A load-dependent voltage would be used to control a parasitic device.
A simplified load-following control scheme has been proposed for a fuel cell power system. The scheme could be used to control devices that are important parts of a fuel cell system but are sometimes characterized as parasitic because they consume some of the power generated by the fuel cells.
The parasitic devices can include the following: a pump for circulating coolant to remove waste heat, pumps for circulating reactant gases and humidifying inlet gases, an electric heater for keeping the fuel cell stack above a minimum operating temperature when the production of waste heat is insufficient for this purpose, and a centrifugal water separator. Operating these parasitic devices steadily at their full power levels would waste power, reducing the overall efficiency of the fuel cell power system. In general, the power demands for optimal operation of the parasitic devices vary with the load (e.g., the optimum coolant-circulation power increases with the load). The power levels of the parasitic devices in fuel cell power systems can be regulated at optimal levels by electronic feedback control systems that include sensors (e.g., current, voltage, temperature, or motor-speed sensors) and power-conditioning subsystems. However, such control systems can sometimes be so complex as to detract from the overall reliability of the affected fuel cell power systems.
In the proposed scheme, a single approximate control signal, generated by relatively simple means, would be used for controlling one or more parasitic devices. The scheme is based on the fact that the terminal voltage of a fuel cell stack decreases with increasing current (in other words, voltage decreases with increasing load) even more strongly than does the voltage of a typical battery having a nominally equivalent current and voltage rating. The figure depicts a simple fuel cell system in which the scheme would be applied to control of a coolant pump. The system would include a primary fuel cell stack and a lower-power secondary fuel cell stack denoted the parasitic-load stack. The two fuel cell stacks would be electrically connected at their positive ends. The coolant pump would be connected between the negative ends of the two stacks.
An increase in the power demand of the load would cause a decrease in the voltage of the primary stack, thereby causing an increase in V2 – V1, the difference between the voltages of the parasitic-load and primary stacks.
This, in turn, would cause an increase in the power supplied to the coolant pump. In a design process, that would entail careful selection of the stack cell areas, the numbers of cells in the two stacks, the electrical resistance of the coolant pump, and other design parameters; it should be possible to make the power supplied to the coolant pump, as a function of the load level, closely approximate the amount required for dissipation of waste heat at that level.
This work was done by Arturo Vasquez of Johnson Space Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-24169-1.