Abstract:
A hybrid power system is comprised of a high energy density element such as a fuel-cell and high power density elements such as a supercapacitor banks. A DC/DC converter electrically connected to the fuel cell and converting the energy level of the energy supplied by the fuel cell. A first switch is electrically connected to the DC/DC converter. First and second supercapacitors are electrically connected to the first switch and a second switch. A controller is connected to the first switch and the second switch, monitoring charge levels of the supercapacitors and controls the switching in response to the charge levels. A load is electrically connected to the second switch. The first switch connects the DC/DC converter to the first supercapacitor when the second switch connects the second supercapacitor to the load. The first switch connects the DC/DC converter to the second supercapacitor when the second switch connects the first supercapacitor to the load.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was made with Government support under Collaborative Technology Alliances Power &amp; Energy Consortium contract DAAD19-01-2-0010 awarded by the Army Research Lab. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A power source supplies a current at a voltage to a load for a period of time. Characteristics of the load typically define the kind of power source that is appropriate. Electronic circuits may demand a relatively small current for an extended period of time. A mechanical device may demand a short burst of relatively high current to generate a powerful motion. Some loads, like an electric vehicle may require smaller currents for motion over a flat surface and a larger current to move up an incline. 
         [0003]    Electric powered vehicles may employ large lead acid batteries to provide energy for their traction systems and operating systems. A battery of this type typically delivers from 24 to 48 volts. A traction system may be powered to move an electric powered vehicle around the workplace under the control of an operator or a computer. Traction systems may draw large currents from the DC bus during acceleration or when moving up an incline, but normally demand lower currents for extended periods of time. Operating systems, such as a lift system, may consume a significant portion of the stored power during normal truck operation. When lifting heavy loads, the operating systems may demand large currents for short periods. 
         [0004]    A conventional lift truck will typically operate from 5 to 6 hours on a fully charged battery. When the battery voltage drops below a certain level the truck is driven to a battery station where the depleted battery is removed and a fully charged replacement battery is installed. This operation typically requires from 20 to 30 minutes during which the truck and operator are nonproductive. 
         [0005]    Efforts have been made to improve the vehicle designs, particularly in ways that will increase the productive period of the battery. For example, the battery may be recharged during truck operation by an alternator, generating charging currents with motions of the traction and lift systems. While this approach does recover some of the energy, lead acid batteries are inefficient energy recovery devices. A large portion of the regenerated energy is dissipated as heat and lost. Periods when large currents are drawn during truck operation significantly limit battery life. 
         [0006]    As can be seen, there is a need for power sources capable of providing large currents in short bursts and lower currents over an extended period of time. A hybrid power source consisting of a high power source and a high energy source can result in a high energy and as well as high power device when the load duty cycle of each component power source is actively managed. For example: a fuel-cell, which is a high energy density device, may be hybridized with a supercapacitor, a high power device, to construct such a source. A supercapacitor or ultracapacitor is an electrochemical capacitor that has an unusually high energy density when compared to common capacitors. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect of the invention, a hybrid power system comprises a power source and a power storage element receiving energy from said power source, wherein said power storage element stores energy received from said power source while simultaneously providing energy to a load. 
         [0008]    In another aspect of the invention, a method of operating a hybrid power system comprises charging a first capacitance bank, charging a second capacitance bank. The first charged capacitance bank is connected to a load and the second charged capacitance bank is connected to an energy source. The first capacitance bank is then disconnected from the load and connected to the energy source. The second capacitance bank is connected to the load. The first capacitance bank is then connected to the load and the second capacitance bank is disconnected from the load. The second capacitance bank is then connected to the energy source. 
         [0009]    In a further aspect of the invention, a hybrid power system comprises a fuel cell, a DC/DC converter electrically connected to the fuel cell and converting the energy level of the energy supplied by the fuel cell. A first switch is electrically connected to the DC/DC converter or some other DC source. A first and second capacitance banks are electrically connected to the first switch and a second switch respectively. A controller is connected to the first switch and the second switch. The controller monitors the charge levels of the first supercapacitor and the second supercapacitor and controls the first switch and the second switch in response to the charge levels. A load is electrically connected to the second switch. When the first switch connects the DC/DC converter to the first capacitance bank, the second switch connects the second supercapacitor to the load. When the first switch connects the DC/DC converter to the second capacitance bank, the second switch connects the first capacitance bank to the load. 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram depicting a hybrid power system in accordance with an embodiment of the present invention; 
           [0012]      FIG. 2  is a block diagram depicting a controlled hybrid power system in accordance with an embodiment; 
           [0013]      FIG. 3  is a block diagram depicting a controlled dual storage hybrid power system in accordance with an embodiment; 
           [0014]      FIG. 4  is a block diagram depicting a converted hybrid power system in accordance with an embodiment; 
           [0015]      FIG. 5  is a block diagram depicting a supercapacitor bank hybrid power system in accordance with an embodiment; 
           [0016]      FIG. 6  is a flow diagram depicting a simplified process of operating a hybrid power system in accordance with an embodiment; and 
           [0017]      FIG. 7  is a flow diagram depicting a process of operating a monitored hybrid power system in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0019]    Broadly, the present invention is a hybrid power system to provide power for electrical vehicles, robots and other electrical devices having varieties of power demands. 
         [0020]    A typical power source optimally provides either low currents or large currents, A capacitor, which is a higher power density but lower energy density device, is typically capable of providing a large current for a short period. A fuel cell, which is a higher energy density but lower power density device, may provide smaller currents for a lengthy period of time. A hybrid power source may include a power source providing low currents for extended periods of time integrated with a power storage element capable of providing large currents for short periods of time. A super-capacitor bank periodically recharged by fuel cells may provide the load with the necessary currents, as needed, resulting in a power source with optimal levels of energy and power densities. 
         [0021]    With reference to  FIG. 1 , a hybrid power system  100  in accordance with an embodiment is shown. A power source  102  may provide energy to power storage element  104 . A fuel cell may be used as the power source  102  for the hybrid power system  100 . Fuel cells may provide a steady source of power for as long as the cells remain fuelled. Batteries such as lithium primary or secondary batteries may be used as a power source  102 . The power storage element  104  may be charged by the energy provided by the power source  102 . Supercapacitors may be used as a power storage element  104 . Power storage element  104  may store the energy provided by the power source  104  until the energy may be demanded by a load  106 . 
         [0022]    With reference to  FIG. 2 , a controlled hybrid power system  200  in accordance with an embodiment is shown. A power source  102  may provide energy to a power storage element  104  where the energy may be stored. Load  106  may draw power as needed from the power storage element  104 . Further power storage  110  such as a battery may be charged. A controller  108  may be connected to the power storage element  104  to monitor charge levels of the power storage element  104 , and connect power source  102  to the power storage element as needed. 
         [0023]    With reference to  FIG. 3 , a controlled dual-storage hybrid power source  300  in accordance with an embodiment is shown. A pair of power storage elements  112  and  114  may be charged by power from a power source  102 . The load  106  may receive power from the power storage elements  112  and  114 . A first switch  116  may route power from the power source  102  to the first power storage element  112  until the first switch  116  may be instructed by the controller  108  to route the power from the power source  102  to the second power storage element  114 . Using the first switch  116 , the controller  108  may regulate the flow of power from the power source  102  to charge the power storage elements  112  and  114  alternately. A second switch  118  may alternately route power from the power storage elements  112  and  114  to the load  106 . The controller  108  may control the switches  116  and  118  so that when the first power storage element  112  may be charged, the load  106  may draw power from the second power storage element  114  and when the second power storage element  114  may be charged, the load may draw power from the first power storage element  112 . The controller  108  may be an oscillator. In accordance with an embodiment of the invention, controller  108  may include a programmed microprocessor. 
         [0024]    Typically, each of the power storage elements  112  and  114  may be identical capacitance banks. Alternately, the power storage elements  112  and  114  may be capacitance banks of various capacitance values. Each capacitance bank  112 ,  114  may include a specified number of capacitor cells in series, where the number of cells may be selected to comply with specified load voltage requirements. In addition, the capacitance banks  114 ,  116  may consist of parallel strings of cells, where the number of cells and strings may be chosen to provide the necessary capacitance value. The specifications may be determined with reference to appropriate capacitor bank weight, volume, cost, charge voltage, current and timing. 
         [0025]    In addition to controlling the switches  116  and  118 , the controller  108  may monitor various system voltages such as the voltage level of the capacitance banks  114 ,  116  and system currents, such as the current level supplied by the fuel cell  102 , of the hybrid power source  300 . The controller  108  may operate a load switch  118  to connect and disconnect the power storage elements  112  and  114  to and from the load  106  so that power may be available to the load at all times. A ‘make before break’ switching scheme may be implemented accordingly. 
         [0026]    With reference to  FIG. 4 , a converted hybrid power system  400  in accordance with an embodiment is shown. A power source  102  may include fuel cells receiving fuel from a fuel storage unit  124 . The power source  102  may provide energy at a given voltage level that may be different from the voltage levels needed to charge the power storage elements  104  and support the load, so the energy from the power source  102  may be sent through a first converter  120 . The first converter  120  may be typically a standard DC/DC converter to convert the input voltage level  134  to an output voltage level  136 . A first switch  116  may route the energy received from the converter  120  to one of the capacitance banks  114 ,  116  in the power storage elements  104  in accordance with control signals received from controller  108 . A load switch  118  may route the energy from one of the capacitance banks  114  and  116  in the power storage elements  104  in accordance with control signals received from the controller  108 . The power storage elements  104  may provide energy at a given voltage level that may be different from the voltage levels needed by the load  106 , so the energy from the power storage elements  104  may be sent through a second converter  122 . The second converter  122  may be typically a standard DC/DC converter to convert the input voltage level  138  to an output voltage level  140 . The controller  108  may receive signals from the fuel storage  124 , power source  102  and power storage elements  104  indicating fuel levels, energy levels and charge levels. 
         [0027]    With reference to  FIG. 5 , a supercapacitor bank hybrid power system  500  in accordance with an embodiment is shown. A fuel cell  102  may generate energy which may be then stepped-up by a DC/DC converter  120 . The energy may be provided to the sources  142 ,  148  of MOSFETs  126 ,  128  (Metal Oxide Semiconductor Field Effect Transistor). The drains  146 ,  152  of MOSFETs  126 ,  128  may alternately provide energy to one of the capacitance banks  130 ,  132 . Likewise, load switch  118  may alternately provide current from one of the capacitance banks  130 ,  132  to the load  106 . The MOSFETs  126 ,  128  may be controlled by controller  108  connected to gates  144 ,  150 . The load switch  118  may be controlled by controller  108 . 
         [0028]    MOSFET switches  126 ,  128  may be used to apply charge energy to individual capacitance banks  130 ,  132  such that while one bank  130  may be providing energy to the load, the other bank  132  may be charged by energy from the fuel cell  102 . While two capacitance banks  130 ,  132  may be shown in the present embodiment, it should be understood that any number of capacitance banks could be implemented in accordance with another embodiment. The switching sequence of the MOSFETs  126 ,  128  may be managed by the controller  108  connected to gates  144 ,  150 . The switches  126 ,  128 ,  118  may be implemented using MOSFETs or any suitable switch compatible with electronic control and providing appropriate resistance. 
         [0029]    With reference to  FIG. 6 , a process  600  of operating a hybrid power system  500  in accordance with an embodiment is shown. Initially, a first capacitance bank  130  may be charged at function block  602  and a second capacitance bank  132  may be charged at function block  604 . The load may be connected the first capacitance bank at function block  606 . After a predetermined time, typically sufficient to charge the second capacitance bank  132  and before the first capacitance bank  130  may be completely discharged, the load  106  may be connected to the second capacitance bank  132  at function block  608 . The first capacitance bank  130  may be disconnected from the load  106  at function block  610 . The first capacitance bank  130  may be connected to the fuel cell  102  and may be charged at function block  612 . After the predetermined time elapses, a fully charged first capacitance bank  130  may be connected to the load  106  at function block  614 . The second capacitance bank  132  may be disconnected from the load  106  at function block  616  and connected to the fuel cell  102  to be charged at function block  618 . The process may cycle after the predetermined period elapses, connecting the load  106  to the second capacitance bank  132  at function block  608 . 
         [0030]    With reference to  FIG. 7 , a process  700  of operating a hybrid power system  500  in accordance with an embodiment is shown. The capacitance bank  130 ,  132  may be initially charged at function block  702 . The first capacitance bank  130  may be connected to the load  106  at function block  704 . The charge level of the first capacitance bank  130  may be monitored by the controller  108  at function block  706 . The charge level of the first capacitance bank  130  may be compared to a predetermined threshold at decision block  708 . Until the charge level reaches the threshold, the process follows the NO path and continues monitoring the charge level. When the charge level reaches the threshold, the process follows the YES path. The second capacitance bank  132  may be connected to the load  106  at function block  710 . The first capacitance bank  130  may be disconnected from the load  106  at function block  712  and connected to the fuel cell  102  to be recharged at function block  714 . The second capacitance bank charge level may be monitored at function block  716 . The charge level of the second capacitance bank  132  may be compared to a predetermined threshold at decision block  718 . Until the charge level reaches the threshold, the process follows the NO path and continues monitoring the charge level. When the charge level reaches the threshold, the process follows the YES path. The first capacitance bank  130  may be connected to the load  106  at function block  720 . The second capacitance bank  132  may be disconnected from the load  106  at function block  722  and connected to the fuel cell  102  to be recharged at function block  724 . The cycle repeats and the first capacitance bank charge level may be monitored at function block  706 . 
         [0031]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.