Improvements in or Relating to the Extraction of Energy from the Wind
The present invention relates generally to the extraction of energy from the wind, and particularly to a method and apparatus for generating power, particularly electrical power, from wind energy.
It is well known that fossil fuel supplies are running out rapidly and although nuclear power is available for generating energy there remain serious concerns about its safety and the long-term risks involved in expanding its use. For this reason attention has been turned to alternative energy sources, one of which is aeolian or wind energy. In many parts of the world the wind blows consistently and strongly, and by appropriate techniques it is possible to extract energy from the wind and use it either directly, as mechanical or kinetic energy, or to store it as potential energy, or as chemical or electrical energy. Traditionally, and, indeed, currently, fixed horizontal axis wind turbines have been used for this purpose, each wind turbine comprising two or more aerofoil-section rotor blades which are driven by the wind to rotate about a horizontal axis when this is aligned with the wind direction. Wind turbines, however, are not popular because of the visual pollution, especially in view of the continuous movement of the rotor blades which, in modern wind turbines, may be up to 80m in diameter on a tower which may be 50m high. In addition, suitable sites on which wind turbines can be located are becoming harder to find, and alternative ways of extracting energy from the wind are needed.
It is known to use kites for extracting energy from the wind, recognising the surge of force which is experienced as a kite is flown across the wind. The basic operation of a kite is well known. An operator, standing with his or her back to the wind can fly a kite over a region which can be defined roughly as a surface approximately one quarter of a sphere demarcated by a horizontal semi-circle from side to side at ground level on either side of the operator and a semi-circle from these end positions on either side
passing through a point directly overhead the operator. In practice a kite may not be able to reach the end positions so the "window" within which a kite may be flown may be slightly smaller in all directions than that hypothetically outlined above. When a kite is close to the edges of the "window" of its potential range of movement the aerodynamic forces which it exerts on its line (or lines in the case of a multiple line kite) are quite low and kite-flyers use this information to "park" their kite in order to rest when tired from the exertion of restraining it when it is flown in the middle of its "window" where the aerodynamic forces are considerably greater and exert a significant tension or force along the line, which must be resisted by the operator. Large aerofoil section kites have now been designed, similar in construction to the crescent-shape parachutes which are used for hang gliding and parascending, and large versions of such kites are used both for sporting purposes where, in appropriate conditions, they can lift the operator entirely off the ground, and for traction purposes where surf boarders and "buggy" riders can use the kite to draw them along. It has even been known to make extremely large versions of such kites to assist in the propulsion of seagoing vessels.
To get the most out of such kites they are "flown" in a closed loop path, generally a figure-of-eight, across the wind window from one side to another, rising (or falling) at the edges in a steep path and then descending (or rising) in a shallow inclined downward (or upward) path from one side of its path to the other. In such a flight pattern the kite is in a low force region whilst in the steep ascending or descending part, and, whilst traversing along the shallow part it exerts a significant force on the tether line. This has been used, for example, in a system described in PCT application WO 2007/034193 to rotate a crankshaft by acting on it in a manner rather similar to the pedal action of a cyclist, applying a force on one part of the revolution, and releasing the force on the other side. A similar arrangement is used in US patent 7,188,080 where two kites are used to rotate a drum on a rotatable platform, but in this case a single kite
is caused to fly up and down a line held aloft by a "support body" or set of support bodies which may be lighter-than-air balloons.
The present invention seeks to provide a system using two or more kites in which the flight pattern of each kite may be different from that of the other, each being independently controlled in dependence on the instantaneous wind conditions. Sensors on each kite enable information on the wind to be gathered more reliably and for an improved energy extraction to be obtained. The system of the present invention is also particularly suitable for use as an energy generator in remote regions where its self-contained nature and flexible characteristics can be exploited profitably.
According to the present invention there is provided apparatus for converting wind energy into another form of energy, comprising at least two steerable kites on respective tether lines and having respective means for controlling the kites to follow substantially independent flight paths, operable to cause them to follow respective paths which include a low-energy part and a high-energy part, the arrangement being such that when a kite is in the high-energy part of its flight path its tether is allowed to pay out whereby to generate energy and when a kite is in the low-energy part of its flight path its tether is drawn in, thereby in effect consuming a part of the energy generated during the pay-out period, the said means for controlling the flight paths of the kites including means for avoiding collision or conflict of the kites in flight. The high energy part of the flight path may be of longer duration than the low energy part so that the "power strokes" of the system overlap ensuring that there is no gap or interruption in the delivery of power.
Although the two kites are flown in such a way that the high-energy or high-force part of one kite's flight path does not coincide with that of the other kite, there is no absolute interconnection of the two which forces one tether to be paid out only when the other is winding in. This allows the control means which determine the flight path of
each kite almost independently to maintain one kite in a relatively stationary position where sensors mounted on or associated with it can provide reliable signals on wind strength and direction to allow the control system quickly to take account ot variations in wind speed and direction to control the flight of the other kite most effectively in the immediately-occurring circumstances. Thus, for example, one kite may be flown to a low-energy position overhead the base station or anchor point and held there while its line is retracted and the other kite is flown for example in a sinuous path comprising the basic figure-of-eight pattern described above, but with simultaneous pay-out of the tether line resulting in the kite flying gradually further and further away from the base station or anchor point. The kites may alternatively be flown in a circular flight pattern (which is, of course, effectively a helical flight path when the pay out of the tether line is take into consideration) and swivel connectors allow this without causing excessive twisting of the tether lines. The kite control system is mounted "above" the swivel, that is on the kite side of the swivel for this purpose. Obviously the control means need to be able to determine where the kites are in relation to one another in order to avoid tangling their tethers and control lines, and for this purpose each kite is provided with sensors, including accelerometers, strain gauges, wind speed and direction sensors as well as, perhaps, magnetic or gyroscopically operated devices for determining the frame of reference and orientation of the kite; further sophistication may be introduced by the use of GPS position detecting systems for determining the instantaneous position of each kite. In this way each kite can be flown to take advantage of the wind conditions while respecting the airspace of the other.
Conveniently, the tether lines of the kites are wound on respective drums, which are allowed to rotate to pay out or driven to rotate to draw in the lines. Each drum may be driven by a respective drive motor, or the drums may be driven selectively by a common drive motor for retracting the respective tether lines when the associated kite is in the low-energy part if its flight path.
The drums may be connectable selectively to the common drive motor via a lay shaft and respective clutches, which allows a non-symmetric connection pattern thereby taking account of variations or shifts in the relative position of the kites around their flight paths.
If the power generated by the system is to be used to generate electricity there may be further provided an alternator for generating electricity, and means for selectively connecting the alternator to each drum alternately, whereby to generate electricity as the tether lines are paid out. Preferably such an alternator is an asynchronous machine with an automatic voltage regulator providing a substantially constant output voltage despite the variations in speed of rotation which will inevitably be encountered as the kite passes across the line of the wind. Typically an alternator for this purpose should be capable of operating in the range from, say, 25 - 75Hz.
The output from the alternator may be connected to a rectifier inverter for output to storage means and/or to an electricity power grid.
As an alternative, or in addition, a proportion of the output from the alternator may be directed to a battery for storage and subsequent use, and, indeed, the stored electricity in the battery may be used to drive the motor connectable to the drums for retracting the tether lines if this is an electric motor.
Configurations can be envisaged in which the motor is supplied at least in part from the alternator or the battery.
The kites are preferably controlled by kite flight control means which include at least one computer for calculating the required tensions on and/or movements of flight control lines of the kites. Such control lines may extend from the relevant corners of the kites themselves to a point part way along the tether lines at which transducers or
actuators controlled by the computer, as well as the computer itself may be located. In this way the control of the kites does not require lines extending all the way from the kite itself to the ground so that only the tether line is present at a low level, thereby minimising the risk of tangling and twisting. This also simplifies the arrangement of the tether line on the drum for paying out and winding in. As mentioned above, a swivel connector between the tether line and the kite control system allows continuous rotations of the kite in relation to the tether when the conditions encourage this flight pattern.
As mentioned above, each kite preferably carries sensors for detecting flight parameters such as the relative wind speed and direction experienced during flight of the kite. One problem which is encountered when trying automatically to control the flight of a kite is that the wind speed and direction may fluctuate rapidly, and whereas a human operator equipped with both sight and tactile senses can react rapidly to control a kite to follow a desired path, automatic control systems do not have these facilities, and the relative wind speed, which is the only one detectable by a kite in motion, is obviously very largely affected by the instantaneous direction of travel and speed of the kite itself. This can be mitigated to a large extent in accordance with an important feature of this invention in which two kites are used since the signals from the sensors can be sampled at a known point in the path of the kite, particularly a part where it is almost stationary, and this information used to control the flight of the other kite so that the information on the wind strength and direction is constantly updated and more reliably sampled. As mentioned above the control means may include a global positioning satellite system (GPS) as well as accelerometers and pressure sensors to assist in determining the position of the kite around its flight path and thus the true wind direction from the moving platform.
The computer can therefore be programmed to determine from the input signals thereto the instantaneous absolute wind speed and direction for use in determining the flight paths of the kites.
Each kite requires to be highly controllable and to provide a significant "lift" force in use. In a preferred embodiment of the invention the kites are controllable aerofoil structures joined to the tether by at least two control lines the tensions on which determine the flight path of the kite. Such kites may be partly or wholly inflatable aerofoil structures, the inflation being achieved by allowing the wind to enter through openings in the structure to inflate the aerofoil to its desired shape. Alternatively, buoyancy may be provided by means of closed chambers containing a lighter-than-air gas. This has the advantage that the kites can remain airborne in nil wind conditions, or when the wind is too light to fly a heavier-than-air kite. And the kites are aloft and in position to take advantage of any small zephyrs or breezes which may blow from time to time.
The flight control system may be carried on a platform in the form of a secondary aerofoil structure located beneath the primary aerofoil structure of the kite and possibly providing additional lift. This secondary aerofoil may provide a rigid platform for support of the sensors and control equipment.
The present invention also comprehends a method of converting wind energy to another form of energy by flying two kites in closed flight paths having high-energy and low-energy components, allowing the tether lines to pay out when the kite is in a high- energy part of its flight path and winding in its tether line when the kite is in a low- energy part of its flight path, the difference in the power consumed by drawing in the tether and that gained as the tether is paid out being applied to do useful work. The high-energy part of the flight path may involve repeated traverses across the direction
of the wind, with the force exerted by the kite on the tether line being used to draw the line out and rotate a drum to perform useful work.
The said useful work may be driving an electric machine to generate electricity or, as mentioned above, direct kinetic motion or mechanical application of means for storing the energy as potential energy. Direct useful work may be driving a mechanical pump. The tether lines may be carried by a ground station fixed to solid ground or floating on water and restrained by anchors.
The tether lines of the two kites may be wound in opposite directions around a common drum, or respective drums, so that the kites can be controlled such that one is located in a high-energy part of its flight path exerting a high force on its tether line at the same time as the other kite is in the low-energy part of its flight path exerting low force on its tether line allowing the rotation of the drum or drums caused by the high tension tether line to wind in the low tension tether line.
The energy extracted from the wind as a kite passes across the wind direction through the high-energy part of its flight path can be used to generate electricity and part of that electricity can be used to power a motor to wind in the tether line of the other kite.
One of the advantages of a wind energy extraction system such as that described above is that it does not have to be maintained in its operating position at all times. A kite system may be periodically deployed and retrieved in a diurnal pattern of use which is different from site to site. For example, at one site the proximity of high density population and/or the presence of aircraft in the vicinity may make it inadvisable to fly the kites during the day although at night, when the visual impact of the kites will not disturb the local population, it may be perfectly safe and satisfactory. Automatic control of the kites avoids the need for them to be visually observed and there is thus no need for daylight. It is also useful to be able to retrieve the kites in dependence on
current or predicted meteorological conditions so that in times of very high winds, for example, the kites may be safely stowed or flown in a pattern or at an altitude which does not expose them to the high speed upper air.
In another aspect of the present invention there is provided apparatus for extracting useful power from the wind by flying a kite in a recirculating flight pattern, successively paying out and rewinding a tether line of a kite during parts of its flight pattern in which the line tension is high and low respectively, thereby generating a nett useful power, and flying a second kite as part of the same system with sensors for detecting wind strength and direction, and using the information from these sensors when the said second kite is in a given part of its flight pattern to control the flight path of the other kite.
Embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram illustrating the range of movement available to a kite, and the positions at which maximum and minimum power is developed;
Figure 2 is a schematic diagram illustrating the paths of two kites forming part of a system according to the invention;
Figure 3 is a schematic diagram illustrating a kite and a control aerofoil;
Figure 4 is a schematic diagram illustrating one form of ground station bearing the equipment for converting the reciprocating longitudinal movement of the tether lines into electricity; and
Figure 5 is a schematic diagram illustrating another form of ground station configuration.
Referring first to Figure 1 , there is shown a volume of space representing one quarter of a sphere centred at point A which is the nominal tether point of a kite. A kite 13 on a tether line 14 is shown in a "power" position on the nominal surface of the quarter
sphere which is drawn with a notional wind direction represented by the arrow B in the direction of the line A-C, which represents the wind direction in relation to the ground. The lines A-D and A-E of Figure 1 represent the right and left perpendicular azimuth directions relative to the wind direction A-C, while point F is located vertically above point A.
When a kite is in line with the wind direction A-C or at a relatively small angle from it represented by the azimuth points C3 and C2 and elevation point Cl the greatest power is generated by the kite, whilst increasing the angle of the tether line 14, by moving the kite further around the wind "window" represented by the quarter sphere the power progressively reduces. Thus, as the azimuth angle increases from AC towards AD or towards AE, or as the elevation angle increases towards AF the tension in the tether line gradually decreases as the force exerted by the wind on the kite falls. In the area represented by the curved surface F-D-C2, for example, the kite can be flown, still under control, but with relatively low tension in the line. By suitably controlling the control lines (not shown in Figure 1 ) the kite can be caused to be moved up and down in altitude or from right to left in azimuth through the central "high power" portion of the window to the "low power" regions at its edges.
Figure 2 illustrates a typical flight path pattern of two kites 15, 16 held by tether lines 17, 18 to respective drums 19, 20 on a base 21 which is itself turnable about a vertical axis in order to align the kites with the wind. The base 21 and structures thereon will be described in more detail in relation to Figure 4. The kites illustrated in Figure 2 are shown without any control lines, and the general control structure for a kite is described in more detail in relation to Figure 3.
For the present purposes the kites 15, 16 are represented in Figure 2 simply as the crescent-shape canopies of a steerable or controllable kite which, in this example, are controlled to follow a flight path having a power section with an axis of symmetry about
α line generally determined by the wind. The power section of the flight paths of the kites 15, 16 thus comprises two lobes, symmetrically located about the wind direction B and generally describable as a figure-of-eight as drawn on the nominal quarter-sphere surface in Figure 2, having four nominal reference points G, H, I, J.
The low-power section of the flight path thus comprises a falling section from point G to point H near the edge of the wind window where the aerodynamic forces on the kite are relatively low. This is then followed by an ascending path from H to I across the main central region of the wind window traversing the high power region where the tension applied to the line is greatest. A further falling section from I to J, on the opposite side of the wind window, again in a relatively low-tension region is followed by a second pass in the opposite direction from the first, from point J towards point G in an ascending curve across the high power region. Once established in this pattern its dimensions can be reduced until the flight path is wholly within the most powerful part of the high-power region of the wind window. This pattern is repeated as the line 17 is drawn off the drum 19 allowing the kite 15 to fly further and further from the base 21 , gaining height as it does so.
While kite 15 is thus flown in its power section, kite 16 is flown to a relatively static low power position almost directly overhead the base station 21 , and the drum 20 is wound in to draw the kite 16 down to its starting position ready for its power section. As the kite 15 progressively flies out on its power section it causes the drum 19 to unwind, thereby rotating an electricity generating machine (typically an alternator) which is not shown in Figure 2 but described in more detail in Figure 4. Correspondingly, as the kite 16 reaches its station above the base 21 it experiences a tension in the tether line 18 which is the lowest value during the entire flight path and while it is here the drum 20 is wound in under the control of an electric motor (not shown but descried in relation to Figure 4) so that the kite 16 is drawn down to a point ready for it to commence the "power stroke" section of the flight path during which, as previously described in relation to the
kite 15, the tether line is allowed to unwind from the drum 20 and rotate the alternator to generate electricity. The path followed by kite 16 from the end of its power section to its "static" position is shown by the broken line 12. This "return" path takes it out to the right side (as viewed in Figure 2), and then curves into a position over the base station 21 where it can be drawn down to be ready to start again.
This pattern of events is then repeated, although to ensure that the tether lines of the two kites do not get tangled together the kite 15 is flown off to the left (as represented by arrow 1 1 in Figure 2) on its way back to the return path where kite 16 was flown off to the right.
Turning now to Figures 3 and 4, there is shown a representation of the kite 15 which is tethered by the line 17. The kite body itself is a flexible aerofoil structure of generally parachute shape with a plurality of openings 25 in the leading edge, and attached by four control lines 26, 27 and 28, 29 from the corners of the aerofoil structure to a rigid, semi-rigid or inflated aerofoil platform 30 which is in turn connected by lines 31, 32, 33, 34 to the tether line 17. The connection to the tether 17 is made via a swivel coupling (not shown) which allows continuous circulatory motions of the kite to take place without causing excessive twisting of the tether 17.
The control lines 26, 27 lead from the opposite front corners of the aerofoil section 15 and the control lines 28, 29 lead from the rear corners. Each line is connected to a respective servo motor or actuator 35, 36 in the case of the front control lines 26, 27 and 37, 38 in the case of the two rear control lines 28, 29. The actuators are controlled to vary the effective lengths of the lines 26, 27, 28, 29 under the control of a computer control system, generally indicated 40 which receives input signals from sensors 41 , 42 located within the body of the aerofoil 30. The sensors 41 , 42 include wind strength and direction sensors, accelerometers, magnetic field sensors, GPS receivers for determining the orientation of the aerofoil 30, the direction it is pointing, the speed and direction of
the apparent wind, that is the relative speed and direction of the wind in relation to the movement of the aerofoil 30 and its position around the intended flight path.
On the basis of the calculations made by the computer 40 the transducers 35, 36, 37, 38 control the tensions in the lines 26, 27, 28, 29 to control the flight of the kite 15.
As can be seen in Figure 4, the tether line 17 is wound around the drum 19 (in a clockwise direction as viewed from the axle in the direction of the arrow Y) and the tether 18 (see Figure 2) is wound around the drum 20 in an anti-clockwise direction as viewed in the direction of the arrow Y. The drums 19, 20 are carried on respective axles 43, 44 (the mechanical bearing arrangements and other such structures are not shown for clarity). The two shafts 43, 44 rotate with the drum 19, 20 and act as input shafts to a twin input bevel drive unit 45 the output shaft 46 of which drives an alternator 47. Within the bevel drive unit 45 are respective clutches 48, 49 which allow one or other of the shafts 43, 44 to drive through the internal gears (not shown) to the output shaft 46. An electric motor 48 on a lay shaft 49 drives the drums 19, 20 alternatively via gears 50, 51 carried on a lay shaft 49 the engagement of which gears 50, 51 is represented schematically by the arrows 52, 53.
The electrical output from the alternator 47 is carried on wire 54, 55 via a rectifier inverter 56 the output of which leads to a battery 57 and, via a switching arrangement 58 to the mains grid on line 59. The arrow 60 represents the electrical connection of the battery 57 to drive the motor 48.
As will be appreciated from the preceding description, as the kite 15 is flown under the control of the computer 40 along the "power" part of the flight path, it experiences greater aerodynamic forces than when in the "holding" part of its circuit, and the tension in the lines 17 increases. The clutch 49 is decoupled and the clutch 48 engaged so that the drum 19 is allowed to rotate driving the output shaft 46 to rotate the
alternator 47 and generate electricity which is used to charge the battery 57 with the surplus being forwarded to the national grid along line 59. At this time, with the clutch 49 disengaged, the motor 48 is driven via the line '60 from the battery 57 and engaged on the lay shaft 49 to drive the gear 51 and thus the drum 18 causing it to wind in whilst the kite 16 is in the low power or "holding" part of its flight path.
When the kite 16 commences the high power part of its flight path the kite 15 is flown to the low power position. The sensors 41 , 42 detect these conditions and pass the information to computer 40 which emits the control signal, either via a wireless or radio transmission or by a cable or line passing down the tether line 17 to the base controller 61 which is connected to the drum control system to cause engagement and disengagement of the clutches 48 and 49 and engagement or disengagement of the motor 48 from the lay shaft 49. This control therefore engages the clutches 49 and 48 and simultaneously transfers the motor engagement from the gear 51 to the gear 50. The drum 48 is now allowed to pay out, thereby driving the shaft 46 and thereby the alternator 47 whilst the motor 48 drives the gear 50 to rotate the drum 19 and cause the tether line 17 to be drawn in.
It will be appreciated that although Figure 3 illustrates one kite 15, the kite 16 is similarly equipped with sensors, actuators, control computers and communication means and the computer program is able to determine the wind speed and direction from the kite at the holding position in the flight path so that any changes in the true wind speed and direction can be detected quickly and accommodated in the control signals which are transmitted to the lines (such as 26, 27, 28, 29) to determine the precise flight path of the kites. The base station may be mounted on the ground, turnable about a vertical axis to follow changes in the wind direction, or may be a floating platform anchored to the sea bed and thus able to orientate itself to take account of the wind direction.
Figure 5 illustrates an alternative base station 64, again in highly schematic form, comprising two winches 65, 66 on which the lines 17, 18 are wound respectively. The winches are connected via respective single stage epicyclic gear boxes 67, 68 and clutches 69, 70 to a bevel drive gearbox 71 the output from which drives an alternator 72.
Each winch in connected by a respective drive train 73, 74 and clutch 75, 76 to a respective lay shaft 77, 78 driven by a winch motor 79 the operation of which is controlled by a control unit 80 under the control of signals received from the kite- mounted sensors described in relation to Figure 3. In order to gain a sufficient reduction in speed between the winch motor 79 and the drums 65, 66 a two-stage epicyclic gear reducer may be used. Likewise, for high power applications each winch drum may have its own motor, thereby minimising the drive train and avoiding the need for a lay shaft.
As an alternative to the figure-of-eight flight path described above the kites may be flown in a continuous circling path (in fact a helix as the tether line is paid out) which has been found to provide a smoother and less variable power stroke during flight. The control signals to the flight control computers may be transmitted wirelessly from a ground station or may be transmitted along the tether (or a component of the tether) which may be electrically conductive. Indeed, an electrically conductive tether line has additional advantages in providing protection against lightning strike. Highly conductive materials such as those utilising nanotubes or fullerenes are considered to be particularly suitable for this purpose. The use of nanotube technology has the advantage of reducing the weight of the tether, and its dimension (therefore achieving a better aerodynamic performance) whilst at the same time offering a greater durability than other potential tether materials such as dyneema.
The computer on the kite control system for each kite may also be provided with its own autonomous safety routines in case of communications failure with the ground station or the other kite.
The use of conductive tether lines also opens up the possibility of using these as aerials for other purposes, such as for mobile (cellular) telephone communications.