If it doesn't work, then what?

(This one was worth a quickie.)

In a comment in "Treating irregularity", Marcos Dumay de Medeiros says this about direct-carbon fuel cells:

It annoys me a lot your insistence in making your calculations with the carbon fuel cell. It is experimental!

All right, for the sake of argument let us assume that the direct-carbon fuel cell scheme has some show-stopping problem and it's not usable.  Not for vehicular power, not in stationary applications, not anywhere.  If it doesn't work, what are the options?

Humor aside, when reality creeps in I am not one to bust it for trespassing.  I always have a plan B, and in this case plan B is...

zinc-air fuel cells!  (Tell me you didn't know that was coming.  I won't believe you, but it'll be good for a laugh.)

Going back to Zinc: Miracle metal? for the chemistry, we pull these properties:

From a mole of carbon (93960 cal/mol), a mole of ZnO and an indeterminate amount of heat, we get a mole of zinc metal (84670 cal/mol) and a mole of carbon monoxide (68560 cal/mol) plus waste heat.  Ignoring the waste heat, the 93960 cal of reactants yields 153230 calories of products.  The question becomes, can these make as much useful output as a DCFC can make of raw carbon?

I believe so.  Zinc metal is convertible to ZnO and electricity with an efficiency of roughly 62%, and CO can be fed to either a molten-carbonate fuel cell or a solid-oxide fuel cell; both can make electricity at an efficiency of roughly 60%.  Here's what we'd get out of a mole of carbon via the two options:

As long as you have a source of heat to drive the zinc reduction, you can get about 24% more total output using the zinc cycle compared to the direct-carbon system.  There's a second fallback too:  if neither the MCFC nor the SOFC are ready for widespread commercial use in time, carbon monoxide makes a perfectly good gas-turbine fuel.  It can probably be converted to work as efficiently as natural gas, or about 55% in a combined-cycle plant.  There's plan C.

Going back to dealing with irregularity, a carbon/zinc cycle helps in this way:

1. It adds another storable fuel, carbon monoxide, to the chain.  CO can be stored in spent gas wells and other gas-tight reservoirs.
2. It adds flexibility.
   1. A direct-carbon fuel cell yields carbon dioxide, which is mainly suitable for sequestration.  The zinc reduction produces carbon monoxide, which is a chemical feedstock as well as an energy source.
   2. A system which depends on carbon as a feedstock halts when it runs out of fixed carbon.  Zinc metal can be produced from oxide either chemically (reduced with carbon) or elecrolytically; this allows wind, solar, nuclear or hydro to substitute for carbon.

There's one more issue to deal with, and that's the dependence of the solar-thermal zinc reduction system (ZnO + C + Δ -> Zn + CO) on cloudless days.  There just aren't many of those in some parts of the country that need energy.  This is not a killer, because solar heat is just the sexiest source of energy to drive the reaction; it could just as easily be driven by surplus wind electricity (turning the immediate supply of wind power into two different storable fuels) or by combustion of part of the carbon (sacrificing the carbon monoxide byproduct).  Either way, there's a reasonable alternative.

Does that address your objections, Mr. de Medeiros?

Update:  ZAFC yield corrected in Table 2 (total was correct already)

¶ 1/10/2006 10:12:00 PM