Patent Application
20100261085
The present invention relates to an electric power source arrangement comprising a fuel cell means having a nominal voltage and a specified voltage-current characteristic, to be connected to a load, and comprising a variable DC-DC voltage converter, a by-pass branch by-passing the DC-DC voltage converter, a switch alternatively connecting the fuel cell to the DC-DC voltage converter or to the by-pass branch, and a method of how to use the arrangement.
An electric power source arrangement as stated above, i.e. a fuel cell system capable of reducing an electric power loss, is known from U.S. Pat. No. 6,255,008 B1. In this system, a secondary battery is connected in parallel to the power source arrangement. Depending on the power demand of a load and the status of charge of the battery, the battery is connected or disconnected to the load, the fuel cell is connected to the load via the DC-DC voltage converter or via the by-pass branch or the fuel cell is connected to both the battery and load via the DC-DC voltage converter.
The characteristic of a fuel cell is that the ideal fuel cell has a comparably high output voltage which decreases when current is drawn. Then for a considerable interval of amperage the output voltage keeps almost constant, decreasing by some percent only, and then, after a certain value of current is passed, the voltage drops significantly. The DC-DC voltage converter serves to see to more or less constant voltage, at least in the steeper parts of the characteristics. It has an efficiency of e.g. 70% to 90%. The by-pass branch serves to avoid the loss caused by the converter as long as the voltage is sufficiently close to the nominal voltage.
It is an object of the invention to provide a useful control for the switch-changing between the DC-DC voltage converter and the by-pass branch.
According to the invention, the electric power source arrangement is characterized in that the control unit comprises a measuring device coupled to the fuel cell means for detecting the operating point thereof and is configured to connect the by-pass branch if the fuel cell means voltage is within a selected range of the voltage-current characteristic of the fuel cell means and to disconnect the by-pass branch in the remaining range of said characteristic.
Preferably, the selected range of the voltage-current characteristic is the range at both sides of the nominal voltage wherein the voltage difference in dependency of current changes is not more than 25%, more preferably not more than 15% and even more preferably not more than 5% depending on the demand of the load.
According to advantageous embodiments the control unit, the switch, the by-pass branch and the DC-DC voltage converter are integral parts of a single unit; the switch is an electronic switch, preferably a MOSFET or an IGBT; and the DC-DC voltage converter is a stabilized closed-loop controlled converter.
According to the invention, a method of how to use the electric power source arrangement is provided, wherein the switch is in the status of connecting the by-pass branch if the fuel cell means voltage is within a selected range of the voltage-current characteristic of the fuel cell means and is in the status of disconnecting the by-pass branch in the remaining range of said characteristic, the selected range being determined to be the range at both sides of the nominal voltage wherein the voltage difference in dependency of current changes is not more than 25%, more preferably not more than 15% and even more preferable not more than 5%.
According to the invention, a method of how to control the electric power source arrangement, wherein the range of the characteristic limiting the selected range towards lower voltages is divided into at least two sections, i.e. a recovering section and a shutdown section, the recovering section being limited by a first operational point representing the minimum fuel cell means output voltage to be still convertible under continuous operation conditions and a second operational point beyond which the shutdown section starts, is provided in that in the recovering section the DC-DC voltage converter issues a sub-nominal voltage, further that in the recovering section the fuel cell means is intermittently switched off and re-started, preferably according to time and/or idle voltage of the fuel cell. If using a DC-DC voltage converter capable of stepping up the voltage, between the selected range and the recovering section, an adapting section is provided wherein the converter issues the nominal voltage.
The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings.
The diagram of
The section 5 extends at both sides of the nominal voltage by a voltage difference of some percent, which needs not be equal at both sides, and constitutes the above mentioned selected range. The selected range is determined so that it covers the allowed voltage fluctuation of the load 1. Sections 4, 6 and 7 constitute the above mentioned remaining range wherein only sections 4 and 6 constitute an adaptable range with continuous operational conditions. The operational point 9 of minimum continuous voltage that narrows section 6 on the lower voltage side is defined by the lowest voltage point wherein the fuel cell could barely run without causing problems e.g. in terms of cooling or efficiency. The operational point 10 of absolutely minimum voltage that borders section 7 on the lower voltage side against section 8 represents an operational point beyond which the efficiency and performance of the fuel cell 2 is too poor and a continuous operation is no more possible due to too much problems with the fuel cell e.g. with the cooling system.
As the load 1 draws different current intensities, the voltage issued by the fuel cell 2 changes along the characteristic. In sections 4 and 6, the voltage is too far away from the nominal voltage and thus, the DC-DC voltage converter 3 adapts the voltage to the nominal voltage as shown by arrows in
In section 7 the voltage measured from the control unit 12 is too far away from the nominal voltage. A continuous power output of the fuel cell 2 is no longer suitable because the low voltage starts causing severe problems e.g. in terms of cooling or efficiency. Thus, the operational point in the recovery section 7 tends to leave this section to section 6 or 5 if recovery is successful or to section 8 if recovery is not possible. As long as the operational point is in this section 7, the adapted DC-DC voltage converter 3 will not issue the nominal voltage but a voltage less than the nominal voltage, as shown by arrows in
If the load 1 is adaptable to a lower voltage e.g. an Ohm resistive load, the current will decrease with the decreased voltage. E.g. if the load is an incandescent lamp, it will work with lower light intensity but in a not suitable operational range. The desired consequence is that the fuel cell 2 can recover due to the lower current e.g. by reactivation of the catalyst, and reach section 6 again wherein a continuous operation is possible. The DC-DC voltage converter 3 can issue the nominal voltage again as described above.
If the chosen load 1 is an active one i.e. a load that demands a constant power, the fuel cell 2 can not recover in section 7 and can not attain section 6. Due to the fact that a continuous operation in section 7 is no longer possible, a signal to the load or respectively the operator behind the load that an adjustment is necessary, will be issued. According to a specific method, a final attempt of recovery cycle will now start. The fuel cell 2 will shutdown by the switch 14 for a certain time interval only. According to a first embodiment, this time interval is a predefined, short time interval of preferably 1 to 20 seconds, most preferably 5 seconds, then the fuel cell will be reconnected. According to a second embodiment, this interval is limited by the idle voltage measured by the control unit 12 which controls the switch 14 in the arrangement of
In section 8 beyond the operational point 10 of absolutely minimum voltage a continuous operation is not possible, problems with the fuel cell 2 are too severe even to start a final recovery cycle like in section 7. A shutdown of the fuel cell will be applied immediately.
The items 3, 11, 12 and 13 are shown as distinct elements, in practise it will be useful to integrate them into a single unit.
In the circuit arrangement of
The switch 13 is shown as a double pole switch, obviously a single pole switch will do. The switch 13 can be an electronic switch capable of switching the occurring current intensities, preferred elements are a MOSFET or an IGBT.
1 load
2 fuel cell
3 DC-DC voltage converter
4 adapted section
5 selected section
6 adapted section
7 recovery section
8 shutdown section
9 operational point
10 operational point
11 by-pass branch
12 control unit
13 switches
14 switch
| Number | Date | Country | Kind |
|---|---|---|---|
| 200910048996.2 | Apr 2009 | CN | national |
