Abstract:
A switching circuit, typically a thyristor bridge, is connected to a source of waste energy. The source can be waste energy occurring as a by-product of man&#39;s activities such as waste heat energy, stored water, or the potential mechanical energy of elevated objects. The source provides d.c. power either directly or through conversion. The switching circuit is also connected to an a.c. load and across a.c. power mains energizing the load. The switching circuit is operable to periodically connect the waste energy source to the a.c. load in a power supplying manner. The a.c. mains impart suitable voltage and frequency characteristics to the electrical power so supplied. In the event the supplied power exceeds the power required by the a.c. load, the difference is taken by the a.c. power mains.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to circuitry which enables waste thermal, electrical or mechanical energy to be utilized. 
     2. Description of the Prior Art 
     The machines and processes used by man in the modern industrial society while of continually increasing efficiency inevitably result in the generation of waste energy. As supplies of conventional energy decrease and costs increase, these losses are of rising concern. 
     Many of these losses are in the form of waste heat. Heat is generated for thermal processes, such as heating and cooling, and so that mechanical or electrical work may be extracted from a heated medium, as with stream engines or the thermal generation of electricity. 
     The heat so generated is never totally used resulting in a residuum of heat which is wasted. This waste heat is usually dissipated in the atmosphere with both an energy loss and a detrimental effect on the environment. 
     Heat may also be generated as a result of mechanical or electrical work resulting from friction or resistive losses and the like. This heat is also largely unused and dumped intof the atmosphere. 
     While attempts have been made to utilize such waste thermal energy, they have not been totally successful. Utilizing the heat directly, as thermal energy, usually is not practical because heat may not be the desired form of energy or because of the necessity to match the source temperatures with the load temperatures. The often sporatic and untimely nature of the availability of thermal energy also renders it difficult to use. Storage of heat energy is difficult. 
     In many circumstances, electrical energy is generated incident to processing, testing, and the like in industry. For example, in the manufacture of storage batteries, it is necessary to charge and discharge the batteries several times prior to shipment. At present, this energy is also largely wasted due to a lack of a utilization technique or means. The energy of the storage batteries is discharged into a resistive bank. The heat so produced is usually dissipated into the atmosphere or in a cooling means. 
     While electrical waste energy does tend to be more usuable than thermal waste energy, it usually does not have the standard voltage and frequency characteristics, such as 60 Hz, 110 volts, necessary to facilitate utilization. Any effort to transform such energy to useful voltage and frequencies has been impeded by the inefficiency, unreliability, and expense of the necessary apparatus. 
     Like thermal energy, waste electrical energy tends to be sporatic and unrelated to available power demands which it might satisfy. Storage of such energy until a demand arises has also been blocked by expense and inefficiency. 
     On the other hand, some forms of energy are easy to store, for example, the potential mechanical energy of an elevated, stored body of water. However, similar to other forms of waste energy, inexpensive, efficient means of utilizing such energy as by converting it to electrical power of standard voltage and frequency characteristics have been lacking. 
     SUMMARY OF THE PRESENT INVENTION 
     It is, therefore, the object of the present invention to provide a method and means by which waste thermal, electrical, or potential mechanical energy produced by industrial or other activities may be utilized. 
     Such utilization is accomplished by converting the waste energy, if necessary, to d.c. electrical power and feeding such power into an existing a.c. power distribution network for supply to electrical loads energized by the power network. The a.c. power distribution network facilitates the power transformation and load matching required to utilize waste energy of the type described above. The economy, simplicity and efficiency of the present invention greatly enhances the practicality of using these energy sources. 
     The d.c. electrical power obtained from the waste energy is periodically supplied to an a.c. load energized by the a.c. power network. The a.c. voltages existing in the a.c. power network are employed to control the power transfer and to transform the power into the same voltage and frequency as exists in the a.c. power network which energizes the a.c. load. The transformation process is thus simplified and rendered highly efficient and economical. The a.c. power distribution network functions as a receiving and redistribution medium for excessive quantities of transformed electrical power. This enables the use of a variable waste energy source without the need for expensive, inefficient storage means. Both the generation and the redistribution of electrical power produced from the waste energy lessens power demands on power stations in the a.c. power network, resulting in a reduction in the amount of conventional fuels consumed. 
     In its apparatus aspects the present invention employs a conversion means, if necessary, to convert the waste energy into a corresponding amount of d.c. electrical power. Such a converter is necessary when the waste energy if, for example, thermal or mechanically potential in form. The converter becomes a d.c. power supply. Where the waste energy is already in the form of direct current electricity no such converter is needed and the waste energy source becomes the d.c. power supply. A switching means, typically a thyristor bride, is interposed between the d.c. power supply and the a.c. load. The switching means is also connected across the a.c. power mains of the distribution network which energizes the load. The switching means is operable to periodically connect the d.c. power supply to the a.c. load. The instantaneous differences between the voltages of the d.c. power supply and the a.c. power mains provide a power supplying current flow from the d.c. power supply through the switching means to the a.c. load and impart suitable voltage and frequency characteristics to this power. 
     In the event the amount of power available from the waste energy utilization circuit is less than that required by the a.c. load, the a.c. power mains provide the necessary difference. If the power available from the utilization circuit is greater than that required by the a.c. load, the excess flows into the a.c. power distribution network for supply to other a.c. loads connected in the network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood with reference to the drawings in which: 
     FIG. 1 is a schematic diagram of one embodiment of the waste energy source utilization circuit of the present invention employing waste electrical energy; 
     FIG. 1a is a partial view of apparatus suitable for utilizing waste thermal energy; 
     FIG. 1b is a partial view of apparatus suitable for utilizing the potential energy of stored water; 
     FIG. 1c is a partial view of another embodiment of apparatus suitable for utilizing potential mechanical energy; 
     FIG. 2 shows the d.c. voltage and one of the a.c. voltages existing in the circuit of FIG. 1; 
     FIG. 3 is similar to FIG. 2 and further shows an additional a.c. voltage existing in the circuit of FIG. 1; 
     FIG. 4 shows currents existing in the circuit of FIG. 1; 
     FIG. 5 shows the operation of the circuit of FIG. 1 in providing power to a load; 
     FIG. 6 shows the voltages of FIG. 3 on an expanded scale; 
     FIG. 7 shows a modification in the operation of the circuit of FIG. 1 in providing power to the load; 
     FIG. 8 is a schematic diagram of a thyristor firing circuit suitable for use in the circuit of FIG. 1; 
     FIGS. 9A through 9G show various electrical signals employed in the operation of the thyristor firing circuit of FIG. 8; 
     FIG. 10 is a schematic diagram of another embodiment of the switching means of the circuit of the present invention; 
     FIG. 11 is a schematic diagram of a polyphase embodiment of the switching means of the circuit of the present invention; and 
     FIG. 12 shows polyphase voltages existing in the embodiment of FIG. 11. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The Circuitry 
     In FIG. 1, the circuitry 10 includes d.c. power supply 12, shown as a bank of storage batteries 14 which have been charged during the manufacturing process. D.c. power supply 12 includes impedance 16 having inductive properties connected between voltage source terminal 18 and output terminal 20. D.c. supply 12 includes a second output terminal 22. 
     A switching means 24 is coupled to d.c. power supply output terminals 20 and 22. The switching means may typically comprise a thyristor bridge having input terminals 26 and 28 and output terminals 30 and 32. Input terminals 26 and 28 are connected to d.c. power supply output terminals 20 and 22 via conductors 25 and 27. Each of the input terminals of the bridge is connected to each of the output terminals by bridge arm conductors containing thyristors to provide a pair of conduction paths in thyristor bridge 24 between input terminals 26 and 28 and output terminals 30 and 32. One such conduction path includes thyristors 34a and 34b while the other conduction path includes thyristors 36a and 36b. Thyristors 34a and 34b and 36a and 36b are fired by firing circuit 100 shown in detail in FIGS. 8 and 9. 
     In the d.c. portions of the circuitry of FIG. 1, current flows in the counter clockwise direction shown by the arrow labelled I. This current is made up of the currents in the two conduction paths in the thyristor bridge 24. Thyristors 34a and 34b and 36a and 36b are poled so that this current alternates between the two conduction paths in thyristor bridge 24. Current I 1  in the conduction path containing thyristors 34a and 34b flows out terminal 30 and in terminal 32. Current I 2  in the conduction path containing thyristors 36a and 36b flows out terminal 32 and in terminal 30. 
     Output terminals 30 and 32 of thyristor bridge 24 are connected via conductors 38 and 40 to terminals 42 and 44 of a.c. load 46, shown as an induction motor in FIG. 1. A conventional a.c. power distribution network is also connected to conductors 38 and 40 and hence to thyristor bridge 24 and a.c. load 46. Such a network may comprise a.c. power mains 47 connected to dynamo or generator 48 in power station 49. A.c. power mains 47 and the power distribution network have a characteristic low impedance property which tends to maintain terminal voltages regardless of the magnitude of currents in the mains. This property renders the mains relatively immune to the effects of removing or inserting power and may be said to impart a &#34;stiffness&#34; to the mains. 
     The Operation of the Circuitry 
     The Supply of Power from D.C. Supply 12 to A.C. Load 46 
     Inasmuch as the operation of the circuitry of FIG. 1 involves the supply and receipt of electrical power, it is important to note the common analytical convention regarding this matter. This convention states that for an element to be a source of electrical power, current must flow out the more positive terminal of the element and in the less positive or negative terminal. For an element to function as an electrical load, current must flow in the more positive terminal of the element and out the less positive or negative terminal. 
     FIG. 2 shows certain voltages existing in the circuitry of FIG. 1 during its operation. Voltage 50 represents the voltage of d.c. power supply 12 applied to terminal 20. For d.c. power supply 12 to function as an electrical source with the counter-clockwise current flow direction of current I indicated in FIG. 1, the voltage at terminal 20 must be of the positive polarity shown in FIGS. 1 and 2. 
     A.c. power mains 47 apply an a.c. voltage to output terminals 30 and 32 of thyristor bridge 24. When thyristors 34a and 34b are conducting current I 1 , this voltage will appear at input terminals 26 and 28 of thyristor bridge 24 as a.c. voltage 52 due to the negligible conduction voltages across thyristors 34a and 34b. A.c. voltage 52 also appears at output terminals 20 and 22 of d.c. power supply 12. A.c. voltage 52 periodically varies from zero to a maximum magnitude of one voltage polarity and through zero to a maximum magnitude in the other polarity. 
     In comparing the magnitudes of d.c. voltage 50 generated by d.c. power supply 12 and a.c. voltage 52 existing at a.c. load 46 and established by a.c. mains 47, there will exist periods during which the alternating voltage will be less positive than the d.c. voltage, due to varying magnitude of the a.c. voltage. Such a period exists from time T 1  to time T 2  in FIG. 2. 
     Inasmuch as electrical current flows from a more positive voltage level to a less positive voltage level, current can be made to flow from the d.c. power supply 12 to a.c. load 46 during this and similar periods. Thyristors 34a and 34b in thyristor bridge 24 are poled to facilitate this current flow. In order to form a current conduction path, the anodes of the thyristors must be more positive than the cathodes. As may be seen from FIG. 2, this condition will occur during the aforementioned time interval T 1  -T 2  in which d.c. voltage level 50 is instantaneously more positive than the a.c. voltage level. During this time interval, for example, d.c. voltage 50 at terminal 20 applied to the anode of thyristor 34a will be more positive than the a.c. voltage 52 at terminal 30 applied to the cathode, permitting thyristor 34a to conduct current I 1  from output terminal 20 of d.c. power supply 12 to terminal 30, conductor 38, and terminal 42 of load 46. 
     Under the conditions shown in FIG. 2, in which the magnitude of d.c. voltage 50 is less than the peak magnitude of a.c. voltages 42, the d.c. voltage 50 applied to the anode of thyristor 34a prior to time T 1  and subsequent to time T 2  is less positive than a.c. voltage 52 applied to the cathode, preventing conduction of thyristor 34a during these times. Corresponding conduction controlling conditions exist on thyristor 34b. 
     It is important to note that the bi-polarity character of the a.c. voltage makes both polarities of voltage available during the time period T 1  -T 2 . Both bridge terminals 30 and 32 and load terminals 42 and 44 are connected across a.c. mains 47 by conductors 38 and 40. Bridge terminal 30 and load terminal 42 will thus simultaneously be of the same polarity by virtue of their connection to conductor 38. By proper selection of the voltage on the bridge terminals and the load terminals, electrical power may be transferred by the current flow from d.c. power supply 12 to a.c. load 46. As shown in FIG. 2, during time period T 1  -T 3  alternating voltage 52 on terminal 30 of thyristor bridge 24 and terminal 42 of a.c. load 46, while lesser in magnitude than d.c. voltage 50, is, nonetheless, positive with respect to neutral axis 54. Current I (I 1 ) is flowing out a positive terminal 30 of thyristor bridge 24 and in a positive terminal 42 of a.c. load 46. In accordance with the analytical power convention noted above, when current flows out the positive terminal of an element, the element is an electrical source. D.c. power supply 12 and thyristor bridge 24 are thus an electrical power source. When current flows in the positive terminal of an element, the element is an electrical load. A.c. load 46 is thus an electrical load receiving power from d.c. supply 12. 
     It is also important to note that at T 3 , the polarity of the a.c. voltage at terminal 30 of thyristor bridge 24 and terminal 42 of a.c. load 46 reverses to the negative polarity. If the flow of current I 1  continues through thyristor 34a and in conductor 38, it will be flowing out a terminal of thyristor bridge 24 which is instantaneously negative. In accordance with the convention noted above, when current flows out the negative terminal of an element, the element is a load. Thus, if the conduction of thyristors 34a and 34b is permitted subsequent to time T 3 , conductors 38 and 40 connected to a.c. power mains 47 become a source supplying power to d.c. power supply 12. Under such conditions, thyristor bridge 24 operates in the manner of a conventional rectifier. Thereforce, in order to insure the flow of power in the direction from d.c. power supply 12 to a.c. load 46, it is necessary to turn off thyristor 34a at or prior to time T 3 . 
     In summary, in order to obtain a supply of power from d.c. power supply 12 to a.c. load 46, thyristor 34a must be rendered conductive at or after time T 1  and turned off at or before time T 3 . While thyristor 34a has been described in detail above, it will be appreciated that complementary thyristor 34b must be rendered conductive in co-ordination with thyristor 34a in order to complete the conduction path through thyristor bridge 24. 
     Circumstances similar to those existing during time T 1  -T 3  exist between times T 4  and T 2  when a.c. voltage 52 returns to the positive polarity but is smaller in magnitude than d.c. voltage 50. If thyristors 34a and 34b are rendered conductive during this time interval, the supply of power from d.c. power supply 12 through thyristor bridge 24 to a.c. load 46 will again result. 
     FIG. 3 is similar to FIG. 2 and further shows a.c. voltage 56 which appears at input terminals 26 and 28 of bridge 24 and output terminals 20 and 22 of d.c. power supply 12 when thyristors 36a and 36b conduct current I 2 . The action of thyristors 34a and 34b and thyristors 36a and 36b in establishing the pair of a.c. voltages 52 and 56 shown in FIG. 3 may be considered analogous to the operation of a reversing switch in performing the same function. Thyristors 36a and 36b will be biased to conduct current I 2  for periods extending from time T 5  to time T 6  and time T 5a  to time T 6a  shown in FIG. 3. The supply of power from d.c. power supply 12 through thyristors 36a and 36b in thyristor bridge 24 to a.c. load 46 occurs during time periods T 5  -T 7 , T 3  -T 6 , T 5a  -T 4 , and T 3a  -T 6a . 
     It will be apparent from FIG. 3 that when considering both pairs of thyristors and current conduction paths in thyristor bridge 24, time periods T 1  through T 6 , T 5a  through T 2  and T 1a  through T 6a  are periods during which a power supplying current I, comprised of the current components I 1  and I 2 , may flow from d.c. supply 12 through thyristor bridge 24 to a.c. load 46. FIG. 4 shows these currents under conditions in which low inductive impedance is present in the circuitry. With greater inductive impedance, the current flow intervals will increase and current flow may become continuous. 
     Any difference between the amount of power supplied by d.c. power supply 12 during such power supplying periods and that required by a.c. load 46 is accommodated by a.c. power mains 47. In the instance in which the power required by a.c. load 46 is in excess of that supplied by d.c. power supply 12, a.c. power mains 47 provide an input power equal to the difference. 
     FIG. 5 shows the power supplying operation of the present invention. The graph shows the power utilized by a.c. load 46. The shaded areas 62 occupying times T 5  -T 8 , T 1  -T 6 , T 5a  -T 2 , and T 1a  -T 6a  indicate the power supplied by d.c. power supply 10. In FIG. 5, the amount of power supplied during these time periods is shown as equal to that required by a.c. load 46. The power requirement of a.c. load 46 during the time periods T 8  -T 1 , T 6  -T 5a , and T 2  -T 1a  is provided by a.c. lines 47. FIG. 5 also illustrates the action of the a.c. power in mains 47 in imparting suitable voltage and frequency to the power 62 supplied by a.c. power supply 12 to a.c. load 46. 
     Considerations in the Operation of Switching Means 24 During Power Supply 
     On a theoretical basis, to provide the maximum supply of power from d.c. power supply 12 to a.c. load 46, thyristors 34a and 34b in bridge 24 should be turned on at time T 1  and turned off at T 3 . Thyristors 36a and 36b in bridge 24 should be turned on at time T 3  and turned off at time T 6 . This would supply power during the entire time period T 1  -T 6  during which the a.c. voltages 52 and 56 are positive in polarity but smaller in magnitude than d.c. voltage 50. 
     As a practical matter, however, in a bridge, such as bridge 24, utilizing thyristors as the current control devices, consideration should be given to using only thyristors 36a and 36b and the time period T 3  -T 6 , to supply power rather than utilizing the entire time period T 1  -T 6  and both sets of thyristors 34a and 34b and 36a and 36b. While the amount of power supplied to a.c. load 46 is reduced, the ease and certainty with which the operation, particularly the turn off operation, of the thyristors may be achieved and the facility which the magnitude of the current I may be controlled is increased. 
     With the use of thyristors 36a and 36b to supply power during the period T 3  -T 6 , the thyristors are automatically biased toward the nonconductive condition at time T 6  as the relative polarity of the voltages on the anodes and cathodes of the thyristors reverses. The control of the magnitude of the current I 2  conducted by thyristors 36a and 36b is simplified both by the automatic turn off of the thyristors and by the decreasing difference in bias voltages applied to the thyristors during the time period T 3  -T 6 . 
     In contrast, the difference in the biasing voltages applied to thyristors 34a and 34b during the time period T 1  -T 3 , are such as to tend to increase conduction and the current I 1  through the thyristors. This presents problems in effecting turn off of thyristors 34a and 34b at time T 3 . The attendant limitations in the firing point of the thyristors, as well as the biasing voltages make control of the magnitude of current through the thyristors difficult. A more serious problem is that should the turn off of thyristors 34a and 34b not be completed by time T 3 , it may be impossible to thereafter turn them off and turn thyristors 36a and 36b on due to the more favorable conduction conditions existing on thyristors 34a and 34b subsequent to time T 3 . 
     Referring to FIG. 6, which shows the portion of FIG. 3 containing the time period T 1  -T 6  on an expanded scale, consider circumstances existing at time T 10  with thyristors 34a and 34b still conducting . Time T 10  is subsequent to time T 3  and, therefore, the voltage on terminal 32 is instantaneously positive and the voltage on terminal 30 is instantaneously negative. This is represented in FIG. 6 by the voltages 52 and 56. Terminal 28 will be at the same potential as terminal 32 since conductive thyristor 34b, to the anode of which is applied the positive voltage 56, has a negligible voltage drop across it. Thyristor 36b thus has the positive potential at terminal 28 applied to its cathode and the negative potential at terminal 30 applied to its anode. It is therefore reverse biased and cannot be rendered conductive. A similar circumstance exists in connection with thyristor 36a so that if the conduction of thyristors 34a and 34b continues past time T 3 , as to time T 10 , it will be impossible to turn on thyristors 36a and 36b. The current flow through thyristors 34a and 34b subsequent to time T 3  causes an undesirable power flow from a.c. power mains 47 to d.c. power supply 12. This power flow will continue until time T 2 . It is to avoid this inability to turn on thyristors 36a and 36b and turn off thyristors 34a and 34b that only thyristors 36a and 36b are employed to conduct current during the portion T 3  -T 6  of the time interval T 1  -T 6 . 
     In the subsequent power supply time interval T 5a  -T 2  the relative polarities of a.c. voltages applied to output terminals 30 and 32 of bridge 24 are reversed so that thyristors 34a and 34b are rendered conductive during the portion T 4  -T 2  of the time interval T 5a  -T 2  and thyristors 36a and 36b are not used. 
     In spite of the deliberate use of only one of the pairs of thyristors, for example thyristors 36a and 36b, during the latter portion, T 3  -T 6 , of the power supplying interval, T 1  -T 6 , there is always the possibility that the other pair of thyristors will be conducting in the power supplying interval. This may be due to the presence of a large inductance in the circuit. For example, the current maintaining properties of the inductance may cause the current flowing in thyristors 34a and 34b during a previous power supplying interval T 7  -T 8  to continue past T 1  of the subsequent power supplying interval T 1  -T 6 . However, as long as thyristors 36a and 36b are fired on prior to time T 3  thyristors 34a and 34b will be commutated off by the voltage conditions existing in thyristor bridge 24 prior to time T 3 . These conditions favor the conduction of thyristors 36a and 36b over the conduction of thyristors 34 a and 34b. 
     Consider, for example, time T 11  in FIG. 6 which is prior to time T 3  under conditions in which thyristors 34a and 34b are conducting. At time T 11  the voltage on terminal 30 is positive and the voltage on terminal 32 is negative. Conducting thyristor 34b applies the negative voltage at terminal 32 to terminal 28 and places that terminal at the same negative potential as terminal 32. Thyristor 36b thus has a negative potential on its cathode and the positive potential at terminal 30 on its anode. It is therefore forward biased and can be easily rendered conductive by an appropriate firing signal. The conduction of thyristor 36b commutates off thyristor 34b by providing the positive voltage at terminal 30 to its cathode. 
     The foregoing suggests that under circumstances in which thyristors 34a and 34b continue to conduct current past time T 1 , that thyristors 36a and 36b must be fired prior to time T 3  if proper operation on bridge 24 is to be maintained. 
     While theoretically a small amount of power supply from a.c. power mains 47 to d.c. power supply 12 will result by the conduction of current through thyristors 36a and 36b prior to time T 3 , no significant amount of reverse power flow occurs if firing occurs close to time T 3 . The initial current build up may be very slow with the impedance 16 of d.c. power supply 12. This also reduces the amount of reverse power flow. Thyristors 36a and 36b will normally be commutated off at time T 6  as the voltage applied to the thyristors reverses to a condition not favoring conduction. If the inductive properties of the circuitry continue current flow through the thyristors past this turn-off time the current is quite effective in supplying power to a.c. load 46 because of the voltages existing subsequent to time T 6 . Thus, any power flow from a.c. power mains 47 to d.c. power supply 12 occurring by firing the thyristors 36a and 36b prior to time T 3  tends to be regained by the conduction of current subsequent to time T 6 . With inductive impedance present, there is also the additional advantage in that the RMS currents tend to be lower because of the longer conducting interval which extends from prior to time T 3  to subsequent to time T 6 . 
     The supply of power to a.c. load 46 through the use of the latter portion of the power supplying intervals is shown in FIG. 7. FIG. 7 also shows conditions in which the amount of power provided by d.c. power supply 12 exceeds that required by a.c. load 46. The numeral 66 indicates the total amount of power supplied by the current flow occurring during the power supplying time intervals. The numeral 62 indicates the portion of the supplied power concurrently used by a.c. load 46. The numeral 68 indicates the remaining excess portion of the supplied power which is taken or stored in the a.c. power distribution network. This excess power may be returned to a.c. load 46 in the time periods subsequent to times T 8 , T 6 , T 2 , and T 6a . 
     The amount of power supplied to a.c. load 46 is determined both by the level of d.c. voltage 50 and the firing angles of the thyristors in bridge 24. The level of d.c. voltage may be determined by appropriate controls associated with batteries 14. The controls may be adjusted so that d.c. voltage 50 is approximately 90% of the RMS value of the a.c. voltage. Considerations relating to the firing angle of the thyristors in bridge 24 are described above. 
     In the instance in which d.c. power supply 12 provides more power than is required by a.c. load 46, the excess power in a.c. power mains 47 may be supplied to other users connected to the a.c. power distribution network, for example, users connected to power mains 64, thereby reducing the power generating requirements of power station 49. 
     The Firing Circuit 
     FIG. 8 shows a firing circuit 100 suitable for firing thyristors 34a and 34b and thyristors 36a and 36b to accomplish the above described operation. Firing circuit 100 includes a pair of firing pulse generators 102 and 104. Firing pulse generator 102 is coupled through pulse transformer 106 to the gate terminals of thyristors 36a and 36b. The firing pulse generator 104 is coupled through pulse transformer 108 to the gate terminals of thyristors 34a and 34b. 
     The pulse generators 102 and 104 are generally similar in construction and employ a unijunction transistor pulse generating circuit. A unijunction transistor is a three element semi-conductor device having a pair of base terminals and an emitter terminal. If the voltage on the emitter terminal is less than a predetermined peak voltage, only a small reverse leakage current will flow in the device. When the emitter voltage exceeds the peak voltage a low resistance path is established for the emitter current. 
     Unijunction transistors are commonly used in simple relaxation oscillators in which a capacitor, connected to the emitter terminal, is charged by a controlled current source. When the capacitor voltage reaches the peak emitter voltage, it triggers the unijunction transistor and the energy of the capacitor is discharged through the device to provide a thyristor gating pulse. Due to the similarity of the pulse generators 102 and 104, only pulse generator 102 is described in detail below. 
     Pulse generator 102 includes unijunction transistor 110. The emitter of unijunction transistor 110 is connected to triggering capacitor 112 through isolating diode 114 and conductor 116. A second triggering capacitor 118 is connected in parallel with capacitor 112 through isolating diode 120. One base of unijunction transistor 110 is connected to transformer 106 by conductor 122. The other base of unijunction transistor 110 is connected to the output of a logic circuitry, hereinafter described, by conductor 124. The logic circuitry determines the time interval during which unijunction transistor 110 may be triggered by capacitor 112. 
     Pulse generator 102 is energized by a d.c. power supply shown diagrammatically as battery 128. The emitter of transistor 130 is connected through bias resistor 132 to d.c. power supply 128. The collector of transistor 130 is connected to capacitor 112 to provide the charging current to the capacitor. The base of transistor 130 is connected to control signal source 134 which provides an adjustable magnitude control signal to the base of transistor 130. The magnitude of this signal determines the magnitude of the current supplied to capacitor 112, the charging rate of the capacitor, and the timing of pulse generation by unijunction transistor 110. The greater the magnitude of the control signal, the more rapid the charging of capacitor 116 and the earlier the triggering and pulse generation of unijunction transistor 110. 
     Step-down transistor 136 has primary winding 138 coupled to a.c. power mains 47 to provide voltages in secondary winding 140 suitable for synchronizing the operation of firing circuit 100 with conditions existing in thyristor bridge 24. The synchronizing voltages are applied to the logic circuitry which determines the period during which unijunction transistor 110 may be rendered operative. 
     Transformer 136 provides a voltage corresponding to voltage 56 in the right hand half of secondary winding 140, as shown in FIG. 9A. FIG. 9A also contains certain times such as T 3  and T 6  previously identified in FIGS. 2, 3 and 4. Voltage 56 is phase shifted in a lagging manner by capacitor 142 and resistor 144 to establish voltage 200 which is applied to diode 146. The diode 146 provides the positive half cycle of voltage 200 as logic input signal 202 shown in FIG. 9B. Inasmuch as it is the presence and absence of logic signal 202, not its magnitude which is important, logic input signal 202 is shown as a simple square wave. 
     Logic input signal 202 is supplied in conductor 148 to one input of NOR gate 150. A NOR gate is a coincident element which provides a logic 0 output (usually the absence of an electrical output signal) when an input signal having a logic state of 1 (usually the presence of an electrical signal) is applied to the first input, or the second input, etc., of the element. The name NOR gate is derived from the &#34;not&#34; or logic 0 characteristic of the output and the &#34;or&#34; characteristic of the input. When the input signals to a NOR gate are all logic 0, a logic 1 signal appears at the output thereof. 
     Disregarding, for the moment, the other input signal to NOR gate 150 in conductor 152, the absence of input logic signal 202 (i.e. logic 0) provides an output signal (i.e. logic 1), shown as signal 204 in FIG. 9C, from NOR gate 150 to unijunction transistor 110 in conductor 124 which biases unijunction transistor 110 into the operative state and permits it to generate a firing pulse when the appropriate voltage is applied to the emitter terminal thereof. The period during which unijunction transistor 110 may generate a firing pulse extends from time T 18  to time T 19 , as shown in FIGS. 9A, B, and C. The firing interval for unijunction transistor 110 and hence for thyristors 36a and 36b thus commences when voltage 56 is somewhat negative and extends well into the positive half cycle. Typically the extension into the positive half cycle is about 50°. The operative period of unijunction transistor 110 thus embraces the time interval T 3  to T 6 . 
     The exact instant of pulse gneration during the time period T 18  - T 19  depends upon the magnitude of the control signal from the control signal source 134. As a practical manner, pulse generation cannot be accomplished at time T 18  since it takes some time for capacitor 112 to charge to the voltage necessary to trigger unijunction transistor 110 so that the generation of a firing pulse and the firing of thyristors 36a and 36b is typically initiated sometime after the negative peak of voltage 56. The firing interval extends at least to time T 6 . 
     As noted above, under circumstances in which current is flowing through thyristors 34a and 34b subsequent to time T 1 , it is necessary to fire thyristors 36a and 36b prior to time T 3 . This is particularly true if this current persists as time T 3  approaches. To insure generation of a firing pulse by unijunction transistor 110 under these circumstances, a logic input signal which alters its logic state shortly before time T 3  is established. This signal is used in conjunction with a logic signal indicating the presence of current in d.c. supply 12 to initiate firing signals to thyristors 36a and 36b  prior to time T 3  if such a signal has not already been generated by the operation of control signal source 134. The required logic function is accomplished by AND gate 160. An AND gate is a coincident element which provides a logic 1 output signal when a logic 1 input is applied to its first input and its second input, etc. 
     The first logic input signal to AND gate 160 is generated from voltage 56 in the right hand half of transformer secondary winding 140. The wave form of this voltage is repeated in FIG. 9D. To this voltage is added a positive d.c. bias voltage from d.c. power supply 128 through resistor 162 to form a voltage wave form 206 which is shifted upward with respect to neutral axis 54. 
     Voltage wave 206 is applied to diode 164 which provides the positive half cycle as logic signal 208 of FIG. 9E. The commencement of logic signal 208 at time T 20  is advanced slightly with respect to time T 3  as shown in FIG. 9D. The amount of this advance may be 20°. 
     The second input to AND gate 160 is responsive to the presence of current in d.c. supply 12 and thyristor bridge 24. A resistor 168 is inserted in the output of d.c. supply 12 as by interposing it at output terminal 22. Conductor 170 is connected to one end of resistor 168 to sense the voltage signal generated by current flow through resistor 168. Conductor 170 is connected to the second input of AND gate 160 to provide input signal 210. 
     In the absense of any current in d.c. supply 12, there is no signal in conductor 170 to AND gate 160 and the AND gate is rendered inoperative. No output signal from AND gate 160 is provided to NOR gate 150 and the commencement and termination of the operative period of unijunction transistor 110 is controlled by logic input signal 204 to NOR gate 150. The instant of pulse generation within the operative period is controlled by the rate of charge of capacitor 112. 
     The presence of current in d.c. supply 12 and in conductor 170 provides an input logic signal 210 shown in FIG. 9F to the input of AND gate 160. This provides an output signal from AND gate 160 to NOR gate 150 in conductor 152 whenever input logic signal 208 is present in conductor 166. This signal commences at time T 20  and terminates at time T 21 . The signal from AND gate 160 to NOR gate 150 does not affect the commencement of the operative period of unijunction transistor 110 at time T 18 . Signal 208 and the output signal of AND gate 160 terminates at time T 21  which is prior to time T 18 . The removal of the second input signal 202 to NOR gate 150 at time T 18  causes the output signal to assume a logic state of 1 energizing unijunction transistor 110. 
     If the magnitude of the input control signal from control signal source 134 is not sufficient to trigger unijunction transistor 110 and fire thyristors 36a and 36b prior to time T 20  and hence prior to time T 3  the operation of pulse generator 102 is as follows. At time T 20 , a second input logic signal 208 is applied to AND gate 160 in addition to the current signal in conductor 170. The pair of input signals to AND gate 160 causes the AND gate to generate an output signal in conductor 152 to the input of NOR gate 150. The application of this input signal to NOR gate 150 removes the output signal of the NOR gate and terminates the operative period of unijunction transistor 110 at time T 20 . Prior to time T 20 , a voltage divider consisting of resistors 154 and 156 has charged capacitor 118 to a voltage slightly less than that necessary to trigger unijunction 110 when same is biased by NOR gate 150. At the end of the operative period concluding at time T 20 , the drop in voltage supplied to unijunction transistor 110 by NOR gate 150 causes a drop in the voltage required to trigger unijunction transistor 110 and the voltage across capacitor 118 is now large enough to cause triggering. A firing pulse is thus generated at time T 20  to thyristors 36a and 36b. Time T 20  is prior to time T 3  and insures that thyristors 36a and 36b will be conducting prior to time T 3  when current is present in d.c. supply 12. 
     The voltage on capacitor 118 also causes unijunction transistor 110 to generate a firing pulse to thyristors 36a and 36b during the normal operation of pulse generator 102 shown in FIG. 9C occurring without current in d.c. supply 12. This causes a firing pulse to be supplied to thyristors 36a and 36b at time T 19  which is the termination of the normal operative period. As time T 19  is subsequent to time T 6 , this has little effect on the operation of thyristor bridge 24. 
     While control signal source 134 has been shown as an independently controllable element in FIG. 8, it will be appreciated that the signal source will typically include reference and feedback regulating circuitry coupled to the energy source utilization circuitry which adjusts the firing angle of the thyristors in bridge 24 to achieve desired operating conditions in the energy source utilization circuitry. For example, the firing angle of the thyristors may be regulated in accordance with the output voltage of d.c. power supply 12 to provide a level of voltage 50 below which the circuitry will not take power from the power supply. The current I in the energy source utilization circuitry may be used to regulate thyristor firing angles so as to control the rise in voltage which takes place as the output of power supply 12 increases from zero to maximum current and to establish a protective current limit if necessary. 
     Modifications and Other Embodiments 
     It will be appreciated that numerous modifications may be made to the circuitry described above. For example, the batteries forming d.c. power supply 12 may be replaced with the thermoelectric conversion means capable of utilzing waste heat energy, as shown in FIG. 1a. Heating coils 300 may be inserted in chimney 302 for extracting heat which would otherwise be lost up the chimney. A fluid, such as water, circulating in coils 300 drives thermal engine 304 to which is coupled d.c. generator 306. The output of generator 306 may be coupled to terminals 18 and 22 of utilization circuit 10. Direct thermoelectric conversion may also be used, if desired. 
     FIG. 1b shows a means of utilizing the potential energy of the elevated water, commonly found in a cooling tower. Heated water is pumped through pipe 308 to pan 310 at the top of cooling tower 312. The water trickles down over slats 314 for exposure to the atmosphere. Water wheel 316 is positioned in cooling tower 312 so as to be rotated by the falling water. Water wheel 316 is connected to a d.c. generator 306 to form a d.c. power supply. 
     FIG. 1c shows another apparatus for utilzing the potential mechanical energy of elevated objects, for example rock from which ore has been extracted or the slag discharged from a smelter or blast furnace. Such slag 318 is discharged from chute 320 into the buckets 322 of conveyor 324. The weight of the descending slag rotates sprocket 326 to which generator 306 is connected. 
     The thyristors 34a and 34b and 36a and 36b shown in FIG. 1 may be replaced with other types of current control devices. For example, transistors or triacs may be employed. Inasmuch as the control afforded by some of these devices includes a positive means of rendering them nonconductive, such devices may be utilized throughout the entire power supply interval T 1  -T 6 , thereby increasing the amount of power supplied to a.c. load 46. 
     Similarly, various configurations of the switching means 24 may be employed. For example, the common Wheatstone of Graetz bridge shown in FIG. 1 may be replaced with the center tapped transformer configuration shown in FIG. 10. Elements found in the circuitry of FIG. 1 bear a similar identifying numeral in FIG. 10. Such a configuration has the advantage to a reduction in the number of parts required. If it is desired to utilize additional amounts of power, polyphase power mains 47a, 47b and 47c and polyphase switching means 24a, shown in FIG. 11, may be employed. The bridge includes conductors connecting each of the a.c. power mains to input terminals 26 and 28 of the bridge. Each of the conductors contains a thyristor 34-1 through 34-6. 
     The operation of polyphase bridge 24a is analogous to that of single phase bridge 24. Sequential conduction by the thyristors in polyphase bridge 24a sequentially applies the line to line voltages between a.c. mains 47a, 47b and 47c to input terminals 26 and 28. 
     FIG. 12 shows two of the sequential line to line voltages developed in thyristor bridge 24b. One of the line to line voltages is identified as the phase C to phase B line to line voltage, conducted by thyristors 34-1 and 34-2. The succeeding line to line voltage is the phase A to phase B line to line voltage. This voltage is obtained by commutating off thyristor 34-1 and commutating on thyristor 34-3 while continuing the conduction of thyristor 34-2. 
     As shown in FIG. 12, a power supplying interval T 4b  to T 2b  is provided when the phase C to phase B line to line voltage is less than d.c. voltage level 50 but positive with respect to the neutral axis so that the operation of polyphase bridge 24a resembles that of single phase bridge 24. 
     In regard to the commutation of the thyristors, it will be noted that the phase A to phase B line to line voltage will be less than the phase C to phase B line to line voltage during the interval T 30  to T 31 . Commutation from the phase C to phase B line to line voltage to the phase A to phase B line to line voltage may occur during this interval and as late as time T 31 . Thus, with a polyphase bridge the interval during which natural commutation may be obtained is at least partially concurrent with the power supplying period. The commutation interval is greater than that found in a single phase bridge, so that the firing control of the thyristors in a polyphase bridge is rendered less critical. A firing circuit, such as that shown in FIG. 8 insures provision of a firing pulse prior to time T 31  by virtue of the firing pulse generated by capacitor 118. 
     While FIG. 8 shows a firing circuit which fires each pair of thyristors independently, it is also possible to utilize a firing circuit which applies a single firing pulse to all thyristors simultaneously. Only the thyristors which are most favorable biased for conduction will actually be rendered conductive. This technique has the advantage of requiring only one pulse generator in the firing circuit. However, precautions must be taken to insure that firing pulses are not applied at the crossing of the a.c. voltages, such as the zero crossing at time T 3  to T 4  in FIG. 3, since this is an indefinite condition in which neither pair of rectifiers is favored for conduction. Also, it is not possible to fire in the positive half cycles of single phase embodiments of the invention since commutation of the thyristors will not be obtained. Firing is limited to the negative half cycles and hence it is preferable to use simultaneous firing of all thyristors with d.c. power supplies, such as d.c. generators which are sufficiently inductive to maintain current flow. 
     Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.