Abstract:
A power converting apparatus for a fuel cell, and a method thereof. The power converting apparatus for a fuel cell comprises: a converting unit for converting a DC voltage outputted from a stack of a fuel cell into a boosted or dropped AC voltage by being switched by a switching control signal; and a controlling unit for comparing the detected AC voltage level with a preset AC voltage level, and outputting a switching control signal for controlling a switching of the converting unit on the basis of the comparison result

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a fuel cell, and more particularly, to a power converting apparatus for a fuel cell capable of maximizing a power conversion efficiency of a fuel cell and a method thereof.  
         [0003]     2. Description of the Background Art  
         [0004]     Generally, a fuel cell system serves to directly convert fuel energy into electric energy.  
         [0005]     The fuel cell system is provided with an anode and a cathode at both sides of a high molecule electrolyte membrane. As a fuel of hydrogen is electrochemically oxidized at the anode and oxygen is electrochemically deoxidized at the cathode, electrons are generated. The fuel cell system generates electric energy as the generated electrons move.  
         [0006]      FIG. 1  is a diagram showing a proton exchange membrane fuel cell (PEMFC), in which a hydrocarbon-based fuel such as LNG, LPG, CH 3 OH, etc. (LNG in drawing) undergoes a desulfurizaton process, a reforming process, and a hydrogen refining process in a reformer, so that only hydrogen is refined thus to be used as a fuel.  
         [0007]     As shown in  FIG. 1 , the conventional fuel cell system comprises a reforming unit  10  for refining hydrogen from LNG; a fuel supply unit  20  for supplying a refined hydrogen to an anode by connecting the reforming unit  10  to the anode; an air supply unit  30  for supplying atmospheric air to a cathode; a stack unit  40  having an anode  41  to which hydrogen is supplied and a cathode  42  to which air is supplied, for generating electric power and heat by electrochemically reacting hydrogen and air; a power output unit  50  connected to an outlet of the stack unit  40  for supplying power to a load; a heat exchange unit  60  for cooling the reforming unit  10  and the stack unit  40  by respectively supplying water thereto; and a controller (not shown) electrically connected to each of the units and controlling an operation of each unit.  
         [0008]     The power output unit  50  comprises a DC-DC converting unit  51  for generating an alternating current (AC) by switching a direct current (DC) generated from the stack unit  40 , and rectifying the generated AC into a DC; and a converting unit  52  for converting a DC outputted from the DC-DC converting unit  51  into an AC thereby generating an AC.  
         [0009]     An unexplained reference numeral  21  denotes a fuel supply line,  22  denotes a fuel supply pump,  31  denotes an air supply line,  61  denotes a water storage tank,  62  denotes a water circulation line,  63  denotes a heat emitter, and  64  denotes a water circulation pump.  
         [0010]     An operation of the conventional fuel cell system will be explained.  
         [0011]     First, a hydrocarbon-based fuel is refined in the reforming unit  10 , thereby refining hydrogen. The refined hydrogen is supplied to the anode  41  of the stack unit  40 .  
         [0012]     The reforming unit  10  supplies air to the cathode  42  of the stack unit  40 .  
         [0013]     An electrochemical oxidation is performed in the anode  41  of the stack unit and an electrochemical deoxidation is performed in the cathode  42  of the stack unit  40 .  
         [0014]     While the oxidation and the deoxidation are performed, electrons are generated. As the generated electrons move to the cathode  42  from the anode  41 , a DC voltage is generated. The generated DC voltage is converted into an AC voltage by the DC-DC converting unit  51  of the power output unit  50 .  
         [0015]     An AC voltage outputted form the DC-DC converting unit  51  is boosted or dropped by a control signal outputted from the controlling unit (not shown). Then, the boosted or dropped AC voltage is rectified to a DC voltage thus to be applied to the converting unit  52 .  
         [0016]     The converting unit  52  converts a DC voltage outputted from the DC-DC converting unit  51  into an AC voltage, and supplies the AC voltage to a load such as a home electric unit.  
         [0017]     However, the conventional has the following problems. First, when a voltage outputted from the fuel cell is to be converted into a commercial voltage, the voltage outputted from the fuel cell is boosted or dropped by the DC-DC converting unit, and then the boosted or dropped voltage has to be converted into an AC voltage. As the voltage outputted from the fuel cell is converted into a commercial voltage by two steps, a power conversion efficiency is lowered.  
         [0018]     Furthermore, the number of components of a semiconductor device for a power conversion is increased, and thus a production cost is increased.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0019]     Therefore, an object of the present invention is to provide a power converting apparatus for a fuel cell capable of enhancing a power conversion efficiency by matching an impedance between a fuel cell and a power line by an impedance matching unit, and by converting a DC voltage outputted from the fuel cell to an AC voltage by boosting or dropping by a converting unit, and a method thereof.  
         [0020]     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a power converting apparatus for a fuel cell, comprising: a converting unit for converting a DC voltage outputted from a fuel cell into a boosted or dropped AC voltage by being switched by a switching control signal; and a controlling unit for comparing the detected AC voltage with a preset AC voltage, and outputting a switching control signal for controlling a switching of the converting unit on the basis of the comparison result.  
         [0021]     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a power converting apparatus for a fuel cell that comprises a stack unit having an anode and a cathode and generating electric power by electrochemically reacting hydrogen and air, the apparatus comprising: an impedance matching unit for matching an impedance of a power line of the fuel cell to an impedance of a substantial commercial power line; a converting unit for converting a DC voltage inputted from the impedance matching unit into a boosted or dropped AC voltage by being switched by a switching control signal; a filter for filtering an AC voltage outputted from the converting unit and thereby outputting an AC voltage of a sine wave; a power detecting unit for detecting an AC voltage outputted from the filter; and a controlling unit for comparing the detected AC voltage with a preset AC voltage, and controlling a conversion of the DC voltage outputted from the fuel cell into an AC voltage on the basis of the comparison result.  
         [0022]     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0024]     In the drawings:  
         [0025]      FIG. 1  is a block diagram showing an example of a fuel cell system in accordance with the conventional art;  
         [0026]      FIG. 2  is a schematic view showing a construction of a power converting apparatus for a fuel cell according to the present invention;  
         [0027]      FIG. 3  is a flowchart showing a method for converting a power of a fuel cell according to the present invention;  
         [0028]      FIG. 4  is a view showing a switching waveform of a converting unit according to the present invention; and  
         [0029]      FIG. 5  is a view showing a waveform of an output voltage from the converting unit according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0031]     Hereinafter, a power converting apparatus for a fuel cell capable of enhancing a power conversion efficiency by converting a DC voltage outputted from a fuel cell to an AC voltage by boosting or dropping by a converting unit without an additional boosting device or a dropping device, and a method thereof will be explained in more detail with reference to the attached drawings.  
         [0032]      FIG. 2  is a schematic view showing a construction of a power converting apparatus for a fuel cell according to the present invention.  
         [0033]     As shown in  FIG. 2 , the power converting apparatus for a fuel cell according to the present invention comprises an impedance matching unit  100 , a converting unit  200 , a filter  300 , a voltage detecting unit  400 , a storing unit  500 , and a controlling unit  600 .  
         [0034]     The impedance matching unit  100  matches an impedance of a power line of the fuel cell to an impedance of a substantial commercial power line.  
         [0035]     The impedance matching unit  100  comprises a first coil L 1  having a front end connected to an output port of the fuel cell, a first capacitor C 1  having a front end connected to the output port of the fuel cell, a second coil L 2  having a front end connected to a rear end of the first coil L 1  and a rear end connected to a rear end of the first capacitor C 1 , and a second capacitor C 2  connected between the rear end of the first coil L 1  and the front end of the second coil L 2 .  
         [0036]     The converting unit  200  converts a DC voltage of the fuel cell inputted from the impedance matching unit  100  into an AC voltage by boosting or dropping, and then outputs the boosted or dropped AC voltage.  
         [0037]     The converting unit  200  comprises a second PNP transistor P 2  having a collector connected to an emitter of a first PNP transistor P 1 , a third PNP transistor P 3  having a collector connected to a collector of the first PNP transistor P 1 , and a fourth PNP transistor P 4  having a collector connected to an emitter of the third PNP transistor P 3  and having an emitter connected to an emitter of the second PNP transistor P 2 . The converting unit  200  outputs a difference value between a voltage (VAN) generated at a connection point between the first PNP transistor P 1  and the second PNP transistor P 2  and a voltage (VBN) generated at a connection point between the third PNP transistor P 3  and the fourth PNP transistor P 4 .  
         [0038]     More concretely as shown in  FIG. 5 ( a ), the converting unit  200  outputs a voltage (VAN) generated at a connection point between the first PNP transistor P 1  and the second PNP transistor P 2 . As shown in  FIG. 5 ( b ), the converting unit  200  outputs a voltage (VBN) generated at a connection point between the third PNP transistor P 3  and the fourth PNP transistor P 4 . As shown in  FIG. 5 ( c ), the converting unit  200  outputs a final voltage (VO).  
         [0039]     A diode is respectively connected to the first to fourth PNP transistors P 1  to P 4  in parallel in order to prevent an inverse current.  
         [0040]     The filter  300  that is an AC filter filters an AC voltage outputted from the converting unit  200 , thereby generating an AC voltage of a sine wave.  
         [0041]     The filter  300  comprises a capacitor C 3  for discharging a charged voltage when the converting unit  200  performs a voltage dropping operation.  
         [0042]     The voltage detecting unit  400  detects a level of an AC voltage outputted from the filter  300 .  
         [0043]     The storing unit  500  stores each RMS value corresponding to a plurality of AC voltage levels.  
         [0044]     The controlling unit  600  compares an AC voltage level detected by the voltage detecting unit  400  with a preset AC voltage level, controls a conversion mode of the converting unit  200  on the basis of the comparison result, and outputs a switching control signal for controlling a switching of the converting unit  200 .  
         [0045]     More concretely, the controlling unit  600  compares an AC voltage detected by the voltage detecting unit  400  with an AC voltage preset by a user As a result of the comparison, if the AC voltage detected by the voltage detecting unit  400  is larger than the preset AC voltage, the controlling unit  600  drops an AC voltage outputted from the converting unit  200 . On the contrary, if the AC voltage detected by the voltage detecting unit  400  is smaller than the preset AC voltage, the controlling unit  600  boosts an AC voltage outputted from the converting unit  200 .  
         [0046]     At the time of a voltage dropping mode, the controlling unit  600  converts an AC voltage level detected by the voltage detecting unit  400  into an RMS value. If the converted RMS value is larger than an RMS value corresponding to the preset AC voltage, the controlling unit  600  increases dead time of a switching control signal for simultaneously turning off the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4 . On the contrary, if the converted RMS value is smaller than the RMS value corresponding to the preset AC voltage, the controlling unit  600  decreases dead time of a switching control signal for simultaneously turning off the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4 .  
         [0047]     At the time of a voltage boosting mode, the controlling unit  600  converts an AC voltage level detected by the voltage detecting unit  400  into an RMS value. If the converted RMS value is larger than an RMS value corresponding to the preset AC voltage, the controlling unit  600  increases overlap time of a switching control signal for simultaneously turning on the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4 . On the contrary, if the converted RMS value is smaller than the RMS value corresponding to the preset AC voltage, the controlling unit  600  decreases overlap time of a switching control signal for simultaneously turning on the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4 .  
         [0048]     An operation of the power converting apparatus for a fuel cell according to the present invention will be explained with reference to  FIG. 3 .  
         [0049]     First, a user sets a level of a commercial AC voltage to be used at a load by an inputting unit (not shown) (SP 1 ).  
         [0050]     Then, the controlling unit  600  compares the AC voltage level detected by the voltage detecting unit  400  with the commercial AC voltage level set by a user (SP 2 ), and controls a switching mode of the converting unit  200  on the basis of the comparison result.  
         [0051]     More concretely, when the AC voltage level detected by the voltage detecting unit  400  is larger than the AC voltage level set by a user, the controlling unit  600  drops an AC voltage outputted from the converting unit  200 .  
         [0052]     On the contrary, when the AC voltage level detected by the voltage detecting unit  400  is smaller than the AC voltage level set by a user, the controlling unit  600  boosts an AC voltage outputted from the converting unit  200 .  
         [0053]     A voltage boosting operation and a voltage dropping operation by the converting unit  200  will be explained with reference to  FIG. 4 .  
         [0054]     As shown in FIGS.  4 ( a ) and  4 ( b ), at the time of a voltage dropping mode, the controlling unit  600  controls a switching control signal for simultaneously turning off the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4  of the converting unit  200  to have dead time. Under the state, the converting unit  200  drops a DC voltage outputted from the fuel cell by a certain level, and outputs the dropped DC voltage (SP 3 ).  
         [0055]     Then, the filter  300  filters the dropped AC voltage outputted from the converting unit  200  and thereby outputs an AC voltage of a sine wave to a corresponding load (SP 4 ).  
         [0056]     The voltage detecting unit  400  detects a level of the AC voltage outputted from the converting unit  200  thus to apply it to the controlling unit  600  (SP 5 ).  
         [0057]     Then, the controlling unit  600  converts an AC voltage level detected by the voltage detecting unit  400  into an RMS value. If the converted RMS value is larger than an RMS value corresponding to a preset AC voltage (SP 6 ), the controlling unit  600  increases dead time of a switching control signal for simultaneously turning off the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4  (SP 8 ).  
         [0058]     On the contrary, if the converted RMS value is smaller than the RMS value corresponding to the preset AC voltage (SP 6 ), the controlling unit  600  decreases dead time of a switching control signal for simultaneously turning off the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4  of the converting unit  200  (SP 7 ).  
         [0059]     As shown in FIGS.  4 (c) and  4 (d), at the time of a voltage boosting mode, the controlling unit  600  Controls a switching control signal for simultaneously turning on the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4  of the converting unit  200  for a certain time to have overlap time. Under the state, the converting unit  200  boosts a DC voltage outputted from the fuel cell by a certain level thereby to output it (SP 9 ).  
         [0060]     Then, the filter  300  filters the boosted AC voltage outputted from the converting unit  200  into an AC voltage having a since wave, and thus supplies it to a corresponding load (SP 10 ).  
         [0061]     The voltage detecting unit  400  detects an AC voltage outputted from the converting unit  200 , and then applies it to the controlling unit  600  (SP 11 ).  
         [0062]     The controlling unit  600  converts an AC voltage level detected by the voltage detecting unit  400  into an RMS value. If the converted RMS value is larger than an RMS value corresponding to a preset AC voltage (SP 12 ), the controlling unit  600  increases overlap time of a switching control signal for simultaneously turning on the first PNP transistor P 1  the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4  of the converting unit  200  (SP 14 ).  
         [0063]     On the contrary, if the converted RMS value is smaller than the RMS value corresponding to the preset AC voltage (SP 12 ), the controlling unit  600  decreases overlap time of a switching control signal for simultaneously turning on the first PNP transistor P 1 , the second PNP transistor P 2 , the third PNP transistor P 3 , and the fourth PNP transistor P 4  of the converting unit  200  (SP 13 ).  
         [0064]     As aforementioned, in the power converting apparatus for a fuel cell and the method thereof according to the present invention, a power conversion efficiency of a fuel cell is enhanced by converting a DC voltage outputted from the fuel cell to an AC voltage by boosting or dropping by the converting unit without an additional boosting device or a dropping device.  
         [0065]     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.