Abstract:
Improvements Relating to Wind Turbine Sensors A sensor apparatus for a wind turbine is described. The apparatus comprises a sensor and a heating system. The heating system comprises an optical fibre arranged to transmit electromagnetic radiation from a light source to the sensor. The sensor is irradiated by the electromagnetic radiation thereby heating the sensor and preventing or reducing ice accretion.

Description:
BACKGROUND 
       [0001]    The present invention relates to wind turbine sensors. 
         [0002]    Modern wind turbines include a number of sensors for monitoring external parameters such as wind speed and wind direction, and internal parameters such as vibration and strain affecting the wind turbine blades and other components. Information from these sensors is used for controlling the wind turbine and for monitoring the health of the wind turbine components. 
         [0003]    Many wind turbine sensors are made from metallic components. However, as wind turbines are very tall structures and these sensors tend to be located externally at the top of the tower or on the rotating blades, they are susceptible to lightning strikes. To counter this problem a sensor made entirely from non-metallic components has been proposed in WO2011/095170. Alternatively, the sensors may comprise galvanic insulation to reduce the risk of lightning strikes. 
         [0004]    Another problem affecting wind turbine sensors is ice accretion. In cold conditions ice may accumulate on the wind turbine blades and other parts of the wind turbine. If this ice covers the sensors it may prevent the sensors from operating effectively. 
         [0005]    Against this background, the present invention aims to provide an improved sensor system that does not suffer from the problems described above. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention there is provided a sensor apparatus for a wind turbine, the apparatus comprising: a sensor; and a heating system, wherein the heating system comprises an optical fibre arranged to transmit electromagnetic radiation from a light source to the sensor and irradiate the sensor with the electromagnetic radiation to heat the sensor and thereby preventing or reducing ice accretion on the sensor. 
         [0007]    The sensor apparatus may further comprise a controller configured to activate the heating system in conditions that are conducive for ice accretion on the sensor. Low ambient air temperatures may cause ice accretion but other factors such as air humidity and air pressure may also influence whether ice accretion will occur. 
         [0008]    In a preferred embodiment, the controller is configured to activate the heating system if a measured temperature falls below a predetermined threshold temperature. In order to prevent ice from forming on the sensor, the predetermined threshold temperature is preferably above ice-forming temperatures. The benefit of only activating the heating system below a threshold temperature is that power is conserved compared to having the heating system constantly active. In other embodiments, the controller may be configured to activate the heating system when the measured temperature falls below the threshold temperature and when the level of humidity exceeds a predetermined threshold. 
         [0009]    Wind turbines usually include temperature sensors to measure the ambient temperature local to the wind turbine and, preferably, ambient temperature is the measured temperature referred to above. 
         [0010]    Alternatively, the measured temperature may be the temperature of the sensor itself. In some cases, the ambient temperature may not accurately reflect the temperature of the sensor itself. Using the temperature of the sensor, it can be more accurately determined whether ice-forming temperatures are present at the sensor compared to when ambient temperature is used. 
         [0011]    Preferably, the controller is configured to increase the power output of the heating system as the measured temperature decreases according to a predetermined relationship between the measured temperature and the power output of the heating system. More power is required to maintain the sensor apparatus temperature above ice-forming temperatures in colder conditions. Increasing the power output of the heating system in colder conditions therefore serves to conserve power by minimising the power required to maintain the sensor apparatus temperature above ice-forming temperatures, whilst at the same time ensuring that the heating system is fully-functional even in extremely cold conditions. 
         [0012]    Alternatively or additionally, the controller may be configured to activate the heating system in response to identifying spurious sensor data indicative of the presence of ice on the sensor. This configuration is used to melt any ice that may have formed on the sensor. Ice accretion on the sensor may disrupt the normal operation of the sensor causing the sensor to provide spurious measurements. Any spurious measurements may be detected by the controller through statistical comparison of the measurements with typical measurements to find any significant outliers in the data. The control software may be configured to activate the heating system on receiving a predetermined number of spurious measurements within a predetermined time period, when the measured temperature is at or below ice-forming temperatures. 
         [0013]    In a preferred embodiment, the light source of the heating system emits electromagnetic radiation with wavelength in the infrared range. Alternatively, the wavelength of the electromagnetic radiation may be in the visible range or indeed any other suitable range capable of providing a heating effect. The light source may comprise one or more LEDs, lasers, halogen, metal halide light sources or any other light source capable of providing a heating effect on the sensor when the electromagnetic radiation is transmitted via the optical fibre. 
         [0014]    The electromagnetic radiation may be distributed over the sensor system from the end of the optical fibre of the heating system using an arrangement of lenses and/or mirrors. The beam of light emitted from the end of the optical fibre may be relatively narrow compared to the size of the sensor. The inclusion of at least one lens may therefore be beneficial in distributing the electromagnetic radiation over a wide area of the sensor in a shorter distance than would be possible without any lens. Further, components of the sensor may block electromagnetic radiation from the end of the optical fibre from reaching other parts of the sensor and at least one mirror could be used to reflect the electromagnetic radiation towards parts of the sensor that may otherwise be over shadowed. 
         [0015]    Preferably all elements of the sensor apparatus that are to be mounted externally on a wind turbine are made from electrically-insulating materials. More specifically, all elements of the sensor apparatus to be mounted externally on the wind turbine are preferably made from non-metallic materials. This reduces the susceptibility of the sensor to lightning strikes. For example, a housing for the sensor apparatus may be constructed from a material such as plastic with reinforcements of glass or carbon fibres; alternatively the material may be a deformable material such as a polymer, for example rubber, natural rubber, polypropylene, polyethylene, nylon, elastomers, Kevlar, or the like. 
         [0016]    It is advantageous if the light source is located remotely from the sensor as the light source contains electronic components and it is desirable not to add any metallic components to an externally mounted sensor apparatus for the reasons described above. 
         [0017]    Similarly, it is preferable for all elements of the sensor apparatus that are susceptible to damage from electrical discharges or induced currents caused by lightning strikes to be protected by electrical shielding. For example, the electrical components could be located inside a Faraday cage to shield them from lightning strikes. 
         [0018]    In embodiments comprising at least one mirror to distribute electromagnetic radiation over the sensor, the mirror is preferably a non-metallic mirror such as a dielectric mirror. 
         [0019]    In embodiments where the measured temperature is the sensor temperature itself, the temperature sensor is preferably located remotely from the sensor and is a non-contact thermometer such as an infrared camera or infrared pyrometer. Again, this ensures that no metallic components are associated with the sensors that may attract lightning. 
         [0020]    Preferably, the sensor has no metallic components or the sensor otherwise comprises galvanic insulation, non-limiting examples include an optical anemometer, an optical light meter or an optical strain gauge. An optical light meter can be used in a system to prevent shadow flicker caused by wind turbines. As each turbine blade rotates around the axis of the shaft, repetitive, strobing shadows may be cast when the sun is low in the sky, this phenomenon is known as ‘shadow flicker’. Shadow flicker may cause undesirable and distracting lighting, especially if the wind turbine is located near residential areas. To counter this problem, some wind turbines include light meters that are able to provide information about the angle and position of the sun. This information is useful for monitoring the relative light levels on opposite sides of the wind turbine tower. Knowing the relative light levels allows the risk of shadow flicker to be calculated and this information can be used to control the wind turbine so as to avoid or reduce shadow flicker. 
         [0021]    Preferably, the sensor apparatus described above is used on a wind turbine. Accordingly, the invention provides a wind turbine comprising the sensor apparatus described above. The sensor apparatus is both resistant to lightning strikes and prevents or reduces ice accretion. The invention also includes a wind farm comprising a plurality of wind turbines as described above. 
         [0022]    Alternatively or additionally, the sensor apparatus described above could be used on meteorological measurement towers since sensors such as anemometers used on measurement towers are also susceptible to lightning strikes and ice accretion. 
         [0023]    The invention further provides a method of reducing or preventing ice accretion on a wind turbine sensor, the method comprising heating the sensor by irradiating the sensor with electromagnetic radiation, wherein the electromagnetic radiation is transmitted from a light source to the sensor by an optical fibre. 
         [0024]    In a preferred embodiment, the method may comprise measuring a temperature and heating the sensor if the measured temperature falls below a predetermined threshold temperature. Preferably, the method comprises increasing the intensity of the electromagnetic radiation transmitted to the sensor as the measured temperature decreases. Alternatively or additionally, the method may comprise heating the sensor in response to the identification of spurious sensor data indicative of the presence of ice on the sensor. Preferably, the method comprises irradiating the sensor with electromagnetic radiation with wavelength in the infrared range, although other wavelengths capable of providing a heating effect may be suitable. In a preferred embodiment of the method, the light source may be located remotely from the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    In order that the invention may be more readily understood, embodiments of the invention will now be described in more detail, by way of example only, and with reference to the following figures in which: 
           [0026]      FIG. 1  is a schematic view of a wind turbine according to the present invention; 
           [0027]      FIG. 2  is a schematic plan view of a wind turbine sensor apparatus according to an embodiment of the present invention; 
           [0028]      FIG. 3  is a schematic side elevation view of the sensor apparatus of  FIG. 2 ; and 
           [0029]      FIG. 4  shows the sensor apparatus of  FIG. 2  in more detail and connected to an optoelectronic equipment suite. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  shows a wind turbine  10  having a sensor apparatus  12  according to an embodiment of the invention. The wind turbine  10  comprises a tower  14  on which a nacelle  16  is supported. A rotor  18  is mounted to the front of the nacelle  16 . The rotor  18  comprises a hub  20  on which three equally-spaced rotor blades  22  are mounted. The sensor apparatus  12  is mounted externally on an upper surface  24  of the nacelle  16 . The nacelle  16  comprises electrical shielding, such as a Faraday cage, to protect its contents against lightning strikes. An optoelectronic equipment suite  26  is located within the electrically shielded nacelle  16  and connected to the sensor apparatus  12  by a bundle of optical fibres, as shown schematically by the line  28  in  FIG. 1 . In alternative embodiments, both the sensor apparatus  12  and the optoelectronic equipment suite  26  could be located elsewhere on the wind turbine  10 . 
         [0031]    Referring now to  FIGS. 2 and 3 , the sensor apparatus  12  comprises a sensor. In this example, the sensor is an optical anemometer configured to determine the speed and the direction of the wind  30  by detecting the motion of dust particles  32  or other such matter carried in the wind. The operation of the optical anemometer will be discussed in more detail later. 
         [0032]    The optical anemometer includes a dome-shaped housing  34  in which three light sources  36  and a light detection apparatus  38  are located. The light detection apparatus  38  is centrally located in the housing  34  and the three light sources  36  are equally-spaced around the light detection apparatus  38 . The light sources  36  emit beams of light  40  that converge in a focal region  42  below the light detection apparatus  38 . 
         [0033]    The sensor apparatus  12  further comprises a heating system  44  to prevent or reduce ice accretion on the optical anemometer. The heating system  44  comprises a lens  46  and an optical fibre  48 . The housing  34  has a first aperture  50  through which a first end of the optical fibre  52  enters the housing  34 . The lens  46  is mounted inside the housing  34 , adjacent to and spaced apart from the first end of the optical fibre  52 . As will be described in more detail later, the heating system  44  functions using the lens  46  to distribute infrared radiation  54  emitted from the first end of the optical fibre  52  over the sensor apparatus  12 . 
         [0034]    In this embodiment, the sensor apparatus  12  does not contain any metallic or electrically conductive components and is therefore less vulnerable to lightning strikes than a sensor apparatus having metallic components. 
         [0035]      FIG. 4  shows the housing  34  in this embodiment with five apertures  50 ,  56  through which five optical fibres  48 ,  58 ,  60  enter the housing  34 . The first aperture  50  and optical fibre  48  are as described above and illustrated in  FIG. 3 . Additionally, each of the three light sources  36  (one of which is not visible in this side view) is connected to its own optical fibre  58 . Similarly, the light detection apparatus  38  is connected to an optical fibre  60 . Outside the housing, the optical fibres  48 ,  58 ,  60  are grouped together to form the bundle  28  referred to above. The optical fibre bundle  28  passes into the interior of the nacelle  16  to the optoelectronic equipment suite  26 . In other embodiments, the optical fibres  48 ,  58 ,  60  may be grouped into a bundle within the housing  12  and pass through a single aperture in the housing. 
         [0036]    The optoelectronic equipment suite  26  inside the nacelle  16  comprises an optoelectronic light source  62 , a light detector  64  such as a photo diode, a controller  66  and an infrared optoelectronic light source  68 . The optical fibres  58  connected to the three light sources  36  in the sensor housing  34  are each connected at their other ends to the optoelectronic light source  62  in the optoelectronic equipment suite  26 . Similarly, the optical fibre  60  connected to the light detection apparatus  38  in the housing  34  is connected at its other end to the light detector  64  in the optoelectronic equipment suite  26 , and the optical fibre  48  of the heating system  44  in the sensor housing  34  is connected at a second end  70  to the infrared optoelectronic light source  68  in the optoelectronic equipment suite  26 . The controller  66  has respective connections  72 ,  74 ,  76  to the optoelectronic light source  62 , the infrared optoelectronic light source  68  and the light detector  64 . The controller  66  is also connected to a temperature sensor  78  located outside the optoelectronic equipment suite  26 . 
         [0037]    In this embodiment, both the optoelectronic light source  62  and the infrared optoelectronic light source  68  comprise a plurality of LEDs. Specifically, the optoelectronic light source  62  emits visible electromagnetic radiation and the infrared optoelectronic light source  68  emits electromagnetic radiation having a wavelength in the infrared range. 
         [0038]    The controller  66  includes a memory on which control software is stored and a processor to run the control software. The control software governs the operation of both the optical wind sensor and the heating system  44 . The controller  66  is also configured to receive ambient temperature information from the temperature sensor  78 . 
         [0039]    Referring to  FIGS. 3 and 4 , the light sources  36  and light detection apparatus  38  are substantially arranged as described in WO2011/095170 to determine the speed and direction of the wind  30  based upon the speed and direction of travel of dust particles  32  carried in the wind. The reader is referred to WO2011/095170 for a detailed description of the mode of operation of the sensor. However, for convenience, a brief description of the operation is provided below. 
         [0040]    Each of the three light sources  36  (one of which is not visible in the side views of  FIGS. 3 and 4 ) emits two beams of light  40  with distinct wavelengths. The light detection apparatus  38  receives flashes of light caused as the dust particles  32  carried in the wind  30  pass through the individual beams of emitted light  40  and reflect light towards the light detection apparatus  38 . The flashes of light are transmitted from the light detection apparatus  38  to the light detector  64  in the optoelectronic equipment suite  26  via the associated optical fibre  60 . The flashes of light are processed by the controller  66  to determine the speed and/or direction of the motion of the dust  32  and hence the speed and/or direction of the wind  30  in which the dust  32  is carried. The controller  66  also uses the order in which the distinct wavelengths of light are reflected to determine the direction of motion of the dust particles  32 . 
         [0041]    Referring still to  FIGS. 3 and 4 , the operation of the heating system  44  will now be discussed in more detail. In addition to the lens  46  and adjacent optical fibre  48  described above, the heating system  44  further comprises the infrared optoelectronic light source  68  in the optoelectronic equipment suite  26 . The heating system  44  is arranged such that infrared light  54  emitted from the infrared optoelectronic light source  68  is transmitted via the optical fibre  48  and distributed via the lens  46  over the sensor apparatus  12 . The sensor apparatus  12  absorbs the infrared light  54  and converts the energy of the infrared light  54  into heat. The heat absorbed by the sensor apparatus  12  is sufficient to prevent or reduce ice accretion on the sensor apparatus  12 . This enables the sensor apparatus  12  to continue to operate even in low ambient temperatures which would usually cause ice accretion on the sensor apparatus  12 . 
         [0042]    In this embodiment, the system is configured to prevent ice from forming on the sensor. Hence when the temperature sensor  78  indicates that the ambient temperature has fallen below a predetermined threshold that is above ice-forming temperatures the controller  66  activates the heating system  44 . In this example, the predetermined threshold is set at +1° C. This leads to the heating system  44  irradiating the sensor apparatus  12  with infrared light  54  to ensure that the temperature of the sensor apparatus remains above ice-forming temperatures, thereby preventing ice accretion on the sensor apparatus  12 . 
         [0043]    The control software is configured to prevent the heating system  44  from interfering with the operation of the sensor apparatus  12 . When the heating system  44  is activated, the emitted infrared light  54  may be reflected or transmitted around the inside of the housing  34  and transmitted to the optoelectronic equipment suite  26  through the optical fibre  60  connected to the light detector  64 . Any infrared light consequentially received by the light detector  64  is filtered out by the control software, which uses algorithms to remove signals resulting from wavelengths in the infrared range. 
         [0044]    The control software further defines a predetermined relationship between ambient temperature and power output of the infrared optoelectronic light source  68  where the power output of the infrared optoelectronic light source  68  generally increases with decreasing ambient temperature so that the sensor apparatus  12  is heated more as temperature falls. The predetermined relationship between ambient temperature and power output of the infrared optoelectronic light source  68  serves to conserve power by minimising the power required to maintain the sensor apparatus  12  temperature above ice-forming temperatures, whilst at the same time ensuring that the heating system  44  is fully-functional even in extremely cold conditions. 
         [0045]    In alternative embodiments, the heating system  44  could be used to melt ice rather than to prevent ice forming. If ice accretion has occurred, the sensor apparatus  12  may provide spurious wind speed and wind direction measurements. Any spurious measurements are detected by the controller  66  through statistical comparison of the measurements with typical measurements to find any significant outliers in the data. The control software is configured to activate the heating system  44  on receiving a predetermined number of spurious measurements within a predetermined time period, if the temperature sensor also indicates ice-forming ambient temperatures. The heating system  44  then irradiates the sensor apparatus  12  with infrared light  54 , melting any ice on the sensor apparatus  12 . 
         [0046]    Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. 
         [0047]    For example, the optoelectronic light source  62  and the infrared optoelectronic light source  68  need not be integrated into the same optoelectronic equipment suite. In alternative embodiments, a standalone sensor heating optoelectronic equipment suite may be connected to the optical fibre  48  adjacent to the lens  46  in the housing  34 . The sensor heating optoelectronic equipment suite may contain the infrared optoelectronic light source  68 , the temperature sensor  78  and a dedicated controller. 
         [0048]    Whilst a temperature threshold of +1° C. is used in the above examples, a different temperature threshold may be employed in other examples. For example, a lower temperature threshold may be employed in cases where air humidity is low. The apparatus may further comprise means for measuring air humidity, and the heating system  44  may only be activated when temperature falls below a predetermined threshold and air humidity exceeds a predetermined threshold. Alternatively, the predetermined threshold temperature below which the heating system is activated may be calculated on the basis of an air humidity value (either measured or otherwise provided) and/or other potentially relevant factors such as air pressure. 
         [0049]    The temperature sensor  78  in the embodiment described above is configured to measure ambient temperature and could include a thermocouple, thermistor, analogue temperature sensor or digital temperature sensor. However, the ambient temperature can only be used to infer whether ice-forming temperatures are present at the sensor apparatus  12  itself. Therefore, in other embodiments, the temperature sensor  78  may be a non-contact thermometer, such as an infrared pyrometer, configured to measure the temperature of the sensor apparatus directly. In such embodiments, the controller  66  uses the sensor apparatus temperature in the same way as ambient temperature as described above to prevent or reduce ice accretion on the sensor apparatus  12 . The benefit of this is that the sensor apparatus temperature is measured directly without increasing the vulnerability of the sensor apparatus  12  to lightning strikes because the temperature measurement equipment does not require metallic parts to be associated with the sensor apparatus  12 . 
         [0050]    In the embodiment described above, one lens  46  is used to distribute the electromagnetic radiation from the first end of the optical fibre  52  of the heating system  44 . However, depending on the configuration of the sensor apparatus  12 , one or more lenses and/or mirrors may be employed to distribute the electromagnetic radiation from the first end of the optical fibre over the sensor apparatus. 
         [0051]    Whilst single optical fibres have been described above for convenience, in practice bundles of optical fibres, such as in a fibre optic cable, may be used to increase the transmission of electromagnetic radiation and for redundancy purposed making the apparatus more robust.