Re: hydrogen data

From: Don W. (dNOSPAMwiddersAThotmail.com)
Date: 11/18/04 

Date: Thu, 18 Nov 2004 10:03:36 -0800

"Don Lancaster" <don@tinaja.com> wrote in message
news:419CBA48.382833E9@tinaja.com...
> Peter Lowrie wrote:
> >
> > Does it take as much energy to make Hydrogen as is produced when it is
> > ignited?
> >
>
> Yes, it takes more energy to make Hydrogen as is produced when it is
> ignited.
>
> Great heaping bunches more when properly amortized.
>
> The only exception is that extremely low level and uneconomical hydrogen
> production can in theory be up to one-sixth endothermic. At useful
> levels, cell losses utterly overwhelm this theoretical capability.
>

Don, the following is something I stole from a PDF file some time ago and
now I don't even remember where I got it, but it says that at best
electrolysis can only be 84.5% efficient. Apparently they have the burning
and unburning of water backwards, but I don't really know enough about this
stuff to see where they went wrong. Would you mind to show me?

"The reaction of the formation of water is:
H2 + ½ O2 -> H2O + energy

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.

The electrolysis reaction is the opposite of the formation of water
reaction:

H2O + energy -> H2 + ½ O2

For purposes of electrolysis in this report liquid water, and not steam, is
electrolyzed to produce hydrogen. So the reaction above is the reverse of
the formation of liquid water. As a result, the amount of energy needed to
create hydrogen from water using electrolysis is 39 kWh/kg. In order to
determine the efficiency of the electrolysis process, the theoretical
amount of energy needed, 39 kWh/kg of hydrogen, needs to be divided by the
actual amount of energy used by the electrolysis unit to create hydrogen.

The reason this distinction is important is because by using the lower
heating value, we misrepresent the efficiency of electrolyzers. If you use
LHV, to calculate the efficiencies, the efficiencies are low. For example,
the Stuart IMET 1000 series electrolyzers are calculated to be 33 kWh/kg
(LHV) ÷ 53.4 kWh/kg (the energy required to produce 1 kg of hydrogen using
Stuart electrolyzer) which equals 64%. Using the HHV of 39 kWh/kg yields an
efficiency of 73%. 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."

Don W.

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