Wind Turbine Rotor Blades

Field of the Invention

This invention relates to wind turbine rotor blades. Background to the Invention

For every rotor blade there is a relationship between the pitch angle of the blade and the efficiency of that blade. In a wind turbine blade, a difference in pitch angle of a few degrees may reduce the efficiency of the blade significantly, thereby reducing the lift, or torque, of the blade and reducing the power produced by the turbine. Typically, the relationship is known to the designer of the rotor blade and the rotor manufacturer and variations from that known relationship produce reduced levels of lift and subsequently produce a less efficient wind turbine. The "fine pitch" angle of a blade is the angle at which maximum lift, and therefore torque, is produced. This is normally the angle at which a turbine will be operated when the wind is below its rated power. For a 2.4MW turbine, the fine pitch angle may be around 20 degrees. At angles above or below the fine pitch angle, the efficiency of the blade is reduced, resulting in no lift at one extreme and stalling in the other extreme. An even torque from all blades of a rotor is desirable so that the turbine runs efficiently and thereby becomes cost-effective. Furthermore, uneven torque produces fatigue in the other parts of the wind turbine, reducing the life-time of the rotor and increasing the cost in terms of maintenance and repairs. Errors in the pitch angle of rotor blades on a wind turbine can cause uneven torque on the rotor. Where the blades are all intended to be identical, it is possible for weaknesses in the blade to occur or for the aerodynamics of the blade to vary slightly but significantly; for example, the thickness of the adhesive on each blade may vary, adjusting the lift coefficient of the blade.

Summary of the Invention

Accordingly, the present invention comprises a method of determining the fine pitch angle of one wind turbine blade of a rotor comprising a plurality of rotor blades, comprising the steps of:

Calculating the lift of the said one rotor blade at a predetermined angle over at least one rotation;

Changing the pitch angle of the one rotor blade;

Calculating the lift of the one rotor blade over at least one rotation at the new pitch angle; and

Repeating the steps until the angle at which maximum lift of the one rotor blade is determined.

Using this method of analysing the wind turbine, the actual fine pitch angle can be determined using information gathered from the wind turbine rotor while in use. It is therefore possible to detect and quantify error in pitch positioning of the individual blades of a rotor.

When running at below-rated power, it is desirable to get the most power from the wind conditions, which is obtained when the rotor blades have maximum lift. By ensuring that the torque provided by each blade of a rotor is maximised, the available energy captured for a given wind condition can be increased so as to obtain efficient generation of power. Therefore, by using the method disclosed herein, the optimum pitch angle to create an increased and/or maximum lift can be determined. There may be times when an increase in the lift is preferred, rather than reaching a maximum lift; for example, it may be necessary to balance the thrust generated by the rotor in preference to obtaining maximum torque. This is especially important if the turbine is running at, or close to, rated power because the thrust will be increased along with the increased torque. It may be more desirable to obtain the optimum balance of torque against thrust and, knowing the torque and thrust characteristics of the blades at different pitch angles from the method disclosed herein, the required balance can be calculated.

The lift characteristics may change over time, for example due to a collision with a foreign object or fatigue that weakens the blade, making it more flexible. The present method allows one to monitor the blade and re-calculate the lift characteristics at different pitches over time.

It may be desirable to use a previously calculated fine pitch angle as the theoretical fine pitch angle when reassessing the performance of the rotor blade.

In an advantageous embodiment, the steps of the method are repeated until the maximum lift of the one rotor blade is determined. By determining the pitch angle that provides maximum lift (the fine pitch angle), an indication of which angle from the fine pitch angle is required for the desired level of lift in the rotor blade can be determined.

It is preferable that the lift is determined using at least one strain sensor. Strain sensors can be used to monitor the strain on the rotor blade at the particular angle and the result used to calculate the input torque to the rotor produced by the individual blade.

Advantageously, the strain sensor is an optical strain sensor. Optical fibre strain sensors are not susceptible to fatigue or electromagnetic interference, thereby making reliable and long-term sensors.

In a preferred construction, the at least one strain sensor is in close proximity to the root of the turbine blade in the flapwise and/or edgewise plane. By positioning the strain sensor near the root of the blade, an overall indication of the bending moment is obtained. The reading may be taken in just one plane, either flapwise or edgewise, however, it may be desirable to measure the strain in both directions in order to produce the most accurate indication. It is unlikely to be necessary to resolve the lift using multiple sensors because the pitch angle will be known. Therefore, it may be preferable to have the strain sensor in one plane, advantageously, the edgewise plane.

Advantageously, when the pitch angle of the one rotor blade is changed, the other rotor blades are retained at their existing pitch angle. By retaining the other blades at the same angle (or sequence of angles over the course of a full rotation of the rotor) the effect of the changed pitch angle for the one rotor blade is more easily identified.

In one construction, it is advantageous that the values of lift are normalised. By normalising the values of the lift, the effect of the wind strength can be removed so that one does not need to factor that into the lift calculations.

Alternatively, the wind speed at which the angle is determined is noted in order that the angle is monitored over comparable wind speeds. By monitoring the wind speed at the time of the strain being measured, pitch angle variations at different wind speeds can be compared.

Preferably, the other rotor blades are set to their theoretical fine pitch angle. By keeping the other rotor blades at their theoretical fine pitch angle, the torque produced is held at a constant and known value during the process. Therefore, the determination of the torque produced by the rotor blade being tested is simplified because the input from the other rotor blades is constant.

It is advantageous that the lift is monitored over a plurality of rotations, and preferably, at least 100 rotations.

The invention further extends to a method of improving the performance of a wind turbine, comprising the steps of determining the fine pitch angle of each rotor blade using the method of determining the fine pitch angle of a rotor blade disclosed herein and adjusting the pitch of the rotor blades to ensure substantially maximum and/or uniform torque input, or thrust input, to the rotor for each rotor blade. The invention also extends to a method of creating an array of data using the method disclosed herein to obtain information on the lift of a wind turbine rotor blade at a particular wind speed and blade pitch angle, recording that information in a suitable format, and creating a reference table for the wind turbine blade being analysed showing the pitch angle providing maximum lift at a given wind speed.

By creating a data array, preferably in the form of a reference table, or 'look-up' table, using the method disclosed herein, the pitch angle that produces the maximum lift in give wind speeds can be determined, thereby allowing the wind turbine adapt to the wind conditions and produce maximum energy. Because of the characteristics in a rotor blade, it may be that when a blade is to 20°, the offset required to reach the fine pitch angle is 2°, but when the blade is set to 10°, the offset to reach the fine pitch angle is 1 °. By having a look-up table that is generated using the process as described, the wind turbine rotor can be adjusted by varying amounts rather than having a predetermined offset for all pitch angles. Where the method described herein is automated, the look-up table may be updated using real-time information from operating the rotor, without requiring the rotor to be specifically stopped.

The data array may be in the form of a table or a graph, wherein the date can be referenced for determining at which angle a wind turbine blade should be pitched at to give a desired level of toque, according to the wind speed.

The invention further extends to a method of controlling a wind turbine blade, comprising the steps of measuring the wind speed, determining the pitch angle of the blade to provide the required torque using the reference obtained using the method disclosed herein, and adjusting the pitch of the blade accordingly. This process may be automated in part or in its entirety. Once the wind speed is known, the pitch angle of the wind turbine blade can then be adjusted to the required angle to produce a desired level or torque. The data being in a reference format allows for quick determination of the required pitch angle. By having the process automated, at least in part, the wind turbine blade can reacted swiftly to changes in the wind speed to improve the performance of the rotor. Detailed Description of Exemplary Embodiments

In a wind turbine to be examined, the rotor blades of the wind turbine are each provided with optical fibre strain sensors, incorporating fibre Bragg gratings, close to the root of the blade, that is, near the end of the blade that connects to the rotor hub.

In one method, the blades of the wind turbine rotor are initially all set to the same predetermined pitch angle. Preferably this is the theoretical fine pitch angle, which should be the same for each blade as they should all be identical. The wind turbine rotor is then monitored over several hundred rotations in below-rated-power wind conditions. During the rotations, the lift, or torque, produced by each blade is calculated using the optical strain sensors in the root of each blade. Numerical processing is then used to determine a quantifiable level of torque for each blade; the mean or inter-quartile average value is calculated.

The calculated quantifiable level of torque for the blades does not need to be the same for each blade, nor does it need to be in standard units. It is likely that the value will not be the same for each rotor blade due to the differences in gauge factors for the sensors and the stiffness of each blade. The identifiable values should be sufficient to provide a relative measurement for each blade, such that the values could be reasonably expected to increase when the lift is increased and decreased when the lift is decreased, with a reasonable level of linearity. This is accomplished using optical fibre strain sensors.

Once the identifiable measurements corresponding to the lift for each blade have been calculated, scale factors should be determined for each blade to enable subsequent readings to be 'normalised' .

As an example, the readings on a three rotor wind turbine may be 500, 400 and 550, which represents a mean shift in wavelength for optical sensors mounted on the leading edge of each rotor. The 'scale factors' for normalising the later results are then 500, 400 and 550. The pitch angle of one of the rotor blades is then offset by one or two degrees from the original setting and the rotor is operated under similar conditions as previously, but with the pitch angle of one rotor blade offset from the others.

The blades are then monitored again to determine the torque produced by each blade. The scaling factors are then applied to the readings to determine the normalised changes in the lift of the rotor blades.

If the wind conditions for the two test periods are considered to be comparable when the pitch angle differs for the two test periods, then the normalised lift of the offset blade should be compared. If the lift has increased, the blade pitch angle should be further offset in the same direction. By contrast, if the normalised lift has decreased, the blade should be rotationally offset in the opposite direction and monitored. If there is no significant statistical change in the torque produced by the blade, the blade is operating close to, if not at, its optimal pitch setting. However, this can be checked by rotating the blade in one direction and/or the other, and this check may be desirable. The blade should be rotated in both directions until the optimal pitch angle is determined.

Once the operation has been performed for one blade, it is then repeated for the others.

Once the actual fine pitch angle has been determined for each rotor blade, these can be used to correct pitch angle errors and improve the performance of the wind turbine. The correction may be applied by adding numerical offsets in the software settings of the programmable logic controller or turbine main controller. This can be done either remotely or by physical adjustment of the mechanical or electrical components of the pitch control system in the hub of the rotor. Where software is used to control the pitch angle of the rotors, it may be desirable to either individually correct the offset angles, for example a correction of 5, 6 and 8 degrees for the three rotor blades, or alternatively, it may be preferably to apply a correction ratio, for example 0, 1 and 3, respectively, that is, take out the common 5 degree offset and apply the relative values.

In an alternative method, the wind speed is taken into account when monitoring the lift produced by a particular blade. The rotor blades of the wind turbine rotor are all set to the same starting pitch angle, preferably the theoretical fine pitch angle. The blade being tested is then monitored over a period of several hundred rotations and the lift produced by that blade is calculated, taking note of the wind speed, and any changes therein, over the same period. The resulting lift measurements are then attached to the corresponding wind speed; for example, a quantifiable level of lift is determined while the speed was at 4 m s to 5m/s. Again, the measurement need only be a relative measurement of the lift for that blade, not necessarily having standard units. The lift increase and decrease should have a reasonable level of linearity such that the lift decreases and increases accordingly.

Once the first measurement is determined for the theoretical fine pitch angle, the test blade is then offset so that the wind turbine is operated under similar conditions, but with the test blade at a different pitch angle. The lift is again determined for the wind speed and a comparison between the lift for the blade at the original angle and offset angle can be made.

Again, if the lift has increased, the pitch angle should be increased in the same direction to determine if the lift if further improved at a greater offset angle. Where it decreases, the blade should be rotated in the opposite direction. Where there is no change in the lift, the blade is close to the actual fine pitch angle and it may be desirable to check this by offsetting the blade in one direction or the other. Offsetting the pitch angle of the blade should be continued until the most efficient fine pitch angle is determined. This may involve monitoring the blade over various pitch angles in a variety of wind speeds.

Once the first blade has been tested, the other rotor blades should also be assessed to determine their actual fine pitch angle. Once the rotor blades have been tested, it may be desirable to return them to the theoretical fine pitch angle or retain them at the actual fine pitch angle prior to testing the next blade.

When all of the blades have been tested, the desired pitch angles of the blades can be applied, which may be the fine pitch angle of each in order to obtain the most efficient results from the wind turbine. As with the first method, an absolute offset may be applied to the software settings of the pitch control, or a relative offset for each blade might be applied. Whilst it may be preferable for the apparatus to be permanently installed in a rotor, one alternative method is to apply the apparatus as and when the blades are to be tested. As an example, in a wind turbine field, all of the blades may be fitted with fibre optic strain sensors and an interrogator applied to the wind turbine to be tested when required, rather than leaving an interrogator attached to each wind turbine rotor permanently. Such an arrangement would allow for periodic testing to be performed without the expense of an interrogator for each turbine. The interrogator may be provided with a battery, a solar panel or another integral power source to reduce the requirement for a power supply and/or extra wiring within the turbine nacelle.

It may be desirable for the method of determining the fine pitch angle to be automatically triggered by the rotor control software at predetermined time periods, or when the wind turbine drops below a pre-set power generation level. Alternatively, it may be triggered manually. Clearly, this should only be undertaken in appropriate conditions where it is safe for the pitch angles of the blades to be altered.

The resulting corrections can then be applied automatically to the wind turbine rotor. The determined fine pitch angle following one test may form the basis for the theoretical fine pitch angle values on a subsequent test, rather than using the factory theoretical angles.

The pitch angles may be altered during normal operation of the wind turbine rotor and the method may take advantage of those normal operating conditions and monitor the lift during those periods. By taking advantage of these conditions, the pitch angle can be monitored without affecting the normal operation of the wind turbine, thereby retaining maximum output from the turbine, it may still be necessary to apply subsequent pitch angles to the rotors, however, at least some of the measurements will have been taken, thereby reducing the 'down time' of the rotor. The system may be fully automated and can be run during normal operation of the wind turbine. By employing the method of monitoring the lift during normal cycles of the wind turbine, an automated on-going optimisation system is effectively built in to the turbine. It may be advantageous for the data to be recorded and then analysed at a later time, possibly away from the wind turbine, and the pitch angles of the blades can then be altered subsequently, either remotely or manually.

Once the pitch angles have been determined, it may be desirable to employ the blades slightly offset from the optimal angle to ensure that an equal torque or thrust is applied by each blade.

Pitch angle differences may be detected by monitoring and taking comparative measurements of strain between the three blades as a function of pitch angle. Once the measurements are recorded, they can be used to diagrammatically show the relationship between the torque in the blades against the pitch angle of the blades. If this plotted on a graph, there will be a peak at the maximum fine pitch angle for all of the blades. If there is a relative 'error' in the fine pitch angles of the blades, each blade will peak at a different position on the said graph.