Penn State engineers have come up with an interesting twist on fuel production, coming up with a cell that very efficiently produces hydrogen
The cell uses microbial fermentation of acetic acid to generate hydrogen, but with a twist. Oh, and the technique also works (not as well) for cellulose, which is the prima plant waste material we all would like to find something useful to do with.
A picture, as they say, is worth a thousand words:
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Credit: Zina Deretsky, National Science Foundation
The basic idea is one of these "duh, wish I'd thought of that".
They have bacteria, which digest some organic matter (acetic acid ideally, cellulose in a pinch), and make hydrogen. Problem is that bacteria are not all that efficient at making hydrogen, it is incidental to their metabolism.
We know how to make hydrogen through electrolysis, but that is a process that costs energy - it is useful if we need hydrogen as a portable fuel, or conceivably to store electricity (make hydrogen, efficiently, through electrolysis, store hydrogen, make electricity efficiently through hydrogen-air fuel cell), but hydrogen is not directly a primary source of energy.
We have known for a long time that some bacteria fart hydrogen, but they are inefficient, so you are looking at large ponds, generating hydrogen diffusely, hard to collect, capital expensive, inefficient.
What the PSU people did, was put the bacteria in a cell, with carbon electrodes and some catalyst, and put a low voltage current across the whole thing.
The voltage drives the efficiency of the hydrogen production from the chemical feedstock to near maximum! Just needed that little extra push.
The process is efficient enough you can bleed off a small fraction of the hydrogen and use it in a fuel cell to generate the current - this is not perpetual motion, you still need the chemical feedstock and it is the oxidation of the feedstock which is the source of the energy.
Nifty. Paper in the PNAS.
They discuss some of the practical aspects in it - including fuel generation for gas turbines, and feedstock for fertilizer production.
This is not going to be a prime mover - you're not going to be able to stack some of these cells in the back of the car where the fuel tank is, feed in some wood chips or white vinegar and keep a turbine running.
The process is just not that efficient - biological systems don't have high energy density. Consider your own metabolism - a 100 kg or so of microbial cells, your body, can conver a few kg of feedstock to energy at a sustained rate of ~ 100 W.
A modern car is more like 200 hp - or 150kW!
Now that is peak power, mean power is maybe 30 kW while cruising, but that'd require many tons of bacteria to sustain fed from chemical feedstock provided at the rate of maybe 10 kg per hour.
Oh, and the metabolic waste heat would kill the cells.
So... this only works for fixed scale production and requires large area and capital, just not as big as it would otherwise; then store or convert the hydrogen and ship it for use off-site.
Hey, its an improvement.
Is there any accounting for that pesky carbon dioxide molecule that is released in the first half of the process?
The carbon dioxide released is neutral because it was captured by the plants from which the vinegar is produced. Its the same as bio fuels, where the carbon is captured from the atmosphere , instead of being released from deep underground. This is what is meant by the phrase "Carbon Neutral".