The Ergosphere
Wednesday, June 29, 2005

Zinc: Miracle metal?

Research at ETH Zürich (News release) have implemented a pilot-scale thermochemical process to capture solar energy using a zinc cycle and a supply of free carbon (diagram) (hat tip: Green Car Congress).  This process converts zinc oxide (ZnO) and carbon to metallic zinc and carbon monoxide; the zinc can be used to make either hydrogen or electricity, and the ZnO product completes the cycle.  The carbon source can be anything from coal to biomass.  The researchers claim 30% conversion efficiency in the pilot plant, with full-scale plants expected to hit 50%-60%.

The adjective "impressive" is, I believe, an understatement.

When I read this, I was initially troubled by the use of carbon (coal!) in what was represented as a solar energy system (bait and switch).  However, the researchers claim that the amount of carbon required to reduce the zinc is reduced to 1/5 of that required by pure chemical processes and that biomass can be used as the carbon source.  It's also obvious that the off-gas (CO) retains much of the energy from the original carbon fuel.  The big questions are:
  1. Is this true?
  2. How much energy is retained?
  3. What are the follow-on possibilities?

Is it true?

This one starts with the chemistry:  specifically, heats of formation.  (Elements have zero heat of formation by definition.)

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

A process with the inputs of elemental carbon, zinc oxide and heat and the outputs of zinc gas and carbon monoxide has the following energy balance:

ZnO + C + 90540 cal/mol -> Zn (gas) + CO

The energy supplied by heat vs. the total is 90540/115940 = 0.78, or 78% (the remaining 22% comes from the oxidation of carbon to CO).  This is close enough to the author's claim of 4/5 to count the latter as correct.  However, it is not the whole truth; out of the chemical energy output of the process, 61% comes from the input carbon and only 39% from the thermochemical additions.  But that's misleading too; the outputs are not equivalent.

How much energy is retained?

The reaction inputs include carbon with a heat of combustion of 93960 cal/mole (plus the thermal energy required to bring the inputs up to reaction temperature, but which are not quantified here).  The outputs include: The chemical energy of the outputs (at room temperature) is 153230 cal/mol, while the chemical energy of the input carbon is 93960 cal/mol.  The thermal process has increased the total energy substantially, but the carbon input still accounts for 61% of the total input energy (chemical and thermal).

A conservative estimate of the sensible and latent heat of the products, assuming the zinc is condensed to liquid and the CO is cooled to 200 C, is 20420 + (7 * 1000) = 27420 cal/mol.  Perhaps 25% of this could be converted to work via a steam turbine, so the net output would be 6855 cal/mol (7.97 Wh/mol); this is about 7.6% of the input solar energy.  This would retain the full chemical energy of the products for other purposes, but this is not the only option.

What are the follow-on possibilities?

The zinc can be used in one of three basic ways, all of which convert it back to zinc oxide:
  1. Use in a stationary zinc-air fuel cell to make electricity for the grid.
  2. Use in a vehicular zinc-air fuel cell for as a motor-fuel replacement.
  3. Reaction with water to make H2.
Examining these possibilities in turn:

Stationary fuel cell:  The carbon monoxide is surplus in this process, and could be burned in a combined-cycle powerplant.  At 50% efficiency, the CO would yield 34280 cal/mol or 39.8 Wh/mol electric output; this electricity would be added to the 7.97 Wh/mol from the thermal output, for a total of 47.8 Wh/mol from the heat and off-gas.

The real surprise is the output from the zinc-air process.  At 1.65 volts per cell and 100% coulomb efficiency (ha!), a Zn-air cell will deliver 91.4 watt-hours per mole of metallic zinc (about 78600 cal/mole).  The total output is 139.2 Wh/mol.  An IGCC powerplant burning carbon (coal) at 40% efficiency would only yield 37580 cal/mol (43.7 Wh/mol).  The total useful energy output is more than tripled.

Mobile fuel cell:  There are already projects to run vehicles such as buses on zinc-air fuel cells.  If these could be moved down to cars, the results could be quite impressive; a vehicle using 250 Wh/mile would require only 179 grams of zinc (2.74 moles) per mile.  Zinc is a reasonably dense metal at 7.14 g/cc; solid zinc would yield about 40 miles to the liter, or upwards of 150 miles per gallon (powdered forms would not be quite so energy-dense).  The carbon monoxide would also be surplus in this scenario.

The actual available energy (electricity) from a Zn-air fuel cell is several times as great as what can be obtained from the same chemical input of gasoline to an internal combustion engine.  The metallic zinc contains about 90% as much energy as the input carbon, and it can be converted to motion with very high efficiency.  It appears likely that a solar-mediated zinc reduction process using coal could power 3.5 times as many vehicle-miles as a conversion of coal to liquid fuel.

Reaction with water to make hydrogen:  This one is interesting, because the efficiency of conversion is relatively high (83% conversion of zinc metal to hydrogen).  But what do you do with the hydrogen?  Among possibilities like the manufacture of ammonia for fertilizer, you could use it to convert half of the carbon monoxide (CO) off-gas to methanol (CH3OH).  Each mole of reactants would yield 19.9 Wh/mol of electricity and 1/2 mole (16 grams) of methanol; methanol has 173.6 kcal/mol heat of combustion (86.8 kcal/mol of reactants).

Methanol is an excellent motor fuel, powering a great many racing vehicles and mixing well with gasoline.  At a density of 0.79, it packs about 55% of the energy of the same volume of gasoline.  A vehicle achieving 30 MPG of gasoline could be expected to get about 16.5 MPG on methanol; each mile of travel would require about 5.66 moles of methanol.

Suppose that a solar power system is set up on a square kilometer of land in a sunny locale.  It receives an average of 5400 Wh/m^2 per day, or 5.4 GWh/day over the whole array.  It converts this thermal energy to zinc with an efficiency of 50%, plus the carbon inputs and outputs.  How much does it process each day, and what could it do?

At 50% capture and 90540 cal/mol captured thermal energy, the process would consume 25.7 million moles of ZnO and carbon (308 tons carbon) plus 5.4 GWh of heat (4.65*1012 cal); it would produce 25.7 million moles of metallic zinc and 25.7 million moles of carbon monoxide.  Byproduct electricity from the thermal output comes to 204 MWh.  Where it goes from there depends on the process options chosen:

Table 2:  Process outputs per day
  Process option     Electric output     Chemical output     Vehicle miles powered  
  All electric   3570 MWh  n/an/a
  MeOH motor fuel   716 MWh   275,000 gallons MeOH 4.54 million 
  Zn fuel cell vehicle   1230 MWh   1680 tons zinc 9.38 million 

Of the three options, the last is by far the most impressive.  At an average daily consumption of 24 kWh, the electric output of such a plant could power 51,000 homes; the zinc output would be sufficient to drive approximately 188,000 vehicles 50 miles a day.  A second such plant devoted to electricity would power an additional 149,000 homes.  Total carbon consumption would be 616 metric tons per day, or about 7.2 pounds per household per day.  (Much of this carbon could probably be obtained from the organic and plastic content in the residential trash, and the rest from biomass.)  That is a very big win.  A car getting 30 MPG driven 50 miles per day would consume 8.7 pounds of carbon in its gasoline alone; producing 24 kWh from carbon at 40% efficiency would release another 14.5 pounds.

A bedroom suburb with .25 acres per house (including streets and parks) would have 2560 homes per square mile, or 988 homes per square kilometer.  Two square kilometers of solar-zinc plants could power 190 square kilometers of such suburb (188,000 homes), with one vehicle each; this is about 1% of the total suburban land area devoted to power production.  This is clearly something people would accept.

Zinc cars?  Getting more than three times as much useful energy out of a ton of coal, and cutting carbon emissions accordingly?  Where do I sign up?

UPDATE 2005-Jul-1:  Two little bits of fallout from this come to mind.  Okay, three:
  1. 25.7 million moles of zinc per day could yield 25.7 million moles (51.4 tons) of hydrogen.  This hydrogen could fix about 240 tons of nitrogen to make about 291 tons of ammonia.  Every day.  At 86 kg/ha/year nitrogen requirement for crops, one day's operation of the plant in nitrogen-fixing mode would yield sufficient nitrate to fertilize 2790 hectares (almost 28 square kilometers) of farmland for a full year.  If you can't make the green algae trick work, here's a backup that only requires a carbon supply like rice straw or corn cobs to drive the process.
  2. There is no requirement that the off-gas be dumped to the atmosphere.  If the carbon monoxide was burned in e.g. a solid-oxide fuel cell, the product CO2 could be captured and compressed to liquid with relative ease; it could be fully sequestered at low cost.  If the input carbon came from the atmosphere, such as biomass or municipal solid waste (which is largely cellulose), the system could become carbon negative.
  3. Solar heat is good, but this process could be driven by anything which supplies heat at a sufficiently high temperature.  If an HTGR could be operated at perhaps 1300°C (maybe during off-peak hours, cranking the power output down and allowing the tempeature to climb), nuclear heat could keep such a process running 24/7 and during the winter in northern climes.  This would permit carbon-negative waste disposal regardless of night or clouds.
It looks like it's going to be harder and harder to keep technological civilization down, oil or no oil.

Edit 2005-Jul-01:  Corrected entry in Table 2; erroneous figure 204 MWh changed to 716 MWh.

Related items:  Fertilize this!, Going negative
I like it. I've always wondered if there would be an efficient chemical process for getting more out of coal other than fancy ways of burning it - it looks like we've found it. Seems to work best however for places like Las Vegas and Phoenix. I suppose Kunstler had better get off the "long emergency" kick and stick to New Urbanism.
For a while I also thought aluminum-air batteries were the most attractive solution, but zinc has the advantage that it can be regenerated electrolytically in aqueous solution; aluminum requires molten salts and the like.  (I have seen articles about cars run on both Zn-air and Al-air batteries, dating back quite a few years; neither presents huge technical hurdles.)

GWB chose hydrogen over both battery-hybrid and metal-air batteries.  You are correct that this is the least attractive, most difficult option with the longest horizon.  Why?  Either he deliberately chose to cripple the US migration away from petroleum (I doubt he's savvy enough about the issues to have done this himself) or he gets all his information from people who effectively chose for him.  Either way, that person or persons have committed something very close to treason against the United States of America.
Here's the problem I have with this: how do we gaurentee the CO2 sequestering? Note that this is a pure cost aspect. The guy who runs this cycle and doesn't sequester his CO2 is going to be more profitable and lower cost than the guy who does. Which means there will always be pressure to not sequester. Or to sequester in cheaper (less effective) methods. This is basically the equivelent of putting scrubbers on the smoke stacks of your coal plant.

This is the advantage of a hydrogen cycle- there's no incentive, no reward, for hurting the environment. Use windpower and/or solar to generate the power, convert the power to hydrogen, then burn the hydrogen to make water. Every step is clean, with minimal impact on the environment. The only problem is an engineering one- storage. While the problem with Zinc as an energy store is a social/political one- making sure that everyone, everywhere, sequesters their CO2, forever. Of the two, I'll take the engineering problem, as once it's solved, it's solved forever.
"how do we gaurentee the CO2 sequestering?"

1.  Have someone else run the injection wells.
2.  Pay only for what's delivered, like any competent set of receiving and accounts-payable departments.

"The guy who runs this cycle and doesn't sequester his CO2 is going to be more profitable and lower cost than the guy who does."

Well, sure.  And as long as the carbon is either from biomass or paid a carbon tax upstream, it doesn't matter; it'll get done where it makes sense, and won't where it doesn't.

There are a lot of stripper wells stretching in a band across Kansas, Oklahoma and Texas, and exhausted oil fields in Pennsylvania.  On the order of half of the oil laid down there is still in the ground, stuck in the pores of the rock.  This oil can be recovered with non-polar solvents like supercritical CO2.

If there was a carbon tax (and sequestration rebate), owners of those wells would be looking at a double bonus:

1.  They could get paid to dump CO2.
2.  The CO2 would bring up oil that would otherwise be unrecoverable.

If people are willing to pay you to sequester your CO2, you're likely to do it.
The only problem is an engineering one- storage.

A difficult problem that puts solutions practically out of reach in the near term. I want to see this country cut its import of oil - it is strangling us. The unsustainable game of trading oil for paper assets has to end and it has to start ending quickly.

Hydrogen vehicles would cost way more than a car with just a zinc-air battery - a huge impediment to progress.

This zinc cycle is only thing I've seen that has a prayer of putting a sizable dent in our addiction to oil and gas.

Ironically it probably won't be the U.S. who adopts this first - it'll be the Chinese or the Europeans.
Well, if it's storage you want....

It's a truism that, if everyone tried to fill up their cars on the same day, all the filling stations in the USA would be out of fuel.  In other words, the storage capacity of the vehicles is (assuming they averaged half-full that day) more than twice the average inventory of the filling stations - perhaps more than the stations' total storage capacity.

Suppose that we were all driving cars with specifications like the Li-ion tzero; ~300 miles range, 60 kWh of battery storage.  The USA has on the order of 200 million registered vehicles (cars and light trucks?); at 60 kWh each, the total storage would come to 12 terawatt-hours.  This is as much as 12 hours' peak production of every generating plant in the USA, 27 hours of average consumption (~440 GW), 65 hours of total transportation energy consumption (including heavy trucks), or 4 days 16 hours of consumption by gasoline-powered vehicles (calculations here).

Conclusion #1:  Vehicles built around 60 kWh battery packs are good enough for most storage needs.
Conclusion #2:  They're good enough to back up the grid too.

If we were going to provide 60 kWh of storage per vehicle using zinc, we'd need 656 moles (43 kg) of zinc each; over a population of 200 million vehicles, it would require 8.58 million tons of zinc in inventory.  That's about 1 year's world production, but we could amass it over a decade or more.  If we could boost world production by 20%, ten years would let us keep 120 kWh/vehicle of inventory on hand.

I don't think this is going to be a huge difficulty.  Whether we use zinc as motor fuel or just as an intermediate in the production of electricity and chemicals, we should be able to get more than enough for our needs.
I came at the efficiency of this demonstration a different way: how much is the chemical potential power of the claimed 45 kg/h of zinc with respect to the claimed solar input power of 300 kW? I get zinc's oxidation yielding 89.0 Wh/mol, 1360 Wh/kg, so efficiency 20.4 percent.

E-P acknowledges that his yield figure for a zinc-air fuel cell assumes 100 percent "coulomb efficiency" -- a measure which I think really applies only to endoergonic processes driven by an externally applied EMF, aluminum electrolysis for instance, for only then are there electrons trying to move on their own, so to speak. What he's putting 100 percent for is just the energy efficiency. A little later he uses this assumption to derive an electric zinc-mobile's zinc requirement to go a mile.

I don't think anyone would want such a vehicle. A zinc burner, maybe, for then the already large amounts of ZnO that have to be retained would not be mass- and bulk-enhanced with wet caustic. I gave some numbers in my zinc footnote. I say zinc is "Boron Hevy".

--- Graham Cowan, former hydrogen fan
boron: fireproof fuel, real-car range, nuclear cachet
If you're going to use biomass for the carbon source, how do you arrive at fully reduced C as in coal?

I've looked at your site.  You've failed to make a case for boron:

1.  Use of boron as a gas-turbine fuel requires purification of oxygen on board vehicles.  Energy budget is one thing, but I didn't see any hint that this function can be performed in the required space or mass constraints.
2.  You have extreme materials handling problems.  Fuel on a spool?  Ash which has to be cast into ingots and stacked precisely?  Just one jam from going over a bump is going to keep buyers away in droves.
3.  Your efficiency is very low.  Small gas turbines appear to be running around 25%; diverting 33% of the output to an oxygen purifier cuts your net output to less than 17% (a boron-burning turbine would probably fare worse, as the combustion turns gas into solid and increases the compressor back-work).  You also have the high idling losses of a turbine, and your scheme requires heat exchangers for its closed gas cycle with the attendant pressure drops and efficiency losses from greater compressor inlet temperature.

Density is an issue.  Boria glass (vitreous B2O3) has a density of 1.812.  Its molecular weight is about 69.62 of which 31% is boron yielding .5627 kg/l of boron in the product, or 52.054 mol/liter.  With the heat of combustion of 590.76 kJ/mol and a conversion efficiency of 17%, you net only 1.45 kWh/liter.  If you handle your product as powder or pellets with a solid fraction of 75% you're down to 1.09 kWh/liter of effluent.  On top of this you have difficulties of costly inputs (boria would require a deposit to make certain people returned it, which would make it valuable enough to steal), corrosive environments (hot, pure oxygen!), materials handling, and engine design issues to deal with a liquid product which can freeze if the temperature drops too low.

Compare zinc-air.  The published cell voltage of Zn-air is 1.65 volts; assuming no current leakage, this represents 88.4 Wh/mol of zinc.  If the product is Zn(OH)2 as powder with a solid fraction of 75%, you get about 2.3 kg/l of product.  This contains 23 mol/liter of zinc, which can produce about 2.04 kWh.  Conclusion;  a zinc-air battery's sludge space represents more available energy per liter than a boron-oxygen engine's "output bin".  On top of this, the battery incorporates the "engine" in its mass and bulk.  The crowning achievement:  people are using them today, on scales from hearing aids to buses.

You'd need at least 2x better energy density to make boron competitive with zinc; instead, you're 2x the wrong way, and you still have all those other problems to solve.  I'm afraid that your boron concept is not going anywhere.
BBM:  Pyrolysis should do it.
They certainly are a little sparse on specifications.  Call them vaporware for now.
If zinc-air batteries' crowning achievement is that they are a present reality, then an engineer who likes them doesn't have to assume 50 percent of contained chemical potential energy is available as DC electricity; he could find out the real deal.

However, it is I who have done this. I used EF-tech-brochure.pdf data. These (88-kg, 78.77-L, 47-cell) modules' direct-current capacity of 325 amp-hours and 17.4 kWh each implies they deliver 53.54 V. Open-circuit voltage should be 78.05 -- 47 times 1.66 -- so at 53.54 V they are 68.6 percent efficient.

Their (minimum voltage, maximum power) point is (40 V, 8 kW) so they're 51.3 percent efficient, chemical to DC, there. Interestingly, the energy per module deliverable at this maximum rate, a datum that appears nowhere in the spec sheet, is 12.9996 kWh.

At what rate is the not-so-round 17.4 kWh deliverable? Maybe 3 kW, but that's just a guess; the spec sheet is evasive.

A perfectly sensible zinc-air EV, i.e. one that no-one would buy, might have three modules on board. Five modules, 440 kg, 40 kW maximum continuous and 15 kW (I guess) at the rated 87-kWh energy capacity, now, that might sell.

The 0.9-litre-per-kWh space requirement for boria lumps that is used to dismiss the boron combustion concept seems about right; based on the supposed 17 percent net efficiency, I get 0.918 L. Along with 0.221 L/kWh for boron pellets that makes 1.14 L/kWh.

The number is more or less right, but the dismissal is not, because as above shown, zinc-air's volume per unit energy, with a bit of power thrown in, is 4.53 L/kWh. Demanding maximum power would raise that to 6.06 L/kWh. The 0.5-L/kWh figure that E-P made up did not, despite his claim, allow for any kilowatts along with the kilowatt-hour ...

--- Graham Cowan, former hydrogen fan
boron: fireproof fuel, real-car range, no emissions
"I used EF-tech-brochure.pdf data.... Open-circuit voltage should be 78.05 -- 47 times 1.66 -- so at 53.54 V they are 68.6 percent efficient."

For someone who went to the trouble to download the PDF, you somehow missed that the OC voltage is specified as 67 volts.  Cell voltage might be lower with a KOH electrolyte.  I won't speculate on efficiency.

"A perfectly sensible zinc-air EV, i.e. one that no-one would buy, might have three modules on board."

Okay, question here:  what makes you think that battery modules designed for use in a municipal bus are suitable for a private car?  The design specs would be very different.

The modules are rated at 200 Wh/kg, while you'd expect about 1.35 kWh/kg of metal fuel.  The obvious conclusion is that 1/5 or less of each module is zinc.  If you were designing for a passenger car, you'd aim for a much higher fraction of fuel and much higher specific power.  Then again, if you were designing for a passenger car you'd have a hundred (a thousand?) times the production volume and could spend a lot more money optimizing your design.

The rest of your piece is about as valid as criticizing a dump truck because it doesn't handle like a Porsche.  It's also about as honest as three-card Monty, because you've done nothing to address the problems which a boron energy system would actually have to solve.

That's easy for you, because you have nothing in the field.  I'll take the good-enough thing that's out there working over the gosh-wow thing that hasn't made it to lab tests.
The GreenCar Congress article you link to mentions

"Straight thermal dissociation of ZnO requires operating temperatures above 1,750ºC. (And PSI is working on a solar reactor for that as well.) However, the use of a carbonaceous material as a reducing agent (e.g., coal, coke, biomass) reduces the required operating temperature to between 1,000ºC–1,400ºC. The SOLZINC process operates at approximately 1,200ºC.[emphasis added]"

So this seems to imply that with advancements in solar concentrators we could even eliminate the need for carbon from the picture. This may not ultimately be desirable as I like the idea of using biomass (essentially recycling co2 from the atmosphere) but it would also solve the carbon sequestration problem or at least provide two options: 1) (likely) more expensive solar concentrators to heat the reaction to above 1,750 C and avoid any carbon inputs and co2 outputs or 2) use carbon to lower the reaction temperature but require more costly carbon inputs and carbon sequestration.

More options are always welcome. Allows more flexibility for plant configuration.
Can this be used with lithium oxide. Which yields lithium that goes into a lithium air batteries, lithium air batteries have greater energy density than zinc air batteries.
What a bunch of retards. No, lithium is not more E-dense than zinc.

6.941 .534 598.73/2: 43.1 kJ/g 23 kJ/cc
10.811 2.34 1273.5/2: 58.9 kJ/g 138 kJ/cc
65.409 7.34 350.46: 5.36 kJ/g 39.3 kJ/cc

Now, what do purification, volume, and overhead hav to do with which element is better on-board a car? Special work is better than densial.

And why not skip the zinc and make carbon the battery's anode?
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