How They Work Hydrogen Sources
How They Work: Hydrogen Sources
The PEM fuel cells in most FCVs use hydrogen to produce electricity. The hydrogen, however, can be supplied in several ways.
Pure hydrogen. FCVs can be fueled with pure hydrogen gas stored in onboard fuel tanks. Since hydrogen gas is diffuse, it must be stored in high-pressure tanks in order to store enough to travel reasonable distances on a full tank of fuel. Currently used tanks, which allow hydrogen to be compressed to 5,000 pounds/square inch (psi) of pressure, can only store enough hydrogen gas to allow FCVs to go about 200 miles before refueling. However, manufacturers are designing and testing tanks that will store more hydrogen at a higher pressure.
In addition to onboard storage problems, our current system for getting liquid gasoline to consumers can't be used for gaseous hydrogen. Therefore, new facilities and systems would have to be built, requiring significant time and resources.
Methanol reformer functions in an FCV: Liquid methanol flows from the fuel tank to a vaporizer where it is vaporized with steam and passed to the reformer. The reformer uses a chemical process that releases the hydrogen from the methane and steam. The carbon combines with oxygen from the steam, producing carbon dioxide (CO2) and carbon monoxide (CO). The mixture of hydrogen, carbon monoxide and carbon dioxide is then passed through a "gas cleaner" that converts the remaining carbon monoxide to carbon dioxide. The H2 and CO2 enter the fuel stack, which uses the hydrogen to produce electricity, with water as a byproduct. The CO2 and excess hydrogen from the fuel stack are passed to a catalytic burner, where they are burned to produce heat for the vaporizer and the reformer. The byproducts of this catalytic burning are air and CO2. Hydrogen-rich fuels. FCVs can also be fueled with hydrogen-rich fuels, such as methanol, natural gas, petroleum distillates, or even gasoline. These fuels must be passed through onboard "reformers" that extract pure hydrogen from the fuel for use in the fuel cell. Reforming does emit some carbon dioxide (CO2), but much less than gasoline engines do.
The fuels mentioned above contain enough hydrogen to allow FCVs to travel the same distance as a conventional vehicle on a single tank of gas; about 300 to 400 miles. Also, unlike hydrogen gas, liquid fuels like methanol and gasoline wouldn't require a completely new system for delivering fuel to consumers.
Although there are advantages to powering FCVs with these fuels, there are also several disadvantages.
* Onboard reformers add to the complexity, cost, and maintenance demands of a vehicle's fuel cell system.
* If the reformer allows carbon oxides to reach the fuel cell anode, it can gradually decrease the performance of the cell.
* Reformers produce small amounts of greenhouse gases and other air pollutants.
It is not yet clear which method of fueling fuel cells will prevail. Research and development continues for all of these fueling options.
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