Abstract:
Airborne system for producing electricity from wind includes at least one shaft, one or more wind turbines rotatably mounted to each shaft, and generators that convert rotation of the turbine(s) into electricity. A lifting section is connected to the shaft(s) and generates a lifting force that causes the system to be airborne. A center of gravity of the system is lower than its center of lift. When the turbines are exposed to wind and rotate (at least one in one direction and another in an opposite direction), torques induced on the shaft(s) of the system balance each other and remaining deferential torque is balanced by returning torque that is generated by the angular deviation of the center of gravity from its lowest position. A magnitude of this returning torque increases as the angular deviation increases until the system reach angular stability. Electricity is generated and conducted for storage or usage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 12/284,046 filed Sep. 18, 2008 now U.S. Pat. No. 7,709,973, and is related to U.S. patent application Ser. No. 12/465,877 filed May 14, 2009 which is a divisional of the &#39;046 application, both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a wind turbine system for generating electricity and more specifically to a wind turbine system for generating electricity which is airborne, and includes both turbine and generator on a common, airborne structure. 
     BACKGROUND OF THE INVENTION 
     Wind energy is one of the most readily available forms of renewable energy, which include solar, hydro, geothermal energy, and is therefore often used to generate electricity. The density of wind energy, in terms of watts per square meter, is one of the highest among other forms of renewable energy. 
     Existing systems for utilizing wind energy to generate electricity have certain disadvantages. For example, since wind velocity generally increases with altitude and a large wind velocity is critical to optimize wind-based electricity generation, tall towers must be constructed to elevate a wind turbine to a high operational altitude in order to maximize the potential to generate electricity from the wind. However, tall towers are expensive to build and once built, are subject to intense vibrations during operation. Furthermore, land for building the towers to support such wind turbines is limited in view of numerous, known factors, including acquisition costs, environmental impact, zoning issues. 
     Although offshore winds, i.e., winds over bodies of water, are stronger than winds over land, the construction of support structures for wind turbines over a body of water is expensive, although there are significantly fewer limitations on the space for building support structures over such bodies of water. The most notable limitation is that the construction of support structures for wind turbines is limited to certain depths of the bodies of water. 
     In view of the problems with installing wind-based electricity generating systems over land or bodies of water, flying windmills have been developed. Generally, such flying windmills do not require an extensive support structure connected to land or otherwise anchored over a body of water. One such flying windmill is the well-known Magenn system, which is lighter than air, and utilizes the Magnus effect. A drawback of this prior art system is that its power generation is very limited and it is not very efficient. 
     Another flying windmill currently under development is a flying electric generator, by Sky Wind Power Company. This system is heavier than the air and attempts to utilize the wind in the upper level of the atmosphere. Among its drawbacks are that it is expensive to construct, includes complex mechanical parts and is not very practical. 
     It would therefore be desirable to provide an airborne wind turbine, electricity-generating system, which overcomes the drawbacks of the systems mentioned above. 
     OBJECTS AND SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a new and improved system and method for generating electricity from wind at different altitudes. 
     It is an object of the present invention to provide a system for generating electricity from wind which will be angularly stable while being airborne. 
     In order to achieve these objects and possibly others, a system for producing electricity from wind in accordance with the invention includes at least one shaft, at least one turbine section each including at least one wind turbine that can rotate relative to a respective one of the at least one shaft when exposed to a wind; and a plurality of generators each associated with one of the wind turbines and arranged to convert rotation of the wind turbine into electricity. The system also includes a lifting section, connected to the at least one turbine section for generating a lifting force to enable the entire system to be airborne at a desired altitude. The system is preferably connected by a tether to an anchoring section. The electricity generated by the generators is conducted to usage or energy storage on the system itself and/or to a ground location via the anchoring system or another cabling system. The anchoring system can include a winch that can control the operational altitude of the system. 
     The amount of power that can produce by the system of the present invention is proportional to the multiplication of the torque that the wind turbine induces on the system by the angular velocity of the turbine; power=torque×angular velocity. As the diameter of a high power wind turbine is very large, the angular velocity of the turbine is relatively low, and therefore, the torque that the turbine induces on the system through the generator is very high. In order to keep the angular stability of the system, it is preferable to provide a balancing torque which will balance the torque that the wind turbine is inducing on the system while operating, this balancing torque must be equal to the wind turbine&#39;s torque, and in an opposite direction. Without the balancing torque, the whole system might be rolling angularly in the direction of the wind turbine rotation and little or no electricity can be generated. 
     An important object of the system of the present invention is to provide the balancing torque to the airborne wind turbine system. 
     In order to continue this description of the present invention, two expressions will be defined:
     CENTER of GRAVITY; and: CENTER of LIFT of stationary floating body;   

     Center of Gravity of a body, defined as the point in or near a body at which the gravitational potential energy of the body, is equal to that of a single particle of the same mass (as the body), located at the same point and through which the resultant of the gravitational forces on the component particles of the body act. 
     Another word, the Center of Gravity is a point that if all the mass elements of the body will be concentrated in this point, the same physical status of the body would result. 
     Center of Lift of a stationary floating body, defined as the point in or near the body, that if all the partial lifting forces acting on the floating body will be concentrated in this point, the same physical status of the body will result. 
     In one version of the invention, the at least one turbine section (which includes the main shaft, the generator and the transmission) is attached to two inflated lighter than air bodies, one in front of the turbine section and the second behind the turbine section. In this version of the invention, the respective shaft of that turbine section and the generator are placed below the center of the inflated lighter than air bodies in a way that the center of gravity of the system is lower than the center of lift of the system. When the turbine is not rotating, the system will positioned itself in a way that the center of gravity is vertically below the center of lift of the system (like a pendulum), and when the turbine start to rotate, torque will induce on the system and the system will start to rotate in the same direction of the turbine. By this rotation, the angular position of the center of gravity will change, and angle will opened between the line that connects the center of gravity point with the center of lift point and the vertical direction; this angle will be called alpha. A returning torque will act on the system (as the returning torque acting on pendulum). The returning torque will be called RT.
 
 RT =mg×d×Sine Alpha;
 
     wherein
         m is the mass of the system;   g is the gravity;   d is the distance between the center of gravity and the center of lift;       

     the maximum returning torque:
 
 RT  max=mg×d×1 (Sine 90=1).
 
     The torque that the wind turbine induces on the system will be called WT; If WT&lt;mg×d, the system will reach equilibrium when 0&lt;angle Alpha&lt;90. 
     In another version of the invention, one end of the at least one turbine section is connecting by a tether to the lifting section; and the other end of that turbine section, by another tether to the anchoring section. The line between the two points of the turbine section that are connected to these two tethers will be called the Line of Tension. The system is constructed in a way that the center of gravity of the system is lower than the Line of Tension. When the wind turbine is not rotating, the system will bring itself to a position where the center of gravity will be vertically in the lowest possible position relative to the Line of Tension, (like a pendulum). When the wind will blow through the turbine, the turbine will rotate and will induce rotational torque on the system, the system will start to rotate to the same direction of the turbine; by this rotation of the system, the angular position of the center of gravity will change; and angle will be opened between the plane that connect the center of gravity point with the Line of Tension, and the vertical plane that is crossing the line of Tension. This angle will be called Alpha. A returning torque will act on the system (as the returning torque that acts on a pendulum). This returning torque will be called again RT, the calculations are the same as the above; the only different is that d will be the distance between the center of gravity point and the Line of Tension.
 
 RT =mg×d×Sine Alpha;
 
 RT  max=mg×d×1; and
 
if WT&lt;mg×d the system will reach equilibrium; when 0&lt;angle Alpha&lt;90.
 
     In another version of the invention, the turbine section will include more than one wind turbine, these turbines are constructed in a way that when the wind is blowing through them they will rotate in opposite directions; therefore each one of the turbines will induce rotational torque on the system in opposite directions. These opposite rotational torques will balance each other; but in order to avoid the rotating of the whole system the result of the torques acting on the system, must be zero, achieving zero torques result, requires a very accurate and complicated angular controller. This angular controller should continuously control the angle of attack of the wind turbines blades, the power that produced by the generators of the system and other things that can influence the magnitude of the torque that each turbine inducing on the system. The alternative to this complicated angular controller is system that utilizing the principle of the present invention in the same way as described above. The only different in this type of system with more than one wind turbine, is that the required returning torque RT needs to balance only the differential unbalanced torque that has not been balanced by the opposite rotating turbines. And, this can be achieved in low cost and in simple and reliable way, using the principle of the present invention. 
     Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawings, wherein like parts have been given like numbers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals identify like elements, and wherein: 
         FIG. 1  is schematic front view partly in cross section of the system of the present invention when the wind turbine of the system is not rotating. 
         FIG. 2  is the system of  FIG. 1  in an equilibrium state when the wind turbine is rotating. 
         FIG. 3  is a side view partly in cross-section, of the first embodiment of a wind turbine system in accordance with the present invention. 
         FIG. 4  is a detailed view partly in cross section, of the turbine section of the system shown in  FIG. 3 , when the system has two turbines. 
         FIG. 5  is a detailed view partly in cross section, of the turbine section of the system shown in  FIG. 3  when the system has one turbine. 
         FIG. 6  is a side view partly in cross section of second embodiment of a wind turbine system in accordance with the present invention. 
         FIG. 7  is a detailed view partly in cross section of the turbine section of the system shown in  FIG. 6  when the system includes one turbine. 
         FIG. 8  is a detailed view partly in cross section of the turbine section of the system shown in  FIG. 6  when the system includes two turbines. 
         FIG. 9  is a cross section view along the line  3 - 3  of  FIG. 8 , this cross section view shown the opposite angle of attack of the blades of the two turbines shown in  FIG. 8 . 
         FIG. 10  is a side view partly in cross section of another embodiment of an airborne wind turbine system in accordance with the present invention. 
         FIG. 11  is a detailed drawing of a version of a lifting section of a wind turbine system in accordance with the present invention. 
         FIG. 12  is a side view of another embodiment of a wind turbine system in accordance with the invention. 
         FIG. 13  is a view of the wind turbine system of  FIG. 12  taken along the line  13 - 13 . 
         FIG. 14  is across sectional view taken along the line  14 - 14  of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the accompanying drawings,  FIG. 1  and  FIG. 2  are schematic drawings of the system of the present invention, many details are not shown in these drawings and are shown elsewhere and explained later, the purpose of these simple drawings is to explain the basic principles of the system of the present invention. 
     Reference  19  is a sealed cylindrical body; body  19  is filled with lighter than air gas  20 , such as helium or hydrogen; 
     The system has two similar bodies  19 , one of them is not shown in the drawings. The two bodies  19  are lighter than air and are generating a lift force  14  (14=L) which causes the system to be airborne in the atmosphere. The two bodies  19  are connected to each other rigidly by a shaft  12 ; the shaft  12  is placed below the symmetrical center of the two bodies  19 ; a wind turbine  13  is rotatably mounted to the shaft  12 , in a way that allow the turbine  13  to rotate relative to the shaft  12 . The turbine  13  is connected through transmission to a generator (the generator is not shown in these drawings), the generator is mounted rigidly and fixed to the shaft  12 . The total weight of the floating system is  15  which is equal to mg. wherein m is the total mass of the system, and g is the gravity.  10  is the Center of Lift of the system, and  11  is the Center of Gravity of the system, (the center of lift and the center of gravity are defined above). 
     The lift force  14 &gt;15, (15 is the weight of the system), a tether, which is not shown in these drawings, controls the system altitude. When the turbine  13  is not rotating, the system will position itself in a way such that the center of gravity  11  will be vertically below the center of lift  10 , (same as a pendulum). The distance between the points  10  and  11  is  21  (21=d). This position is shown in  FIG. 1 . 
     When the wind is blowing through the turbine  13 , the blades of the turbine  13  will start to rotate in direction  16  as shown in  FIG. 2 ; the turbine  13  will transfer the rotational motion to the generator, the generator while generating electricity will resist the transferred rotational motion, and as the generator is rigidly mounted to the shaft  12 , the rotational torque of the turbine  13  will be transferred to the whole system through the shaft  12 . The system will start rolling angularly in the same direction of the rotation of the turbine  13 , and angle  18  (18=Alpha) will be opened between the line that connects point  10  to point  11 , and the vertical direction. A returning torque will act on the system, this torque will be RT; the distance between the center of gravity point  11  and the vertical line that passes through the center of lift point  10 , is 17 (17=d×Sine Alpha);
 
Therefore:  RT =mg×d×Sine Alpha;
 
 RT  max=mg×d×1(Sine 90=1);
 
The torque that the turbine  13  is inducing on the system is: WT, if WT&lt;RT max, the system will reach equilibrium when: 0&lt;Alpha&lt;90;
 
     If WT&gt;RT max, the system will keep rolling angularly and no electricity can be produced. 
       FIG. 3  is a side view partly cross section of the first embodiment of the present invention;  FIG. 4  and  FIG. 5  are two versions of this first embodiment. 
     In  FIG. 3 , element  114  is a sealed inflatable front floating body; this body  114  is filled with lighter than air gas  115 , this gas  115  can be helium, hydrogen, hot air, or any lighter than air gas;  134  is the rear inflatable floating body that is also filled with gas  115 . These two floating bodies  114 ,  134  generate a combined lifting force  109 ; ( 109 =L); the combined lifting force  109  can be consider as acting on the system through the center of lift point  110 ; the center of lift point has been defined above. 
     The total weight of the system is 112 (112=mg); the total weight of the system can be considered as acting through the center of gravity point  111 ; the center of gravity point has been defined above; the vertical distance between  110  and  111  is 132, (132=d); when the system&#39;s turbines  113  are in rest status. The system is floating in the air, therefore  109 &gt; 112  (L&gt;mg), the access lift force is balanced by the tethers  126  and  125 ; the tether  126  is connected to the anchoring base  128  through winch  127 ; winch  127  can adjust the operational altitude of the system. Element  122  is a structural beam that supports the body  114  and connects the system to the tethers  125  and  126 ;  122  is also connected, preferably rigidly, to the supporting wall  119 ; another supporting wall  119  is connected rigidly to the rear structural beam  123  that supports the rear floating body  134 . A shaft  121  is mounted rigidly and fixed between the two supporting walls  119 ; the shaft  121  is placed vertically below the center of the supporting walls  119 . The shaft  121  connects the front body  114  to the rear body  134 . 
     Around the shaft  121 , one or more than one, wind turbines  113  are rotatably mounted.  FIG. 4  shows a detailed side view cross section of a version of this embodiment which includes two wind turbines; (more than two turbines can be included in this embodiment of the invention).  FIG. 5  shows a detailed side view cross section of version of this embodiment which includes one wind turbine; the turbines  113  are assembled around the shaft  121  through bearings  131 ; the bearings  131  allow the turbines  113  to rotate around the shaft  121 . 
     Generators  118  are mounted to the supporting walls  119 ; the generators  118  are connected to the turbines  113  through gear wheels  117  that are mounted rigidly to the generators shaft, and gear wheels  116  are mounted rigidly to turbines  113 ; the electricity produced by the generators while rotating passes by conductive wire  130  through the hollow shaft  121  to transformation unit  129 , and from transformation unit  129  to the tether  126 , and attached to tether  126  to the anchoring base  128 , for usage or to energy storage. The electricity produced may also be used by the system, i.e., by one or more components on the system or stored on the system for future use by such components, the electricity may also be directed to the ground to a usage or storage system independent of the tether  126 , i.e., via a cable that is not coupled to the tether  126 . 
     The role of the transformation unit  129  is to collect the electricity from all generators  118  and to unify them to one output at a desired voltage. When the system includes one turbine, one or more than one generator can be used, this version is shown in  FIG. 5 . 
     When the turbines of the system are not rotating, the system will position itself in a way such that  111  (the center of gravity) is approximately vertically below  110  (the center of lift) When turbines  113  of this embodiment are exposed to wind, the airborne system will drift downwind in a way such that the shaft  121  will be substantially parallel to the wind direction. In the version shown in  FIGS. 3 and 4 , the two turbines will start to rotate relative to the shaft  121 ; each turbine will rotate in an opposite direction from the other turbine, as their blades have opposite angles of attack relative to the wind direction; each turbine will induce a rotating torque on the system through the generators. These torques will be WT 1  and WT 2 , the two torques will act on the system in opposite directions and therefore they will balance each other. The difference between the two torques, will be WT 3 ; (WT 1 −WT 2 =WT 3 ). The whole system will start to rotate in the direction of WT 3 ; angle Alpha will be opened between the vertical direction and the line between  110  and  111 . As explained above, returning torque will act on the system in opposite direction of WT 3 ; (same as in a pendulum). If WT 3 &lt;mg×d; (mg=112; d=132), the system will reach equilibrium when: 0&lt;angle alpha&lt;90. 
     If the version of this embodiment includes one turbine as shown in  FIG. 5 , the torque that will be induced on the system by one turbine will be WT, and in this case, the system can reach equilibrium just if WT&lt;mg×d; (mg=112; and d=132). 
       FIG. 6  is a side view partly in cross section of the second embodiment of the system of the present invention;  FIG. 7  and  FIG. 8  are two detailed side view partly in cross section versions of this second embodiment;  FIG. 9  is a detailed cross section of the turbines blades shown in  FIG. 8 . 
     In this second embodiment of the invention, the wind turbine section includes at least one wind turbine, but can include a few wind turbines wherein part of them will rotate in the opposite direction from the rest. The turbine section of this embodiment is connected by a tether to the lifting section, and by another tether to the anchoring section. 
     The lifting section:  214  is a lifting body that is filled with lighter than air gas  215 , like helium, hydrogen, hot air or any other lighter than air gas. The lifting body  214  has an aerodynamic shape so when the wind  240  is blowing over the lifting body  214 , there is an additional lifting force acting on the lifting body  214 . The lifting body  214  is connected to the turbine section of the system through a tether  225 . 
     In the turbine section,  221  is the main shaft, this is preferably a cylindrical hollow tube that gives a constructional strength to the turbine section; it is made preferably from composite material or from other structural light material. The turbines  213  are rotatably mounted around main shaft  221  through a bearing assembly  231 , in a way such that they can rotate around the main shaft  221 . A beam  222  is mounted and fixed, e.g., rigidly and substantially perpendicular, to the rear end of the main shaft  221 ; the other end of beam  222  is connected to the tether  225  and through tether  225  to the lifting body  214 . Element  220  is a rear supporting element that is mounted rigidly and fixed to the main shaft  221 ;  219  is a front supporting element that is mounted rigidly and fixed to the main shaft  221 ; on these two supporting elements, two electricity generators  218  are mounted. The generators  218  are connected to the turbines  213  through transmission wheels  217  that are mounted rigidly to the generators shafts, and  216  that are mounted rigidly to the turbines  213 . When the turbines  213  are rotating, the rotation is transmitted to the generators  218  through the transmission wheels  217  (any other transmission type, like transmission belts can be used in order to transmit the rotation of the turbines  213  to the generator  218 ). Element  224  is a connecting element at the front end of  221 ; the tether  226  is connecting the turbine section to the anchoring base  228 , through the winch  227 . The winch  227  can control the operational altitude of the system. Attached to the tether  226  is the wire  230 . Wire  230  conducts the electricity produced by the generators  218  and passes through the transformation unit  229 . The role of the transformation unit  229  is to collect the electricity that has been generated by all generators  218  and to unify them to one electrical output at a desired voltage. 
     The center of gravity of the turbine section is  211 ; the lift that is generating by the lifting section is  209 ; the line of tension  210  is the line between the two points that connect the turbine section to the tethers  225  and  226 , (the center of gravity and the line of tension has been defined above). The distance between the center of gravity of the turbine section  211  and the line of tension  210  is 232, (232=d). 
     When the turbines  213  are not rotating, the system will position itself in a way such that the center of gravity  211  will be approximately vertically below the line of tension  210  (as a pendulum). 
     When turbines  213  are exposed to wind  240 , the system will drift downwind and the turbines  213  will start to rotate. In the version of the embodiment that is shown in  FIG. 6  and  FIG. 8 , the two turbines  213  will rotate in opposite directions because the blades of the two turbines  213  have an opposite angle of attack relative to the wind direction  240 , this is shown in  FIG. 9 , when the front turbine will rotate to direction  241  and the rear turbine will rotate to direction  242 . 
     Each turbine  213  will induce rotational torque on the shaft  221  through the generators  218 . These torques will be WT 1  and WT 2 ; these two torques act in opposite direction from each other and therefore they partially balance each other. The differential in the opposite torques will be WT 3 . As explained before, the turbine section will start rolling angularly around the line of tension  210 ; angle Alpha will be opened between the vertical plane that passes through line of tension  210  and the plane that passes through point  211  (the center of gravity) and line of tension  210 . Returning torque will act on the turbine section of the system in the opposite direction of WT 3 . This returning torque will be RT, equal to:
 
 RT =mg×d×Sine Alpha; (mg=212 ; d= 232);
 
     If WT 3 &lt;mg×d×1, (Sine 90=1), the system will reach equilibrium when 0&lt;Alpha&lt;90. 
     If WT 3 &gt;mg×d, no equilibrium can be achieved and the system will keep rolling angularly, and electricity cannot be produced. 
     It is important to understand that the turbine section can include any number of turbines and if, after balancing each other&#39;s torque, the resultant torque will be WT 3 , the above calculation is valid. 
       FIG. 7  shows a version of this embodiment of the invention, where the turbine section includes one wind turbine; the torque that this turbine is inducing on the shaft  221  through the generator  218  is WT. 
     As explained above, if WT&lt;mg×d×1, equilibrium can be achieved when: 0&lt;Alpha&lt;90; if WT&gt;mg×d, no equilibrium can be achieved and the system will keep rolling angularly around line of tension  210 . In this case, no electricity can be produced by the system. To make the above description clearer here are some calculations for the version of the embodiment shown in  FIG. 8 . 
     In the following example, the front turbine  113  has an output of 1.5 megawatt and the rear turbine  113  has an output of 1.25 megawatt, (the total output of the system=2.75 MW). The turbines are rotating at a velocity of 10 rpm. 
     The total weight of the turbine section is 10 metric tons; the distance  232  is 5 meters (d=5 meters), therefore: 
     The angular velocity of the turbines is: 10×2×3.14/60=1.047 radian/second; 
     The torque that the front turbine is inducing on the shaft  221  is WT 1 ; 
     As: torque×angular velocity=power;
 
 WT 1=1.5 megawatt/1.047=1.43 mega Newton×meter
 
     The torque that induced on the shaft by the rear turbine  113  is WT 2 ;
 
 WT 2=1.25 megawatt/1.047=1.19 mega Newton×meter;
 
     As the two torques act in opposite directions, the net torque acting on the shaft  221  is WT 3 ; WT 3 =WT 1 −WT 2 =(1.43−1.19) mega Newton×meter=0.24 mega Newton×meter. When this torque is acting on the shaft  221 , the turbine section of this version of the invention, will rotate in the torque direction, angle Alpha will be opened between the vertical plane that passes through line  210  and the plane that passes through point  211  and the line  210 , resulting returning torque; this returning torque will be RT; RT=mg×d×Sine Alpha; as mg=10,000×9.8=98000 and d=5 meter; 
     At equilibrium position WT 3 =RT=0.24 mega Newton×meter=240,000 Newton×meter. 
     Therefore, sine Alpha at the equilibrium position of the turbines section will be:
 
Sine Alpha(at equilibrium= RT/mg×d=WT 3=240,000/490000=0.49;
 
     Therefore, the angle at which the system will reach equilibrium will be approximately 30 degree;
 
(Sine 30=0.5)
 
       FIG. 10  is a side view partly cross section of the third embodiment of the present invention. 
     In this embodiment, there are a plurality of turbines sections that are connected by tether  226  in a serial formation. The upper turbines section is connected to the lifting section, and lower turbines section is connected to the anchoring base  228  preferably through winch  227  that can control the operational altitude of the plurality of turbine sections. 
     Each turbine section, the lifting section, and the anchoring base are similar to those shown in  FIGS. 6-8 . 
     The advantages of this embodiment are: harvesting of wind energy in multiples altitudes by the same system; the cost of electricity output will be lower. 
     The disadvantage is: the system is more complicate to control and to maintain. 
       FIG. 11  shows a possible lifting section that can generate the lifting force to the airborne system, in different embodiments of the invention. 
     Element  327  is an air foil lifter kite;  314  are inflatable sealed balloons,  315  is lighter than air gas that is filled the balloons  314 . 
     The combined lifting force of the balloons and the kite are generating the lifting force for the turbine sections of the systems of the present invention through the tether  326 . 
     Using a kite alone as the source of the lifting force of the system, or using lighter than air balloons alone as the source of the lifting force of the system is possible. 
     The advantage of combined lifting section like the one shown in  FIG. 11  is that it will operate better in all level of wind velocities. When the wind velocity is low, the balloon will generate the main lifting force, when the wind velocity is higher, the balloons will drift stronger downwind and the system can lose altitude, but the air foil kite will generate more lifting force at high wind velocity and will make the control of the system altitude, and the angle of the tether  326  more controllable. A unify body which include the aerodynamic properties of a kite, with the floating properties of lighter than air balloon is possible and shown as item  214  in  FIG. 6 . 
       FIG. 12  shows another embodiment of the invention wherein there are two turbine sections that are connected together by a torque transfer mechanism  235 , such as a mechanical connection, a connecting member, a connecting bar or other torque transferring connection. The torque transfer mechanism  235  may be made of metal, rubber, plastic, combinations thereof, or other any material or combination of materials that enables torque to be transferred between one turbine section connected to one side thereof and another turbine section connected to the other side thereof. The torque transfer mechanism  235  may be made as a single piece of material or from a number of different pieces and attached together. 
     A first turbine section includes a shaft  221 A about which one or more turbines  213 A rotate (only one turbine being shown), and a generator  218 A coupled to the turbine  213 A to produce electricity as the turbine  213 A rotates about the shaft  221 A. Similar to the embodiment shown in  FIG. 6 , a beam  222  is mounted and fixed to the rear end of the shaft  221 A; the other end of beam  222  is connected to one or more tethers  225  and through tether(s)  225  to the lifting body  214 . Beam  222  could be, but is not required to be, mounted rigidly and substantially perpendicular to the rear end of the shaft  221 A. 
     A second turbine section includes a shaft  221 B about which one or more turbines  213 B rotate (only one turbine being shown), and a generator  218 B coupled to the turbine  213 B to produce electricity as the turbine  213 A rotates about the shaft  221 B. The lower turbine section is connected to the anchoring base  228  preferably through a tether  226  controlled by a winch  227  that can thus control the operational altitude of the plurality of turbine sections. 
     In other respects, each turbine section, the lifting section, and the anchoring base are similar to those shown in  FIGS. 6-8 . For example, a single transformation unit may be provided to conduct electricity from the generators of the turbines sections though one or more common conductors or wires for usage and/or storage. Although only a single turbine is shown on each shaft  221 A and  221 B, any number of turbines may be mounted on each shaft. 
     Moreover, although  FIG. 12  shows only two turbine sections, a wind turbine system in accordance with this embodiment of the invention may include three or more turbine sections, each including a shaft, one or more turbines, etc. In each case, a torque transfer mechanism  235  is arranged between adjacent turbine sections. A series of wind turbine sections could therefore be provided with a single lifting section connected, for example, to the uppermost turbine section and single anchoring section connected, for example, to the lowermost turbine section. Thus, a wind turbine system with three turbine sections would include two torque transfer mechanisms. 
     However, in this embodiment, the shafts  221 A and  221 B are offset from one another by the torque transfer mechanism  235  to increase the exposure of the turbines  213 A and  213 B to the wind  240 . 
       FIG. 13  shows the effect of the offset of the rotational axes of the shafts  221 A and  221 B as reflected by significantly increased viewing of the rotation of the turbines  213 A,  213 B. A larger length of the torque transfer mechanism  235  will result in a larger offset in the rotational axes of the shafts  221 A,  221 B by the torque transfer mechanism  235  and thus more exposure of the turbines  213 A,  213 B to the wind. 
     The torque transfer mechanism  235 , or any similar structural connecting member or members that connect the shafts  221 A,  221 B, may be a solid rod as shown in  FIG. 14  or have other forms. In one embodiment, the shafts  221 A,  221 B and torque transfer mechanism  235  may be formed in an integral manner as a single piece. Otherwise, if formed as separate components, the shafts  221 A,  221 B and torque transfer mechanism  235  are connected together in such a manner to ensure the capability of the torque transfer mechanism  235  to transfer torque from one turbine section including shaft  221 A at one side of the torque transfer mechanism  235  to another turbine section including shaft  221 B on the opposite side of the torque transfer mechanism  235 . Collectively, the shafts  221 A and  221 B will therefore be referred to as a shaft section that provides at least two offset shaft portions. 
     Also, in this embodiment, the turbine(s)  213 A on the shaft  221 A and the turbine(s)  213 B on the shaft  221 B are mounted to rotate in opposite directions, and otherwise configured, to improve the angular equilibrium of the system in that a resultant torque arising from the torque induced on the shaft  221 A by rotation of turbine(s)  213 A and the torque induced on the shaft  221 B by rotation of turbine(s)  213 B may be of substantially equal magnitude and opposite in direction. As such, the torque generated by rotation of the turbine(s)  213 A will balance the torque generated by rotation of the turbines  213 B thereby facilitating angular stability of the system. The number of turbines sections, the size of the turbines, the number of turbines and the offset between the rotational axes of the shafts of the turbines sections may be adjusted as desired to optimize exposure of the turbines to wind while providing for and ideally maintaining angular stability of the system. 
     Any of the systems described above can be constructed to operate over land or offshore, over a body of water. Moreover, the systems are easily can operate in various altitudes to maximize the exposure of the systems to high velocity winds. An offshore operation in which the system is mounted to a floating rig on a body of water is likely to be cheaper and easier to implement than existing offshore wind-based electricity generating systems which are mounted on towers because the system in accordance with the invention requires only an anchoring point on the ocean floor, or on a floating rig, with no other constructions. 
     It is to be understood that the present invention is not limited to the embodiments described above, but include any and all embodiments within the scope of the following claims. While the invention has been described above with respect to specific apparatus and specific implementations, it should be clear that various modifications and alterations can be made, and various features of one embodiment can be included in other embodiments, within the scope of the present invention.