Abstract:
A system for deicing a wind turbine blade includes an electrically powered active plasma actuator applied to a desired portion of a wind turbine blade. The activated plasma actuator energizes the air in the vicinity of the plasma actuator to increase the surface temperature of the wind turbine blade in the vicinity of the plasma actuator sufficiently to reduce or eliminate the collection of ice on a desired portion of the wind turbine blade.

Description:
BACKGROUND 
       [0001]    The invention relates generally to wind turbines, and more specifically to use of a plasma actuator that operates without an external source of air other than ambient air to modify wind turbine boundary layer separation to energize retarded flows and to reduce the collection of ice over wind turbine blades. 
         [0002]    Boundary layer separation takes place over highly curved surfaces for flows at high-angles of attack. This characteristic contributes to loss of pressure and hence a decline in aerodynamic efficiency of a wind turbine. In cold areas, collection of ice on the wind turbine blades not only contributes to loss of pressure, but also is hazardous. 
         [0003]    Different techniques have been employed to modify the boundary layer interaction to control flow characteristics. Many of these well known techniques use passive methods and devices, while some others use piezo electric surface modifications for flow control. One known technique employs Dielectric Bather Discharge (DBD) devices to modify boundary layer interaction to control flow characteristics associated with an air induction system for an aircraft. Another known technique employs surface cavities to modify the boundary layer growth to mitigate flow losses. 
         [0004]    Currently, there are no techniques used to overcome boundary-layer separation and simultaneously deice wind turbine blades. Vortex generators have been used to passively delay flow separation; but a major disadvantage to this solution is the vortex generator enhanced flow, even in situations where delaying flow separation is no longer desired. 
         [0005]    In view of the foregoing, it would be advantageous to provide a method to actively control flow separation and deice wind turbine blades. The method would employ a plasma actuator that is compact with no exposed moving parts, that operates without an external source of air other than ambient air, that requires little power to operate, that provides more versatility than passive techniques, and that can be applied to existing devices such as, without limitation, fan blades and turbine blades, with only minor modifications(s). 
       BRIEF DESCRIPTION 
       [0006]    Briefly, in accordance with one embodiment of the invention, a method of deicing a wind turbine blade comprises applying an active plasma actuator to a desired portion of the wind turbine blade, and electrically energizing the plasma actuator to ionize the air in the vicinity of the plasma actuator such that the surface temperature of the wind turbine blade in the vicinity of the plasma actuator is increased sufficiently to reduce or eliminate the collection of ice on the desired portion of the wind turbine blade. 
         [0007]    According to another embodiment, a system for deicing a wind turbine blade comprises an active plasma actuator applied to a desired portion of the wind turbine blade. An electrical power supply energizes the plasma actuator such that the air in the vicinity of the plasma actuator is ionized to increase the surface temperature of the wind turbine blade in the vicinity of the plasma actuator sufficiently to reduce or eliminate the collection of ice on a desired portion of the wind turbine blade. 
     
    
     
       DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0009]      FIG. 1  is a cross-sectional view of an actively controlled plasma actuator for controlling boundary-layer separation and wind-turbine blade deicing; 
           [0010]      FIG. 2  illustrates the actively controlled plasma actuator illustrated in  FIG. 1  configured as a tape that is applied to an airfoil surface according to one embodiment of the invention; 
           [0011]      FIG. 3  illustrates a top view Infra Red (IR) image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 5 KHz for a period of 3 seconds; 
           [0012]      FIG. 4  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 5 KHz for a period of 4 seconds; 
           [0013]      FIG. 5  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 5 KHz for a period of 7 seconds; 
           [0014]      FIG. 6  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 25 KHz for a period of 1.5 seconds; 
           [0015]      FIG. 7  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 25 KHz for a period of 3.5 seconds; and 
           [0016]      FIG. 8  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 25 KHz for a period of 7 seconds. 
       
    
    
       [0017]    While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0018]    The embodiments described herein with reference to the figures are directed to methods for controlling boundary layer separation and collection of ice on wind-turbine blades using an active plasma actuator. An active plasma actuator is a device that uses electricity to ionize air. The gradient in electric field results in a body force which acts on the external flow and imparts momentum to the fluid particles. It can also provide a surface temperature increase, which can be controlled by adjusting the operation frequency of the device. The plasma actuator device applications described herein can advantageously modify the boundary layer separation through ionization of air to mitigate flow losses and increase the surface temperature to enhance deicing of wind-turbine blades, among other things. 
         [0019]    Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). 
         [0020]      FIG. 1  is a cross-sectional view of an actively controlled plasma actuator  10  for controlling boundary-layer separation and wind-turbine blade deicing according to one embodiment. Plasma actuator  10  comprises a base electrode  12  that may be, for example, and without limitation, a copper foil, having a top planar surface  14  and a bottom planar surface  16 . A suitable insulating layer may be, for example, without limitation, a Kapton film, that is attached to or disposed atop the top planar surface  14  of the base electrode  12 . A second electrode  20 , that may also be, without limitation, a copper foil, may be attached to or disposed atop the insulating layer opposite the top planar surface  14  of base electrode  12 . According to another embodiment, the base electrode  12  may be embedded within a desired portion of a wind turbine blade  18 , while the second electrode  20  may be attached or disposed on a desired outer surface portion of the wind turbine blade  18 , such as depicted in  FIG. 1 . 
         [0021]    According to one embodiment, plasma actuator  10  advantageously can be manufactured as a tape that can be attached to a desired surface such as the surface of a wind turbine blade  18 . Once attached to the surface of the wind turbine blade, plasma actuator  10  operates to energize retarded flows and simultaneously reduce the collection of ice over the surface of the wind turbine blade  18 . Plasma actuator  10  thus negates the efficiency decline associated with loss of pressure and ice collection on wind turbine blades. 
         [0022]    With continued reference to  FIG. 1 , airflow  22  having a high angle of attack relative to the curved surface of a wind turbine blade is ionized via the active plasma actuator  10 . Active plasma actuator  10  is energized via an electrical power source  28 . The resultant gradient in electric field  24  yields a body force, which acts on the external flow  22  and imparts momentum to the fluid particles to energize retarded flows and modify wind turbine boundary layer separation. The gradient in electric field  24  also results in a surface temperature, which can be controlled by adjusting the operation frequency of the device  10  for wind turbine blade applications. This feature advantageously promotes deicing of wind turbine blades and inhibits collection of ice on wind turbine blades in cold regions that may otherwise cause loss of pressure and reduced wind turbine efficiency. 
         [0023]    In summary explanation, boundary layer separation takes place over highly curved surfaces for flows at high-angles of attack. This contributes to loss of pressure and hence a decline in aerodynamic efficiency of a wind turbine, while collection of ice on wind turbine blades in cold areas also results in hazardous wind turbine operating conditions. An actively controlled plasma actuator  10  is provided for simultaneously controlling boundary-layer separation and wind-turbine blade deicing. The plasma actuator  10  is manufactured according to one embodiment, in the form of a tape, similar to electrical tape that can be applied to a desired portion of a curved surface, such as a portion of a wind turbine blade. The plasma actuator  10  operates in the absence of an external source of air such as air jets or air stream generators, other than ambient air and has no exposed moving parts. Because the actuator  10  can be manufactured as a tape that can be attached to the blade surface(s), each operation can be conducted on demand. Thus, wind turbine blades can be designed to have reduced surface area, and thus reduced extreme loads during parked conditions. 
         [0024]      FIG. 2  illustrates the actively controlled plasma actuator  10  illustrated in  FIG. 1  configured as a tape that is applied to an airfoil surface  30  such as a wind turbine blade surface, according to one embodiment of the invention. Plasma actuator  10  may have a thickness of about 1 mm according to one embodiment when manufactured as a tape. 
         [0025]    Plasma actuator  10  may be connected to a power source  28  including a waveform controller  34  that is configured to control an input voltage level and pulsing, variable or AC voltage frequency, duty cycle and shape, such that air located in the electric field region enumerated  24  in  FIG. 1  is ionized in a desired fashion to create a region of discharge plasma. In this manner, the plasma actuator  10  exerts a force upon the ionized particles capable of changing the path of motion of the particles against other forces, such as inertia, which tends to maintain the particles in their normal path. Air flow represented by arrows  22  and  26  is thus energized with increased momentum in a near-surface region such that flow separation can be delayed or prevented. If the flow has, for example, been previously separated, it can be re-attached. 
         [0026]    Simultaneously, the gradient in electric field enumerated  24  in  FIG. 1 , results in a surface temperature increase, which can be controlled by adjusting the operation frequency of the device  10  for wind turbine blade applications, to promote deicing of wind turbine blades and/or inhibit collection of ice on wind turbine blades in cold regions that may otherwise cause loss of pressure and reduced wind turbine efficiency, as stated herein. 
         [0027]    With continued reference to  FIG. 2 , an orientation of a plasma actuator  10  is defined herein as the direction in which it imparts momentum. The plasma actuator  10  in one embodiment is oriented to impart momentum generally parallel with the direction of flow  36  and accelerate the boundary layer in the near-surface region, although momentum can be added in any direction parallel to the surface in which plasma actuator  10  is attached. 
         [0028]      FIG. 3  illustrates a top view Infra Red (IR) image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 5 KHz for a period of 3 seconds. 
         [0029]      FIG. 4  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 5 KHz for a period of 4 seconds. 
         [0030]      FIG. 5  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 5 KHz for a period of 7 seconds. 
         [0031]    A fairly uniform temperature distribution can be seen along the surface of the plasma actuator. The maximum temperature obtained is approximately 38 degree Celsius after 7 seconds of operation of the actuator at the 5 KHz operating frequency. 
         [0032]      FIG. 6  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 25 KHz for a period of 1.5 seconds. 
         [0033]      FIG. 7  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 25 KHz for a period of 3.5 seconds. 
         [0034]      FIG. 8  illustrates a top view IR image of a plasma actuator temperature distribution along the surface of the plasma actuator during operation at about 25 KHz for a period of 7 seconds. 
         [0035]    Plasma streamers with high temperatures are evident in  FIGS. 6-8 . The maximum temperature obtained is approximately 208 degree Celsius after 7 seconds of operation of the actuator  10  at the 25 KHz operating frequency. 
         [0036]      FIGS. 3-8  demonstrate that one can increase the surface temperature by using the plasma actuators  10 . Additionally, one can also control the temperature by adjusting the operation frequency of the plasma actuator device  10 . According to one embodiment, operation frequency of the plasma actuator device  10  can be adjusted between about 5 KHz and about 30 MHz. 
         [0037]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.