Electrical Power Systems
Section 4.3.3.
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Hydrogen-Oxygen Fuel Cells for Lunar Habitat Energy Storage

Initial Exploration Base Deployed on the Moon 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 separately.

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.

Lunar oxygen separation plant 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 reserve.

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 hydrogen produces 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 be done.

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 provide roughly 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) may suffice.

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 from Earth.

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.

Untried Items

The following items need research:
  1. Continued cyclic use of fuel tanks to hold relatively high pressures for long periods
  2. Availability of a suitably low maintenance electrolytic cell that captures hydrogen and oxygen
  3. Pumping hardware to compress the gasses.
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.

Electrical Power Systems

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