From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Re: Energy production from non fossil fuel sources
Date: Fri, 05 Jul 1996 17:41:58 GMT
I previously wrote:-
>>In article <...> firstname.lastname@example.org (Scott Nudds) writes:
>> McCarthy is apparently unaware of the fact that with an appropriate
>> metal, hydrogen adsorption can produce a hydrogen density greater than
>> that of liquid hydrogen.
>One would assume that it would be quite hard to get a
>higher density of hydrogen than liquid hydrogen, a bit
>like trying to get a higher density of water....
Rethinking, perhaps Nudds is talking about *volume* density of hydrogen
( g H2 / ml hydride ), not * weight* density of the hydride
( g H2 / g hydride). It is weight density that is important for transportation
systems, such as cars, trucks, and planes. Certainly a wide range of hydrides
can store a greater volume of hydrogen in the same space as liquid hydrogen,
however their mass is significant *and* they still need to be pressurised, but
the pressure is much lower than needed for compressed gaseous systems.
There have been hydride storage systems tried in vehicles, mostly using
relatively cheap hydride materials such as iron/titanium, rather than the
expensive systems such as palladium ( remember cold fusion? - that required
careful loading the palladium with deuterium, and the heat balances involved
in those stages have to be accounted for when measuring "excess heat" ).
With a density of 12.0 g/ml, and a high price, palladium isn't really a viable
storage medium, so other metals or mixtures are used. For example, Pd can
hold about 0.7% by mass of hydrogen, so...
1 ml PdH= 12 x 0.007 = ( 0.084 / 2.016 ) x 22,400 = 930 mls of H2(g) from 1 ml,
and 78 mls H2(g) from 1 g of hydride.
1 ml H2(l) = (0.071 / 2.016) x 22,400 = 790 mls of H2(g) from 1 ml,
and 11,100 mls H2(g) from 1 g of liquid.
Other metals offer better volumes of hydrogen per gram, eg
Fe-Ti ( with approx of 0.5% Mn to prevent decrepitation ) = 1.9 % mass
( can range from 1.6 - 2.3 % mass )
Mg2-Ni = 3.6 % mass
Mg = 7.6 % mass ( requires high temperature to release the hydrogen, so
not really viable for automotive use ).
La-Ni5 = 1.5 % mass.
It should be noted that refuelling hydrides is relatively slow, and a cooling
system is usually required to remove the heat of reaction generated during
refuelling. Hydrides have been shown to be very durable, provided the
hydrogen is pure - something that is easy to do.Syracuse university was
researching using activated carbon granules instead of metal hydrides.
The Fe-Ti system has been trialled in a bus. The system was 220 psi
operating pressure, 490 psi charge pressure, weighing 1429 kg
( of which 1016 kg was hydride ) and holding 12.7 kg of hydrogen.
If we assume a fuel tank of 50 litres, a petrol density of 0.74 g/ml, and
a calorific value of 43MJ/kg, then an automotive fuel storage system
of 1590MJ is required ( ignoring different engine efficiencies ).
E.M.Goodger ( Alternative Fuels p 134) contains such a comparison
and, although dated ( 1980 ), little new information has changed the
Fuel Type Fuel Fuel + Tankage
kg l kg l
Motor Gasoline 37 50 46 57
H2(l) Cryogenic 13.30 187 105 276
Metallic Hydrides 183 208 214 234
H2(g) @ 13.8 MPa 13.25 1136 710 1700
Obviously, if the hydrogen or gasoline is used in more efficient
engines or fuel cells the actual numbers will change.
I should note that the production of hydrogen in the USA in
1993 was around 15.8 million kg / day, mainly for petroleum
refining or ammonia production, but some is marketed. Even the
small % that is traded is sufficient to power a 100,000 vehicles
on hydrogen fuel.
Hydrogen as an automotive fuel was another of the many
casulties of the demented CARB Zero Emissions Vehicle
policy. When burned in air, hydrogen produces some NOx,
but those emissions can be reduced by the injection of water,
and/or using direct cylinder injection. In the future, it is
likely that catalysts will be developed to reduce NOx either
during combustion or in the exhaust gases. CARB refused to
consider exempting H2 from the NOx requirement, and so
minimal research ( when compared to EVs ) was conducted
on the use of H2 as an automotive fuel in engines or fuel cells.
Working out the various costs is always tricky, especially as
the cheapest sources of hydrogen are from fossil fuels, but
there is no doubt that fossil-derived hydrogen is very
competitive with gasoline. Unfortunately, electrolysis
production is more expensive, with a recent study for a
H2-hybrid EV ( an unusual coupling of technologies just to
create a ZEV ) giving 5c/mi for Gasoline, 4c/mi for H2 from
truck delivery to service station, 5.5c/mi for steam electrolysis,
7c/mi for alkaline electrolysis, and 10c/mi for home electrolysis
( Science and Technology Review - March 1996 p30 ). These
are just the fuel costs, not the purchasing and operational
costs of the vehicle. As usual, the author cites the mass of
hydrogen required, and the mass(1,140kg) of the "body and
frame" of the 80mpg concept vehicle, not the kerb weight
of a functional vehicle.