Abstract:
A glider, a system and methods for electric power production from wind are disclosed. The glider includes an airfoil, onboard steering means for pitching, rolling and yawing the glider when airborne, sensor means that provide a first signal related to an absolute position of the glider, a second signal related to an air speed of the glider and a third signal related to an acceleration of the glider, a control device connected to the steering means for controlling autonomous flight of the glider based on the signals provided by the sensor means, and a connection means for a tether connecting the glider to a ground-based electrical machine constructed for converting a lift force generated upon exposure of the airfoil to wind and transferred to the ground via the tether into electric power. The system includes the glider, the ground-based electrical machine and tether.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/EP2013/002446, filed Aug. 14, 2013, and claims priority to EP 12181506.2, filed Aug. 23, 2012. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The invention relates to a glider for electric power production from wind. The invention further relates to a system for electric power production from wind. 
         [0004]    2. Brief Description of Related Art 
         [0005]    The production of electric power from wind is generally accomplished by airfoils or structures with an aerodynamic profile, which produce lift forces upon exposure to wind. Thereby, energy is extracted from the wind, which can be converted into electricity, for example by exploring said lift forces to drive an electrical generator. Well known wind turbines for instance comprise a rotor with aerodynamically profiled rotor blades, wherein the lift forces of the rotor blades cause the rotor to turn. The rotor is mounted to an electrical generator, which is for example located on top of a tower, for production of electricity. 
         [0006]    In order to explore wind energy resources at altitudes above a few hundred meters over ground, where the average wind is stronger and steadier due to less disturbing interaction with the earth&#39;s surface, it has been proposed to use airborne airfoils. These concepts are often referred to as airborne wind energy or airborne wind energy production. 
         [0007]    One of the challenges of airborne wind energy production is the transferal of energy extracted from the wind at high altitudes to the ground. Two general approaches are proposed, the first providing for an airborne generator and subsequently a relatively heavy flight object, and the other providing for a ground based generator, wherein the energy extracted from the wind has to be mechanically transferred to the ground. 
         [0008]    An example of the latter approach is the so-called pumping kite concept. A kite flies downwind of a ground-based generator connected to its steering lines, thereby pulling the lines and driving the generator as the kite departs from the generator. In order to recover the lines, the generator is driven as a motor to pull back the kite. During this phase, the kite is steered to exert less pull on the lines, so that power consumption for pulling back the kite is less than the power produced by the kite pulling the lines before. 
         [0009]    The underlying problem of the invention is to provide for electric power production from wind using an airborne airfoil, wherein in particular the integrated energy yield is to be improved with respect to the prior art described above. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    According to the invention, this problem is solved by a glider for electric power production from wind, said glider comprising an airfoil, onboard steering means for pitching, rolling and yawing the glider when airborne, sensor means providing a first signal related to an absolute position of the glider, a second signal related to an air speed of the glider and a third signal related to an acceleration of the glider, a control device connected to the steering means for controlling autonomous flight of the glider based on the signals provided by the sensor means, and a connection means for a tether connecting the glider to a ground-based electrical machine constructed for converting a lift force generated upon exposure of the airfoil to wind and transferred to the ground via the tether into electric power. 
         [0011]    A glider or sailplane in terms of the invention in particular is a fixed wing aircraft, especially without propulsion means such as propellers or jet engines, wherein on-board steering means allow for full flight maneuverability of the glider around its longitudinal axis, its lateral axis and its vertical axis. In terms of the invention, these three principle axes form a Cartesian coordinate system, wherein the origin of said coordinate system is defined to be at the center of gravity of the glider. 
         [0012]    In general terms, with reference to straight and level flight, the longitudinal axis relates to the direction of motion, the vertical axis relates to the direction of lift and the lateral axis is essential horizontal to complete a Cartesian coordinate system. 
         [0013]    The glider for instance comprises a fuselage and a main wing, wherein the main wing constitutes or comprises an airfoil. In this configuration, the longitudinal axis is essential parallel to the fuselage, the lateral axis is essential parallel to the main wing and the vertical axis is perpendicular to both the longitudinal and the lateral axis. Those skilled in the art will appreciate that the glider can have another airplane configuration, for instance an all-wing aircraft, with appropriate definitions of the principle axes. 
         [0014]    In terms of the invention, rolling refers to a rotation of the glider around its longitudinal axis, pitching refers to a rotation of the glider around its lateral axis and yawing refers to a rotation of the glider around its vertical axis. 
         [0015]    A glider provides the advantage of low aerodynamic resistance or drag and a high aerodynamic lift due to the fixed wing with rigid aerodynamic profile or airfoil, respectively. This is in particular beneficial, because the energy effectively extracted from the wind strongly depends on lift and drag, in particular on the so-called lift-over-drag-ratio. 
         [0016]    The sensor means and control device of the glider according to the invention allow for unmanned flight, which reduces the total weight of the glider. Therefore, a larger amount of the total lift force generated by the airfoil is available for electric power production and thus increases the integrated energy yield. 
         [0017]    For enhanced safety of the glider, the connection means in particular are arranged for releasable connection of a tether to the glider, wherein the tether is connecting or arranged for connecting the glider to a ground-based electrical machine. 
         [0018]    The sensor means and control device also allow for automated optimization of the flight, in particular in order to maximize the lift force during the energy production phase and in order to minimize the pull on the tether during the recovery phase. Also, the flight during the recovery phase can be optimized for minimum duration. 
         [0019]    In terms of the invention, a signal related to a specific parameter in particular is a measurement value or a set of measurement values, which is continuously or repeatedly taken during the flight and allows determination of the specific parameter. 
         [0020]    The position of the glider in particular is the absolute position relative to the ground, which for instance is given in world coordinates, i.e. by longitude, latitude and height above sea level. 
         [0021]    A signal related to the position for instance is the ground speed of the glider, which allows the iterative determination of the position of the glider starting from a known initial position. The ground speed in particular is the movement or velocity, respectively, of the glider relative to the ground. 
         [0022]    In a preferred embodiment of the invention, the sensor means comprise a first position sensor, in particular a GPS sensor, i.e. a sensor according to the standard of the well known Global Positioning System. A position sensor in particular provides a direct measurement signal of the absolute position, which often is more precise than the iterative position determination. Those skilled in the art will appreciate that a position sensor can be a sensor according to the standards of any satellite based positioning systems, for instance the Galileo project, or can be based on other navigation technologies, such as RADAR. 
         [0023]    Preferably, the sensor means comprise a second position sensor, in particular a GPS sensor, wherein the second position sensor is located at a given distance to the first position sensor. This allows determining the orientation of the virtual line between the first position sensor and the second position sensor and thus gives the orientation of the glider relative to the world coordinate system. 
         [0024]    In contrast to the ground speed, the air speed is the movement or velocity, respectively, of the glider with respect to the surrounding air. In particular due to the presence of wind, the air speed in general differs from the ground speed. However, the air speed can be derived from the ground speed and the wind speed, i.e. the velocity of the air relative to the ground, wherein the ground speed for instance can be determined from the change in position of the glider with time. 
         [0025]    It is preferred that the sensor means comprise an air speed sensor, in particular a pitot tube. Here, the signal related to the air speed is a direct measurement signal and generally more precise than the indirect determination of the air speed from the ground speed and the wind speed. 
         [0026]    A pitot tube is a well-known instrument for determining the speed of an aircraft based on a measurement of a pressure difference, for instance the difference of an air pressure in a direction of flight (dynamic pressure) and an ambient air pressure in a direction perpendicular to the direction of flight (static pressure). 
         [0027]    For instance, a pitot tube comprises a cylindrical tube oriented along the longitudinal axis of an airplane with a hole at the tip and a hole at the side, wherein the two holes are connected via internal passageways with a differential pressure sensor. 
         [0028]    Preferably, the air speed sensor is a directional air speed sensor, in particular a multichannel pitot tube. For instance, a left-right pressure difference and a bottom-top pressure difference are measured in addition to the dynamic-static pressure difference described above. 
         [0029]    For instance, a multichannel pitot tube comprises a cylindrical tube with a dome-shaped tip oriented with the longitudinal axis of an airplane, said tube comprising five holes at the tip for determining the dynamic pressure and at least one hole at the side of the tube for determining the static pressure. It can be provided for more than one hole for determining the static pressure, for instance four or even twelve holes evenly distributed along a circle around the side of the tube. The five holes at the tip are arranged with one hole at the center of the dome-shaped tip and the other four holes arranged at equal distance to the center hole, wherein these four holes are pair-wise oriented with the lateral axis and vertical axis of the air-plane, respectively. In this configuration, the left-right pressure difference is the pressure difference from the two holes oriented with the lateral axis, the bottom-top pressure difference is the pressure difference from the two holes oriented along the vertical axis, and the dynamic-static pressure difference is the pressure difference from the center hole at the tip and the average pressure from the holes at the side of the tube. Alternatively, the absolute pressure at each of the nine holes can for instance be measured independently, wherein the left-right pressure difference, the bottom-top pressure difference and the dynamic-static pressure difference are calculated from these measurements, respectively. 
         [0030]    An acceleration of the glider can be a translational acceleration or, for a rotational movement is an accelerated movement, a rotational velocity and is induced by forces acting on the glider as a whole. A signal related to acceleration for instance is the second derivative with time of the position in case of a translational acceleration and the first derivative with time of the orientation in case of a rotational velocity. 
         [0031]    In a preferred embodiment of the invention, the sensor means comprise an inertia sensor, which in particular provides for a direct measurement of a translational acceleration and/or rotational velocity. For instance, the inertia sensor measures the translational acceleration in three different directions and the rotational velocity around three different axes. 
         [0032]    An appropriate inertia sensor includes in particular an accelerometer for measurement of a translational acceleration and/or a gyroscope for measurement of a rotational velocity. 
         [0033]    The steering means preferably comprise at least one aerodynamically active control surface. 
         [0034]    Aerodynamically active control surfaces are used to exert torque on the glider around one or more of the glider&#39;s principle axes. These control surfaces for instance comprise at least one aileron to mainly induce rolling and/or at least one elevator to mainly induce pitching and/or at least one rudder to mainly induce yawing. However, those skilled in the art will appreciate that other control surfaces known in aviation technology are also appropriate steering means in terms of the invention. In particular, a particular control surface can induce a rotation around an arbitrary axis, which does not correspond to one of the principle axes of the glider. 
         [0035]    Besides control surfaces, the steering means of the glider for instance further comprise actuators, such as electric motors or hydraulic systems with pumps and cylinders, for moving the control surfaces. These actuators are for instance powered by an on-board power source, such as a battery. Alternatively, the connection means can include a power plug for connecting the glider to a ground-based power source via the tether, which significantly reduces the weight of the glider. In this configuration, the glider may still comprise a small emergency battery for continued safe flight in case of loss of connection to the ground. 
         [0036]    A further embodiment of the invention is characterized in that the control device comprises a data storage unit for storing data related to flight characteristics of the glider and a data processor unit for deriving control signals for the steering means based on the stored data and on the signals provided by the sensor means. 
         [0037]    Here, data related to flight characteristics for instance is a plane model, which in particular comprises a set of measured or simulated response curves for the correlation between the operation or change in operation of the steering means and the resulting state or change in state of the glider. 
         [0038]    Preferably, the control device implements a Kalman filter. By this, the effect of measurement uncertainties on the control of the steering means and consequently on the flight of the glider is reduced. 
         [0039]    It is further preferred that the control device implements an un-scented Kalman filter, for an unscented Kalman filter in particular allows for non-linear dependencies and correlations. 
         [0040]    For optimized electric power yield, the control device preferably provides for a first operation mode for pulling on a tether connecting the glider with a ground-based electrical machine and wherein the control device provides for a second operation mode for approaching the electrical machine. 
         [0041]    The two operation modes in particular differ by the intended flight path or flight pattern, respectively. For instance, the flight pattern of the first operation mode is a high lift flight pattern with mainly crosswind flight of the glider, while the flight pattern of the second operation mode comprises a mainly straight flight path of the glider against the wind. 
         [0042]    In a further preferred embodiment of the invention, the glider comprises at least one aerodynamic control surface for varying a lift coefficient of the airfoil and/or for varying a drag coefficient of the airfoil and/or for varying a drag coefficient of the glider. This can for instance optimize lift and/or drag of the glider optimized with respect to the current operation mode. In particular, high lift and low drag, as is beneficial for the first operation mode, could delay descent of the glider and thus result in a slower return during the second operation mode. It is therefore of advantage, if the lift could be reduced and/or the drag could be increased during the second operation mode. 
         [0043]    Suitable control surfaces are so-called spoilers located on top of the airfoil, so-called slats located at the leading edge of the airfoil, so-called flaps located at the trailing edge of the airfoil and so-called air brakes, which affect only the drag coefficient of the whole glider. 
         [0044]    Additionally or alternatively, the airfoil may comprise a variable aerodynamic profile, which is another way for varying the lift coefficient and/or the drag coefficient. An airfoil with variable aerodynamic profile for instance is semi-rigid and can be modified in its curvature. 
         [0045]    The underlying problem of the invention is also solved by a system for electric power production from wind comprising a glider according to the invention, a ground-based electrical machine and a tether connecting the glider with the electrical machine, wherein the electrical machine is constructed for converting a lift force generated upon exposure of the airfoil to wind and transferred to the ground via the tether into electrical power. 
         [0046]    The problem is further solved by the use of a glider according to the invention for production of electric power from wind. 
         [0047]    Further characteristics of the invention will become apparent from the description of the embodiments according to the invention together with the claims and the included drawings. Embodiments according to the invention can fulfill individual characteristics or a combination of several characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]    The invention is described below, without restricting the general intent of the invention, based on exemplary embodiments, wherein reference is made expressly to the drawings with regard to the disclosure of all details according to the invention that are not explained in greater detail in the text. The drawings show in: 
           [0049]      FIG. 1  schematically a glider according to the invention; 
           [0050]      FIG. 2   a  schematically the operation of a system according to the invention in a first operation mode; and 
           [0051]      FIG. 2   b  schematically the operation of a system according to the invention in a second operation mode. 
       
    
    
       [0052]    In the drawings, the same or similar types of elements or respectively corresponding parts are provided with the same reference numbers in order to prevent the item from needing to be reintroduced. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0053]      FIG. 1  shows an exemplary embodiment of a glider  10  for electric power production from wind  50  according to the invention. 
         [0054]    The glider  10  is designed as a fixed wing aircraft comprising a fuselage, a main wing  14 , a tailplane  16  and control surfaces  20 ,  22 ,  24 . Also depicted in  FIG. 1  are the longitudinal axis  32 , the lateral axis  34  and the vertical axis  36 , which meet at the center of gravity of the glider  10  and which constitute the intrinsic coordinate system of the glider  10 . 
         [0055]    In the example shown, the fuselage comprises a tube constructed from fiber reinforced composite material as mechanical backbone  11  between the main wing  14  and the tailplane  16  and a nacelle  13 , which is mounted in front of the main wing  14 . 
         [0056]    The main wing  14  can for instance be constructed from a single wing, as in the embodiment depicted in  FIG. 1 . However, alternative designs, for instance with a separate main wing  14  on either side of the fuselage are within the scope of the invention. 
         [0057]    In flight, the glider  10  is maneuvered by control surfaces, which in the exemplary embodiment comprise ailerons  20  at either side of the main wing  14 , as well as elevators  22  and a rudder  24  at the tailplane  16 . The control surfaces  20 ,  22 ,  24  for instance are hinged surfaces used to induce torque around the principle axes  32 ,  34 ,  36  of the glider  10  by aerodynamic means. 
         [0058]    Torque around the longitudinal axis  32  is induced by means of the ailerons  20 , which can be or are operated simultaneously and in opposite directions. Here, opposite directions means that when the left aileron is moved upwards with respect to the main wing  14 , the right aileron is moved downwards. By this, lift is enhanced on the right side of the main wing  14  and reduced on the left side of the main wing  14 , causing a torque around the longitudinal axis  32 . The resulting movement of the glider  10 , a rotation around its longitudinal axis  32 , is referred to as rolling. 
         [0059]    A rotation of the glider  10  around its lateral axis  34 , which is referred to as pitching, is achieved by the elevators  22 , which are used to increase or decrease the lift at the tailplane, thereby inducing a torque around the lateral axis  34 . 
         [0060]    The rudder  24  induces rotation of the glider  10  around its vertical axis  36 , which is referred to as yawing. 
         [0061]    In addition to the control surfaces  20 ,  22 ,  24 , the glider  10  comprises spoilers  26  on either side of the main wing  14 , which can be raised to decrease the lift coefficient and at the same time increase the drag coefficient of the main wing  14 . Further control surfaces at the main wing  14  could be foreseen for affecting the lift coefficient and/or drag coefficient of the main wing  14 . In particular, these could be control surfaces at the leading edge of the main wing  14 , so called slats, and/or at the trailing edge of the main wing  14 , so-called flaps. Similar effects can be achieved with a wing with variable aerodynamic profile, for instance a semi-rigid wing where the curvature of the aerodynamic profile can be varied. 
         [0062]    Additionally or alternatively, air brakes at the fuselage could be foreseen, which increase the drag coefficient of the whole glider  10  without changing the lift coefficient of the main wing  14 . 
         [0063]    The operation of the control surfaces  20 ,  22 ,  24  is controlled by a control device located in the nacelle  13 , which for instance generates steering signals for moving the control surfaces  20 ,  22 ,  24  according to an intended flight path or flight pattern  52 ,  54 , respectively. 
         [0064]    The intended flight path, to which the flight of the glider  10  is controlled, can be externally set or derived by the control device according to an operation mode of the control device. In particular, the flight path may be controlled and adopted continuously, for instance to account for unsteady conditions of the wind  50 . 
         [0065]    For instance, the control device determines an estimate of the current state of the glider  10  and compares this with a desired state defined by the intended flight path  52 ,  54 . In case the estimated state and the desired state differ, the control device determines steering signals for the control surfaces  20 ,  22 ,  24  taking into account the known flight characteristics of the glider  10 . 
         [0066]    The state or state vector of the glider  10  is a set of parameters containing enough information to describe the momentary flight of the glider  10  and the differential evolution thereof. The state vector of the glider  10  for instance comprises the position of the glider  10  in world coordinates, the velocity vector of the glider  10  relative to the surrounding air and the translational acceleration and rotational velocity in three dimensions each of the glider  10 . 
         [0067]    The state vector is continuously determined from measurement signals of two position sensors  17 ,  17 ′ mounted on the mechanical backbone  11 , an air speed sensor  18  mounted at the tip of the nacelle  13  and an inertia sensor with a three-direction accelerometer and a three-axis gyroscope housed inside the nacelle. 
         [0068]    To limit the influence of measurement uncertainties on the flight of the glider  10 , the control device implements a Kalman filter, more specifically an unscented Kalman filter. In particular, the control device comprises a data storage unit, a data processor unit and appropriate algorithms implemented in hardware or software. 
         [0069]    For production of electric power, the glider  10  is connected to a ground station  40  via a tether  44 , which is attached to or connected with the glider  10  at a connection means, which is preferably arranged close to the centre of gravity of the glider  10 . This way, varying loads on the tether  44  do not significantly impair the balance of the glider  10  in flight. 
         [0070]    At the ground station  40 , excess length of the tether  44  is stored on a reel  42 , which is connected to an electrical machine  46 . The electrical machine  46  is connected to an electricity storage and/or distribution system (not shown) such as a power grid, a transformer station or a large-scale energy reservoir. Those skilled in the art will appreciate that the power storage and/or distribution system can be any device or system capable of receiving electricity from and delivering electricity to the rotating electrical machine. 
         [0071]    The system comprising the glider  10 , the tether  44  and the ground station  40  is alternately operated in a first operation mode for production of electric power, illustrated in  FIG. 2   a , and a second operation mode for system recovery, illustrated in  FIG. 2   b.    
         [0072]    In the first operation mode, which in particular is an energy production operation mode, the glider  10  is, by means of the control device, controlled to follow a high lift flight pattern indicated by line  52  downwind of the ground station  40 . In the figures, the direction of the wind is indicated by arrow  50 . During crosswind flight, in particular fast crosswind flight, the airfoil or the main wing  14 , respectively, of the glider  10  generates a lift force much larger than required to keep the glider  10  at a given altitude. As a consequence, the glider exerts a pull on the tether  44 , which is correlated to the excess lift force. 
         [0073]    The pull on the tether  44  is used for reeling out the tether  44  from the reel  42  in direction of arrow R, thereby inducing a rotation of the reel  42 . The resulting torque, which in particular depends on the diameter of the reel  42  and the force with which the tether  44  is pulled, is transmitted to the electrical machine  46 , where the mechanical energy is transformed to electric power. Optionally, a gearbox is arranged between the reel  42  and the electrical machine  46 , which is not shown in the figures for reasons of simplicity. 
         [0074]    As long as the tether  44  is reeled out, the glider  10  flies away from the ground station  40 . Thus, the overall length of the tether  44  limits maintaining the first operation mode. 
         [0075]    For recovery of the tether  44 , the glider  10  is, again by means of the control device, controlled to fly towards the ground station  40 . As the glider  10  approaches the ground station  40 , the free length of the tether  44  is shortened and the tether  44  is reeled in onto the reel  42  as indicated by arrow R′ by operating the electrical machine  46  as a motor rather than as a generator. The necessary power for instance is provided or delivered by the electricity storage and/or distribution system. 
         [0076]    In the second operation mode, it is preferred that the pull on the tether  44  is as low as possible in order to minimize power consumption for reeling in the tether  44  and as fast as possible in order to minimize the dead time, i.e. the period of time where no electric power is produced. The glider  10  therefore is controlled to follow a low lift flight pattern  54 , which for instance is a descent or a fast dive of the glider  10  against the wind  50  towards the ground station  40 . However, the low lift flight pattern  54  can also be an approach of the glider  10  towards the ground station  40  without loss in altitude, including a slight gain in altitude. 
         [0077]    If the approach runs slow, for instance because a high lift coefficient of the main wing  14  delays a descent of the glider  10 , the lift could be decreased and/or the drag could be increased by means of the spoilers  26  or the equivalent measures discussed above. This way, the return of the glider  10  towards the ground station  40  can be sped up and the time where the system does not produce electric power is reduced. 
         [0078]    An optimization of the lift and/or drag can also be achieved by modified operation of the ailerons  20 . Instead of anti-parallel operation for rolling the glider  10 , both ailerons  20  in parallel can be moved upwards for decreased lift or downwards for increased lift. 
         [0079]    If the glider has two control surfaces on either side of the main wing  14 , for instance an aileron  20  and an additional flap, drag can be increased without or with almost no change in the lift by moving the ailerons  20  up and the flaps down or vice versa. Here, flap in particular refers to a hinged control surface at the trailing edge of the main wing  14 , i.e. a control surface which is structurally similar to an aileron  20 . 
         [0080]    All named characteristics, including those taken from the drawings alone, and individual characteristics, which are disclosed in combination with other characteristics, are considered alone and in combination as important to the invention. Embodiments according to the invention can be fulfilled through individual characteristics or a combination of several characteristics. 
       LIST OF REFERENCES NUMBERS APPEARING IN THE ACCOMPANYING DRAWING FIGURES 
       [0000]    
       
           10  glider 
           11  mechanical backbone 
           13  nacelle 
           14  main wing 
           16  tailplane 
           17 ,  17 ′ position sensor 
           18  air speed sensor 
           20  aileron 
           22  elevator 
           24  rudder 
           26  spoiler 
           32  longitudinal axis 
           34  lateral axis 
           36  vertical axis 
           40  ground station 
           42  reel 
           44  tether 
           46  electrical machine 
           50  wind 
           52  high lift flight pattern 
           54  low lift flight pattern