Abstract:
A generally toric lighter-than-air gas bag-type airfoil is tethered to the ground at a plurality of angularly widely distributed points about the periphery of the gas bag. A wind turbine is mounted at the entrance to the axially central vent. The tether lines are entrained about individually operable power winches, preferably controlled by a microprocessor which takes in wind direction and tether line tension data and operates the winches and inflation gas inlet and outlet valves to orient the wind turbine into the wind for maximum power output.

Description:
BACKGROUND OF THE INVENTION 
     There is shown schematically in FIG. 1 an elevational view of a first-generation tethered airfoil wind energy conversion system, which was designed by the present inventor and which is included here for illustrative purposes. Referring to that FIGURE, the following derivation is made. 
     Consider the tethered airfoil wind energy conversion system (hereinater &#34;TAWECS&#34;) of FIG. 1 to be a large circular airfoil of planform surface area S, and wind passing over it produces lift L. The portion of L that is effective in reducing the air pressure over the TAWECS vent (in which is mounted a conventional wind turbine and electric generator) should be (A v  /S)L, where A v  is the area of the vent. Then the RMS decrease in air pressure over the TAWECS vent caused by the wind, 
     
         Δp=(A.sub.v /S)L≠A.sub.V =L/S                  (lb/ft.sup.2) 
    
     The RMS head due to wind input to the TAWECS, 
     
         h=Δp/ρ=L/Sρ                                  (ft) 
    
     where ρ is the air density (0.0023 slug/ft 3 ). From O. W. Eshbach, &#34;Handbook of Engineering Fundamentals&#34;, 2nd Ed., Wiley, 1952, p. 7-75 to 7-77, 
     
         C.sub.L =L/(1/2ρV.sub.w.sup.2 S)=(3/20)a 
    
     where 
     C L  =coefficient of lift 
     V w  =RMS velocity of wind over the airfoil surface (ft/sec) ##EQU1## Then 
     
         L=3/40 aρV.sub.w.sup.2 S                               (lb) 
    
     
         h=3/40 a ρV.sub.w.sup.2 =Δp/ρ                (ft) 
    
     The above derivation, which will be useful in the detailed description to follow, does not provide for the local variations in pressure over the airfoil surface. Instead, these calculations were for gross, first order feasibility estimates, using overall or average airfoil properties, and RMS values for the time-varying parameters. (A more detailed analysis using partial differential equations would be required to predict local pressure variations.) 
     In literature searching prior to the preparation of this document, the present inventor has become aware of the following prior art: 
     Prior U.S. Patents 
     
         ______________________________________Patentee       Pat. No.    Issue Date______________________________________Bolie          4,017,205   April 12, 1977Carson, et al  4,018,543   April 19, 1977Bolie          4,116,581   Sept. 26, 1978______________________________________ 
    
     Bolie, in U.S. Pat. No. 4,116,581, has described a wind turbine (hereinafter &#34;WT&#34;) employing airfoil principles to enhance the pressure drop across the WT, i.e., reduced pressure on the suction side. The Bolie structure employs a fixed hemispherical shell containing the WT in the lower half of the structure, and a rotating hemispherical shell in the upper half, with a vent oriented into the wind, and curved deflectors to direct the wind into the WT. The Bolie structure is rigid, implying size limitation, dictated by strength of materials considerations, on WT radii of probably 50 feet or less. Since the Bolie structure is approximately a sphere, with two opposing suction sides and camber of approximately 0, the reduced pressures on the top and bottom hemispheres should approximately cancel each other, leaving only the wind flowing through the aperture being effective in driving the WT, thus providing no aerodynamic advantage over a conventional horizontal axis WT, and a considerable economic disadvantage. 
     In both of the Bolie patents, the angle of attack of the airfoil α=0; an angle of attack α&gt;0 is impractical with the Bolie embodiments. Further, the Bolie structure in U.S. Pat. No. 4,116,581 must rotate into the wind, implying mechanical wear problems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a tethered airfoil wind energy conversion system which holds promise for being more efficient than those of the prior art. In particular, the device of the invention preferably has an optimum angle of attack α of up to at least 30° in the preferred embodiment, which increases the coefficient of lift by a factor of at least 6-8 with respect to an airfoil with α=0. See Eshbach, supra, p. 7-78. Further, the device of the invention preferably is deployed into the wind without a requirement for being rotated and does not have the size limitation implied in prior art structures. 
     Other distinctions and advantages will become apparent in the following description. 
     The principles of the invention will be further discussed with reference to the drawings wherein preferred embodiments are shown. The specifics illustrated in the drawings are intended to exemplify, rather than limit, aspects of the invention as defined in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawings 
     FIG. 1 is a diagrammatic elevational view of a first generation TAWECS device, discussed in the &#34;Background&#34; section hereinabove; 
     FIG. 2 is a diagrammatic elevational view of a first embodiment of the TAWECS device of the present invention; 
     FIG. 3 is a perspective projection showing the geometry of a tether control system therefor; 
     FIGS. 4a and 4b are plots of predicted performance data for the TAWECS of FIG. 2 at two different average wind velocities; 
     FIG. 5 is a top plan view of a modified gas bag base for the TAWECS, in effect a second embodiment thereof. 
    
    
     DETAILED DESCRIPTION 
     A first embodiment of a tethered airfoil wind energy conversion system, TAWECS, is shown in FIG. 2 at 10. In this embodiment, a gas bag airfoil 12 of approximately hemispherical shape is filled with enough lighter-than-air gas, e.g., hydrogen to lift the TAWECS components and provide a moderate tension on the three tethers 14. The WT is mounted in the bottom center of the gas bag 12, with a cylindrical vent 16 above the WT. 
     In a typical such TAWECS configuration, by requiring that the radius of the gas bag (R GB ) be at least three times the radius of the wind turbine (R WT ), the cylindrical vent may be tall enough to produce and sustain a confined vortex by lining the interior sidewall 18 of the cylindrical vent with spiral ridges 20 to impart an angular acceleration to the air flowing vertically in the cylindrical vent, as in the vortex generator (VG) described by Yen in U.S. Pat. No. 4,070,131. The vortex thus produced further reduces the pressure above the WT in addition to the pressure reduction caused by the suction side of the hemispherical airfoil shape of the gas bag. 
     The gas bag, being flexible, should prevent ice and snow buildup, by being free to deflect in six modes: roll, pitch, yaw, heave, sway, and surge. Since gas bags may be of almost arbitrary size, it is likely that the size limitation is determined by the WT vanes, and 150-foot vanes are now under development by others. A hemispherical airfoil produces maximum camber/chord ratio, maximizing pressure drop across the WT, and thus maximizes power produced. 
     The hemispherical hydrogen-supported gas bag is tethered by three or more lines 14 (shown in the elevation view of FIG. 2 as if there were four), which are adjusted by winches driven by motors 21 controlled by a tether control system (TCS) to control the tilt of the gas bag 12 into the wind, such that the angle of attack α is optimum to maximize the pressure drop across the WT. Each tether 14 ends at the outer end of the frame 22 that supports the WT and the generator (GEN) driven by the WT. The gas bag is attached to this frame to lift the TAWECS components. In addition to those components already mentioned, a service line 24 is provided from the bottom of the GEN to the ground, and along the ground a distance greater than R GB , to provide hydrogen replenishment from a hydrogen generator 26 on the ground, power for aircraft warning lights, if required, control signals for a hydrogen release valve 28 in the top of the gas bag, control signals for pitch control of the WT vanes, lightning ground cable 32 30 and a power output cable. Service line 24 may be led along one of the tether lines 14, instead of vertically to the ground as shown in FIG. 2. This will enable placing two or more TAWECS one above the other at particularly desirable (windy) sites. The power output cable of service line 24 may be replaced with a microwave transmitter in the TAWECS and a microwave receiver on the ground. A wind direction indicator 34 is required on the ground to supply a wind direction-representing signal to the TCS, which calculates the required lengths of the three tethers 14 to tilt the TAWECS into the wind at its optimum angle of attack α  to maximize power output. Control signals are then sent to the three tether winch motor controllers 36 to set the three tether lengths in accordance with these calculations. In effect, as each tether line is shortened, e.g. by reeling in on the respective winch, the two ends of that tether line are brought closer to one another. Since the tether line extends between its end aloft with the airfoil and its ground-anchored end, the act of effectively shortening it pulls downwards on the corresponding side of the aloft airfoil, thus correspondingly tilting the airfoil. 
     To obtain the equations that must be solved in real time by the TCS to control the three tether winches 21, consider FIG. 3, in which the circle at elevation H above the ground at its center represents the bottom of the gas bag 12. Let 
     
         ∠AOC=∠COB=∠AOB=2π/3 
    
     
         OA=OB=OC=R.sub.GB, the three frame arms 
    
     
         O&#39;D=O&#39;E=O&#39;F=R.sub.B in the ground plane 
    
     Then 
     
         a.sub.1 =(a.sub.2.sup.2 +a.sub.3.sup.2).sup.1/2 =(a.sub.2.sup.2 +(H-a.sub.5).sup.2).sup.1/2 
    
     
         b.sub.1 =(b.sub.2.sup.2 +b.sub.3.sup.2).sup.1/2 =(b.sub.2.sup.2 +(H-b.sub.5).sup.2).sup.1/2 
    
     
         c.sub.1 =(c.sub.2.sup.2 +c.sub.3.sup.2).sup.1/2 =(c.sub.2.sup.2 +(H=c.sub.5).sup.2 L.sup.1/2 
    
     where 
     
         a.sub.2 =R.sub.B -a.sub.4 =R.sub.B -a.sub.6 =R.sub.B -(R.sub.BG.sup.2 -a.sub.5.sup.2).sup.1/2 
    
     
         b.sub.2 =R.sub.B -b.sub.4 =R.sub.B -b.sub.6 =R.sub.B -(R.sub.GB.sup.2 -b.sub.5.sup.2).sup.1/2 
    
     
         c.sub.2 =R.sub.B -c.sub.4 =R.sub.B -c.sub.6 =R.sub.B -(R.sub.GB.sup.2 -c.sub.5.sup.2).sup.1/2 
    
     and 
     
         for θ=0 to 2π/3, 
    
     
         a.sub.5 =R.sub.GB sin α cos 3θ/2 
    
     
         b.sub.5 =-R.sub.GB sin α cos 3θ/2 
    
     
         c.sub.5 =-R.sub.GB sin α 
    
     
         for θ=2π/3 to 4π/3, 
    
     
         a.sub.5 =-R.sub.GB sin α 
    
     
         b.sub.5 =-R.sub.GB sin α cos 3θ/2 
    
     
         c.sub.5 =R.sub.GB sin α cos 3θ/2 
    
     
         for θ=4π/3 to 2π, 
    
     
         a.sub.5 =-R.sub.GB sin α cos 3θ/2 
    
     
         b.sub.5 =-R.sub.GB sin α 
    
     
         c.sub.5 =R.sub.GB sin α cos 3θ/2 
    
     These equations are readily solvable by many commercially available, inexpensive microprocessors, for given values of R B , R GB , H, and α, with θ obtained from the wind direction indicator 34. 
     The hydrogen generator 26 may be one of several known types, which produce hydrogen by electrolysis of water (possibly obtained from condensed water vapor in the air), or by the process described by Pangborn, et al in U.S. Pat. No. 4,075,313, or by other means. (The Pangborn process for generating hydrogen requires use of at least one of several high temperature subprocesses, one of which is preferably carried out at 750° C.-950° C. For those subprocesses requiring temperatures exceeding that obtainable from a rotary mechanical heat generator, either a double convex lens may be used when the sun is available (or some other means of concentrating the sun&#39;s rays), or some of the hydrogen generated may be burned to produce the required temperature. Electric heating could be used in the Pangborn process for hydrogen production, and probably would be preferable, connected to the WT-driven electric generator (either resistance, arc, or induction heating). In fact, electric heating may be preferable for all the subprocesses requiring heat, to avoid burning any of the hydrogen produced by the hydrogen generator.) 
     The hydrogen generator 26 must have a storage tank 38 and reserve capacity enough for a heavy snow or ice load accumulating during a period of no wind, to provide the extra lift required to keep the gas bag aloft with moderate tension on the tethers. The hydrogen generator may supply hydrogen not only to the airfoil, but elsewhere as an end product of the TAWECS. The hydrogen generator may be borne aloft by the airfoil. The TCS must sense tension on the tethers, and open the hydrogen supply valve 40 when tension becomes too low. The TCS must also sense excessive tension on the tethers, and open a release valve 28 in the top of the gas bag to release excess hydrogen to lower the tension on the tethers. The TCS must also provide for manual control of the tether winch motors for use in lowering the TAWECS for maintenance, which would require sufficient control to settle the GEN and frame gently into a special cradle 41. 
     Leading and trailing edge flaps 42, 44, which increase the camber, coefficient of lift, and optimum angle of attack of an airfoil, can be added to the hemispheric gas bag airfoil by adding a metal ring 46 around the base of the gas bag of sufficient strength to support the flaps, and attached to the frame 22 that supports the WT and GEN and connects to the tethers 14. Such a metal ring may be desirable for another reason-to provide tie points for hold-down lines (not shown) or a net (not shown) over the gas bag. 
     In lieu of an electric generator GEN, the WT may drive a flexible shaft (not shown) connected to a mechanical load (not shown) on the ground. 
     The equation set forth hereabove in regard to the simplified system shown in FIG. 1, relating pressure across the WT, airfoil geometry, and wind velocity, may be rewritten 
     
         Δp/V.sub.w.sup.2 =0.075 a ρ 
    
     and compared with test data for an airfoil approximating a hemisphere, to confirm the derivation. In &#34;Aerofoil Sections&#34;, F. W. Riegels, Butterworth, London, 1961, the airfoil with a section closest to that of a hemisphere is the Go 625, tested at Gottingen in 1941, for which a =10, pressure coefficient C p  =-1.6 on the suction side and 0 on the pressure side at 20% of chord (length from leading edge) for a design coefficient of lift C L  * of 0.075, and α=14° for C L  max of 1.5. From Riegels, p. 275, C p  =Δp/q=Δp/(1/2ρV w   2 ) Δp/V w   2  =1/2ρC p   
     Comparing the derivation with regard to FIG. 1 for Δp/V w   2  with test data for this airfoil, 
     
         0.075 aρ=1/2ρC.sub.p 
    
     
         0.15 a=C.sub.p. 
    
     1.5≈1.6 and the derivation is approximately confirmed. In &#34;Handbook of Airfoil Sections for Light Aircraft&#34;, by M. S. Rice, Aviation Publications, Milwaukee, 1971, p. 73 shows the test data for a USA-35A airfoil, with a section closest to that of a hemisphere. No test date is given, but most of the 125 airfoils in the handbook were tested between 1918 and 1937. The USA-35A airfoil has a value of a of 9.35 at 30% of chord, with α=20° for maximum value of C L . No data on C p  are given in Rice. From the data for these two airfoils, a hemispherical airfoil with a cylindrical central vent, and R GB  =3R WT , should have a value of a of about 20 at 50% of chord, and an α of about 30°. (a max  for a hemisphere=(1/2r/2r)100=25). 
     Having confirmed from test data that Δp/ρ=0.075 a V w   2 , the derivation with regard to FIG. 2 for RMS power obtainable from the WT then follows: ##EQU2## where w=mass rate of air flow through the WT in lb/sec 
     c p  =power coefficient of the WT, assumed to be 0.4 from &#34;Wind Machines&#34;, by F. R. Eldridge, Mitre Corp., Publication No. MTR-6971, October 1975, p. 55. 
     A v  =elliptical area facing the wind of the vent in which the WT is located (i.e., the vertical projection of the circular vent)   (ft 2 ) 
     V v  =vertical air velocity in the vent (ft/sec) 
     g=gravitational acceleration=32.2 ft/sec 2  ##EQU3## 
     To obtain estimates for plant cost per unit of power produced, the volume of hydrogen required to lift the TAWECS components must first be found from the equation 
     
         Lift-W.sub.T =ε ##EQU4## Then ##EQU5## W.sub.T =total weight of the components lifted by the gas bag ε=an arbitrary moderate lift, found by 
    
     successive approximation by varying R GB , to ensure moderate tension on the tethers. An ε≈10 R WT  was used in the following calculations. For each successive approximation of R GB , an approximate value of a is first assumed, ε is calculated and a revised value of a is found from 
     
         a=(1/2(R.sub.GB -R.sub.WT)/2R.sub.GB)100=25(1-R.sub.WT /R.sub.GB) 
    
     Then ε is recalculated using this revised value of a and this process is iterated until ε meets the above criterion. Then the component costs are estimated and summed, and the sum $ T  is divided by P. Let 
     
         W.sub.T =W.sub.GB +W.sub.WT +W.sub.GEN +W.sub.FRAME +W.sub.TETHERS +W.sub.LINE +W.sub.FLAPs 
    
     and let d be the unit weight for each component to be lifted by the gas bag. Then 
     
         W.sub.GB =d.sub.GB A.sub.GB =d.sub.GB (2πR.sub.GB.sup.2 +πR.sub.GB.sup.2 -2π(1.1R.sub.WT).sup.2 +2π(1.1R.sub.WT)R.sub.GB)3 
    
     where the factor 3 is included to account for the compartmentation of the gas bag necessary to prevent one puncture causing catastrophic damage. Then ##EQU6## and let C be the production model unit costs for each component of the TAWECS system, both those to be lifted by the gas bag, and the ground supported components. Then ##EQU7## Let $ Control  Sys =$300; Let C Winch  and Motor =4 R WT  $/Winch and Motor ##EQU8## Using these equations provides the data in FIGS. 4a and 4b, which indicate that this TAWECS configuration should produce power economically, but there may be little or no economic advantage to large values of R WT . However, all the assumed values for component unit weights and costs are subject to revision, and the increase in $ T  /P is probably due to too large an assumed unit cost (and/or unit weight) for at least one component. Even if the unit costs are too low by a factor of 1/2-1/5, $ T  /P should be economic for either power grid or remote site applications when V w  ≧15 mph. 
     FIG. 5 relates to a second embodiment of the invention in which there is provided an alternative to the metal ring 46 of the embodiment of FIGS. 2-4. FIG. 5 shows the second embodiment in schematic top plan view of the level of the gas bag base. Two metal rings 60, 62 may be used, made of shaped steel or aluminum (e.g., tubular), with spokes 64 in tension between the rings to support the inner ring 60. The outer ring 62 supports the flaps 42, 44. The inner ring supports a three-arm frame 22&#39; which supports the WT and GEN. The gas bag is connected to both rings to support the TAWECS components borne aloft. The gas bag should be constructed in sections of about 30° per section, or less, each section self-contained, to minimize damage and gas loss from one puncture. The section vertical sides and bottoms may be black plastic or fabric, with transparent hemispherical section tops, to warm the hydrogen inside by solar radiation and greenhouse effect, and increase lift by decreasing the density of the hydrogen. The gas bag sections connect at the top center to a circular member such as a tube (not shown) which supports the hydrogen release valve, the aircraft warning lights, if required, and the lightning rods. With these structural changes in the TAWECS, the previous calculations are modified as follows: ##EQU9## Then 
     
         W.sub.T =W.sub.GB +W.sub.OR +W.sub.FLAPS +W.sub.IR +W.sub.Spokes +W.sub.FRAME +W.sub.WT +W.sub.GEN +W.sub.TETHERS +W.sub.LINE ##EQU10## Using these modified equations, significant decreases in $.sub.T /P are obtained when compared with FIG. 4 (approximately 23% for V.sub.w =10 mph and 12% for V.sub.w =15 mph). 
    
     For the TAWECS of FIGS. 2 and 5, with R GB  =3 R WT  and a confined vortex generated and sustained, the volume of hydrogen necessary for ε=10 R WT  may be found by solving for it in 
     
         Lift-W.sub.T =ε 
    
     Let a=20, α=30°, and P=twice the power produced by a TAWECS without a confined vortex. Let air at a pressure slightly greater than ambient fill the volume of the gas bag not required by the hydrogen. Making these changes, 
     
         P=0.001837 R.sub.WT.sup.2 V.sub.w.sup.3 
    
     
         A.sub.GB =π(3 R.sub.WT (27 R.sub.WT -6.19 R.sub.WT)-2.42 R.sub.WT.sup.2)=60πR.sub.WT.sup.2 
    
     
         W.sub.T =0.1 A.sub.GB +12.55 R.sub.WT.sup.2 +5 P=(10.37+12.55) R.sub.WT.sup.2 +5 P=22.92 R.sub.WT.sup.2 +5 P 
    
     
         Lift=0.0751 Vol H.sub.2 =W.sub.T +10 R.sub.WT 
    
     
         Vol H.sub.2 =13.32(22.92 R.sub.WT.sup.2 +5 P+10 R.sub.WT) 
    
     
         $.sub.T =C.sub.GB A.sub.GB +0.02 Vol H.sub.2 +0.05 R.sub.WT.sup.3 +R.sub.WT (14.25 R.sub.WT +15)+300+50 P 
    
     Using these equations, the data obtained show a decrease in $ T  /P when compared with the TAWECS of FIGS. 2 and 5 of approximately 22% for V w  =10 mph and 7% for V w  =15 mph. Whether or not this case is in fact more economic than that of the first embodiment will be determined by the strength of the vortex (if any) that is generated and sustained, which can only be determined by further experiment. 
     In FIG. 2, the TAWECS is shown disposed over a flat terrain surface. It should be apparent that a hilltop site is included in the purview of the invention, even preferred, since wind velocity is generally greater at such sites. 
     The TAWECS could be assembled on a relatively flat terrain surface, inflated, and then moved to a relatively inaccessible site for installation, such as a hilltop, marsh, or over water by using one helicopter per tether. 
     To decrease the weight and cost of the generator connected to the wind turbine, a synchro generator could be used, connected electrically to a synchro motor on the ground at some convenient distance away, which is then connected to the load (electrical or mechanical). 
     It should now be apparent that the present invention provides a tethered airfoil wind energy conversion system preferably incorporating the following features and advantages: 
     A flexible gas bag support which: 
     a. Provides a maximum ratio of camber/chord, which produces a maximum pressure drop across the vent containing the wind turbine, thus maximizing the power produced by the wind turbine 
     b. Provides a means of eliminating ice and snow buildup 
     c. Allows wind turbines probably limited in size only by the wind turbine vanes 
     d. May provide in one configuration a confined vortex above the wind turbine to further increase the pressure drop across the wind turbine; and 
     A tether control system to keep the hemispheric gas bag airfoil oriented into the wind from any direction at an optimum angle of attack to maximize power output. 
     There are many forms of airfoils for aircraft applications employing reduced pressure on the suction side and increased pressure on the pressure side (as described in any aerodynamics text). A variety of gas bag shapes employing these basic principles may be used for wind turbine applications in place of the hemispherical form shown. 
     It should now be apparent that the tethered airfoil wind energy conversion system, as described hereinabove, possesses each of the attributes set forth in the specification under the heading &#34;Summary of the Invention&#34; hereinbefore. Because it can be modified to some extent without departing from the principles thereof as they have been outlined and explained in this specification, the present invention should be understood as encompassing all such modifications as are within the spirit and scope of the following claims.