Abstract:
Energy from a naturally recurring source of variable mechanical energy is captured and transmitted to an electrical generator causing it to rotate at speeds proportional to the amount of energy captured. The output voltage produced by the generator is a function of the rotational speed imparted to the generator and a load impedance which is coupled via controllable switching circuitry to the generator output. The effective load impedance is varied and controlled by means of a control mechanism responsive to the rotational speed of the generator and the output voltage of the generator which selectively turns the controllable switching circuitry on and off.

Description:
This application claims the benefit of U.S. Provisional Application 60/084,096 filed May 4, 1998 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the generation of electric power from a source of energy whose energy output is highly variable and, in particular, to the efficient generation of electrical power even when the amplitude and frequency of the energy supplied by the source of energy vary over a wide range. 
     There is growing interest in obtaining electrical power from “clean” sources of energy such as ocean waves and/or air currents. However, these “natural” sources of energy produce energy whose amplitude and frequency vary widely. As a result of these variations, even where a system exists for capturing energy present in ocean waves and/or air currents, a problem exists in how to efficiently transform the captured energy into electric power. 
     For purpose of illustration, the generation of power from ocean waves will be used in the description to follow. Capturing the energy present in ocean waves is problematic because the amplitude of the waves is constantly varying and the frequency (or period) of the waves also varies constantly. An additional problem in capturing the constantly varying energy present in ocean waves is to do so efficiently because, in typical power conversion systems, the efficiency of power conversion falls off rapidly when the system is operating outside of a relatively narrow range of power conversion rates. 
     SUMMARY OF THE INVENTION 
     An electric power generating system embodying the invention includes a mechanical means for capturing energy available from a natural source of energy, where the energy available from the source varies in rate, amplitude and frequency. The variable mechanical energy thus captured is used to drive a generator at a variable rate of rotation. It is known that, dependent upon the parameters of the generator used, for each speed of rotation of the generator, there exists a corresponding preferred output voltage of the generator at which the efficiency of conversion of mechanical energy to electrical energy is a maximum. 
     In accordance with this invention, both the speed of operation of the generator and the output voltage of the generator are sensed. Then, in response to these sensed values, the impedance of the load into which the generator output power is fed is varied for driving the voltage thereacross towards that preferred output voltage of the generator corresponding to the maximum efficiency operation of the generator at the actual sensed speed of operation of the generator. 
     In a preferred embodiment, the generator load comprises a capacitor. The generator output power (in d.c. form; either directly from a d.c. generator or rectified from an a.c. generator) is fed directly into the capacitor, and the voltage across the capacitor, corresponding to the output voltage of the generator, is continuously monitored. Simultaneously with the feeding of power to the capacitor, power is extracted from the capacitor during spaced apart short time intervals. By varying the rate of power extraction from the capacitor relative to the rate of power fed thereto by the generator, the voltage across the capacitor is driven towards a preferred voltage corresponding to the instantaneous sensed speed of operation of the generator. The preferred generator output voltage is obtained based upon the known speed versus preferred generator output voltage characteristic of the generator being used, e.g., by calculation or by the use of a look-up table or the like. The rate of power extraction from the generator is controlled in response to an error signal obtained by comparing the sensed output voltage across the capacitor against the looked-up preferred generator output voltage corresponding to the sensed speed of operation of the generator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a semi-block, semi-circuit diagram of an electric power generating system embodying the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention has utility in a large variety of embodiments using various components for converting various sources of variable energy into electrical energy with a high degree of conversion efficiency. One example of a complete system using the present invention is first described. 
     In the embodiment shown in FIG. 1, a buoy  12 , located in a body of water, is used to capture energy present in ocean waves and/or in the movement of a body of water and to produce mechanical forces coupled to a hydraulic cylinder  14 . The hydraulic cylinder is coupled to a hydraulic rectifier  16  whose output is fed to an accumulator  18  whose output is coupled to, and drives, a hydraulic motor  20 . The motor  20  has a shaft  21  which is mechanically coupled to the shaft of an electric a.c. generator  22 . 
     To the point described, the system for capturing the naturally occurring mechanical energy (via the float) and for using the captured energy for driving an electrical generator can be in accordance with known systems. In the present example, energy is being captured by water motion, e.g., waves on the surface of a body of water. The amount of energy arriving with such waves is, as is typical with most “natural” sources of mechanical energy, e.g., moving fluids, randomly variable from energy levels too small to overcome the inertia of an energy conversion system to energy levels (e.g., during ocean storm conditions) so high as to require at least partial shutdown of the conversion system to avoid system damage. Thus, in general, energy conversion systems, including preferred embodiments of the present invention, are designed to operate in and to capture energy from only a range of energies available from the particular energy source being used. 
     In many prior known conversion systems, not only must the range of available energies be limited, particularly against damage from excessive energies, but various means must be employed for conditioning the power being transmitted such that the power arriving at the mechanical energy to electrical energy converter or transducer, e.g., an electrical generator, is properly matched with the energy converter. As previously explained, the conversion efficiencies of, for example, electrical generators, are typically a function of the operating speeds of rotation of the generators. Thus, speed control mechanisms are typically employed in known systems for regulating the generator speeds in response to varying input energy levels. Such speed regulating mechanisms reduce the efficiency of the systems. A significant advantage of the present invention, however, is that within a selected range of randomly arriving energies, the range being limited only for preventing damage to the system from excessively high energy levels, all the varying amounts of arriving energies are applied “directly” to the electrical generator, i.e., without regulation. Accordingly, the speed of generator rotation is essentially determined by and directly proportional to the amount of captured and transmitted mechanical energy. Little or no energy is lost to speed regulating means and, most significantly, at all levels of energy being transferred, the generator functions at optimal energy conversion efficiencies. 
     Returning to a consideration of the illustrative power drive train shown in FIG. 1, the motor  20  drives the electric generator  22  for converting the mechanical energy to electrical energy. Operation of the illustrative system requires generation of a d.c. voltage and, using an a.c. generator  22 , the a.c. output from the generator is coupled to a full wave rectifier  24 . A d.c. voltage V 1  is produced across the output terminals  23 , 25  of the rectifier  24 . A capacitor C 1  is connected across the output terminals  23  and  25  of the full wave rectifier  24 . The output of the rectifier is applied to the input of a switching regulator  26  whose output provides power to a load  28 . 
     The switching regulator  26  includes a controllable switch T 1 . In this embodiment, T 1  is a bipolar PNP translator with its emitter connected to terminal  23  and its collector connected to a terminal  30  at which is produced a positive going output voltage. An inductor L 1  is connected between terminals  30  and  31 . A diode D 1  is connected at its cathode to node  30  and at its anode to terminal  25 , and a storage capacitor C 2  is connected between terminals  31  and  25  to store the output voltage. 
     The base of T 1  is connected to an output of a control circuit  32 . The turn on and turn off of T 1  is controlled by the control circuit  32  which is responsive to an error signal determined by a comparison of the amplitude of the voltage V 1  and a target voltage which is a function of the speed at which the motor  20  causes the generator  22  to rotate. The amplitude of the voltage V 1  is applied via line  34  to an input of the control circuit  32 , and the rotational speed of the motor shaft  21  is sensed by a sensor  35  to produce a corresponding signal which is applied via line  36  to the control circuit  32 . The functioning of the control circuit  32  is described hereinafter. 
     The problem faced by Applicant may be expressed as follows: 
     It is known that, for any given speed of rotation for most types of electrical energy generators, maximum efficiency of power conversion occurs when the output voltage of the generator falls within a relatively narrow range of values dependent entirely upon the physical parameters of the generator. Typically, in most large power generating systems, variations in loading of the generator are accommodated by variations of the mechanical power applied, e.g., by a steam driven engine, to the generator; the rotational speed of the generator thus remaining substantially constant and at a speed resulting in the generator operating at maximum power conversion efficiency. 
     In situations where the input power is variable, e.g., from power sources such as the wind and ocean waves, the preferred practice in the past has been to somehow sense the actual rate of arrival of the input power and to control the transmission of the power to the generator so as to maintain the speed of rotation of the generator within the preferred range. A problem with prior art power transmission speed control systems, however, is that they are complex, costly, and introduce efficiency losses. 
     In accordance with the present invention, little or no control need be provided over the rate of transmission of the input power to the generator which is thus operated at variable rates of operation. Rather, and based upon the recognition, previously discussed, that maximum efficiency of operation of the generator occurs when the generator output voltage is at a preferred voltage dependent upon the rate of rotation of the generator, the output power from the generator is loaded into a storage element (herein, the capacitor C 1 ) having a variable impedance which is a function of the time average amount of power within the storage element. As known, the output voltage of a generator is a function of the generator output current, which is dependent upon the impedance load on the generator. Here, the impedance load is provided by the capacitor C 1 . Thus, by varying the rate at which power is removed from the storage element, the impedance of the storage element is maintained, for any conditions of operation of the generator, at that average value resulting in the voltage across the element (the output voltage of the generator) being the preferred generator voltage for the actual condition of operation of the generator. 
     Stated slightly differently, for every given generator, it is known what are the preferred output voltages, for maximum efficiency of operation, corresponding to different rotational speeds of operation of the generator. Known means are used for making available such information in real time. For example, preferred voltages versus operational speeds are stored in a look-up table. Accordingly, for any rotational speed of the generator, as determined by the power then being generated by the natural energy power source, a preferred output voltage of the generator is known. Then, by comparing the actual output voltage of the generator, as measured across the storage element, against the desired output voltage, as determined from the look-up table in correspondence with the detected actual speed of rotation of the generator, the rate of removal of power from the storage element is either increased or decreased as necessary to drive the output voltage to the desired output voltage. 
     In other embodiments, the preferred output voltage for the sensed operating speed is found, in real time, by use of an appropriate equation or by hardware parameters in an analog system. Using an equation, i.e., a mathematical expression describing the known speed versus output voltage relationship, the desired voltage is calculated for every reading of the operating speed. 
     In the system illustrated in FIG. 1, operation is as follows: 
     As previously described, the rectified, d.c. output power from the generator  22  is fed directly into the capacitor C 1 . As the power is fed into the capacitor, the voltage V 1 , corresponding to the output voltage of the generator, begins to rise. Simultaneously with the feed of power into the capacitor C 1 , power is removed from the capacitor by the switching regulator  26 . For any given rate of power generation by the generator, the average voltage across the capacitor C 1  is a function of the average ration of the power fed into the capacitor C 1  by the generator and the power extracted from the capacitor by the regulator  26 . While the power being generated by the generator  22  (and being fed to the capacitor C 1 ) is a function of the power available to the generator, the power being extracted from the capacitor is under the control of the switching regulator  26  (it being assumed that all the extracted power is used, e.g., by being fed directly into a storage battery or being fed into a power grid). 
     As mentioned, the rate of rotation of the generator is variable, as determined by the amount of power instantaneously available from the power source, and the speed of rotation of the generator is constantly measured. A known speed sensor  35  is used to measure the generator speed and to generate a signal voltage indicative of the generator speed. With the generator speed known, the preferred generator output voltage Vp for the then used speed of generator rotation is determined as previously described. 
     Simultaneously, the actual output voltage of the generator is determined by measuring the voltage (V 1 ) across the capacitor C 1 . Ideally, the measured voltage V 1  should be equal to the preferred voltage Vp. The two voltages V 1  and Vp are compared within the control circuit  32  and an error signal V E  is generated. The output error voltage V E  is fed to a pulse width modulator circuit. If the error voltage indicates that the capacitor voltage is low, the output of the pulse width modulator is reduced so that the turn-on time of switch T 1  is reduced. This causes less charge to be drawn out of the capacitor C 1  allowing its voltage to rise. Concurrently, the load presented to the generator is changed; in this case the value of the load impedance is effectively increased. Conversely, if the error voltage indicates that the capacitor voltage is high, the output of the pulse width modulator is increased so the turn on time of switch T 1  is increased. This causes more charge to be drawn out of capacitor, reducing its voltage. In this case, the value of the load impedance is effectively decreased. 
     The switching regulator  26 , shown in FIG. 1, is but one example of known switching regulators which can be used. Basically, when the switch T 1  is turned on, current (and power) is drained from the capacitor C 1  and flows through the inductor L 1  and into the load. At this time, the diode D 1  is reverse biased and non-conductive. As the current begins to flow through the inductor L 1 , energy is stored therein. When the switch T 1  is turned off, the energy stored within the inductor L 1  is converted back into current and driven, by the voltage now generated across the inductor L 1 , into the load in the same direction of current flow as when the switch T 1  was turned on. At this time, the current flows through a circuit including the diode D 1  which is now forward biased. 
     Based upon the foregoing description, design of suitable arrangements for practicing the invention will be evident to persons of skill in the power generating arts. Variations from the specific arrangement shown in FIG. 1 are possible. For example: 
     The types of generators which may be used to practice the invention can include any of the following: a DC permanent magnet generator; a three phase AC permanent magnet generator and rectifier; a single phase AC generator and rectifier; a DC controlled field generator; an AC controlled field generator; and various hybrid types of generators. 
     The types of rate sensors which may be used to practice the invention include any of the following: a DC tachometer; an encoder (optical, magnetic, etc.); a gear and sensor combination; or any other appropriate sensing device. 
     The collection capacitor C 1  must have sufficient capacitance such that the DC level is stable at low generator rates and ripple peaks do not exceed the voltage of the power extraction electronics. Effective Series Resistance (ESR) is a consideration due to high ripple current. 
     The switching regulator switch T 1  is shown to be a PNP bipolar transistor. However, any other appropriate solid state device such as an NPN bipolar transistor, an FET, an IGBT, or a DMOS may be used instead. 
     The inductor L 1  may be any type of inductor which is suitable for use in a switching regulator application. 
     The error signal could be used to feed a constant pulse width, rate adjustable circuit instead of a PWM circuit. Likewise, a linear control circuit could be used; but efficiency must be considered. 
     The output load may include any device that can store or consume the energy transferred to it from the generator. Typical devices to store energy may be batteries or capacitor banks. Typical devices to use (consume) the energy may be heaters, lights, etc. (resistance); or inverters (AC output constant frequency and voltage).