Monday, October 22, 2012

Fuel from air or just hot air?

From a few sources, I’ve had a story about a UK company that claims they can make petrol out of air and water (actually hydrogen extracted from water and carbon dioxide extracted from air).

I perused their web site and one critical figure is missing: energy return on energy invested (EROEI: which has to less than unity because a hydrocarbon fuel made from hydrogen and CO2 is an energy store, not source; plants consume solar energy to do this trick). If you make a hydrocarbon fuel any other way, it costs energy, but I have my doubts that this can be better than e.g., using a feedstock like biomass because CO2 density in the atmosphere is so low, about 400ppmv (and biomass starts from chemicals closer to what you want, created with the aid of photosynthesis – so there’s some chance of over-unity EROEI).

Here are some numbers: petrol has an energy density of about 35MJ/litre which is about 10kW-h/litre. They claim they can produce:
1,200 litres of liquid hydrocarbon fuel per day using approx 3 MW (nominal installed capacity) of renewable electricity
Be generous and assume this is in only 8 hours, which means they are using 24000kW-h of electricity to produce a fuel with total capacity of 12000 kW-h so they their energy cost is double the energy out (EROEI = 0.5). If their plant actually takes all day to produce this amount of fuel, EROEI is much worse (0.17, or less than 20% of the energy put in). Add in the inefficiencies of internal combustion engines (as I recall, about 5% of energy produced gets on the road) and this is not terribly exciting.

The only way it can make any sense is if you use up waste energy, e.g., if you have a wind farm that’s producing electricity at a time when you don’t need it. If you are going to do that I would rather just make hydrogen. Hydrogen can be used to operate a fuel cell, and can also be burnt directly as a fuel. While the energy density of hydrogen is much lower (especially in terms of energy per unit volume) than a hydrocarbon fuel, a large vehicle like a bus can operate efficiently off a hydrogen fuel cell, and hydrogen could also be a useful fuel in stationary applications.

The most promising alternative to fossil liquid fuels currently is algal biofuels. Algae use photosynthesis efficiently to produce biomass; that makes a lot more sense to me than this proposal.

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