Fuel cell automatically throttles its power -

A fuel cell that efficiently regulates its own power output based on the amount of hydrogen it is fed has been developed by US researchers. The simple control mechanism could extend the range of devices that can practically be powered using fuel cells.
Fuel cells generate electricity by combining hydrogen and oxygen to produce electricity and waste water. They are a cleaner alternative to petrol and diesel but currently remain experimental.
Although the system sounds simple enough, controlling a fuel cell's power output by feeding in more or less hydrogen has not been practical until now, says Jay Benziger, the chemical engineer at Princeton University in New Jersey, US, who developed the new fuel cell.
Engineers have tended to feed a steady supply of hydrogen and oxygen into their cells, in part to ensure that the gases will force waste water out of the system. But this causes some of the hydrogen to flow through the cell unused, meaning it must then be captured and recycled.
It also means the power output cannot be throttled back by simply lowering the input of gases, unlike a simple petrol engine. If it is necessary to lower the power output, conventional systems simply shunt current to attached resistors, which is less efficient.
Waste not
The new cell instead harnesses its own waste water to ensure that the hydrogen fed is matched by the power output. The reaction chamber, in which hydrogen and oxygen combine to generate water and produce electricity, is connected to a reservoir containing water. Gravity pulls waste water produced by the cell's reaction down into this chamber.
When more hydrogen is fed to the cell, pressure in the reaction chamber increases, which pushing more water out of the reservoir. This in turn leaves more of the anode exposed to react with the hydrogen, generating more power.
Similarly, when less hydrogen is fed into the system, pressure drops and more water is drawn back into the reaction chamber from the reservoir, covering more of the anode and throttling back the chemical reaction.
The water at the bottom of the pan also keeps the fuel cell humidified, which prevents damage that can occur as it begins to dry out.
"It's actually simpler than most fuel cells," Benziger says. He says it could be more efficient for powering anything that needs about a kilowatt of power, like a lawnmower. As with a conventional gas-powered lawnmower, the user would be able to vary the speed simply by controlling the gas.
Journal reference: Chemical Engineering Science (Vol 62, p 957)