Reply To: would nuclear energy really be accessible to all?

[dennisp]

Photosynthesis is an interesting counterexample but doesn’t seem all that relevant, to me. We’re not relying on diffuse sunlight for energy, and we’re making oil, not sugar.

It’s hard to see how Los Alamos could estimate $4.60/gal at only 6% efficiency.

Using older processes:

We could start with the Sabatier reaction, which turns CO2 and hydrogen into methane and water, using a catalyst.
http://en.wikipedia.org/wiki/Sabatier_reaction

This is a very efficient reaction…this paper claims 95% efficiency:
http://www.ncbi.nlm.nih.gov/pubmed/11892751

The next step is steam reforming, which converts methane to carbon monoxide and hydrogen, using a nickel catalyst. This is between 70% and 85% efficient:
http://en.wikipedia.org/wiki/Steam_reforming

The final step for liquid hydrocarbons is Fischer-Tropsche:
http://en.wikipedia.org/wiki/Fischer–Tropsch_process

Using Fischer-Tropsch to generate liquid fuel from biomass has about 40% efficiency. I haven’t found numbers yet for pure syngas.
http://www.greencarcongress.com/2011/05/manganaro-20110516.html

The Los Alamos approach uses an entirely different process, see the link posted above for details. Initially, carbon dioxide is absorbed into a potassium carbonate solution to form bicarbonate ions. An electrolytic processes pulls the CO2 out of solution, and water electrolysis provides hydrogen. From there, they synthesize methanol, followed by a methanol-to-gasoline conversion developed by Exxon.

These numbers aren’t nailed down as well as I like but 6% efficiency seems unduly pessimistic. The initial collection of CO2 doesn’t take that much energy, according to the sources I’ve mentioned, and after that it’s just a matter of applying industrial processes that have been in commercial use for decades. The Germans used Fischer-Tropsch in WWII.

Waste heat is a good point…people have talked about using the waste heat from thorium reactors to desalinate seawater. It could just as well be used to make fuel.