Hydrogen-Oxygen Fuel Cells for Lunar Habitat Energy Storage
This document describes the energy storage techniques used by the lunar
habitat while it is on the moon. Because the habitat is well insulated, the
power will be required as electricity rather than heating.
In fact, cooling systems may well require much of the power.
The technology must be implemented in the field on the first flight, and so
requires here-and-now technology.
What's the Energy For?
Because Artemis is an on-going project, our lunar habitat has to maintain its
systems, run experiments, possibly recharge the robotic rover as it buries the
hab, allow remote operations from Earth, and so on. To stay alive it
requires electrical power, and the most promising method for the initial
missions is to cycle water between its liquid state and constituent
gasses (hydrogen and oxygen) in a fuel cell.
Electricity is generated using a solar power plant during the 14-Earthday-long lunar day. The electricity is used to split water into
hydrogen and oxygen gas by electrolysis. The oxygen and hydrogen are stored
During the 14-day-long lunar night, a type of battery called a fuel cell
recombines the hydrogen and oxygen to form electricity and water. The water
is stored in a tank inside the hab (where it will not freeze) until
there is enough sunlight to power the solar plant once more.
In a standard fuel cell, 2.2 lb (1 kg) of H2 gets you
86 MJ of energy; it's about 70% efficient. So, for every 1 watt of power per
second for the 14-day lunar night we need 14*24*60*60 watt-seconds. (One
watt-second is a Joule). That's 1,209,600 joules or roughly 1.2 MJ.
To get that you need 1.2/86 kg or about 1/2 oz (14 grams) of hydrogen. One
gram of hydrogen combines with exactly 8 grams of oxygen to form 9 grams of water.
So we need to take 4 oz (112 grams) of oxygen too, but hopefully, we'll be able to
extract our own oxygen before long. This would let us take excess hydrogen
and save the mass.
Calculating Energy and Container Requirements
Until the power requirements for all the equipment become clear, no details
can be worked out. We do know that the solar power plant will have to
supply a minimum of enough energy to power the equipment, recharge the fuel
store, run the compressors to do this, and have some safety margin. If we
assume that electrolysis and fuel cells are similarly efficient (70%), we
need to put in twice as much energy as we get out plus the running of
equipment. This works out to be three times the storage recharging requirement, plus
The volume of the water storage tank does not significantly affect the
spacecraft design: the habitat is quite large and 1/2 oz (14 grams) of
a mere 0.22 pints (126 cm3) of water.
The hydrogen and oxygen gas could best be stored in the descent
stage fuel tanks. Even without a liquefaction plant, this can
1/20th oz (1 gram) of hydrogen gas fills 0.39 cu ft (11 litres) at
0oC and 1 ATM. At 3.5 ATM (the pressure of the proposed
descent stage fuel tanks), this volume is reduced
to 0.11 ft3 (3.08 litres).
Assuming we can maintain a temperature in the fuel tanks of
-100oC by using simple shielding and insulation,
the formula PV=nRT shows us
that the volume becomes 0.11*(273-100)/273 = 0.07 ft3 (1.95 liters).
So one cubic meter of our fuel tank could hold enough hydrogen to store
1.1 lb (0.51 kg) of gaseous (not liquid) hydrogen which would in turn
36.4 watts of continuous power for the lunar night. Our hydrogen fuel tanks are
estimated to be approximately 445 ft3 (12.6 m3),
providing adequate storage
for over 460 W continuous power without the need for liquefying the hydrogen
or enlarging the fuel tanks.
The volume of oxygen required is approximately half the volume of the
hydrogen tanks, which may prove to be a limiting factor (our oxygen tanks
are only 141 ft3 (4 m3), allowing only
282 ft3 (8m3) of pressurised H2
to react in the fuel cells). However,
oxygen liquefaction may well be possible and this would maintain hydrogen
storage as our limiting factor.
Exotic Storage Media
On Earth, it is preferable to transport hydrogen absorbed in
metals called hydrides. This is a mechanically safer technique than
pressurized containers or liquefied gas. New technologies involving the use
of carbon nanotubes (stretched buckeyballs) are proving promising as
methods of retaining hydrogen gas. These methods are apparently causing the
hydrogen to form layers up to three atoms deep on the carbon substrate,
though the mechanism is not fully understood as yet. It may be possible to
"upgrade" our storage capabilities by introducing these compounds to the H2
tanks on later missions, or it might be simpler to use large balloons
buried beneath an inflatable toroid for micrometeorite protection.
Obtaining the Fuel
The residual fuel from the descent stage will most likely be the fuel
source, though water (perhaps derived from shuttle flights or even urine)
As the site becomes established, and perhaps from the outset, lunar oxygen
would be a useful addition allowing less oxygen and more hydrogen to come
So What's Wrong With Batteries?
The very latest (as of 6-Mar-1997) nickel-metal hydride (NiMH) batteries
from Sony's labs that compete with lithium-ion technology store 6.8 KWh per
ft3 (240 Wh per liter).
To supply the hab with, say 500 W to keep the sums easy, would
require 2 liters of battery per hour = 2*24*14 = 672 liters (24 ft3)
of battery weighing 2 tonnes, not counting spares.
Volumes and Weights of the Cells
Commercial fuel cells producing 100 W cost about US$3,000 each; a pittance
compared to the transport cost as they weigh 19.8 lb (9 kg) each.
Although 5 cells
would fulfill our requirements, 8 will give some redundancy at a weight of
158 lb (72 kg) plus some support structure. They are approx 4"x4"x12"
(10 cm x 10 cm x 30 cm) each.
Total volume for all 8 is .85 cu ft (24 literes) and will need a
few pounds of control gear.
Electrolysis cells using 1 kW quoted at 90% efficiency fit in a 2 ft (60 cm) cube.
Weight data are not currently available, and it should be borne in mind
that commercial units do not usually recycle the oxygen.
The following items need research:
There may also be a benefit in making the fuel tanks heavier but capable of
holding a higher pressure, so increasing our energy storage capability.
Copyright © 2007 Artemis Society International, for the
contributors. All rights reserved.
- Continued cyclic use of fuel tanks to hold relatively high
pressures for long periods
- Availability of a suitably low maintenance electrolytic cell that
captures hydrogen and oxygen
- Pumping hardware to compress the gasses.
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