Would Cheap, Clean Hydrogen Make Fuel Cells Superior to Batteries?

[mheslep]

How does a fuel cell perform at part load?

This Australian SOFC model reaches peak efficiency at about 80% rated load, falls off slightly at 100%, and falls off drastically at below ~40% load.

http://www.fuelcellmarkets.com/images/articles/443/graph.gif

It's not clear it this company is actually in production mode yet.

 

[russ_watters]

Thanks. Put another way, at 25% ouptut, the efficiency drops by 40%. For the turbine I linked above, the efficiency drops by 30% at 25% load. The curves are not the same shape, though, and it looks like their curves cross somewhere around 10% load (ie, below 10%, the fuel cell isn't losing as much of its peak efficiency).

 

[mheslep]

..
Your second link claims an all-up efficiency of 80%, which is exactly the same as a GTE operating at the lower heating value*. http://www.capstoneturbine.com/prodsol/solutions/chp.asp

Also, one can actually buy a micro CHP from Capstone and obviously larger scale co-generation has long been available. That Siemen's SOFC on the other hand is not in production.

The chemistry of these things is is exactly the same:

[CH4 + 2 O2 => 2 H2O + CO2 + E]

They do the same thing: they both burn methane in air.

Not quite. Don't forget the combustion side reactions due to high temperature in the presence of Oxygen which, in addition to their resultant pollution, consume energy. E.g.
O2 + 2 C + E => 2 CO
Same for NOx, etc.

 

[Topher925]

Did you see slide 14?

Yes I did. I didn't include it because slide 14 refers to electrical power generation while the engine is driving the plane. In other words, the engine isn't only working to produce electricity, its producing thrust and electricity. This will give a value of apparent efficiency making it not comparable to electrical or co-generation only power generation.

But you do have that first part backwards: since fuel cells are more efficient, the waste heat is lower than in a GTE

I think I got it forwards, this is a good characteristic to have. In most applications its always more favorable to produce more electricity than heat since electricity is a very versatile form of power and heat (especially at lower temperatures) is not.

The chemistry of these things is is exactly the same:

[CH4 + 2 O2 => 2 H2O + CO2 + E]

They do the same thing: they both burn methane in air.

The chemistry is most definitely not the same. SOFCs don't "burn" anything. What you posted is the IDEAL global reaction and not what you would find in a real application.

For a SOFC, a more realistic equation would look something like this,

CH4 + 2 (O2 + 3.72 N2) => 2H2O + CO2 + 7.52 N2

For a GTE, especially a higher efficiency one (higher temp), it would look something more like this,

CH4 + a (O2 + 3.75 N2) => b H20 + c CO2 + d N2 + d CO + e NO + f NO2 + g H2 + etc.

http://en.wikipedia.org/wiki/Dissociation_(chemistry)

 

[mheslep]

http://en.wikipedia.org/wiki/Dissociation_(chemistry)

As I recall and the Wiki indicates Dissociation refers to ionic bonds, not the covalent bonds broken and made in combustion.

 

[russ_watters]

Not quite. Don't forget the combustion side reactions due to high temperature in the presence of Oxygen which, in addition to their resultant pollution, consume energy. E.g.
O2 + 2 C + E => 2 CO
Same for NOx, etc.

[and for Topher]

Granted, but just how much are we talking about? According to GE, all of those are in the PPM range, with a total of all of them of less than 1 part in 1000, so the efficiency hit you take is several significant digits away from the decimal point in the efficiency calculation. It isn't enough to be relevant.

 

[russ_watters]

Yes I did. I didn't include it because slide 14 refers to electrical power generation while the engine is driving the plane. In other words, the engine isn't only working to produce electricity, its producing thrust and electricity. This will give a value of apparent efficiency making it not comparable to electrical or co-generation only power generation.

The efficiency number does not include the propulsion of the plane. It says that explicitly on the slide: ""40-45% Jet-A to electrical efficiency"

If that bothers you, just drop this special case and compare the fuel cell to a similar product such the capstone turbine I linked. 15% is not a fair/realistic efficiency for a typical commercial gas turbine generator.

I think I got it forwards, this is a good characteristic to have. In most applications its always more favorable to produce more electricity than heat since electricity is a very versatile form of power and heat (especially at lower temperatures) is not.

No, I was referring specifically to your claim that more waste heat is available with a fuel cell. This is simply wrong.

The chemistry is most definitely not the same.

From the post above: when the difference in the chemistry is on the order of hudredths or thousandths of a percent, it isn't useful to call it different (at least when efficiency is what you are comparing).

As far as what to call what a fuel cell does, different sources say different things. Some sources say any reaction that involves oxidation and releases energy (such as rust) is a combustion reaction. Some say it must happen fast and/or involve heat and light as the energy released. Whatever. I'm less concerned with the label than I am about the similarity between the chemistries.

 

[mheslep]

[and for Topher]

Granted, but just how much are we talking about? According to GE, all of those are in the PPM range, with a total of all of them of less than 1 part in 1000, so the efficiency hit you take is several significant digits away from the decimal point in the efficiency calculation. It isn't enough to be relevant.

Yep, that makes sense, should be small.

 

[Topher925]

If that bothers you, just drop this special case and compare the fuel cell to a similar product such the capstone turbine I linked.

Ok.

No, I was referring specifically to your claim that more waste heat is available with a fuel cell. This is simply wrong.

Sorry, I should have been more clear. What I meant was that the amount of energy that can be utilized per unit of fuel is greater. This is ultimately because you are not forming products which are generated by an endothermic reaction removing usable energy nor do you have to deal with them after your process.

From the post above: when the difference in the chemistry is on the order of hudredths or thousandths of a percent, it isn't useful to call it different (at least when efficiency is what you are comparing).

I'll agree that the amount of heat lost isn't significant but in many cases dissociation can drop an ambient flame temperature by a hundred degrees or so. Granted this isn't a big deal for turbines as flame temperatures are usually as high as alloys can take but the pollutants should be considered. After all, things such as catalytic converters on cars do reduce engine efficiency (and increase cost).

As far as what to call what a fuel cell does, different sources say different things.

Fair enough. I'm not concerned about semantics.

 

[russ_watters]

Sorry, I should have been more clear. What I meant was that the amount of energy that can be utilized per unit of fuel is greater.

Ok, fine.

This is ultimately because you are not forming products which are generated by an endothermic reaction removing usable energy nor do you have to deal with them after your process.

No, as I pointed out, the differences in the chemical reaction are less than one part in a thousand. Essentially all of the difference in efficiency is due to the thermodynamics of a brayton cycle heat engine vs direct electrical conversion of a fuel cell.