The Ergosphere
Tuesday, January 10, 2006

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:

Table 1:  Heats of formation
  Compound     ΔH, gram  
  ZnO (from zinc solid)    -84670
  ZnO (from zinc gas)    -115940
  CO    -25400
  CO2    -93960
  H2O (liquid)    -70600

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:

Table 2:  Yield comparison
  Reactant     ΔH, gram  
  Converter     Efficiency     Yield, cal/mol  
  C    93960   DCFC     80%    75168
  Zn    84670  Zn/air 
 fuel cell 
  62%    52495
  CO    68560  SOFC or MCFC    60%    41136
 TOTAL   153230    93631

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)

It does partialy address. I surely would prefer to see calculations made with technology we can deal with today (termal engines). But the most experimental options, the best.

I also think that fuel cells are the way to go. But the problem is that we can't be sure about how long we'll need to wait until they can be used.
What? No Boron?
Marcos said: "But the problem is that we can't be sure about how long we'll need to wait until they can be used."

Nissan provided their assessment of future engine technologies in the December issue of Automotive Engineering International magazine. I suppose this is based on their own R&D/marketing projections. Anyway:

In 20 years - 65% ICEs, 35% gas-electric hybrids

In 30 years - 43% ICEs, 42% g-e hybrids, 14% diesel-electric hybrids, and some trace amount of fuel cell vehicles.

In 40 years - 26% ICEs, 39% g-e hybrids, 19% d-e hybrids, 16% FCVs.

Personally I think this breakdown is crap but the notable figure for me is that even 40 years into the future, a major automotive manufacturer predicts a mere 16% market penetration for FCVs (and you have to assume they are talking hydrogen here).
E-P said: "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."

Don't we have to consider the cost/efficiency of the source of heat in the equation? The advantage to the carbon cycle is that the heat required to generate carbon is minimal compared to that of the zinc cycle.

In the presence of carbon, solar energy can be stored in zinc. The source of this solar energy is a solar thermal device, probably a parabolic mirror, needed to generate the high temperature. What is the efficiency of this piece of the process? I would assume it's higher than that of a a parabolic mirror linked to a Stirling engine.
I'm sure this has occured to everyone, but in the worst case scenerio, the leftover carbon could be simply burned, at a much lower efficiency of course.

Another option would be to bury a portion of this carbon, if a main goal is to remove C from the atmosphere. The charcoal powder is probably easier to sequester than the C from the pyrolysis gases, though this could also be done.
Jonathan:  No, no boron.  That's George's hobby-horse; my calculations indicate that it's far more difficult than he thinks and may not work at all.

hamerhokie:  You're right about the heat, but given the very different cost of heat from various sources and even the same source at different sites (solar), a calculation to that level of detail is way beyond what I could do for a quickie.  I'm not sure I'd be able to find free data for that, period.

BBM:  Carbon-negativity is definitely high on my list of desirable characteristics for a new energy system.
Boron may be George's hobby-horse; it certainly is mine too, plus there has been some recent ORNL/Dave Beach talk, although their approach seems to me to divide the leap over the dragon-filled crevasse into easy stages.
Okay, I admit it.  I typed "George", but I meant "Graham".
As to figures for solar energy that reaches the earth in many places, I found a book/paper while writing a research paper a while back that attempts to give some fairly detailed numbers:

B. de Jong. “Net Radiation Received by a Horizontal Surface at the Earth”
Delft University Press, 1973

It happens to be available at my University's library; I don't know how you might come by a copy otherwise...
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