are measured).
The following variables are calculated based on said parameters and the
T1, T2 and Tia values:
- Calculation of the heat flows through the outer blade surface in points 1
and 2 (variables q1 and q2), which are calculated based on the gradient
between the air temperature in the interior of the blade and the inner
blade surface temperature measured in said points 1 and 2 of the blade
control station (Ti-Tia and T2-Tia respectively).
- Calculation of external convection resistance (Rconve). This value is
calculated based on the thermal gradient between the temperature of
the air in the interior of the blade (T,a) and the ambient temperature
(Tea), and on the value calculated previously for q2. It is assumed to be
the same for both points of the blade profile in points 1 and 2 of the
blade control station.
- Calculation of the thermal resistance of the ice on the leading edge
based on the thermal gradient between the temperature of the air inside
the blade and the ambient temperature, and on values calculated for ql
and Rconve=
- Calculation of the temperature on the outer blade surface on the
leading edge of the blade based on the previous values.
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The calculation of the variables q1 and q2 is performed by means of the
following mathematical formulas based on the values of the foregoing
predetermined parameters (RcondKl, Rc0ndK2 y Rconvi) and on the temperature
measurements T1, T2 and -La:
Taz ¨ 7-1 r¨ ae
ql ______________________
= Reonc: Rconc: + Rconci.ba + heo 4- PCO.il
T42:.¨ T2 rj-
J
a? = _________________________ = _____________________
Rconcz RCOPZ RCOl?diC2 RCO.il e
Once the value of the heat flow towards the exterior has been calculated in
point 2 (q2), the value of the term of thermal external convection resistance,
Rconve
is calculated. Lastly, based on the previously calculated value of q1 and
using the
value of the term of thermal external convection resistance, Rconve, the
thermal
resistance value provided by the ice, Rice, in point 1 of the blade control
station is
calculated.
Afterwards, the thickness of the ice layer is calculated considering the
additional thermal resistance in the iced surface as a consequence of the ice
and
considering the conductivity of the ice:
e = Rice * Kice
In order to obtain a signal indicative of the presence of ice, a measurement
indicative of the average power consumed can alternatively be used. The value
of
the average power consumed can be obtained in two ways:
- Indirectly: By calculating the difference in temperature between
the air
at the inlet/outlet of the aero heater (after having passed through the
interior of the blade). As ice increases thermal resistance, the
transmission of heat to the exterior for the same thermal gradient
(between the internal air temperature and external air temperature) and
external wind speed conditions is reduced. In this manner the average
power consumed to maintain a reference air temperature is reduced.
- Directly: By measuring the average power consumed.
Furthermore, the present invention also discloses a wind turbine removing
ice method, the wind turbine of the type comprising:
- a rotor having at least two blades;
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- a wind turbine control system; and
- means for detecting the presence of ice on the blades;
- a heating system having at least one heater element connected to the
control system and configured to carry out a heating stage of at least
one of the blades, said method comprising the following stage:
- detection of the presence of ice on at least one of the blades;
and being characterised in that, upon detecting the presence of ice, it
comprises activating at least one ice removal routine comprising, in turn, the
following stages:
- a heating stage of at least one of the blades;
- a mechanical ice removal stage on at least said blade.
The heating stage of at least one of the blades comprises the activation of
at least one heating element.
The mechanical ice removal stage is carried out when the heating stage
has been carried out at least partially. In particular, a minimum duration of
the
heating stage prior to the activation of the mechanical ice removal stage is
established on at least one of the blades.
Preferably, the minimum duration of the heating stage is a predetermined
time. However, in particular embodiments of the present invention, said
minimum
duration of the heating stage can be calculated by the control system in
accordance with the amount of ice detected and/or the weather conditions.
In addition, the present invention envisages that, in a preferred
embodiment, the heating stages and mechanical ice removal stages of at least
one of the blades are carried out simultaneously in at least part of their
duration.
Therefore, once the heating element has been left turned on for a certain
time, the
mechanical ice removal stage is started in at least one of the blades such
that, at
least for a certain period of time, both stages occur simultaneously. However,
in
other embodiments of the present invention, the mechanical ice removal stage
on
at least one of the blades is carried out upon completing the heating stage.
Since, preferably, this method is iterative until the ice is removed from the
blade, after carrying out the mechanical ice removal stage on at least one of
the
blades, the method can be executed again until determining the non-presence of
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ice during the stage wherein the presence of ice is detected in at least one
of the
blades or the substantial reduction of ice on the blade or wind conditions
which
allow wind turbine operation to be resumed to generate power despite the
presence of ice.
5 Likewise,
the routine may comprise sequences wherein blade heating
stages are alternated with movement induction stages in said blades, wherein
the
duration of each stage is predetermined.
The mechanical ice removal stage comprises the execution of a movement
(or sequence of movements) in at least one blade and is executed by means of
at
10 least one of the following two types of orders sent by the control
system:
- wind turbine rotor rotation orders, which can in turn comprise braking
and rotor acceleration orders, or
- blade pitch angle adjustment orders through the actuation of the blade
pitch adjustment system.
In this regard, the blade pitch angle adjustment movement may comprise a
sequence of movements of the blade pitch system, between blade positions
preferably in a range between 90 and 30 .
The initial minimum duration of the heating cycle after which the
mechanical ice removal stage is executed in at least one blade is calculated
in
accordance with the ambient temperature. Alternatively, it may be carried out
for a
predetermined minimum time.
In addition, wind speed and/or ice layer thickness are also taken into
account to determine the minimum initial duration of the heating cycle. Said
calculation can be performed, for example, by means of a table wherein the
duration of the cycle is specified in accordance with environmental parameters
(temperature, wind speed and/or ice layer thickness), wherein the specified
duration has been obtained from field experiments or through simulations.
Preferably, the duration of the heating cycle is progressively reduced in the
subsequent heating cycles.
Preferably, the heating cycle keep running during the blade movement
execution stage.
The mechanical ice removal stage is performed for a predetermined time
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after which it is verified that the ice has been removed or has been
substantially
reduced and, if it has not yet been removed, one of the following actions is
carried
out:
- a new heating cycle is initiated, or
- the heating cycle continues but without executing the blade movement.
If, after the blade movement, the presence of ice on the blades is still
detected, various heating cycles are alternated with various movement cycles.
The detection of the presence of ice on the blades can be carried out by
initiating
a wind turbine starting sequence verifying, for example, rotor acceleration
and
comparing it with acceleration under ice-free conditions.
In one embodiment, executing a blade movement implies modifying the
blade angle by means of controlled acceleration and deceleration by actuating
the
blade pitch system in accordance with the blade pitch angle adjustment orders
sent by the control system. The acceleration and deceleration are preferably
high
to induce vibrations in the blade, thereby contributing to the breakup and/or
detachment of the ice layer.
In one embodiment, executing a rotor rotation movement comprises
performing a wind turbine rotor starting sequence, such that the rotor starts
rotating after the heating stage in accordance with the wind turbine rotor
rotation
orders sent by the control system. This implies that the heating stage was
carried
out with the rotor stopped, an unnecessary aspect according to the invention.
The blade pitch angle movements are performed with the machine
stopped, such that the pieces of ice which are detached from the blades fall
in the
vicinity of the wind turbine and are not projected far from it due to the
rotation of
the rotor. To this end, the blade pitch angle movements are preferably
performed
between blade pitch angle positions which allow the rotor to remain stopped
(for
example, between pitch positions of 90 and 30 ) and leaving at least one of
the
blades feathered (90 ).
Likewise, the method can additionally comprise a stage wherein the blade
is positioned in a predefined azimuth angle and which is carried out prior to
the
stage of executing a blade movement modifying blade pitch by means of
controlled acceleration and deceleration. In one embodiment, in said stage the
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blade is disposed in a position oriented substantially downwards. In another
embodiment, in said stage a blade is disposed in a position oriented
substantially
towards one side.
In one example of embodiment, the controlled acceleration and
deceleration is repeated continuously.
The method additionally comprises a stage wherein the presence of ice is
detected on said blade before generating the controlled acceleration and
deceleration. In one embodiment of the method, said controlled acceleration
and
deceleration is carried out by actuating one or more actuators of the wind
turbine.
The wind turbine of the present invention may comprise at least one sensor
for detecting the presence of ice in at least one of the blades, may comprise
an
azimuth angle detector for detecting the azimuth angle of at least one blade
that is
going to be subjected to the heating stage and to the mechanical ice removal
stage and may comprise a rotor actuator for controlling the azimuth position
of the
blade during said stages.
In one embodiment, the activation of the blade heating routine is carried
out with the wind turbine stopped.
In addition, in order to improve the safety of the procedure, the method of
the present invention comprises a stage wherein at least one of said blades is
disposed in an azimuth angle predetermined prior to carrying out the stage d).
Said predetermined azimuth angle may be such that the blade substantially
points downwards (180 ) or towards one side (270 ) before carrying out stage
d).
Another object of the invention is a wind turbine control method comprising
a blade heating system. Said method comprises a stage wherein the
effectiveness
of the heating system is estimated (for ice removal, i.e. de-icing or
preventing the
appearance of ice, i.e. anti-icing mode, as applicable) in accordance with one
of
the environmental conditions measured or estimated prior to activating the
blade
heating system.
Therefore, the stage wherein the effectiveness of the heating system is
estimated according to the method includes carrying out the following
substages:
- a stage wherein certain wind speed and temperature conditions
envisaged for the heating cycle are determined;
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- a stage wherein the wind speed and temperature conditions envisaged
for the heating cycle are compared with respect to wind speed and
temperature conditions whereunder the heating system is effective.
If the stage wherein the effectiveness of the heating system is estimated
results that the heating system is effective for the purposes pursued, the
method
comprises carrying out a heating system activation stage.
Through the heating system effectiveness estimation stage, the possibility
of removing or preventing ice under turbine operating conditions in order to
decide
whether or not the blade heating routine is activated. The activation of the
blade
heating system will only take place if the heating can be effective for the
purpose
pursued (preventing the formation of ice or removing it after adhering to the
blade). This makes it possible to avoid ineffective power consumption by the
heating system components.
The environmental conditions whereunder the system is effective (for both
modes, i.e. as anti-icing or as de-icing) depend on whether or not the wind
turbine
is operating. Thus, for example, if the environmental conditions whereunder
the
system is effective are represented in a table or graph or in any other way,
the
control algorithm includes a table or graph in its program for de-icing or
anti-icing
operation. The fact that the wind turbine is operating implies a rotor
rotation speed
which makes the apparent wind speed in the blades seem much greater than the
free wind speed, reducing the effectiveness of the heating system.
Said method also comprises a stage wherein the presence of ice or
conditions given to the formation of ice on the blades such that, if said
conditions
are detected, the heating system effectiveness estimation stage is carried
out.
Weather conditions given to the formation of ice on the blades, i.e. that can
influence the formation of ice on the blades, include, for example, humidity
or the
percentage of water in the air, wind speed, temperature, etc. Preferably, the
weather conditions considered in this case are humidity and ambient
temperature.
Thus, for the heating system to operate in anti-icing mode, the heating
system effectiveness stage according to the method includes carrying out the
following stages:
- a
stage wherein the wind speed and temperature conditions envisaged
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for the heating cycle are determined;
- a stage wherein the envisaged wind speed and temperature conditions
are compared with wind speed and temperature conditions whereunder
ice formation can be prevented;
- a heating system activation stage if the comparison between wind
speed and temperature conditions envisaged for the heating system
with respect to the wind speed and temperature conditions whereunder
the formation of ice can be prevented conclude that the heating system
is effective.
In one embodiment, the conditions whereunder the formation of ice can be
prevented are predetermined, having evaluated under simulation the temperature
and/or wind speed conditions whereunder the heating system can prevent the
formation of ice depending on whether or not the wind turbine is in operation.
Thus, the method comprises a wind turbine status verification stage and a
stage wherein wind speed and temperature conditions whereunder the formation
of ice can be prevented in accordance with wind turbine status are determined.
In the event that the presence of ice has not yet been detected but the
conditions are given to its formation and the effectiveness of the heating
system in
preventing the formation of ice has been positively valued, it will be
activated in
anti-icing mode.
However, if after carrying out a stage wherein the effectiveness of the
heating system as an anti-icing system is estimated, it is demonstrated that
for the
turbine operation status, given the weather conditions envisaged for the
heating
cycle (conditions estimated for the next instants in which the heating system
will
be potentially activated) the heating system is not effective (for example,
with a
lower envisaged ambient temperature for the envisaged wind conditions than
that
which would allow an outer blade surface temperature higher than a threshold,
for
example, higher than -2 C), it is decided that the heating system will not
activated
in anti-icing mode. In this example, the wind turbine would continue its
normal
operation until the formation of ice is detected, in which case the
effectiveness of
the heating system would be newly evaluated, in this case as an anti-icing
system.
In this case, if the heating system cannot be effective as an anti-icing
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system and if the conditions given to the formation of ice persist, ice will
accumulate on the blade surface. Once this occurs and the wind turbine control
system detects ice, a stage wherein the effectiveness of the heating system
for
operation in de-icing mode is estimated will be carried out, i.e. it will
evaluate the
5 activation of the heating system for operation in de-icing mode.
Thus, the state wherein the effectiveness of the heating system is
estimated according to the method includes, for operation in de-icing mode,
carrying out the following stages:
- a stage wherein wind speed and temperature conditions whereunder
10 ice can be removed are determined;
- a stage wherein the wind speed and temperature conditions
envisaged
for the heating cycle are determined;
- a stage wherein the wind speed and temperature conditions envisaged
for the heating cycle are compared with the wind speed and
15 temperature conditions whereunder ice can be eliminated;
- a stage wherein the heating system is activated if the comparison
between the wind speed and temperature conditions with respect to
the wind speed and temperature conditions whereunder it can be
removed, indicate that the heating system would be effective.
The stage wherein the wind speed and temperature conditions envisaged
for the heating cycle (i.e. for the next instants in which the heating cycle
will be
potentially executed) are determined, either as an anti-icing or de-icing
system,
comprises one of the following methods for obtaining said conditions:
- measurement of current wind speed and ambient temperature and
assumption that the wind speed and temperature conditions in the next
instants will be similar to the current conditions;
- measurement of current wind speed and temperature, and estimation of
the wind speed and temperature values envisaged for the next instants
based on wind speed and temperature trends observed in the previous
instants (past hours, days, etc.);
- receipt of predictions on the wind speed and temperature
envisaged for
the next instants, for example, from a remote control centre of the wind
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farm or from a weather station.
Thus, the wind speed and temperature conditions of the next instants may
be estimates based on the measurements of said variables made in previous
instants such as, for example, based on the calculation of the average for the
previous 10 minutes, previous 20 minutes, previous 60 minutes, etc. This
estimation can be corrected applying trends of previous days in the same time
zone or using wind predictions received remotely.
The wind and temperature conditions whereunder the system will be
effective depend, to a certain extent, on the thickness of the ice layer.
Thus, in one
example of embodiment, the method comprises previously carrying out an ice
layer quantification stage to increase the effectiveness of the method for
decreasing consumption.
In this case, the method includes:
- a stage wherein the ice layer is quantified,
- a stage wherein the wind speed and temperature conditions
whereunder the ice can be removed in accordance with its thickness
are determined.
In this manner, in accordance with the result of the comparison stage of the
wind speed and temperature conditions envisaged for the heating cycle with
respect to the conditions determined in accordance with the thickness of wind
speed and temperature in which ice can be removed, the heating system will or
will not be activated in the heating system activation stage.
As of a high ice layer thickness threshold (or power losses) and if the
system is not capable of removing the ice with the wind turbine in operation,
the
wind turbine rotor is stopped to activate the heating system. This is so
because if
the rotor is rotating, the relative speed of the air that surrounds the blade
is much
higher and this reduces heating effectiveness. Above this threshold, if the
system
is effective, the system is activated with the wind turbine in operation, i.e.
with the
rotor rotating.
In one embodiment, the wind turbine control system has different curves
(or tables) implemented in its algorithm, wherein wind speed conditions are
defined with respect to the temperature in which the heating system is
effective.
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Said curves are different depending on whether the wind turbine is in
operation or
stopped and depending on ice layer thickness, using the wind speed and
temperature conditions envisaged for the heating cycle with respect to the
wind
speed and temperature conditions whereunder the system is effective, in order
to
determine whether or not the heating system will be effective.
DESCRIPTION OF THE DRAWINGS
In order to complement the description being made and with the object of
helping to better understand the characteristics of the invention, in
accordance
with a preferred embodiment thereof, said description is accompanied, as an
integral part thereof, by a set of drawings where, in an illustrative and non-
limiting
manner, the following has been represented:
Figure 1 shows a perspective view of a wind turbine of the type of the
present invention;
Figure 2 shows a graph wherein the ice mass generated on a blade in
different radial positions is determined;
Figure 3 shows a cross-sectional view of a wind turbine blade with the
temperature sensors of the control station and the values used to calculate
the
thickness of the ice on the blade surface;
Figure 4 shows a graph representing a table of results of the suitability of
activating or not activating the heating system with the wind turbine in
operation;
and
Figure 5 shows a graph representing a table of results of the suitability of
activating or not activating the heating system with the wind turbine stopped.
PREFERRED EMBODIMENT OF THE INVENTION
Following is a description, with the help of figures 1 to 5, of embodiments
of the present invention.
As described previously, one of the problems of wind turbines is that,
under certain weather conditions, they tend to generate ice masses on the
blades (3). In order to remove the ice already deposited on the blades (3),
the
present invention envisages a wind turbine with capacity to remove ice from
the
blades and a method for removing ice from wind turbines.
Figure 1 shows an example of a wind turbine of the type used in the
CA 02948017 2016-11-08
18
present invention. Specifically, this wind turbine comprises a rotor having at
least two blades (3), a control system, means for detecting the presence of
ice
on the blades and a heating system having at least one heating element (31)
configured to carry out a heating stage wherein at least one of the blades is
heated.
The control system comprises at least one ice removal routine that
comprises the following stages:
- a heating stage of at least one of the blades (3);
- a inducing movement stage on at least one of the blades, hereinafter
referred to as mechanical ice removal stage.
The rotor comprises at least one hub whereto the blades (3) are joined
by means of bearings which allow the adjustment of the blade pitch angle by
means of actuators.
Preferably, the heating stage of at least one of the blades (3) is partially
carried out prior to the mechanical ice removal stage, and at least for a
certain
time. Once said time has elapsed, the mechanical ice removal stage is carried
out, contributing to the removal of the traces of ice that give rise to
vibrations in
the blade (3). By inducing a movement in the blade (3), the ice is fractured
perpendicularly to the blade profile surface and ice becomes detached from the
critical zone wherein at least part of the ice mass nearest the heated surface
has melted or softened.
As shown in figure 2, there are certain zones on the blades (3) that are
more susceptible to the generation and/or accumulation of ice. It is
convenient
to conduct the heat generated by the heating element (31) of the heating
system to these zones. The heating element may be, for example, heating
resistors or any heating means known in the state of the art.
Figure 1 shows a critical zone wherein the convenience of disposing a
heating element (31) has been determined. This critical zone is the distal
zone
of the blade (3), as it has been concluded that more ice is deposited on zones
having a higher relative wind speed.
However, thermal resistance throughout the blade (3) surface varies
depending mainly on the thickness of the shells and of the materials used in
the
CA 02948017 2016-11-08
19
manufacture thereof, due to which there are zones wherein the ice layer in
contact therewith melts and zones of the same profile wherein it continues
frozen. The presence of said still frozen zones prevents the ice from becoming
detached, despite the existence of other zones wherein the layer in contact
with
the blade has melted.
The mechanical ice removal stage can be carried out by two types of
orders sent by the control system:
- wind turbine rotor rotation adjustment orders sent by the control
system, which can be rotor acceleration and braking orders,
- blade pitch adjustment orders sent by the control system.
The wind turbine comprises means for detecting the presence of ice on
its blades, which are selected from among:
- an algorithm of comparison between a signal indicative of the power
generated and a signal indicative of the power expected at the wind
speed measured (or of incoherence between the wind speed and
power signals measured),
- equipments for estimating the natural frequency to each blade (3)
based on accelerometers measurements and comparisons with
respect to inherent frequency without ice,
- ultrasound or capacitive ice detection sensors disposed in the interior
of the blade (3) or, for example, an infrared sensor for detecting the
presence of a mass on the distal part of the blade (3),
- an algorithm for estimating the presence of ice and thickness thereof,
- means of comparison between a predetermined power consumed
and an average power consumed by the heating system.
In another embodiment, the detection of the presence of ice by means of
a wind turbine starting sequence wherein the acceleration achieved is
compared with the current status of the blades (3) with a reference
acceleration
taken with the blades in normal conditions (due to the effect of the presence
of
ice on the rotor blades, it has less aerodynamic efficiency and acceleration
will
be lower).
In one embodiment of the invention, the wind turbine comprises the
CA 02948017 2016-11-08
following sensors in the control station:
- an inner blade wall temperature sensor in a zone of the leading edge
to determine a temperature value (Ti) in the interior of the blade on the
leading edge;
5 - an internal air temperature sensor to determine a temperature value
(Tia) in the interior of the blade; and
- a blade wall temperature sensor in a zone where ice is not usually
deposited in order to determine a temperature value (T2) on the blade
wall in an ice-free zone.
10 In the embodiment wherein the presence of ice is detected by means of an
algorithm for estimating the presence of ice on the blades, said algorithm
uses a
mathematical program based on the electrical similarity of the thermal problem
of
heat transmission that uses at least the following parameters:
- Conduction resistance (Rcond) through the blade walls. It is
calculated in
15 the points in which the previously described sensors are disposed
(points 1 and 2 shown in figure 3) to obtain (RcondK1) and (Rc0ndK2).
- Internal convection resistance (Rconvi). It is considered known
(characterised based on a finite element calculation model, such as for
example a CFD (computational fluid dynamics) model (considered to
20 have identical value in points 1 and 2).
The following variables are calculated based on said parameters ad non
the measured values of (T1), (T2) and (Taia):
- Calculation of the heat flows throughout the outer blade surface in
points 1 and 2 (variables q1 and q2) based on the gradient between the
air temperature in the interior of the blade and the temperature of the
inner blade surface measured in said points 1 and 2 of the blade control
station ((Ti-Tia) and (T2-Tia) respectively).
- Calculation of external convection resistance (Rconve). This value is
calculated based on the thermal gradient between the air temperature
in the interior of the blade (Tia) and the ambient temperature (Tea), and
the previously calculated value for (q2). It is assumed to be identical for
both points of the blade profile in points 1 and 2 of the blade control
CA 02948017 2016-11-08
21
station.
- Calculation of the thermal resistance of the ice on the leading edge
based on the thermal gradient between the air temperature in the
interior of the blade and the ambient temperature, and on the values
calculated for (q1) and (Rconve),
- Calculation of the temperature on the outer surface of the blade on the
leading edge based on the foregoing values.
The calculation of (Rice) is performed using the following formulas to
calculate the two parameters dependent on the temperature conditions on the
blade:
Tea Ti To: ¨ Tae
q= ____________________________
Fcon = Reonr: Eco.qc1K1+ Rhzeo + Rc3p.:-e
Taz ¨ T2 Tca ¨
Tc
¨
Reon re
Figure 3 represents a cross-section of a wind turbine blade to show the
arrangement of the different sensors on said blade. It also shows the
different
parameters required to calculate the thickness of the ice layer. More
specifically, ice layer thickness is calculated using the following formula:
e = Rice * Kice
wherein (Kice) is the ice conductivity constant and (R,c) is the thermal
resistance of a blade wall given to ice formation (generally the leading edge
of the
blade).
Likewise, an object of the present invention is a method for removing ice
from a wind turbine of the type comprising:
- a rotor having at least two blades;
- a wind turbine controller system; and
- means for detecting the presence of ice on the blades;
- a heating system having at least one heating element (31) connected to
the control system and configured to carry out a heating stage of at
least one of the blades,
said method comprising the following stages:
- detection of the presence of ice on at least one of the blades;
CA 02948017 2016-11-08
22
and, upon detecting the presence of ice, it comprises activating at least one
ice
removal routine that comprises in turn the following stages:
- a heating stage of at least one of the blades; and
- a mechanical ice removal stage on at least said blade.
The heating stage comprises at least the activation of at least one heating
element. The mechanical ice removal stage on at least said blade is preferably
carried out after the heating stage.
Therefore, it has been determined that implementing an ice removal
method which, firstly, heats at least one zone of the blade (3) to reduce the
adherence of the ice mass to the blade (3) and, subsequently, carries out a
mechanical ice removal stage on said blade, is particularly advantageous since
it
consumes less energy than heating until the ice melts and, in turn, is more
effective than exclusively executing movements for mechanically removing the
ice.
The ice removal method is destined to be implemented in a wind turbine of
the type comprising a rotor having at least two blades, a control system,
means for
detecting the presence of ice on at least one of the blades (3) and a heating
system having a heating element (31) connected to the control system and
configured to carry out a heating stage of at least one of the blades.
The heating element (31) is a part of the heating system and can be any
heating element (31) of those known in the state of the art and is preferably
performed by means of an order given by the control system.
It is important that the heating of, at least, the critical zone of the blade
(3)
is maintained for a sufficient time required to thaw a part of the ice mass.
The
longer the time in which the heating is maintained, the less the adherence of
the
ice to the blade (3).
As regards the established time, this time may be previously determined
and stored in the controller or, alternatively, taking into account the data
obtained
in stage a) and, depending on the amount of ice disposed on the blades (3),
the
time that the heater and/or the heat output to be applied must remain
activated
can be determined. In one embodiment, the heating stages of at least one of
the
blades and the mechanical ice removal stage on at least said blade are
performed
CA 02948017 2016-11-08
23
simultaneously, i.e. the heating system remains activated and a mechanical
movement of the blade is induced at the same time. In another embodiment, the
stage in which the movement of the blade is induced is carried out after the
stage
wherein the heating system is activated when said heating system has already
been deactivated.
The mechanical ice removal stage on the blades can be executed by
means of two types of orders sent by the control system:
- wind turbine rotor rotation orders, which can in turn comprise rotor
braking and acceleration orders, or
- blade pitch angle adjustment orders by means of the actuation of the
blade pitch adjustment system.
These orders can be selected from among:
- Performing a blade movement adjusting the blade pitch angle by
means of controlled acceleration and deceleration. Deceleration is high
to simulate a quick blow that induces vibrations in the blade, thereby
contributing to breaking up and/or detaching the ice.
- Performing a wind turbine rotor starting sequence, such that the rotor
starts rotating after the heating stage.
- Performing a blade movement adjusting the blade pitch angle, said
movement being, preferably, a reiterative movement of the pitch blade
between pitch angles of 90 and 30 .
The present invention also proposes a method for controlling a wind turbine
that comprises a rotor having at least two blades, a wind turbine control
system
and a heating system having at least one heating element connected to the
control system and configured to carry out a heating stage of at least one of
the
blades. The method comprises a stage wherein the effectiveness heating system
is estimated.
In one embodiment, the heating system continues active provided that the
presence of ice is detected on the corresponding blade and the blade surface
temperature in a zone with ice (Ti) is higher, for example, than -2 C,
preferably
higher than 0 C.
The presence of ice can be determined in the different manners described
CA 02948017 2016-11-08
24
previously. When the presence of ice is detected in at least one of the wind
turbine
blades, the heating system effectiveness estimation stage comprises a substage
for determining the thickness of the ice layer.
During the heating system effectiveness estimation stage, the activation of
the heating system when the thickness of the ice layer is greater than a
predetermined thickness is determined. Likewise, during the heating system
effectiveness estimation stage, the deactivation of the heating system when
the
thickness of the ice layer is less than a predetermined thickness is
determined.
In another embodiment, the activation of the heating system is determined
when the ambient temperature and wind speed incident on the blade coincides
with an ambient temperature and wind speed given to the formation of ice on
the
blade.
According to the method, the heating system effectiveness estimation
stage includes carrying out the following substages:
- a stage wherein the wind speed and temperature conditions envisaged
for the heating cycle are determined;
- a stage wherein the conditions envisaged for the heating system are
compared with respect to wind speed and temperature conditions
whereunder the heating system is effective.
If the heating system effectiveness estimation stage reveals that the
heating system is effective for the purposes pursued, a heating system
activation
stage is carried out.
The heating system effectiveness estimation stage evaluates the possibility
of removing or preventing ice in the wind turbine operating conditions to
decide if
the blade heating routine is activated or not. The activation of the heating
system
only occurs if the heating can be effective for preventing the formation of
ice or
removing it after its adhesion to the blade, in accordance with the purpose
pursued.
The weather conditions whereunder the system is effective (for both
modes, i.e. for de-icing and anti-icing) depend on whether or not the wind
turbine
is in operation, since if it is in operation the effectiveness of the heating
system is
reduced. For example, the weather conditions whereunder the system is
effective
CA 02948017 2016-11-08
are provided in table or graphic form or in any other form. The control
algorithm
includes a table or graph in its program for operation in de-icing or anti-
icing
mode.
The stage wherein the presence of ice or conditions given to the formation
5 of ice on
the blades is detected such that, if said conditions are detected, the
stage wherein the effectiveness of the heating system is carried out.
Figure 4 shows different curves representing ambient temperature values
in accordance with wind speed above which the heating system is effective when
the wind turbine is in operation. As mentioned earlier, the speed and
temperature
10 conditions
whereunder the system will be effective depend, to a certain extent, on
the thickness of the ice layer. Thus, each of these curves corresponds to the
wind
speed and temperature conditions above which the heating system is effective
for
different conditions of ice deposited on the profile. Wind speed is
represented on
the x-axis and temperature is represented on the y-axis.
15 In order
to operate the heating system in anti-icing mode, the stage
wherein the effectiveness of the heating system is estimated according to the
method includes carrying out the following stages:
- a stage wherein wind speed and temperature conditions whereunder
the formation of ice can be prevented are determined; and
20 - a stage
wherein the wind speed and temperature conditions envisaged
for the heating cycle are compared with wind speed and temperature
conditions whereunder the formation of ice can be prevented;
- a stage wherein the heating system is activated if the comparison
between the speed and temperature conditions with respect to the wind
25 speed and
temperature conditions whereunder the formation of ice can
be prevented results that the heating system is effective.
The first curve that can be observed in figure 4 (represented with a dotted
line) is used to determine whether or not the system is effective as an anti-
icing
system when the wind turbine is in operation and there is no ice deposited on
the
profile but there are conditions given to ice formation.
The heating system is activated in de-icing mode when it has been
detected that it would not be effective as an anti-icing system but the
conditions
CA 02948017 2016-11-08
26
given to ice formation continue and the presence of ice is finally detected.
In this
case, a stage wherein the effectiveness of the heating system for operation in
de-
icing mode (for ice removal) is estimated will be carried out.
Thus, the stage wherein the effectiveness of the heating system is
estimated according to the method includes, for operation in de-icing mode,
carrying out the following stages:
- a stage wherein the wind speed conditions and temperature in which
the ice can be removed are determined;
- a stage wherein the wind speed and temperature conditions envisaged
for the next instants are determined;
- a stage wherein the speed and temperature conditions are compared
with respect to the wind speed and temperature conditions whereunder
the ice can be removed;
- a stage wherein the heating system is activated if the comparison
between the speed and temperature conditions with respect to the wind
speed and temperature conditions whereunder it can be removed
results that the heating system is effective.
The remaining curves of figure 4 correspond to the effectiveness of the
heating system in de-icing mode when the wind turbine is in operation (with
the
rotor rotating). The different curves represent different ice conditions on
the blade
surface (the curves correspond to a minor amount, a moderate amount and a
severe amount of ice, respectively represented by a dashed line, dash-dot line
and continuous line). Once the amount of ice has been estimated, the most
representative curve is selected and the weather conditions envisaged for the
heating cycle with the conditions whereunder the system is effective are
compared.
Alternatively, a single representative average curve for all cases of ice can
be selected and this single curve can be used if ice is detected and there are
no
means to quantify the amount of ice.
Figure 5 shows the curves corresponding to the effectiveness of the
heating system when the wind turbine is paused (the rotor stopped). The curves
that correspond to conditions of non-presence of ice, minor presence of ice,
CA 02948017 2016-11-08
27
moderate presence of ice and severe presence of ice have been represented,
identified in the same manner as in the graph of figure 4.
The stage wherein the wind speed and temperature conditions envisaged
for the heating cycle are determined (i.e. for the next instants wherein the
heating
cycle will potentially be executed), either for anti-icing or de-icing,
comprises one
of the following methods for obtaining said conditions:
- measuring the current wind speed and temperature conditions and
assuming that the wind speed and temperature conditions in the next
instants will be similar to the current conditions;
- measuring current wind speed and temperature, and estimating the
wind speed and temperature values envisaged for the next instants
based on the wind speed and temperature trends observed in previous
instants (past hours, days, etc.);
- measuring the wind speed and temperature log of the previous instants
and estimating the wind speed and temperature values envisaged for
the next instants based thereupon; and
- receiving predictions on the wind speed and temperature envisaged for
the next instants, for example, from a remote control centre of the park
or from a weather station.
As explained earlier, the speed and temperature conditions whereunder
the system would be effective depend, to a certain extent, on the thickness of
the
ice layer. In this case, the wind turbine would comprise means for estimating
the
thickness or amount of ice deposited on the blades and the method includes:
- a stage wherein the thickness of the ice layer is quantified,
- a stage wherein the wind speed and temperature conditions
whereunder the ice can be removed in accordance with thickness.
The heating system will be activated if the comparison between the speed
and temperature conditions with respect to the wind speed and temperature
conditions results that the heating system is effective.
As of a high threshold thickness of the ice layer (or power losses) and if the
system is not capable of removing the ice with the wind turbine in operation,
the
wind turbine rotor is stopped to activate the heating system. Above this
threshold,
CA 02948017 2016-11-08
,
28
if the system is effective, the system is activated with the wind turbine in
operation,
i.e. with the rotor activated.
