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Name: Johnny
Status: educator
Grade: 9-12
Location: MO 
Country: N/A
Date: 12/21/2005


Question:
What is a reasonable guess to what it cost per mile to operate a subcompact car on hydrogen?


Replies:
I do not think a satisfactory answer exists because it depends upon what the inputs of the cost model is. For example: 1. At present there are few (if any) processes/devices for producing hydrogen in large amounts required at a cost that is even approximately equal to hydrocarbon fuels. Is the cost per mile going to include the cost of reconfiguring the auto engine and/or designing one. 3. There are only beginning to become available systems to store adequate amounts of hydrogen on/in and automobile -- the technology so far just is not there yet. At present storing hydrogen as a liquid (theoretically the best way) is not feasible because it boils at 20 kelvins (about -253 C.). This presents material design problems since many metals absorb hydrogen and become brittle, and of course insulation of the storage vessel would be a challenge as would trying to store it at high pressure. 4. The combustion temperature of hydrogen is very high. Assuming air is used as the oxidant, the nitrogen which is carried along will no doubt produce NOx gases which would have to be removed from the exhaust as it is today.

s It is possible to look up the heats of combustion reactions of: 2 H2 + O2 = 2 H2O and 2 C8H18 + 25 O2 = 16 CO2 + 18 H2O (C8H18 = octane, being a prototype hydrocarbon in gasoline) and compare the heat produced on a weight basis of fuel. However, that is not a valid comparison because all of the costs of hydrogen fuel have to be input. It is the overall cost/benefit that matters. Heat / unit mass of fuel is only one of the factors involved.

Vince Calder



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