Abstract:
The invention is directed to a method, system and device for converting direct current (DC) electrical voltage from a fuel cell to an alternating current (AC) voltage. The inventive method regulates power drawn from the fuel cell and from a battery to maintain a substantially constant DC voltage across a DC bus, and inverts the DC voltage from the DC bus to the AC voltage. The method may further electrically isolate the fuel cell from the load. Also, the inventive method may prevent current from flowing to the fuel cell. The inventive method may also provide a charging current to the battery.

Description:
TECHNICAL FIELD 
   The invention relates generally to fuel cell power generation systems, and more particularly to efficiently inverting direct current (DC) voltage from a fuel cell unit to an alternating current (AC) voltage. 
   BACKGROUND OF THE INVENTION 
   Recently, as environmental concerns have moved to the forefront, there has been a push to provide a more efficient and cleaner form of energy. One proposed solution has been the fuel cell. The fuel cell is an electrochemical energy conversion device that converts fuel and oxygen into electricity, heat, and innocuous by-products such as water vapor. Emissions from the fuel cell system typically are significantly smaller than emissions from the cleanest fuel combustion processes. 
   A fuel cell system is made up of a number of individual fuel cells that form a fuel cell “stack.” Fuel can be supplied to the fuel cell stack in a number of ways. For example, a proton exchange membrane (PEM) fuel cell can be fed directly from a source of hydrogen, or it can operate from hydrogen that is being supplied from a fuel reformer. Typically, the air required by most fuel cells is pumped to the fuel cell stack at a rate that varies with the load and/or various operating conditions. These other devices necessary to operate the fuel cell system (e.g., pumps and fans) are called the system&#39;s “balance-of-plant.” The balance-of-plant components and the reformer typically cause a fuel cell stack to respond to the inevitable load changes much slower than batteries (i.e.,—the balance-of-plant and reformer are unable to keep up with instantaneous changes in the load). 
   Each individual cell in a fuel cell stack produces direct-current (DC) energy, typically with a high current and a low voltage (e.g., 0.7 volts). The low DC voltage produced by the fuel cell varies with the operating conditions such that the voltage is highest at no load and lowest at full load. A typical ratio between full-load and no-load voltage may be 2 or more. The DC energy produced by the fuel cell may be used both in stationary and mobile applications. Certain applications, for example residential and commercial loads, require an alternating current (AC) output. As a result, fuel cells, like many other alternative DC energy sources (e.g., solar energy) require an inverter to convert the DC voltage into AC voltage. Once converted, this AC source may be used as a stand-alone source and/or in parallel with the electrical power transmission grid that currently provides power to residential and commercial loads. 
   Fuel cells place many unusual constraints on the inverter device that is responsible for converting the fuel cell system&#39;s output to a regulated AC voltage. For example, the inverter must be able to adapt to the varying output voltage of the fuel cell. Also, the inverter must protect the fuel cell from a reverse current or an unstable input current, both of which could destroy the fuel cell. In addition, because the fuel source typically is incapable of instantaneously responding to the varying demands of the load, the inverter must be able to cooperate with a DC storage source (e.g., a battery) as well as the fuel cell. As a result of these differences, traditional power inverters can not satisfy the requirements of the fuel cell system, particularly in stand-alone applications (i.e., where the fuel cell inverter directly powers the load). 
   Therefore, it would be advantageous to provide a high efficiency DC-to-AC inverter suited to accommodate the unique operation of the fuel cell powered system. 
   SUMMARY OF THE INVENTION 
   The invention is directed to a method, system and device for converting direct current (DC) electrical voltage from a DC power source that provides varying DC voltage (e.g., a fuel cell) to an alternating current (AC) voltage. The inventive method controls DC power drawn from the fuel cell, and controls DC power drawn from a battery based on power available from the fuel cell. In so doing, the method maintains a substantially constant DC voltage on a DC bus. The inventive method further inverts the DC voltage from the DC bus to the AC voltage. The method further may provide the AC voltage to a load, and may electrically isolate the fuel cell from the load. Also, the inventive method may prevent current from flowing to the fuel cell. The inventive method may provide a charging current to the DC voltage source, as well as maintaining a constant DC voltage. 
   The inventive system includes a DC-to-AC inverter, a DC bus coupled to the DC-to-AC inverter (e.g., an H-bridge inverter), and a battery coupled to the DC bus via a charge/discharge controller. Also, the system includes a converter (e.g., a boost converter) coupled to the DC bus and to the fuel cell. The inventive system also may include an isolation device, for example an electrical transformer that is coupled to the DC-to-AC inverter. Also, the system may include a protection device coupled to the fuel cell that is designed to prevent current from flowing into the fuel cell. The inverter may be designed to operate with a low voltage input. When an increase in load demand occurs, the inverter draws power from the battery equal to the increased demand until the fuel cell is able to support the increased load demand. Also, when other types of load transients occur or when load demands exceed a capacity of the fuel cell, the controller regulates power drawn from the battery and the fuel cell. The system may be used to provide power to a load directly, or to provide power to a load via an electrical power transmission grid. 
   The inventive device includes an inverter (e.g., an H-bridge inverter) that converts DC power, which may have a low voltage, from the fuel cell to an AC power. The device further includes a battery and a boost converter. The boost converter maintains a substantially constant DC voltage to the DC bus by regulating power from the fuel cell. The boost converter also is designed to provide a charging current to the battery. The diode that forms a fundamental part of the boost converter prevents current from flowing to the fuel cell. An optional electrical transformer provides electrical isolation between the fuel cell and the load. When an increase in load demand occurs, the boost converter draws power from the battery equal to the increased demand until the fuel cell is able to support the increased load demand. Also, during load transients and when the load demand exceeds the capacity of the fuel cell, the inverter draws power from the battery via the charge/discharge controller and from the fuel cell via the boost converter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features of the invention are further apparent from the following detailed description of the embodiments of the invention taken in conjunction with the accompanying drawings, of which: 
       FIG. 1  provides a block diagram of a fuel cell inverter circuit, in accordance with the invention; 
       FIG. 2  provides a component-level block diagram of the fuel cell inverter circuit shown in  FIG. 1 ; and 
       FIG. 3  provides a block diagram of certain applications of the fuel cell inverter circuit shown in  FIG. 1 , in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  provides a block diagram of a fuel cell inverter circuit  100 , in accordance with the invention. As shown in  FIG. 1 , a fuel cell  101  is coupled to the remainder of circuit  100  via a switch  110  that operates to disconnect fuel cell  101  from the remainder of circuit  100 . The input of a boost converter  103  is coupled to input filter  102  and it is also coupled to fuel cell  101  via switch  110 . A charge/discharge controller  113  and a battery  104  (coupled in series to each other) are coupled to the output of boost converter  103 . Battery  104  is sized to produce a voltage greater than or equal to the maximum operating voltage of fuel cell  101 . A DC bus filter  112  and the input to a DC-to-AC inverter  105  are both coupled to the output side of boost converter  103 . The output of boost converter  103 , charge/discharge controller  113 , and the input of DC-to-AC inverter  105  are coupled to a DC bus  108 . DC bus  108  typically is designed to operate at voltages slightly above the voltage level of battery  104 . Also, circuit  100  optionally can be coupled to a ground potential  109 . 
   An optional isolation circuit  106  is coupled to the output of DC-to-AC inverter  105 . Isolation circuit  106  also is coupled to an AC load  107 . AC Load  107  may be any energy-consuming device (e.g., motor, lighting) that can operate with AC current. AC Load  107  may be an electrical power transmission grid (as discussed with reference to FIG.  3 ), or other AC voltage source. 
   Fuel cell  101  produces a low DC voltage at a high current. The voltage produced by fuel cell  101  varies with load and operating conditions. Also, the requirements of AC load  107  tend to vary over time. The varying power required by AC load  107  tends to create a fluctuating voltage at the output of fuel cell  101 . However, boost converter  103 , charging/discharging controller  113  and battery  104  operate to provide a nearly constant bus voltage to DC bus  108 , despite the fluctuating voltage provided by fuel cell  101 . For example, when a positive load step change occurs (e.g., when AC load  107  draws a greater quantity of power), battery  104  provides power to DC bus  108  (through charge/discharge controller  113 ) equal to the step change until fuel cell  101  is able to support the entire quantity of load  107 . 
   The amount of power provided by fuel cell  101  to DC bus  108  is determined by boost converter  103 , which allows full control of the power provided by fuel cell  101 . When the available power from fuel cell  101  begins to decrease (e.g., because of a lack of fuel supply), boost converter  103  draws less power from fuel cell  101  and charge/discharge controller  113  draws additional power from battery  104 . Boost converter  103  permits the power drawn from fuel cell  101  to be increased gradually as it becomes capable of providing the full power requirements of AC load  107 . When the available fuel cell power exceeds the load power (plus power consumed by inefficiencies of the inverter), the boost converter  103  is responsible for maintaining the voltage provided to DC bus  108 . If, however, the available fuel cell power is lower than the required load power, then the voltage on DC bus  108  is regulated by battery  104  and charge/discharge controller  113 . Battery  104  operates to provide power both during load transients and during peak loads that exceed the rating of fuel cell  101 . 
   When fuel cell  101  has enough reserve power to both charge battery  104  and to supply the power demanded by AC load  107 , fuel cell  101  provides power to DC bus  108 . In this case, charge/discharge controller  113  operates to stop the flow of current from battery  104  to DC bus  108 , and provide the flow of current from DC bus  108  to battery  104 . As a result, boost converter  103  operates to maintain a nearly constant voltage on DC bus  108 . 
   DC-to-AC inverter  105  converts the DC voltage on DC bus  108  to an AC voltage, suitable for AC load  107 . DC-to-AC inverter  105  is designed to operate with a low voltage input, like that provided by fuel cell  101 . Isolation device  106  provides electrical isolation between AC load  107  and DC-to-AC inverter  105 . Therefore, fuel cell  101  and the remainder of circuit  100  may be protected from any electrically adverse conditions (e.g., power surges) initiated on the load side of the system. The isolation device  106  also allows for the possibility of connecting battery  104  and fuel cell  101  to an earthed ground (e.g., for safety reasons). 
     FIG. 2  provides an example of a component-level block diagram of fuel cell inverter circuit  100 , shown in FIG.  1 . Although  FIG. 2  provides specific components within the elements shown in  FIG. 1 , it should be appreciated the components of  FIG. 2  are not exclusive, and other similar components may be used. 
   As shown in  FIG. 2 , input filter  102  includes a capacitor C 1 . Charge/discharge controller  113  includes a small MOSFET Q 2  coupled in anti-parallel with a diode D 2 . It should be appreciated that diode D 2  can be the body diode of MOSFET Q 2  or a separate diode, like a Schottky diode. Use of a separate diode allows for battery discharge current to be much greater than battery charging current. MOSFET Q 2  operates to permit the flow of current from DC bus  108  to battery  104  (i.e., charging battery  104 ). Diode D 2  operates to permit the flow of current from battery  104  to DC bus  108  (i.e., discharging battery  104 ). Boost converter  103  includes a MOSFET Q 1  a diode D 1 , and an inductor L 1 . DC bus filter  112  includes a capacitor C 2 . 
   DC-to-AC inverter  105  includes a combination of components that form an H-bridge inverter, and associated filtering components. In particular, one half of the H-bridge inverter includes MOSFETs Q 3  and Q 4 , inductor L 2 , and capacitor C 3 . MOSFETs Q 3  and Q 4  form one-half of the H-bridge, and inductor L 2  and capacitor C 3  provide filtering. The other half of the H-Bridge inverter includes MOSFETs Q 5  and Q 6 , inductor L 3 , and capacitor C 4 . MOSFETs Q 5  and Q 6  form one-half of the H-bridge, and inductor L 3  and capacitor C 4  provide filtering. The output of the H-Bridge Inverter is coupled to isolation device  106 , which may be a transformer T 1 , for example. In this instance, transformer T 1  is coupled on its primary side to the H-bridge inverter, and on its secondary side to AC load  107 . 
   In one embodiment, semiconductor switches Q 1 , Q 3 , Q 4 , Q 5 , and Q 6  are 100 volt MOSFETs. As compared to other semiconductor devices that have a nearly constant voltage drop regardless of current flow, for example, Insulated Gate Bipolar Transistors (IGBTs), the MOSFETs are selected so as to reduce losses when the output load is a fraction of the inverter&#39;s full-load rating. 
   In operation, fuel cell  101  provides a low-voltage, high-current power source to the remainder of circuit  100 . The precise value of the available voltage and current from fuel cell  101  may be varied with the number of fuel cells stacked together, based upon the required demand of load  107 . The power generated by fuel cell  101  then passes through a closed switch  110 . Capacitor C 1  acts as source of high-frequency current. Although capacitor C 1  is shown separate from boost converter  103 , it should be appreciated that capacitor C 1  may be incorporated within boost converter  103 . 
   Because fuel cell  101  may not be able to satisfy the demand of AC load  107  at various times throughout the operation of circuit  100 , boost converter  103  operates to regulate power provided by fuel cell  101 . More specifically, diode D 2  operates to detect whether fuel cell  101  can meet the power demanded by AC load  107 . When the average power provided by fuel cell  101  can not meet the average required demand of AC load  107 , the voltage on DC bus  108  drops below the battery voltage and diode D 2  becomes forward biased. The forward biased diode D 2  permits current to flow from battery  104  to DC bus  108 . If, on the other hand, fuel cell  101  provides sufficient power on DC bus  108  to operate AC load  107 , and if battery  104  needs to be charged, MOSFET Q 2  can be operated in the active region to maintain a constant float voltage across battery  104 . 
   Using MOSFET Q 2  allows a constant current to flow into battery  104  by absorbing and preventing a ripple voltage present on DC bus  108  (as discussed below with reference to DC-to-AC inverter  105 ) from appearing across battery  104 . Notably, the DC bus voltage is nominally higher than the battery voltage, so that the voltage across MOSFET Q 2  is small (e.g., 1 to 5 V). In effect, therefore, charge/discharge controller  113  operates to conduct the unregulated discharging flow of current from battery  104  to DC bus  108  using D 2 , while properly regulating the flow of charging current to battery  104  using Q 2 . 
   Boost converter  103  operates to regulate the amount of power provided by fuel cell  101 . As a result, boost converter  103  permits battery  104  and fuel cell  101  to cooperate so as to maintain a substantially constant DC voltage on DC bus  108 . Fuel cell  101  is protected from reverse current (e.g., current from DC bus  108  back to fuel cell  101 ) by diode D 1  in boost converter  103 . Typically, for low voltage sources (like fuel cell  101 ) that require reverse current protection, a series-connected diode&#39;s voltage drop can introduce a significant loss, especially at partial loads. Because of the operation of boost converter  103 , however, diode D 1  provides reverse current protection at a reduced current (as compared to placing the diode directly in series with fuel cell  101 ), thus increasing the overall efficiency of the circuit. Capacitor C 2  filters the high frequency current on the output of boost converter  103 , as well as filtering the AC current required by inverter  105 . 
   Inverter  105  uses an H-bridge Inverter configuration to convert the voltage from DC provided by DC bus  108  to AC voltage that feeds AC load  107 . Therefore, the H-bridge Inverter facilitates controlled power flow between DC and AC circuits. The H-bridge Inverter includes two half-bridges (Q 3 /Q 4  and Q 5 /Q 6 ) and two corresponding filters (L 2 /C 3  and L 3 /C 4 , respectively). Inverter  105  typically draws power from DC bus  108  at a frequency that is twice that of the inversion frequency. For example, power drawn from DC bus  108  will have a significant 120 Hz ripple component if the inverter produces 60 Hz power. DC bus  108  will therefore have a voltage with a 120 Hz ripple component. 
   As is well known to those skilled in the art, an inherent feature of the MOSFET is that it acts as a diode (i.e., a “body diode”) for current flowing in the reverse direction. During normal operation, the load current flows through a MOSFET in each half-bridge for a period of time, and a MOSFET body-diode in each half-bridge for a period of time. Notably, the period of time that the current flows through the body-diode will increase if the voltage on DC bus  108  increases above its minimum designed operating level. However, in order to obtain efficient operation during partial load situations, boost converter  103  and battery  104  in conjunction with charge/discharge controller  113  will operate to keep the voltage on DC bus  108  nearly constant (as discussed above with reference to boost converter  103 ), so to beneficially minimize the duration of current flow through the body diodes. 
   The H-bridge inverter converts the DC voltage from fuel cell  101  to AC voltage for AC load  107  by designing the filters (L 2 /C 3  and L 3 /C 4 ) to pass the desired frequency of the line voltage (e.g., 60 Hz or 50 Hz), while removing the high-frequency switching component (e.g., 20 kHz) of voltage. The MOSFETs are pulse width modulated to provide the respective half-bridge filter components with voltages that are 180° out of phase with each other, so as to create a sinewave across transformer T 1 . The voltages across C 3  and C 4  are sinewaves that are 180° out of phase with each other so that the sinewave applied to the primary of transformer T 1  has twice the amplitude of the sinusoidal voltage across either C 3  or C 4 . 
   The filtering components create fluctuating voltage waves with a small amount of high-frequency ripple created by the pulse wave modulation. Because the voltage between either leg of the primary on transformer T 1  and ground  109  has only a very small high-frequency voltage component, the emitted electromagnetic radiation is significantly reduced. 
   Transformer T 1  provides isolation between load  107  and circuit  100 . Transformer T 1  also may be designed such that the sum total kVA rating of its secondary windings is greater than the kVA rating of its primary winding. Such design accommodates the possibility that either secondary may carry the greater current at any particular time. Therefore, transformer T 1  beneficially provides a method to power unbalanced loads without increasing the rating of the semiconductor switches. Such capability is especially relevant for stand-alone split-phase loads (e.g., residential applications). 
   It should be noted that the circuit configuration shown in  FIG. 2  permits operation in grid-parallel and/or stand-alone mode.  FIG. 3  is a block diagram showing the use of circuit  100  coupled to a customer premise  301  (i.e., stand-alone mode) and/or a power transmission network  303  (i.e., grid-parallel mode). Power transmission network  303  is a network of high-voltage transmission lines that connect producers of electric power to the end customer (e.g., customer premise  301 ). In the United States, there are ten regional networks or “grids” (e.g., Mid-America Interconnected Network and Western System Coordinating Council) collectively serving the power needs in the United States. Power transmission network  303  may receive power from at least one power generation source  302 , such as a nuclear power plant or hydroelectric power generation plant. 
   When coupled to power transmission network  303 , the network causes a sinusoidal voltage to appear across filter capacitors C 3  and C 4  of DC-to-AC inverter  105 . Pulse-width modulation may be used to control the half-bridges of circuit  100  to produce a substantially sinusoidal current through filter inductors L 2  and L 3 . The resulting substantially sinusoidal current may have a frequency substantially similar to the voltage of power transmission network  303 . When coupled to customer premise  301 , the voltage across filter capacitors C 3  and C 4  in circuit  100  may be monitored by a separate device (not shown) so as to maintain a sinusoidal voltage at the desired frequency of customer premise  301  (e.g. 60 Hz for residential premises). Furthermore, by monitoring the current entering the residence, it is possible to modify the current produced by inverter  100  to provide overall power factor correction and/or to prevent net power generation by the residence. 
   The scope of protection of the following claims is not limited to the embodiments described above. Those skilled in the art will recognize that modifications and variations of the specific embodiments disclosed herein will fall within the true spirit and scope of the invention. 
   While the invention has been particularly shown and described with reference to the embodiments thereof, it will be understood by those skilled in the art that the invention is not limited to the embodiments specifically disclosed herein. For example, although the invention was described using certain electronic components with specific ratings, it should be appreciated that those components may be replace or rearranged without exceeding the scope of the invention. Those skilled in the art will appreciate that various changes and adaptations of the invention may be made in the form and details of these embodiments without departing from the true spirit and scope of the invention as defined by the following claims.