Re: hydrogen data

From: Fred McGalliard (frederick.b.mcgalliard_at_boeing.com)
Date: 11/18/04 

Date: Thu, 18 Nov 2004 20:40:28 GMT

"Don W." <dNOSPAMwiddersAThotmail.com> wrote in message
news:PcOdnY5QcZnhfgHcRVn-hg@comcast.com...
...
> At 25 C and 1 atm, the heat of formation of liquid water, or the energy
> released when water is formed in the reaction above is 39 kWh/kg of
> hydrogen. This value is the higher heating value (HHV) of hydrogen. The
> heat of formation of steam is 33 kWh/kg of hydrogen, and is the lower
> heating value (LHV) of hydrogen.

I would be careful using the LHV value. Clearly this is 39-(the heat needed
to turn water into steam). Might make sense if we were running a steam
engine, but not for general design of heat engines, fuel cells, etc.

...
> Another way to look at it is if the actual energy
> required to create 1 kg of hydrogen, 39 kWh/kg, is divided into the LHV
> value, the maximum efficiency of the electrolysis process is 33.3/39.4 =
> 84.5%. That is to say, that an electrolyzer that converts every kWh of
> input energy into hydrogen energy will have only 84.5% efficiency, even
> though there are no losses5."

I don't see why you are using the LHV to establish this ratio? This is not a
fixed number, but depends on the conditions of the water. Go to a higher
pressure and temperature, and at (isn't it the triple point) the difference
between liquid and steam becomes zero, so your ration would be 1. Actually,
using HHV values to establish the energy required to create H2 is implied by
the reversibility of the chemical reaction, which implies the same kind of
thermo that a reversible heat engine implies. When you plug the two ends of
the machine together, you get 1 (or 100%), if you haven't put any non
reversible steps in that just make heat. If there is a reason to put a top
efficiency limit on this stuff it can't be found in the reversible
reactions.

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