Re: PEM fuel cell efficiency

[Don Lancaster]

Don W. wrote:
> "G. R. L. Cowan" <gcowan@xxxxxxxx> wrote in message
> news:42E2B887.BFE8F85D@xxxxxxxxxxx
>
> > A real fuel cell, the Ballard Nexa,
> > does 41 percent of delta 'G' at sea level at beginning of life.
> > That's 39 percent of LHV.
> >
> > A vehicle prime mover made of ganged Nexa cells might
> > include 30 of them, mass 390 kg, rated aggregate power
> > 36 kW.
> >
> > Actual fuel cell vehicles use other versions with higher
> > specific power (more kW/kg) and therefore lower
> > conversion efficiency.
> >
> > We know it's somewhere far south of 39 percent.
> > That's all we know; manufacturers never tell.
> >
> > (Actually, there is one other thing we know:
> > the longest ranges attained by internal hydrogen combustion
> > car prototypes in the 70s -- 
> > http://www.hydrogen.org/h2cars/overview/main01.html
> > -- no fuel cell car prototype has equalled or got anywhere
> > near.)
> >
> >
> >
> > > This probably corresponds to a range of load
> > > conditions encountered during driving, but I am
> > > not sure.  A tradeoff for higher efficiency vs
> > > larger FC size would be possible. The optimum
> > > may be to increase the size to achieve an efficiency
> > > in the 60-75% range.
> >
> > No chance of that. As above said, 40-percent-efficient
> > PEM fuel cells are way too big and heavy for cars,
> > and the tradeoff can only go the other way.
> >
> > --- Graham Cowan, former hydrogen fan
> >                                 www.eagle.ca/~gcowan/Paper_for_11th_CHC.html">http://www.eagle.ca/~gcowan/Paper_for_11th_CHC.html --
> > boron as energy carrier: real-car range, nuclear cachet
>
>
> So what's your take on this Siemens submarine FC?:
>                                 www.hdw.de/pdf/broschueren/Fuel_Cell_Plants_for_Non-nuclear_Submarines__en.pdf">http://www.hdw.de/pdf/broschueren/Fuel_Cell_Plants_for_Non-nuclear_Submarines__en.pdf
>
> On the page marked '7' it shows 100% of the "fuel energy" divided up as
> follows:
> 5% -- fuel cell auxilliary
> 65% -- electrical energy
> 15% -- desorption and evaporation of the hydrogen and oxygen
> 15% -- lost energy
>
> Even if they wrongly used the LHV as 100%, that would only bring them down
> to 48% electrical efficiency (and bring up the "lost energy" figure to
> 32%.)
>
> I'd like to know definitively which figure should be used for calculating
> electrical efficiency of a fuel cell.  This fuel cell supposedly operates
> at about 80 degrees C -- well under the boiling point of water, but at the
> actual point where oxygen and hydrogen are combined is the product steam or
> liquid water?  If steam, then about 17% of the potential energy in the
> hydrogen cannot be recovered as electricity because it's 'lost' as heat in
> creating higher energy water molecules than liquid water.  Even if the
> water is condensed before leaving the fuel cell, the energy is still
> disipated as heat and not as electricity.  Is my understanding correct?  Is
> there any logical justification for using LHV for electrical efficiency
> calculation?
>
> I wonder about the "5% fuel cell auxilliary" energy.  Is this heat energy
> that goes to some constructive use in the fuel cell?
>
> Don W.

The lower value should be used.

Electrolysis can be up to one sixth endothermic.
Thus, a hydrogen fuel cell at most MUST be at least one sixth exothermic.



--
Many thanks,

Don Lancaster
Synergetics   3860 West First Street  Box 809  Thatcher, AZ 85552
voice: (928)428-4073 email: don@xxxxxxxxxx

Please visit my GURU's LAIR web site at                                 www.tinaja.com">http://www.tinaja.com