I don't think we really
want cars for the long term. I don't think we can
afford them.
But if we do want them, and we manage somehow to afford them, it's increasingly clear that gasoline from oil and FT fuels from coal cannot not be sustained for very long.
Recently I've been reviewing the latest in thermochemical hydrogen producing cycles. Most people who are familiar with my thinking, recognize, that I am not fond of the "hydrogen economy," especially the dangerous idea of putting hydrogen in automobiles, which is just short of completely crazy.
However, I do view hydrogen as a useful reaction intermediate for the hydrogenation of carbon dioxide (or carbon monoxide) and so the question is, where would hydrogen come from?
Wind and solar power can be modified to produce hydrogen of course, via electrolysis, but electrolysis is a dirty and inefficient means of producing hydrogen in general, and probably such an effort would be characterized by low return on investment owing to fixed costs being constant while production is intermittent.
Nuclear thermochemical hydrogen is much more promising. Plants can run at close to 100% of capacity loading and thermal efficiencies of close to 60% can be achieved.
The National Academy of Sciences has published a review on the "hydrogen economy" that is available on line. I will point out some salient remarks. First I refer readers to figure G-6 which gives graphically the amount of energy that must be used to produce 1.0 kg of hydrogen at various temperatures. We see at the low end of the temperatures available to High Temperature Gas Cooled reactors, 280 MJ of energy is required for each kg of hydrogen.
http://darwin.nap.edu/books/0309091632/html/213.htmlNext we see the thermal efficiencies of hydrogen cycle reactors given on page 215 in figure G-8.
http://darwin.nap.edu/books/0309091632/html/215.htmlI will take the efficiency as 55% for rough purposes.
Now crudely, we see the claim that a plant should make 1,200,000 kg of hydrogen per day to power 2 million cars.
http://darwin.nap.edu/books/0309091632/html/45.htmlFrom the above figures, ignoring the energy losses that may come from further chemical processing, such as might be involved in the hydrogenation of carbon dioxide to make DME - a safer fuel - we see that the powering of 2 million cars in this scheme is about 340 trillion joules. Translated into power (86400 seconds a day) we see that is roughly equivalent to a
thermal output of 4000 MW. This is slightly larger than the largest of modern nuclear reactors, which more typically are about 3000 MW(th).
To fuel 100 million cars, we would need 200,000 MW of thermal energy to make the hydrogen. Assuming 55% of efficiency for the reactors, 360,000 MW. This means that about 120 would be required.
The calculation is "back of the envelope," but I thought it might be useful for a frame of reference. I note that this is not something that could be accomplished in a very short time frame, a frame in which the severe consequences of global climate change will come home to roost, but I thought I'd throw it out there.