Abstract:
An electrical generation system is based upon sail craft that move between towers through wind power. The system includes at least two support towers (or support structures on buildings, for example), and upper and lower cables extending from one tower to another. At least one sail craft is coupled to the upper and lower cables such that wind moves the sail craft along the cables. Each sail craft is coupled to the upper and lower cables with respective upper and lower modules, one or both of which includes a wheel that rotates as the craft moves along the cables. The wheel is coupled to an electrical generator that that feeds one or both of the cables for further distribution through at least one of the towers. A structure at each tower causes the craft to reverse its direction and travel back and forth between the towers in such a way that sail craft travelling in opposite directions to not block the wind to other craft.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/728,941, filed Nov. 21, 2012, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to wind power and, in particular, to an electrical generation system based upon sail craft that move between towers through wind power. 
       BACKGROUND OF THE INVENTION 
       [0003]    The advantages of wind power are many. It is widely available and does not require flowing water or sources of fuel. Harnessing the wind dates back to the first sailboat. Wind-powered machines have ground grain and pumped water for hundreds of years. With the development of electricity, wind power found new applications in lighting buildings remote from centrally-generated power. Throughout the 20th century, small wind plants evolved for farms and residences, while larger utility-scale wind generators were connected to electricity grids for power in remote locations. 
         [0004]    Today, wind powered generators range in size from small plants for battery charging at isolated residences, to expansive offshore wind farms that provide electricity to national electrical networks. Multi-megawatt turbine technologies in use today include advanced aerodynamic, structural, and acoustic engineering design features such as steel tube towers, variable-speed generators, composite blade materials and partial-span pitch control. In 1987, the MOD-5B was the largest single wind turbine operating in the world with a rotor diameter of nearly 100 meters and a rated power of 3.2 megawatts. 
         [0005]    However, not all the energy of blowing wind can be harvested, since conservation of mass requires that as much mass of air exits the turbine as enters it. Betz&#39; law gives the maximal achievable extraction of wind power by a wind turbine as 59 percent of the total kinetic energy of the air flowing through the turbine. Further inefficiencies, such as rotor blade friction and drag, gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine. Commercial utility-connected turbines deliver about 75 percent of the Betz limit of power extractable from the wind, at rated operating speed. 
         [0006]    Conventional wind turbines face a number of obstacles, including intermittency, space requirements, complaints from homeowners; as such, alternative technologies are being given more serious consideration. One alternative is the airborne generator, the basic premise of which is to tether a device to the ground and let it fly around in the strong winds like a kite, either generating power and sending it down a tether to the ground or using the tether itself to produce electricity at its base. 
         [0007]    One example, the Ampyx&#39;s PowerPlane, is a glider that generates electricity by pulling on its tether, which is connected to a ground-mounted generator. The PowerPlane glides around between 1,000 and 2,000 feet; the next iteration of this design should generate 250 to 500 kilowatts continuously. There are also inflatable designs, as well as a soft-wing kite design from North Carolina-based Windlift that uses a 40-square-meter wing flying at a maximum altitude of 500 feet, with the controls and generator on the ground. 
         [0008]    But scaling up airborne prototypes will not be easy without strong government support. In order to be viable, airborne devices would need to stay aloft for long periods of time with little maintenance required. Another challenge is regulation, since airborne systems are so large and consume such a large volume of airspace. 
       SUMMARY OF THE INVENTION 
       [0009]    This invention resides in an electrical generation system based upon sail craft that move between towers through wind power. A wind energy generator constructed in accordance with the invention comprises at least two support towers extending up from the ground and upper and lower cables extending from one tower to another. At least one sail craft is coupled to the upper and lower cables such that wind moves the sail craft along the cables. Each sail craft is coupled to the upper and lower cables with respective upper and lower modules, one or both of which includes a wheel that rotates as the craft moves along the cables. The wheel is coupled to an electrical generator that that feeds one or both of the cables for further distribution through at least one of the towers. 
         [0010]    A structure at each tower causes the craft to reverse its direction and travel back and forth between the towers. In the preferred embodiment, the system includes four cables between the towers, including an upper pair of cables and a lower pair of cables. The structure at the end towers causes each craft to reverse its direction is a spiral track interconnecting the upper and lower cables, causing the sail craft to travel to one tower on the upper pair of cables and travel to the other tower on the lower pair of cables. In this way, sail craft travelling in opposite directions to not block the wind to other craft. There may also be support towers like the center tower shown in  FIG. 14 . There may be one or more cable support towers that are placed between the end towers that do not cause the cable sailor to turn around. 
         [0011]    Each craft further preferably includes a mast extending between the upper and lower modules, with at least one having a forward edge connected to the mast. In the preferred embodiment, each sail is an elliptical sail, and more preferably, each craft uses upper and lower mirrored elliptical sails connected to the mast. A boom may be provided which extends outwardly from the mast and terminates in a distal end, and wherein a portion of the sail is fastened to the boom. In this configuration, upper and lower trailing modules that ride along the upper and lower cables, with a vertical cable extending between the upper and lower trailing modules. A mechanism disposed at the distal end of the boom includes a pulley for engaging a portion of the vertical cable and a motor for operating the pulley to change the angle of the boom relative to the upper and lower cables. 
         [0012]    The system may further include one or more sensors for determining wind direction or wind speed, and an electronic controller operative to adjust the angle of the boom as a function of wind direction or wind speed. One or more sensors may be used for determining the power output of each generator, with the electronic controller being operative to adjust the speed of the sail craft or generator loading to maximize power output. In preferred embodiments, the generator functions as a motor/generator, with the electronic controller being operative to switch between motor and generator modes of operation, including the ability to stop the movement of a sail craft or cause a sail craft to move to a tower for maintenance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a drawing of a sail craft constructed in accordance with the invention; 
           [0014]      FIG. 2A  is a system overview from an orthographic perspective; 
           [0015]      FIG. 2B  is a side view of the system; 
           [0016]      FIG. 2C  is a top view of the system 
           [0017]      FIG. 3A  is an orthographic illustration of a motor/generator module; 
           [0018]      FIG. 3B  is a right side view of the motor/generator module; 
           [0019]      FIG. 3C  is a front view of the motor/generator module; 
           [0020]      FIG. 3D  illustrates a motor/generator module with an open side; 
           [0021]      FIG. 3E  is a view of a motor/generator module seen from a different perspective; 
           [0022]      FIG. 4  depicts a motor/generator assembly; 
           [0023]      FIG. 5A  is a frontal, detail drawing of a motor/generator assembly; 
           [0024]      FIG. 5B  is a side detail drawing of the motor/generator assembly of  FIG. 5A ; 
           [0025]      FIG. 6  illustrates a boom deployment system; 
           [0026]      FIG. 7  is a detail drawing of a sail craft; 
           [0027]      FIG. 8  is another detail drawing of a sail craft; 
           [0028]      FIG. 9  depicts a motor/generator aero pod; 
           [0029]      FIG. 10  shows a boom and pulley assembly; 
           [0030]      FIG. 11  illustrates turn-around towers also shown in  FIGS. 14 and 15 ; 
           [0031]      FIG. 12  is a graph that shows the cable sailor power output versus its speed along the tower cables; 
           [0032]      FIG. 13  shows a graph comparing the cable sailor power output and the wind turbine output with the cable sailor sail area and the wind turbine swept area being equal; 
           [0033]      FIG. 14  shows a service station, a tower, main tower cables and tower bases; and 
           [0034]      FIG. 15  illustrates a turnaround tower. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    This invention is directed to a wind energy system referred to herein as the “cable sailor.” The cable sailor craft ( FIG. 1 ) is a wind energy device that travels along cables suspended above the ground by support towers. A minimum of two parallel cables ( 11 ) are required to support the cable sailor craft and carry electrical power generated or received by the cable sailor craft to the power grid. The two cables are attached to the cable sailor craft and separated vertically by a distance equal to the mast ( 12 ) of the cable sailor craft plus some additional distance needed for compliance to account for uneven cable sag. 
         [0036]    In the simplest form, the cable sailor system requires only two support towers ( FIG. 2 ). The support towers ( 21 ) would nominally be positioned such that the cables ( 22 ) suspended between the support towers are perpendicular or at least transverse to the prevailing winds. The suspended cables are attached to the support towers by a round section of steel ( 23 ) or other high-strength material that guides the cable sailor in a 180 degree turn at the end of each cable sailor system. The helical turn rails ( 24 ) redirect the cable sailor craft 180 degrees and raise or lower the cable sailor craft altitude. The change in altitude is intended to prevent wind blockage by passing cable sailor craft. The suspended cables will typically be a high tensile stainless steel to prevent corrosion and carry electrical power to or from the cable sailor craft. 
         [0037]    The cable sailor craft generates electrical power through two or more teardrop shaped motor/generator (MG) modules ( 31  shown in  FIG. 3 ). The MG modules contain one or more alternating or direct current, radial or pancake electrical motor/generators ( 32 ) ( FIGS. 4 and 5 ). In one embodiment, each motor/generator is rated for between 10 and 20 horsepower, or in alternative units, 7.5 to 15.0 kW. Multiple motor generators within each MG module may be used to increase the power output of an individual cable sailor craft while not compromising the tear drop shape of the MG module. 
         [0038]    As an example, consider a single cable sailor craft in a 30 mile per hour crosswind, with two of its MG modules each containing four motor/generators producing electrical power at a rate of 15 horse power. By simple arithmetic we see that the cable sailor craft is producing nearly 94 kilowatts of electrical power. Furthermore, by considering that multiple cable sailor craft may occupy the same suspended cable and support tower system, we see that nearly 1 mega-watt of electrical power may be produced by as few as 11 cable sailor craft. 
         [0039]    Each cable sailor craft will actively monitor and control the generated electrical voltage phase such that the electrical power is transferred to the upper and lower suspended cables. If the electrical voltage phase is not actively monitored, high circulating currents may flow within the suspended cables and dissipate the generated electrical power as wasted heat. This voltage monitoring and control will be carried out by a system central, or cable sailor craft embedded computer that controls power switching devices such as insulated-gate bipolar transistors. 
         [0040]    The trailing MG modules contain idler pulleys ( 14 ) which ride along the suspended cables and are connected to smaller diameter cables ( 61 ) which run to the Boom Deployment (BD) system ( 15 ) ( FIG. 6 ). The BD system is composed or an electrical servo motor ( 62 ) that drives a cable spindle separated into two sections ( 63 ). Each section of the cable spindle is fastened to and contains the spooled cable that runs to the upper and lower trailing MG modules. The electrical servo motor is controlled by an embedded computer that accounts for various input variables such as electrical power generation output, apparent wind speed, apparent wind direction, cable sailor craft speed, and inter cable sailor craft distance. 
         [0041]    The embedded computer calculates the appropriate boom angle and directs the electrical servo motor to let out or spool the cable accordingly. When executing a turn at the end of the cable sailor system, the BD system will act to spool the cable, bringing the boom to mid craft and preventing the boom from swinging violently from one side to another due to the change in apparent wind direction. To perform these actions, the cable sailor embedded controller will operate in two distinct modes. 
         [0042]    The first mode will be used as the cable sailor travels between supporting towers and is generating power. During this time, a vane anemometer determines the apparent wind direction and speed with respect to the moving cable sailor craft and feeds this information to the embedded controller. The embedded controller will then process the apparent wind direction and speed using an algorithm tailored to the particular locale and actuate the BD system to achieve the appropriate boom angle. The position of the BD system electrical servo motor, and corresponding angle of the boom, will typically be detected using a commercially available optical shaft encoder or Hall Effect sensor. These particular sensors are well suited to this application due to their nearly solid-state design, aside from the bearings and shaft of the optical encoder or Hall Effect sensor. 
         [0043]    In addition, voltage and current sensors will be used to determine the power output conditions of the electrical generators and the embedded controller will adjust the cable sailor craft speed and generator loading to provide peak power output without overloading any of the electrical or mechanical systems. In the event of a failure by the vane anemometer, the voltage and current sensors may be used to determine the cable sailor craft&#39;s speed and bring the craft safely to a stop or a location designated for maintenance. Ideally, the cable sailor craft will be equipped with redundant sensors for the most important functions. 
         [0044]    The second mode will be used as the cable sailor rounds the turn at the end of each cable sailor system. Here the supporting cables terminate and the cable sailor craft is transitioned to a pair of helical turn rails which allow the cable sailor craft to execute a 180 degree turn before returning in the direction from which it came. During this turn, the embedded controller will no longer attempt to maximize electrical power generation by the electrical generators, but will instead instruct the BD system to swing the boom in a controlled manner to a position suited for traveling in the opposite direction. This motion is analogous to a sail boat executing a “tack” maneuver. 
         [0045]    The supporting towers would typically be at least 300 feet tall to reach the more quickly moving air beyond the air boundary layer at ground level. Alternatively, the cable sailor system may be suspended between tall neighboring buildings. Depending on their particular geometry, the buildings may act to accelerate the nearby wind which in turn drives the cable sailor craft, increasing the maximum possible electrical power output. Furthermore, by situating the cable sailor system near electrical power consuming communities, the voltage drop (IR-drop) that would normally occur from long distance electrical power transmission may be mitigated. The overall benefit would be to lower energy cost and reduce electrical energy dissipated into waste heat. 
         [0046]    The cable sailor craft ideally has mirrored elliptical sails ( FIG. 7 ). The elliptical sail shape was determined to be the most effective at capturing wind energy as compared to other traditional sail shapes. Compared to a triangular shaped sail of equal area, the elliptical shaped sail was shown to be 27 percent more efficient at capturing wind energy in wind tunnel testing. Furthermore, the combination of the twin sails yields 115 percent the power of a single sail and reduces the number of motor/generators, masts, booms, and idler pulleys. The overall effect of the mirrored sail design is to reduce the complexity of the cable sailor craft by a factor of nearly two. 
         [0047]    In cold climates, it is conceivable that ice my form on the stationary supporting cables by which the cable sailor craft travel and transmit the generated electrical power. Such ice formation could be detrimental to the cable sailor craft and power transmission by insulating the electrical contact between the cable sailor craft and the supporting cable. To mitigate this, one or more methods may be employed. The supporting cables maybe slightly heated by an induction device or a small gas flame housed within the MG modules prior to encountering the oncoming cable sailor craft. Alternatively, a scrapper device may run along the supporting cable within or attached to the MG modules. 
         [0048]    The cable sailor system, composed of one or more cable sailor craft, the supporting cables, and supporting towers, will be electrically interfaced with the commercial power grid through one or more electrical transformers which will boost the cable sailor generated voltage to the appropriate level. In the United States of America, the commercial power grid is typically operated at 117 volts alternating current at 60 Hertz or at 220 volts alternating current at 50 Hertz in many foreign countries. As an alternative to grid interfacing, industrial or residential installations may connect directly to a dedicated cable sailor system if the situation so warrants. 
         [0049]    In  FIG. 8 , items  800 ,  806 ,  808  and  810  are the motor/generator housings that can be seen in more detail in  FIG. 9 . Items  801 ,  805  are cables that are used for both mechanical and electrical connections between the mast and the motor/generator pods  800  and  806 . These cables may be a combination of materials such as copper and stainless steel to have both good electrical conductivity and mechanical strength. Boom/mast connector  802  may be made of a combination of wood and fiberglass, or a composite material such as carbon fiber. 
         [0050]    Reference  803  shows the bottom of the over/under elliptical sail. This can be made of common sail cloth, or of photo/voltaic material which can be added to the motor generator voltage to further increase the power output of the cable sailor. Mast  804  may be made of an aluminum tube. If the sails are photo-voltaic, the mast is also used to carry the wire cables connecting to the photo-voltaic cells. 
         [0051]    Cables  807  and  811  connect to the towers shown in  FIGS. 11 ,  14  and  15 . As mentioned, the cables serve a dual purpose, one to provide mechanical support for the cable sailors and second to carry electricity generated by primarily the cable sailors, but in some cases the power grid that the cable sailor towers are connected to. These cables may be a combination of materials such as copper and stainless steel to have both good electrical conductivity and mechanical strength. Electrically conductive cables  809  serve a dual purpose. The first is a mechanical support for the boom to the tower cables  807  and  811 . The second function is to electrically connect the motor/generators contained in aero-pods  808  and  810 . 
         [0052]      FIG. 9  shows the motor/generator aero pod. Reference  901  shows the top access cover in the open position.  902  is an air inlet used for cooling the motor/generators contained in the pod, and  907  is the air exit point. Item  903  is the tower main cable that the motor/generator pulley rides on. Reference  904  is the motor/generator, and  905  is the cable that connects to the mast for both mechanical support and electrical connectivity. Tower cables  900 ,  906  provide for both mechanical support and carrying the voltage and current to and from the motor generators to the towers. 
         [0053]    The boom and its pulley assembly are shown in  FIG. 10 . The pulley  1001  is turned by the motor gearbox assembly  1002  to control the boom position.  1003  is the boom and  1000  and  1004  are the cables that connect to the motor/generator assembly.  FIG. 11  shows the turn-around towers, also shown in  FIG. 14 . Item  1100  is a support rail for the cable sailors  1101 ,  1105  to both turn 180 degrees and also change altitude to prevent wind blockage to cable sailors traversing in the opposite direction. The support rail is also shown on  FIG. 15  as  1503 . Service cables  1102  are connected between the service station  1103  and the switch assembly  1104  and  1106 . The service cables  1102  provide both mechanical support and electrical energy to the cable sailor motor/generator traversing from the Service station  1103  and the tower. 
         [0054]      FIG. 12  is a graph that shows the cable sailor power output versus its speed along the tower cables. The three different curves show two different versions of the cable sailor in the University of Michigan 5×7 foot wind tunnel and also a Computational Fluid Dynamics (CFD) simulation.  FIG. 13  is a graph comparing the cable sailor Power output and the Wind Turbine output with the cable sailor sail area and the wind turbine swept area being equal. The cable sailor exhibits a factor of 2.23 advantage over the average wind turbine power output at 28 MPH wind speed.  FIG. 14  shows the service station  1402 , a tower  1400 , main tower cables  1401  and tower bases  1403 .  FIG. 15  shows a turnaround tower showing the cable sailor reversing directions as well as changing heights.