Abstract:
A method of hybrid power management is provided in the present invention, comprising steps of: providing a hybrid power output device being coupled to a load and comprising a fuel cell module and a secondary cell module; determining a plurality of threshold values, each representing one of output power modes of the hybrid power output device respectively; and monitoring a characteristic value output from the fuel cell module and comparing the characteristic value with the threshold values to determine one of the output power modes to supply power to the load. Moreover, the present invention further provides a system of hybrid power management using the foregoing method to control switches to select from the output power modes such as supplying power from the fuel cell module only, from both the fuel cell module and the secondary battery, or cutting off power supply to the load according to the power state of the fuel cell module.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to a power control method and a power control system using the same and, more particularly, to a method of hybrid power management and a system of hybrid power management, in which one of different output power modes is selected according to a characteristic value output from a fuel cell module. 
         [0003]    2. Description of the Prior Art 
         [0004]    The fuel cell has attracted tremendous attention as an alternative energy because it is free of pollution, high-efficiency, low-noise, low-vibration, fast start-up and long duration. Especially, the awakening in environmental conscience has made the fuel cell a potential power source for use in other fields. The fuel cell has advanced so that the power generation performance is improved with reduced material cost is to make the fuel cell commercialized. 
         [0005]    However, the power supply process of the fuel cell is restricted to the reaction mechanism therein. For example, it is hard for the fuel cell to provide large power to the load due to slow oxidation and fuel delivery, which leads to failure of the fuel cell because of insufficient momentary power supply. Moreover, the unstable transient state occurs due to reduced fuel concentration during fuel supply. 
         [0006]    In order to avoid momentary high-power demand when the load changes or unstable power supply due to the operation of the fuel cell, a capacitor or a secondary cell module is conventionally used. For example, in Taiwan Patent Pub. No. 200518371, a fuel cell device with a secondary cell is disclosed to reduce power consumption during power conversion by adjusting the output voltage of the fuel cell. Moreover, in Taiwan Patent Pub. No. 200735444, the warm-up time for the fuel cell is reduced so that the power generated will not be wasted. In this technique, the fuel cell and the secondary cell generate hybrid power for various applications. During the operations of the secondary cell, the operation of the secondary cell is insufficient to cause lowered output power when it is over-loaded or has operated overtime. Meanwhile, aging of the over-used fuel cell results from high current and low voltage because there is no mechanism in the secondary cell module to avoid reduced output power. Moreover, there is no cut-off voltage for the secondary cell to get recovered in the prior art when the output power of the secondary cell is reduced. 
         [0007]    Therefore, there is need in providing a method of hybrid power management and a system of hybrid power management using the method to overcome the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to provide to a method of hybrid power management and a system of hybrid power management, in which one of different output power modes is selected to supply power to the load according to the power state of fuel cell module so as to prevent the output voltage of the fuel cell from dropping due to unstable transient state. Therefore, the fuel cell module can be recovered at unstable transient state to prolong the lifetime of the fuel cell and keep the load operating normally. 
         [0009]    In one embodiment, the present invention provides a method of hybrid power management, comprising steps of: providing a hybrid power output device being coupled to a load and comprising a fuel cell module and a secondary cell module; determining a plurality of threshold values, each representing one of a plurality of output power modes of the hybrid power output device respectively; and monitoring a characteristic value output from the fuel cell module and comparing the characteristic value with the threshold values to determine one of the output power modes to supply power to the load. 
         [0010]    In another embodiment, the present invention further provides a system of hybrid power management, comprising: a hybrid power output device comprising a fuel cell module and a secondary cell module being electrically coupled to the fuel cell module through a first switch; a load being electrically coupled to the hybrid power output device through a second switch; a sensor unit being capable of monitoring a characteristic value output from the fuel cell module to generate a sensor signal; and a control unit being capable of determining a plurality of threshold values, each representing one of a plurality of output power modes of the hybrid power output device respectively and being capable of receiving the sensor signal and comparing the sensor signal with the threshold values to control the first switch or the second switch to determine one of the output power modes to supply power to the load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The objects, spirits and advantages of the preferred embodiments of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein: 
           [0012]      FIG. 1  is a flowchart of a method of hybrid power management according to the present invention; 
           [0013]      FIG. 2A  is a schematic diagram of a system of hybrid power management according to one embodiment of the present invention; 
           [0014]      FIG. 2B  is a schematic diagram of a system of hybrid power management according to another embodiment of the present invention; 
           [0015]      FIG. 2C  is a schematic diagram of a system of hybrid power management according to still another embodiment of the present invention; 
           [0016]      FIG. 3  is a table showing the relation between threshold values and switch operations according to one embodiment of the present invention; 
           [0017]      FIG. 4A  is a graph showing the characteristic value of a fuel cell module with a load as a function of time according to one embodiment of the present invention; 
           [0018]      FIG. 4B  is a graph showing the characteristic value of a fuel cell module with a load as a function of time according to another embodiment of the present invention; 
           [0019]      FIG. 5  is a table showing the relation between threshold values and switch operations according to another embodiment of the present invention; and 
           [0020]      FIG. 6  is a graph showing the characteristic value of a fuel cell module with a load of  FIG. 2A  as a function of time. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The present invention can be exemplified but not limited by the preferred embodiments as described hereinafter. 
         [0022]    Please refer to  FIG. 1 , which is a flowchart of a method of hybrid power management according to the present invention. The method of hybrid power management  2  comprises steps described hereinafter. First, Step  20  is performed to provide a hybrid power output device. The hybrid power output device is coupled to a load and comprises a fuel cell module and a secondary cell module. The fuel cell module can be a direct methanol fuel cell (DMFC) or a solid oxide fuel cell (SOFC), but not restricted thereto. The secondary cell can be a lithium cell, nickel-metal hydride (NiMH) cell or any other rechargeable cell. 
         [0023]    Then, in Step  21 , a plurality of threshold values are determined. Each threshold value represents an output power mode of the hybrid power output device respectively. The output power mode in this step represents the power simply from the fuel cell module to the load or the power from the fuel cell module and the secondary cell module to the load or the power being cut off instead of being supplied to the load. For example, in one embodiment, the plurality of threshold values comprises a first threshold value, a second threshold value and a third threshold value. More particularly, the first threshold value is 8.6 volts, the second threshold value is 8 volts, and the third threshold value is 7.6 volts. The threshold values are determined based on the fuel cell module and secondary cell module, but are not restricted thereto. More particularly, the first threshold value of 8.6 volts represents the output power mode that the fuel cell module supplies power to the load. The second threshold value of 8 volts represents the output power mode that the fuel cell module and the secondary cell module supply power to the load. Third threshold value of 7.6 volts represents the output power mode that power to the load from the hybrid power output device is cut off and the fuel cell charges the secondary cell module. 
         [0024]    In another embodiment, four threshold values are used. In other words, the plurality of threshold values comprises a first threshold value, a second threshold value, a third threshold value and a four threshold value. More particularly, the first threshold value is 8.6 volts, the second threshold value is 8.2 volts, the third threshold value is 8 volts and the fourth threshold value is 7.6 volts. The threshold values are determined based on the fuel cell module and secondary cell module, but are not restricted thereto. More particularly, the first threshold value of 8.6 volts represents the output power mode that the fuel cell module supplies power to the load. The second threshold value of 8.2 volts and the third threshold value of 8 volts represent respectively one selected from the output power mode that the fuel cell module supplies power to the load and the output power mode that the fuel cell module and the secondary cell module supply power to the load according to the trend of the characteristic value. The fourth threshold value of 7.6 volts represents the output power mode that power to the load from the hybrid power output device is cut off. 
         [0025]    After the threshold values are determined, Step  22  is performed to monitor a characteristic value output from the fuel cell module and compare the characteristic value with the threshold values to determine one of the output power modes to supply power to the load. The characteristic value represents the output voltage, current or power of the fuel cell module. Since the aforesaid threshold values are voltage values, the characteristic value in this step is the voltage of the fuel cell module. In other words, the output voltage is monitored during the operation of the fuel cell module and is compared to the threshold values defined in Step  21  to determine the output power mode defined in Step  21 . Certainly, when the characteristic value is the output current, the plurality of threshold values are current values, as is well-known to those with ordinary skills in the art capable of making modifications within the scope of the present invention. Therefore, the characteristic value of the present invention is not restricted to the output voltage. 
         [0026]    The method of the present invention is implemented as described hereinafter. Please refer to  FIG. 2A , which is a schematic diagram of a system of hybrid power management according to one embodiment of the present invention. The system of hybrid power management  3  comprises a hybrid power output device  30 , a load  34 , a sensor unit  35  and a control unit  36 . The hybrid power output device  30  comprises a fuel cell module  301  and a secondary cell module  302 . The secondary cell module  302  is electrically coupled to the fuel cell module  301  through a first switch  31 . The fuel cell module  301  can be a direct methanol fuel cell (DMFC) or a solid oxide fuel cell (SOFC), but not restricted thereto. The secondary cell module  302  can be a lithium cell, nickel-metal hydride (NiMH) cell or any other rechargeable cell. 
         [0027]    The load  34  is electrically coupled to the hybrid power output device  30  through a second switch  32 . The sensor unit  35  is capable of monitoring a characteristic value from the fuel cell module  301  to generate a sensor signal. The sensor unit  35  is capable of determining the characteristic value to be detected. For example, if the characteristic value is a voltage value, the sensor unit is a voltmeter; if the characteristic value is a current value, the sensor unit is a current meter. In the present embodiment, the sensor unit  35  is a voltmeter. The control unit  36  is capable of determining a plurality of threshold values. Each threshold value represents one of a plurality of output power modes of the hybrid power output device  30  respectively. The control unit  36  receives the sensor signal through the sensor unit  35  and compares the sensor signal with the threshold values according to the trend of the sensor signal to control the first switch  31  or the second switch  32  to determine one of the output power modes to supply power to the load  34 . Moreover, a DC/DC converter  33  is further disposed between the hybrid power output device  30  and the load  34 , as shown in  FIG. 2B . In the system of hybrid power management in another embodiment of the present invention, the DC/DC converter  33  is directly coupled to the fuel cell module  301 . Alternatively, as shown in the embodiment in  FIG. 2C , each of two DC/DC converters  33  is coupled respectively to the fuel cell module  301  and the secondary cell module  302 . 
         [0028]    The system of hybrid power management of the present invention is implemented as described hereinafter. Please refer to  FIG. 3  and  FIG. 4A , where  FIG. 3  is a table showing the relation between threshold values and switch operations according to one embodiment of the present invention and  FIG. 4A  is a graph showing the characteristic value of a fuel cell module with a load as a function of time according to one embodiment of the present invention. In other words,  FIG. 4A  shows curve obtained by monitoring the voltage (representing the output voltage of the fuel cell module) at a monitor point  90  using a sensor unit  35 . In the present embodiment, there are three threshold values, namely, the first threshold value of 8.6 volts, the second threshold value of 8 volts and the third threshold value of 7.6 volts, respectively, in  FIG. 3 . 
         [0029]    The system of hybrid power management of the present invention is implemented as described hereinafter. Referring to  FIG. 2A , the output voltage monitored at the monitor point  90  of the fuel cell module  301  is 8.3 volts. Then, since the power required to be supplied to the load  35  is not large, the fuel cell module  301  is capable of supplying the power to the load  35 . Therefore, the first switch  31  is off and the second switch  32  is on, while only the fuel cell module  301  supplies power to the load  35 . 
         [0030]    As the power required to be supplied to the load  35  increases, the output voltage of the fuel cell module  301  detected by the sensor unit  35  drops to 8 volts (point A). In order to prevent insufficient power supply from the fuel cell module  301 , the control unit  36  controls the first switch  31  to be on so that both the fuel cell module  301  and the secondary cell module  302  supply power to the load  34  to keep normal operations. If the power required to be supplied to the load  34  increases, the output voltage of the fuel cell module  301  continuously decreases. If the output voltage of the fuel cell module  301  detected by the sensor unit  35  drops to 7.6 volts (point B), the control unit  36  controls the second switch  32  to be off so that the fuel cell charges the secondary cell module  302  to recover the power therein because the output power is insufficient for the load. As the output voltage of the secondary cell module  302  rises to 8 volts (point C), the control unit  36  controls the second switch  32  to be on so that both the fuel cell module  301  and the secondary cell module  302  supply power to the load  34  to keep normal operations. 
         [0031]    In another case as shown in  FIG. 4B , if the power required by the load is smaller, the output power of the fuel cell module  301  may rise to 8.6 volts (point D). Meanwhile, the control unit  36  controls the first switch  31  to be off and the second switch  32  to be on so that only the fuel cell module  301  supplies power to the load  34 . This happens because the fuel cell module  301  charges the secondary cell module  302  as the power required by the load gets smaller so that the output power of the fuel cell module  301  returns to point D once the secondary cell module  302  is fully charged. 
         [0032]    Please refer to  FIG. 5  and  FIG. 6 , wherein  FIG. 5  is a table showing the relation between threshold values and switch operations according to another embodiment of the present invention, and  FIG. 6  is a graph showing the characteristic value of a fuel cell module with a load of  FIG. 2A  as a function of time. In the present embodiment, four threshold values are used, namely the first threshold value of 8.6 volts, the second threshold value of 8.2 volts, the third threshold value of 8 volts and the fourth threshold value of 7.6 volts in  FIG. 5 , respectively. The system of hybrid power management in  FIG. 2A  is implemented as described hereinafter. In the beginning, the output voltage of the fuel cell module  301  is 8.6 volts. Since the power required by the load  34  is not large, the power supplied from the fuel cell module  301  to the load is sufficient so that the first switch  31  is off and the second switch  32  is on. Then, as the power required by the load  34  increases, the output voltage of the fuel cell module  301  drops accordingly. When the voltage of the fuel cell module  301  drops and passes through the second threshold value (point A) the first switch  31  and the second switch  32  remain their previous states because the first threshold value decreases. In other words, the first switch  31  is off and the second switch  32  is on. 
         [0033]    When the output voltage of the fuel cell module  301  detected by the sensor unit  35  drops to the third threshold value (point B), the control unit  36  controls the first switch  31  to be on and the second switch  32  to be on so that both the fuel cell module  301  and the secondary cell module  302  output power to keep the load  34  operating normally. As the output voltage of the fuel cell module  301  drops to the fourth threshold value of 7.6 volts (point C) since the power required by the load  34  increases, the control unit  36  controls the second switch  32  to be off to protect the fuel cell module  301 . 
         [0034]    After the second switch  32  is switched to off, the fuel cell module  301  charges the secondary cell module  302  to recover its power. Meanwhile, the first switch  31  and the second switch  32  are switched according to the state corresponding to the voltage increasing from the fourth threshold value. For example, when the output power of the fuel cell module  301  detected by the sensor unit  35  is at the third threshold value 8 volts (point D), the second switch  32  is kept off in order for the fuel cell module  301  to supply sufficient power to the load  34 . When the output power of the fuel cell module  301  is at the second threshold value (point E), the second switch  32  is switched to on so that both the fuel cell module  301  and the secondary cell module  302  supply power to the load  34 . If the output voltage of the fuel cell module  301  increases to the first threshold value of 8.6 volts, the control unit  36  switches the first switch  31  to be off and the second switch  32  to remain on. In other words, only the fuel cell module  301  keeps charging the load. In this manner, the characteristic value of the fuel cell module  301  is repeatedly detected to control the output power according to the relation between threshold values and switch operations in  FIG. 5 . 
         [0035]    According to the above discussion, it is apparent that the present invention discloses a method of hybrid power management and a system of hybrid power management, in which one of different output power modes is selected according to a characteristic value output from a fuel cell module. Therefore, the present invention is novel, useful and non-obvious. 
         [0036]    Although this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art. This invention is, therefore, to be limited only as indicated by the scope of the appended claims.