Abstract:
An electrical storage device comprises a capacitor or capacitor bank capable of storing significant quantities of electricity. An inverter in circuit with the capacitor converts direct energy of the capacitor into alternating current. A variable ratio transformer is in circuit with the output of the inverter to produce an alternating current output of controlled voltage. The impedance of the transformer acts to prolong discharge of the capacitor over a significant time period. To further control the rate of discharge of the energy storage capacitor, an additional capacitor may be provided in the transformer circuit.

Description:
This application is based on Provisional Application Ser. No. 61/000,122, filed Oct. 24, 2007, the priority of which is claimed. 
    
    
     This invention relates to an energy storage device incorporating a capacitor and a circuit allowing slow discharge of the capacitor. 
     BACKGROUND OF THE INVENTION 
     The standard approach to store electrical energy is by using electrochemical batteries. Although batteries have undergone several centuries of development, deficiencies remain particularly for applications which recently have become practical or desirable. For example, the general consensus is that a practical, inexpensive electrically driven automobile awaits the development higher capacity, less expensive batteries which can be charged sufficiently to provide a practical radius of operation. Current electrically driven vehicles are not close to being competitive, in cost or performance, with internal combustion engine driven cars and trucks. Although electrically driven vehicles have recently enjoyed considerably improved performance, internal combustion engines have also improved, meaning that the relative advantage of combustion engine vehicles remains substantial. 
     It is known to use a capacitor to store direct current electrical energy, particularly in smaller capacity sizes. A major problem with capacitors as energy storage devices is they discharge immediately, producing a relatively large burst of energy over a very short time. Often, this does meet the requirements of the device to be driven, i.e. often the driven device requires delivery of energy over a prolonged period of time. In other words, the discharge rate of capacitors is often not matched with the energy rate requirement of a device that is desired to be powered. 
     Another major problem with capacitors is the voltage declines as energy is discharged. This also produces a mismatch of the characteristics of capacitors compared to the requirements of a device to be driven. The amount of energy stored in a capacitor is a function of the square of the voltage, as follows:
 
energy stored= W= ½×C×E 2  
 
where W is the energy stored in joules, C is the capacitance of the capacitor in Farads and E is the voltage of the capacitor in volts. As the energy stored in a capacitor is used, the voltage declines so that electrical motors, for example, normally cannot be driven by capacitors for a prolonged length of time.
 
     Disclosures of interest may be found in U.S. Pat. Nos. 5,920,469 and 7,323,849 and Printed Patent Application 2008/0021602. 
     SUMMARY OF THE INVENTION 
     This invention uses a capacitor or capacitor pack of an appropriate size in conjunction with an inverter to convert direct current stored by the capacitor into alternating current. The inverter is in circuit with a variable ratio transformer so the output voltage of the device can be controlled in a suitable manner, for example to be more-or-less constant. In other words, as the capacitor discharges and produces less voltage, the transformer can be manipulated to produce an output voltage that is matched with a driven device, such as an electrical motor. 
     In some embodiments, the inverter used in this invention is a mechanical inverter in order to handle high voltages that currently available solid state inverters are either incapable of handling or are very expensive, it being understood that high voltage solid state inverters could be used in this invention. 
     Interestingly, the impedance of the transformer subjected to the alternating current acts to prevent immediate discharge of the capacitor thereby prolonging the time the capacitor can drive its work producing device. Control of the capacitor discharge can also be ensured by adding one or more capacitors in the primary transformer coil circuit. 
     It is an object of this invention to provide an improved technique for storing electrical energy. 
     Another object of this invention is to provide a technique for storing electrical energy using a capacitor and a circuit to control the discharge of the capacitor. 
     A further object of this invention is to provide an improved electrical storage device using a capacitor and a circuit to produce a more-or-less constant voltage output. 
     These and other objects and advantages of this invention will become more fully apparent as this description proceeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partly schematic view of an energy storage device of this invention; 
         FIG. 2  is a side view, partly in section, of a mechanical inverter usable in this invention; 
         FIG. 3  is an enlarged cross-sectional view of the inverter of  FIG. 2 , taken substantially along line  3 - 3  of  FIG. 2 , as viewed in the direction indicated by the arrows; 
         FIG. 4  is an enlarged cross-sectional view of the inverter of  FIG. 2 , taken substantially along line  4 - 4  thereof, as viewed in the direction indicated by the arrows; 
         FIG. 5  is a diagram showing voltage patterns of one of the embodiments of this invention; 
         FIG. 6  is a cross-sectional view of a three phase mechanical inverter; and 
         FIG. 7  is a schematic view of another type transformer usable in this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an energy storage device or circuit  10  comprises, as major components, a capacitor circuit  12 , an inverter  14  having an input in circuit with the circuit  12  and an output in circuit with a transformer circuit  16 . 
     The capacitor circuit  12  comprises one or more capacitors or capacitor packs  18  of suitable size connected to leads  20 ,  22  which connect to a terminal  24 ,  26  of switches  28 ,  30  so the capacitor  18  may be isolated during recharging. High Farad capacitors are not currently in great demand and the capacitors available commercially are low voltage, meaning they store little power. Conversely, high voltage capacitors that are now commercially available are used to correct power factors and have low Farad design values. However, the construction of high voltage, high Farad capacitors is well within the skill of the art. 
     The other side of the switches  28 ,  30  connect to leads  32 ,  34  one of which provides a varistor  36  therein. The leads  32 ,  34  terminate in an input to the inverter  14 . The parallel adjustable resistors and capacitors in the circuit  12 , and in the circuit  16  as will be more fully apparent hereinafter, serve to optimize the circuit performance regarding frequency response, losses and the like. 
     In some embodiments, as shown in  FIG. 2 , the inverter  14  is an electromechanical inverter in order to accommodate very high voltages that might not be possible with solid state inverters. In some embodiments, solid state inverters are technically and economically feasible. There is a definite advantage to high voltage capacitor banks  18  because the amount of energy that a capacitor can store is proportional to the square of the voltage. 
     It will be apparent that the electromechanical inverter  14  may be of any suitable design. In some embodiments, the inputs comprise contact brushes  40 ,  42 . The inverter  14  comprises a body  44  mounted for rotation in any suitable manner, as by the provision of bearings  46 ,  48  mounted on aligned shafts  50 ,  52  connected to insulated end caps  54 ,  56  on the ends of the body  44 . The body  44  comprises a pair of conductive disks  58 ,  60  in contact with the input brushes  40 ,  42 . The disks  58 ,  60  are isolated by a pair of insulating disks  62 ,  64 . 
     The output end of the inverter  14  can comprise a pair of conductive semi-cylindrical elements  66 ,  68  separated by an insulating layer  70 . The conductive disk  58  is electrically connected to one of the elements  66 ,  68  by an insulated path  72  while the other disk  60  is electrically connected to the other element  66 ,  68  by an insulated path  74 . As shown best in  FIG. 4 , the insulated paths  72 ,  74  can comprise metal conductors  76 ,  78  surrounded by an insulating sheath  80 ,  82  and terminating in the metal elements  66 ,  68 . The outputs of the inverter  14  can comprise contact brushes  84 ,  86 . Insulated partitions  88 ,  90  can be provided on the inside of an insulated housing (not shown) to suppress arcing across the insulating partitions  62 ,  64 . 
     The inverter body  44  may be rotated in any suitable manner, as by the provision of a small motor  92  connected to one of the shafts  50 ,  52 . The inverter  14  converts direct current from the capacitor bank  18  into alternating current because the conductive element  66  is always in electrical contact with one side of the capacitor bank  18  and the other conductive element  68  is always in electrical contact with the other side of the capacitor bank  18  while the output brushes  84 ,  86  alternately contact the conductive elements  66 ,  68 . Those skilled in the art will recognize that the shape of the alternating current created by the inverter  14  tends to be “squarish” on an oscilloscope while conventional alternating current tends to be analogous to a sine wave. It will also be apparent to those skilled in the art that the shape of the alternating current can be modified in any suitable manner. 
     It will be apparent that the inverter  14  produces single phase alternating current. It will likewise be apparent that the inverter body  44  may be redesigned to produce multiphase alternating current if the requirements of the work producing device so dictate as explained in connection with  FIG. 6 . 
     The transformer circuit  16  includes leads  94 ,  96  connecting the brushes  84 ,  86  to a variable ratio transformer  98  which normally converts high voltage in a primary coil  100  to a lower voltage in a secondary coil  102 . The transformer  98  may have its variable ratio feature provided in any suitable manner, such as having a movable contact on the primary coil, a movable contact on the secondary coil as shown in  FIG. 1 , a movable contact on both the primary and secondary coils, stepped contacts or multiple taps on the primary and/or secondary coils with either a movable contact arm or solid state switches or any other suitable approach. In  FIG. 1 , the transformer  98  includes a primary coil  100  in circuit with the inverter output brushes  84 ,  86 , a secondary coil  102 , a fixed contact  104  and a movable contact  106  on the secondary coil  102 . An alternating current driven work producing device  108  is connected to the contacts  104 ,  106  and is driven thereby. 
     In some embodiments, the voltage delivered by the transformer coil  102  is automatically controlled in any suitable manner, as by a sensor  110  measuring the voltage across the device  108  and operating a servo circuit  112  to move the contact  106  to produce a desired voltage pattern over time. In many embodiments, such as where the work producing device  108  is an alternating current motor, it is preferred to provide a more-or-less constant voltage across the contacts  104 ,  106 . In other embodiments, the contact  106  may be moved in response to a signal from the sensor  110  to a data processor or computer  114  in combination with instructions from a data base or source  116  to produce a voltage pattern other than more-or-less constant. As shown graphically in  FIG. 5 , the voltage appearing in the capacitor bank  18  during discharge is illustrated as curve  118 , i.e. the voltage falls off over time. In contrast, the voltage appearing on the transformer outputs  104 ,  106  is, in some embodiments, more-or-less constant as shown by dashed line  120 . 
     An interesting feature of this invention is that the impedance in the transformer  98  inherently prolongs the duration of discharge of the capacitor banks  18  and the design of the transformer  98  may be modified to adjust the duration of discharge of the capacitor bank  18 . In some embodiments, one or more capacitors  122  may be provided in one of the transformer leads  94 ,  96  in order to ensure that the energy in the capacitor bank  18  does not discharge too rapidly, but is rather used as needed by the work producing device  108  served by the transformer  98 . The capacitor  122  tunes the transformer circuit  16  to the frequency defined by the capacitor  122 , the inductance of the transformer  98  and the resistance of the circuit  16 . It will be evident that the capacitor  122  passes alternating current but not direct current. The capacitor  122  accordingly avoids “stalling” or rapid discharge of the capacitor  18  if there were a failure of the motor  92  to turn the mechanical inverter  14 . In addition, the capacitor  122  insures there will be no direct discharge of the primary energy storage capacitor  18  and its resistance adds to the impedance of the primary transformer coil  100 . In some embodiments, the capacitor  122  may be trimmed by the addition of a subcircuit  124  including a variable resistor  126  in parallel with the capacitor  122 . The variable resistor  126  allows the frequency of the transformer circuit  16  to be modified to meet conditions that may be variable from one application to the next. 
     Another interesting feature of this invention is the amount of energy that can be stored and then withdrawn in a prolonged manner. Table I shows a selection of capacitors of different size and their capacity to store electrical energy. 
                                                 TABLE I                   W = ½ * C * E 2              size capacitor,   voltage,   energy stored,       in Farads C   in volts E   in joules W                    1   100    5,000       1   1000   500,000       1   2000    2 × 10 6         1   5000   12.5 × 10 6         1   10000    50 × 10 6         1   20000   200 × 10 6         2   100    10,000       2   1000    1 × 10 6         2   2000    4 × 10 6         2   5000    25 × 10 6         2   10000   100 × 10 6         2   20000   400 × 10 6         10   10000   500 × 10 6         10   100000        5 × 10 12                      
To place these numbers in perspective, an 8.6 Farad capacitor charged to 10,000 volts stores sufficient energy to drive a 20 horsepower motor for 8 hours in an ideal situation with no losses or inefficiencies. Normal losses and inefficiencies, such as wind resistance, tire friction, power transmission losses, circuit resistance, winding losses and the like, reduce this output significantly.
 
     In order to recharge the capacitor  18 , the switches  26 ,  28  are opened and a direct current source (not shown) connected to the terminals  24 ,  26 . When the capacitor bank  18  is recharged, the source is disconnected and the switches  26 ,  28  closed. In some embodiments, a source of high voltage is desirable to charge the capacitor  18 . In order to do this economically, suitable switches (not shown), a plug (not shown) and a rectifier (not shown) can be provided to use the transformer  98  to convert available alternating current into high voltage direct current to charge the capacitor  18  as opposed to providing a separate transformer at a charging location. 
     Referring to  FIG. 6 , there is illustrated one approach, out of many, for designing a multiphase electromechanical inverter  128 . The inverter  128  is identical to the inverter  14  except there are three outlet brush contacts  132 ,  134 ,  136  spaced 120° apart. Rotation of the inverter body  138  connects the brush contacts  132 ,  134 ,  136  to the conductive elements  140 ,  142  which are insulated from each other by the insulating partition  144  in such a manner to produce three phase alternating current. It will be apparent to those skilled in the art that many other designs of multiphase alternating current inverters are also operable in this invention. 
     Another interesting feature of this invention is shown in  FIG. 1 . In some embodiments, a subcircuit  146  comprising a pair of leads  148 ,  150  connect to the output of the secondary transformer coil  102  to provide a direct current output. In some embodiments, this may be accomplished by providing a rectifier  152  in one of the leads  148 ,  150 . 
     Referring to  FIG. 7 , there is illustrated another embodiment of a variable ratio transformer  154  having a primary coil  156  connected to an inverter for delivering alternating current from the capacitor  18  and a secondary coil  158  providing an alternating current output of suitable voltage. The secondary coil  158  includes a series of taps  160  and a movable contact arm  162  mounted for movement in a path intersecting the taps  160  to provide a reduced output voltage on the leads  164 ,  166  as will be recognized by those skilled in the art. In some embodiments, a servo circuit  168  automatically adjusts the position of the movable contact arm  162 . To this end, a controller  170  determines the voltage output of the secondary coil  158  through leads  172 ,  174  to control the position of the movable contact arm  162 . In addition or in the alternative, the controller  170  may determine the reference voltage of the primary coil  156  through a lead  176  and may communicate with other voltage instructions through a lead  180  connected to a data base or software instructions to control the position of the movable arm  162 . 
     It will be evident there are many applications for the energy storage device  10  of this invention such as a power source for golf carts, over the road or off road motor vehicles, a replacement for large battery installations, capturing energy from lightning by recharging the capacitor during a storm and later discharging the energy into the power grid, and the like. It is also apparent that this invention is useful in installations of a wide range of capacities. 
     Although this invention has been disclosed and described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred forms is only by way of example and that numerous changes in the details of operation and in the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.