Abstract:
An arrangement is provided for supplying electrical energy to a load from a direct electrical energy converter that optimizes converter power generation efficiency. The arrangement for optimizing converter power generation efficiency includes an impedance transformation circuit coupled between the energy converter and load for regulating current delivered by the energy converter so as to maximize power delivered to the load.

Description:
FIELD OF THE INVENTION 
     The present invention relates to vehicular power systems, and more particularly to an optimization arrangement for both primary and direct energy converters. 
     BACKGROUND OF THE INVENTION 
     In most DC electrical power systems for automotive, aerospace and stationary applications, the electrical power requirements have been increasing dramatically over the last several years. There is an ongoing trend to move to a 42-volt power system which is now being deployed in the automobile industry in order to meet the increased electrical parasitic loads. The increasing use of electrical systems in automobiles and aircraft is driven by the introduction of new functionality which will be provided by these systems, and an inherently higher level of control when engine-driven loads are replaced with electrically-powered versions. 
     One arrangement for addressing this rise in electrical power requirements uses direct energy converters (DECs) to recover heat and waste energy and augment the current power plants in vehicles. DECs provide electrical power over an extremely broad range of voltages, nominally 1 mV to several volts DC, but are typically stacked up in series to provide voltages in excess of 300 volts DC. The load currents typically range from 1 milliamp to 300 amps DC, as the power demand in DC electrical systems can vary widely depending upon the mode of operation and upon parasitic subsystems which randomly come on line. 
     If as stated above, DECs are utilized to augment the engine or power-plant, and as such, improve their overall efficiency, it is further desirable that the energy converter itself be optimized to operate at high efficiencies. The proposed system is introduced in order to provide a control scheme (hardware and software) necessary to achieve these higher efficiencies. In addition, the proposed system could also be used to optimize or maximize the lifetime and stability of the DEC energy source. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a system supplies electrical energy to a load from a direct electrical energy converter using an arrangement for optimizing converter power generation efficiency. The arrangement for optimizing converter power generation efficiency includes an impedance transformation circuit coupled between the energy converter and load for regulating current delivered by the energy converter so as to maximize power delivered to the load. 
     In accordance with another aspect of the invention, a method is provided for optimizing power generation efficiency of a direct electrical energy converter applying electrical current to a load. The optimization method includes monitoring output current and output voltage of the direct electrical energy converter and monitoring current through and voltage across the load. Next, an impedance transformation circuit is placed between the direct electrical energy converter and the load. Then, the optimization method involves adjusting the impedance of the impedance transformation circuit as a function of monitored energy converter current and voltage and load current and voltage so as to maximize power delivered to the load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a prior art circuit diagram of a typical system involving a direct energy converter where a DC/DC converter is used to regulate the energy to the load; 
         FIG. 2  is a circuit diagram of a prior art series stacked generator for use with the source optimization system; 
         FIG. 3  is a graph of the power, voltage and efficiency of a typical thermoelectric generator employed as a direct energy converter for a single thermoelectric device where conventional means are used to regulate the energy to the load; 
         FIG. 3   a  is a graph of the power, voltage and efficiency for ten thermoelectric devices in series, where conventional means are used to regulate the energy to the load; 
         FIG. 4  is a circuit diagram of the optimization system according to the principles of the present invention; 
         FIG. 5  is a detailed circuit diagram of the source power optimization system as shown in  FIG. 4 ; and 
         FIG. 6  is a flowchart of a method implemented by the controller of  FIG. 4  for maximizing the efficiency-power product of the optimization system of FIG.  4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a conventional arrangement  10  is shown. The conventional arrangement  10  has a DEC  12  which creates a current I S  and a voltage V S . The DEC  12  is coupled to a DC/DC converter  14 . DC/DC converter  14  is coupled to a load  16 . Current I S  flows from the DEC  12 , through the DC/DC converter  14  to the load  16  and back to the DEC  12 . The DC/DC converter  14  is used in situations where the energy to the load voltage V S  of the DEC  12  is different from the voltage required by the load  16 . The DC/DC converter  14  typically has an efficiency in the range of 90-99%, for power levels less than 1 kilowatt and 85-95% for power levels between 1 to 50 kilowatts. The idea behind the conventional optimization arrangement  10  is to apply a constant terminal voltage across or constant current through the load  16 . However, this solution only addresses regulation of the load  16  and does not consider the internal efficiency or operating current of the DEC  12 . Furthermore, the arrangement  10  does not necessarily optimize the power being drawn from the DEC  12  or delivered to the load  16  through impedance matching of the DEC internal resistance and the load resistance (real part of the load impedance). 
     As shown in  FIG. 2 , most DECs  12  are comprised of a number of individual DEC circuits  18  in order to generate the appropriate system power and voltage requirements (up to several hundred individual DEC circuits  18  in some cases). These individual DECs circuits  18  are usually configured in “stacks” of either series or parallel circuit configurations to achieve different voltage conditions depending upon the specific technology. The DC/DC converter  14  shown in  FIG. 2  does not optimize the DEC  12  according to which stacked configuration of the DEC  12  works best for a given situation; series, parallel or some hybrid of the two. 
       FIG. 3  shows the problems with the conventional optimization system  10 . The voltage-current characteristic  20  of a typical thermoelectric generator (TEG) are displayed. For this curve, the thermoelectric devices are connected directly to a load and hence the load current is equal to the source current. The thermal efficiency  22  and the power delivered to the load  24  as a function of current are also plotted. The load power  24  is calculated from the power (I 2 R L ) developed in the load resistance, (R L ). The thermal efficiency is defined by the power (I 2 R S (T)) lost in the source internal resistance, (R S (T)), the Seebeck power (kΔT) produced by a thermal gradient applied across the device, and the Peltier effect (αTI S ). It can be seen that the current  28  at which maximum TEG efficiency is achieved is not equal to the current  30  at which maximum power is delivered to the load  32 . 
     Furthermore, when ten thermoelectric devices are connected in series, as shown in  FIG. 3   a , the discrepancy becomes even more pronounced. This is shown by the difference between the current  28  at which maximum TEG efficiency is achieved and the current  30  at which maximum power is delivered to the load  32  when compared to FIG.  3 . 
     With reference now to  FIG. 4 , an optimization system  100  for an electrical power conversion system is shown. The optimization system  100  includes a DEC  102 . The DEC  102  is coupled to a source power optimization system (SPOS)  104 . The SPOS  104  is further coupled in parallel to an energy storage device  106 . The energy storage  106  device is also coupled in parallel to a load regulator  108 , in this example a DC/DC converter. The load regulator  108  is connected in parallel to a load  110 . 
     The DEC  102  is a generator which may be any voltage or current source such as a thermoelectric or thermoionic device, electrochemical battery, solar cell or photovoltaic converter, thermophotovoltaic system, fuel cell, plasma power generator, ferroelectric device, piezoelectric device, electrohydrodynamic generator and the like, which produces a voltage, (V S ), and results in a source current (I S ). The DEC  102  could also function as a current generator, (I S ), with a subsequent compliance voltage, (V S ), such as is the case with a photovoltaic device. The current I S  flows from the DEC  102  to the SPOS  104 . 
     In an exemplary embodiment, the SPOS  104  includes a control circuit  112  and a switch mode rectifier circuit  114  as best shown in FIG.  5 . The control circuit  112  includes a current sensor  116  coupled to the DEC  102  and a voltage sensor  118  also coupled to the DEC  102 . A second set of voltage and current sensors  116 ′,  118 ′ measure values from the load  106 . The current sensors  116 ,  116 ′ could be an ammeter or a multi-meter. The voltage sensors  118 ,  118 ′ may be a voltmeter or a multi-meter. Alternatively, a pair of multi-meters could be used to measure both the voltage and the current from the DEC  102  and the voltage and the current from the switch mode rectifier circuit  114 . The current sensors  116 ,  116 ′ and the voltage sensors  118 ,  118 ′ are coupled to a controller  120 . The controller  120  uses the current and voltage measurements from the sensors  116 ,  116 ′,  118 ,  118 ′ to drive the switch mode rectifier circuit  114 . 
     The switch mode rectifier circuit  114  includes a gate drive circuit  122  which is coupled to the controller  120 . The gate drive circuit  122  generates the pulses for a power semiconductor switch  124  within the switch mode rectifier circuit  114 . The DEC  102  supplies the current I S  to the power semiconductor switch  124  which may comprise power metal-oxide semiconductor field effect transistor (MOSFET). It is to be understood that other types of switching devices  124  can be used within the scope of the invention, such as an insulated gate bipolar transistor (IGBT), bipolar transistor or power field effect transistor. 
     The power semiconductor switch  124  is coupled to a power diode  126  and an inductor  128 . The inductor  128  is used to store excess energy during the on cycle of the power semiconductor switch  124 . In the example of  FIG. 5 , an inductance of approximately 10 milli-Henries was used, but the inductance can vary depending upon system requirements. The power diode  126  is coupled in series to an output filter  130 . The output filter  130  reduces ripple current and smoothes the DC output, (V out ). In the example of  FIG. 5  the output filter  130  includes a resistor  127  with a resistance of approximately 20 ohms and a capacitor  128  with a capacitance of approximately 470 mirco-Farads. The output filter  130  is coupled in parallel to the energy storage device  106  of FIG.  4 . 
     The storage device  106  is coupled to the SPOS  104  to provide some load balancing and to meet the load power demand by providing an energy reserve. In the example of  FIG. 4 , the storage device  106  shown is an ultra-capacitor, however any other mechanism for storing energy such an electrochemical battery could be employed. The storage device  106  is also coupled to a DC/DC converter  108  for providing load regulation. 
     The load regulator  108  regulates the current flowing to the load  110 . Further load leveling can also be achieved by incorporating the appropriate battery or capacitance across the load  110  if necessary. 
     The load  110  presents a complex impedance to the SPOS  104  (which can be written as Z L =R L +X L , where, R L  is the resistive or real part, and X L  is the inductive/capacitive part). The load  110  could also be one of a fixed resistance, capacitance or inductance, Z L . The second current and voltage sensors  116 ′,  118 ′ measure the current to the load I L  and the voltage across the load V L . The second sensors  116 ′,  118 ′ transmit the current and voltage measurements to the controller  120 . 
     The optimization system  100  functions by using the switch mode rectifier circuit  114  to perform an impedance transformation based on input from the controller  120 . In general terms, the controller  120  sends a pulse-width modulated (PWM) signal based on an optimized value of the source current Is from the DEC  102  to the gate drive circuit  122 . The gate drive circuit  122  sends a signal to the power semiconductor switch  124 , which then switches on and off at a rate determined by the controller  120 . High efficiency power transfer is achieved by modulating the power semiconductor switch  124 , which is turned on and off at frequencies in the 10 kiloHertz range. The PWM signal created by the controller  120  has a duty cycle, d which is calculated based upon the voltage and current measured by the sensors  116 ,  116 ′,  118 ,  118 ′. This results in the power diode  126  going into conduction and non-conduction in a complementary manner. 
     Assuming that the current I S  is relatively constant over a PWM cycle, then the local average value of the voltage,            V 1           , is given by,          V 1           =(1−d)·V LOAD  and the local average of the output current,          I L           , is given by,          I LOAD           =(1−d)·I S . By controlling the duty cycle ratio, d, one can vary the local average voltage,          V 1           , to any value below V L . Thus, the switch mode rectifier circuit  114  optimizes the current I S  from the DEC  102 .
     An exemplary routine for the controller  120  is shown in FIG.  6 . The controller  120  begins the optimization in step  200 . Next, in step  202 , the controller  120  measures the voltage and the current of the DEC  102  and the load  110  from the sensors  116 ,  118 ,  116 ′,  118 ′. In step  204 , the controller  120  calculates the source power P S , the load power P L , the source efficiency η and the load power transfer β. For a typical thermal electric generator, the source efficiency η is given by 
         η   T     =       I   ⁢     R   Load           S   2           K   ⁢           ⁢   ΔT     +     α   ⁢           ⁢     T   H     ⁢     I   S       +       1   2     ⁢   I   ⁢     R   Internal           S   2               
 
where K is the thermal conductivity, ΔT is the thermal gradient across the device and T H  is the hot side temperature. The load power transfer β is defined as 
         β   =       R   L       R   I         ,       
 
where R I  is the combined impedance of DEC  102 , regulator  108 , SPOS  104  and DEC  102  as seen from the load  110 .
 
     Next, at step  206  the controller  120  sets the PWM to yield a source current I S  one preselected increment up or down in step  206 . In step  208 , the controller  120  re-measures the voltage and the current of the DEC  102  and the load  110  from the sensors  116 ,  118 ,  116 ′,  118 ′. The controller  120 , in step  210 , recalculates the source power P S , the load power P L , the source efficiency η and the load power transfer β. In step  212 , the controller  120  determines if the product of the source efficiency η and the load power transfer β has changed. If the product of the source efficiency η and the load power transfer β has not changed, then the controller  120  goes to step  214 . In step  214 , the controller  120  reverses the step direction of the source current I S  (up to down, or down to up) and sets the PWM to the gate drive circuit  122  to yield a source current I S  one increment up or down from the previous value. The controller  120  then loops to step  208 . 
     If the ηβ product has changed, then the controller  120  goes to step  216 . In step  216 , if the product has increased and I S  was incremented up, then the controller  120  goes to decision block  218 . In step  218 , the controller  120  sets the PWM to gate drive to yield I S  one increment down from its previous value. Then the controller  120  loops to step  208 . 
     If at block  216  the product did not increase with I S  incremented up, then the controller  120  moves to step  220 . In step  220 , the controller  120  sets the PWM to gate drive to yield I S  one increment up from its previous value. The controller  120  then loops to step  208 , and the polarity of the incrementation remains unchanged. 
     The optimization system  100  increases the efficiency of the DEC  102  by about 50% for typical loads under continuous operation. The optimization configuration  100  for the DEC  102  also enables both source and load regulation, resulting in optimum power delivered to the load  110 . Furthermore, the design of the switch mode rectifier circuit  114  is versatile enough to achieve superior performance especially for high power and hybrid vehicle applications, however, other designs are possible such as a conventional buck-boost or Cuk non-isolated DC/DC converter. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.