Why Bring up Hydrogen Again?
31-Dec-04
http://www89.pair.com/techinfo/Aerostation/22_1/22_1b.pdf
Why Bring up Hydrogen Again?
F. Marc de Piolenc
ABAC
It seems like a reasonable question. After all, hydrogen
as lifting gas is the public-relations bête noire of LTA.
Didn’t it cause the destruction of the Hindenburg “and
all on board?” Isn’t it “explosive?” What do we need it
for, anyway? We have helium, after all. It’s safe,
non-explosive…generations of publicists have told us
so.
Three Reasons
There are three reasons to consider hydrogen: efficiency,
cost and availability.
Hydrogen, the simplest atom in the Universe, is also the
most abundant element in the universe. It is the second
most abundant element on Earth, and with a reliable
source of energy is extractible from water by electrolysis
or other convenient means. A supply of hydrogen is there-
fore potentially no farther away than a source of water.
The abundance of helium, on the other hand, is much less,
and in any case has little bearing on its availability. It occurs
as a tiny mass fraction in natural gas; some deposits have
more than others. Its production is therefore a bye-product
of natural gas extraction—-if its extraction is implemented.
Only a few natural gas fields—-those richest in helium—-are
exploited for their helium content due to the cost of the
liquid-air plants used for helium recovery; the helium from
others is vented as an inert minor constituent of the gas
that is burned in furnaces, heaters, stoves, boilers and
engines all over the world. When helium-yielding natural
gas wells are exhausted, helium users may not have the
option, like purchasers of minerals, of exploiting, at higher
cost, lower-yield deposits of helium, because those are
being burned off simultaneously.
The cost and availability of helium both suffer from the
problems inherent in any bye-product, in that both the price
and the availability of helium depend on the demand for
the primary product, natural gas. In times of high natural
gas demand, helium is abundant (natural gas production
may exceed the extractive capacity of the helium plant,
forcing well operators to bypass the helium plant and send
precious helium up the chimney) and helium must be
stored or dumped at low price to make room. If natural gas
demand is low, helium is scarce and of course expensive.
The bottom line is that helium is much more expensive than
hydrogen and—-ignoring momentary fluctuations—-the gap
will widen with time. Eventually, exploitable sources of
helium will be exhausted. It won’t be soon—-natural gas is
a very abundant resource—-but it must happen someday.
This poses a problem that LTA advocates seldom recog-
nize, namely that a resurgence in construction and
operation of large airships, if predicated on the use of
helium for sustentation, could be throttled either by ex-
haustion of helium-rich natural gas deposits or by
insufficient extraction capacity. This was already a concern
in the 1930’s, as noted in Rich van Treuren’s article in this
issue.
Supposing helium to be equal in price to hydrogen and
available in unlimited quantity, there is still one paramount
reason to prefer hydrogen, and that is efficiency. The
difference between the gross lift per unit volume of pure
helium and pure hydrogen is very small, but that small
difference is very significant when it comes to the commer-
cial airship’s “bottom line”—-payload. Quoting Burgess:
Effect of the Lift of the Gas Upon the Performance of
Airships
1
There is much misunderstanding and confusion regard-
ing the loss in performance of airships resulting from
decrease in the lift of the gas, especially from the use of
helium instead of hydrogen. It is common practice in
America to take the unit lifts of hydrogen and helium as
.068 and .060 lb./ft
3
in the standard atmosphere at sea
level. Both units are conservative. From these figures,
11.8% of the gross lift is lost by the use of helium instead
of hydrogen; but the percentage losses of useful and
military or commercial load are much greater because the
weight of the ship empty is a fixed quantity, and the
absolute losses of gross and useful lifts are therefore
equal.
Let q = ratio of unit lift of helium to unit lift of hydrogen
u = ratio of useful to gross lift with hydrogen
m = ratio of military or commercial load to gross lift with
hydrogen
Then the fraction of useful load lost by the use of helium
instead of hydrogen is (1—-q)/u; and similarly, the fraction
of military or commercial load lost is (1—-q)/m.
Example. In an airship inflated with hydrogen, it is given
that
u = .38
m = .20
Find the percentage losses of useful and commercial
loads due to the substitution of helium having 88.2% as
much lift as hydrogen.
2 AEROSTATION: MARCH 1999
1
Source: Charles P. Burgess: Airship Design (The Ronald Press Company, New York, 1927), Chapter 2—-Size and
Page 2
Loss of useful load is (1—-.882)/.38 = 31.0%
Loss of commercial load is (1—-.882)/.20 = 59.0%
A further loss in performance from the use of helium
follows from the necessity of starting flight with only partial
inflation in order to avoid valving the costly gas as the
altitude is increased. With hydrogen, it is customary to
start a long voyage fully inflated, and gas is valved as fuel
is consumed and the ship gains altitude. This disadvan-
tage of helium may be overcome through the use of
coal-gas in ballonets filling the waste air space, and used
as fuel in flight.
Please note that the proposed “solution” to helium’s eco-
nomic disadvantage requires huge quantities of flammable
gas to be housed aboard the ship—-exactly what helium
was adopted to prevent!
Other comparisons are of course possible, based on equal
gross lift or equal payload, or even (to make the compari-
son more directly applicable to commercial concerns) at
equal productivity—-payload times commercial range times
number of trips per year.
At equal payload, the helium ship will be larger. If the two
ships are to maintain the same speed (which will be
necessary if equal productivity is the basis of comparison),
the helium ship must have the more powerful powerplant,
which entails a weight penalty which is a further deduction
from payload. Keeping payloads equal forces the helium
ship to be made larger still, and so on until convergence is
reached at some larger (perhaps much larger) size. Size
is important, as it determines the cost of all the support
infrastructure as well as the first cost of the ship. If that is
not trouble enough, consider that productivity is the air-
ship’s weakness vis–vis heavier-than-air transport, due to
its low flight speed, need for a relief crew and other factors.
All of the foregoing suggests very strongly that no factor
that could give LTA an edge should be neglected; yet every
major LTA project and study is predicated on the use of
helium, because the danger of using hydrogen for susten-
tation is held (usually tacitly) to outweigh any economic
advantage.
One Objection: Safety
Which brings us to the only sustainable objection to using
hydrogen, namely safety. Inasmuch as hydrogen is usually
dismissed as a lifting gas by reference to the Hindenburg
disaster, this issue contains two important pieces on that
subject: The US Department of Commerce’s excellent
summary of the conclusions of the two official inquiries into
the events of May 6, 1937 and Rich van Treuren’s article
presenting Addison Bain’s alternative explanation.
The second piece mentioned, though making up a good
chunk of the volume of this month’s issue, is almost a
digression from the theme of hydrogen safety in airships.
The basis for that astonishing comment? Even if we accept
the assumption of the two commissions that the accumu-
lation and subsequent ignition of a combustible
hydrogen/air mixture was the root cause of the disaster to
the Hindenburg, a careful reading of the commissions’
findings gives a rather startling result, namely that in order
for the disaster to take place as it was thought to have
unfolded, an amazing and highly improbable sequence of
events had to take place in exact order and with very
precise timing, depending in turn upon an equally improb-
able combination of circumstances. An unbiased reader is
forced to the conclusion that in an airship designed for it,
with a suitably trained crew, hydrogen is very safe. That
this is not the conclusion usually drawn from LZ-129’s final
voyage is obvious, and that alone should give pause for
thought.
Addison Bain’s work, which implicates the outer cover as
both the original ignition source and as the major propaga-
tor and damage mechanism, and emphasizes the role of
fuel in this and other airship accidents, serves not only to
explain certain features of the calamity ignored or dis-
missed by the official conclusions, but also to explain the
fact that adoption of helium as a lifting gas did not stop the
occurrence of fire in airships—-far from it.
It is worth reviewing what is known about hydrogen and
considering the uses to which it has been put since it was
all but abandoned as a lifting gas.
As every reader of this magazine knows, hydrogen is not
explosive or combustible in and of itself. No fire or explo-
sion can occur unless an oxidizing agent is present, usually
oxygen. We are interested in the limits of combustibility in
air.
When mixed into a mass of air, hydrogen can burn at
hydrogen concentrations ranging from 9% to 64%,
2
a very
wide range indeed. Flame propagation speed at atmos-
pheric pressure is strongly dependent on concentration,
peaking at more than 7 feet/second when the concentra-
tion is in the mid-40s.
3
Compared to other gases in air, this
is very fast, but it is three orders of magnitude slower than
a detonation wave in a high-explosive compound. There-
fore it is safe to conclude that detonation of a
hydrogen/oxygen mixture can only occur if the mixture is
confined so that pressure and temperature can increase
rapidly, promoting transition from deflagration to detona-
tion.
4
When the opposite occurs and an otherwise pure mass of
hydrogen is contaminated with air, the lower limit seems to
AEROSTATION: MARCH 1999 3
2
Charles deF. Chandler and Walter S. Diehl: Balloon and Airship Gases (New York: The Ronald Press Company, 1926).
Other authorities give an even wider range.
3
Don Overs: Flammable Gases (Balloon Federation of America, 1981)
4
The present writer has in his collection a monograph of some 600 tersely written pages on this topic alone; the complexity of
the subject accounts for the different figures for flame initiation, propagation and detonation given by various competent
Page 3
be about 15% air, the mixture becoming “explosive” at 36%
air.
5
The lower concentration sustains only a very slow,
rising flame—-and then only under ideal conditions, while
the higher one is easily ignited and burns violently.
Offsetting hydrogen’s combustibility in air is the difficulty of
forming and maintaining a combustible mixture. Hydro-
gen’s low density and high diffusivity make it a climber and
a seeker of wide open spaces, in stark contrast to gasoline
fumes, for instance, which tend to accumulated in low
places and persist in combustible concentrations for ex-
tended periods.
Other properties of hydrogen have made it of continuing
interest to various fields of engineering. Its specific heat
per unit volume is almost identical with that of air, but its
heat conductivity is eight times that of air.
6
In addition, its
viscosity (a measure of the drag that the gas exerts on solid
bodies in contact with it) is about half that of air. This makes
hydrogen the ideal cooling medium for large electrical
generators where its good conductivity ensures rapid heat
removal and its low viscosity keeps windage losses low.
Hydrogen cooling is used on all large 1,800- and 3,600-
rpm machines and on synchronous condensers and large
frequency-changer sets in order to reduce windage and
provide better cooling…
…If the hydrogen pressure is maintained slightly above
atmospheric pressure so that leakage shall be to the
outside air from the inside of the casing, and if the
hydrogen has a small percentage of heavier gas as
impurity, the windage friction loss may be taken as 10%
of that in air at atmospheric pressure.
7
The above quotation from a standard electical engineering
manual is significant. Despite the naïveté of the assump-
tion that air can be prevented from diffusing inside the
casing by maintaining a higher absolute pressure inside
(diffusion is driven by differences in partial pressure, not
absolute pressure), it is clear that millions upon millions of
kilowatt-hours are generated daily with high-power electri-
cal machinery bathing in the “deadly” gas credited with
downing the Hindenburg. When did any reader last hear of
a commercial power generating station “blowing up,” or
suffering an explosion in its generators?
Astronautics is probably the hydrogen application best
known to the public. The low molecular weight of hydrogen
and its combustion products and its high combustion en-
ergy per unit mass are the attractions here, and an engine
using hydrogen as fuel and fluorine as the oxidizer would
develop the highest specific impulse (a measure of thrust-
producing efficiency) achievable in a chemical rocket. For
reasons of safety and ease of handling, fluorine is not the
oxidizer of choice, but the liquid hydrogen/liquid oxygen
combination has progressed from exotic experimental
status a few decades ago to become a standard fuel/pro-
pellant combination for man-rated spacecraft. NASA’s
Space Transportation System, better known as the Space
Shuttle, uses that combination in its main engines. Inter-
estingly, although the entire STS program has suffered
much adverse criticism over the years—-much of it justi-
fied—-the basic fuel/oxidizer choice has rarely been
questioned, even in the wake of the Challenger accident.
Fortunately for STS, for hydrogen, and perhaps for space
flight in general, the investigators of that calamity were not
forced to make assumptions about its root cause; Chal-
lenger was probably the best-documented and
best-instrumented mishap in the history of transportation.
The solid rocket boosters were shown to be at fault, the
design errors responsible were identified and corrected,
and the system was put back into service.
Nor does what we’ve already seen exhaust the uses of
hydrogen. It is used in numerous welding processes, in-
cluding
the
exotic,
high-temperature
recombinant-hydrogen process. It is being advocated as
the key component in what some are calling the “hydrogen
economy,” one in which hydrogen will become the energy
storage medium of choice and water vapor the only waste
product exhausted to the atmosphere.
The point of this long digression from LTA is that hydrogen
has many uses today and may well have many more
tomorrow, and the workforce of any developed country
therefore already possesses a diffuse but identifiable
group of people experienced in the safe handling, storage
and use of that gas, not all of them doctors of science
working on the frontiers of technology.
As hydrogen gradually enters the daily life of much of the
world’s population, its advantages and dangers will be
better understood, and by a wider constituency. It’s time
that our industry—-Lighter Than Air—-gave this diatomic
gas another look.
4 AEROSTATION: MARCH 1999
5
Chandler and Diehl, op. cit.
6
Chandler and Diehl, op. cit
