Copyright © 2003 by Robert Dyck. Published by The Mars Society with permission.
ARTIFICIAL PHOTOSYNTHESIS: A LOW POWER RECYCLING LIFE SUPPORT SYSTEM
Robert B. Dyck
Ardeco Consulting Ltd.
170 Dearborn Ave
Winnipeg, Manitoba R2L 0M3, Canada
The challenge of any life support system is to create a closed system that includes the metabolism of astronauts to recycle the requirements for life. Apollo carried bottled oxygen and used lithium-hydroxide to remove carbon dioxide. The space shuttle uses the same system. This is fine for a few days or a couple weeks, but a trip lasting months or years would require too much to make the trip practical. The Mir space station used a water electrolysis system to extract oxygen from water, and recycled water from the dehumidifier and urine collection tube. Carbon dioxide was removed with a reusable sorption bed, and the CO2 dumped. The International Space Station uses the same system, but will improve water recycling to include all sources. NASA hopes to achieve 97% water recycling closure.
However, only half of the oxygen breathed to metabolise carbohydrates ends up as water; the other half is incorporated into carbon dioxide. Photosynthesis does the reverse: it releases all of the oxygen from water and half of the oxygen from carbon dioxide, and combines the hydrogen from water with the carbon and the other half of the oxygen from carbon dioxide to form sugar. An electrolysis system will only release oxygen from water. This means half of the oxygen breathed by astronauts is not recycled, but dumped as carbon dioxide. Water has to be delivered to the ISS to replace the lost oxygen. The water recycling system does not have to be perfect since even dehydrated food contains a fair amount of water, but a trip to Mars will require a lot of water to replace lost oxygen.
Adding a sabatier reactor to the electrolysis system would combine all of the hydrogen with half of the carbon dioxide to form methane. The formula is 4 H2 + CO2 → CH4 + 2 H2O. The resulting water would go back into the electrolysis tank. This doubles the amount of water that must be split by electrolysis, but all oxygen breathed is recycled. Currently, the life support system must produce enough oxygen for the astronauts to breathe, so the electrolysis tank already splits twice as much water as human metabolism produces. The Russian system just lets it consume more water than is produced, and relies on shipping water from Earth with each load of supplies. A sabatier reactor would produce water to replace what is consumed by electrolysis. This is actually quite similar to photosynthesis. The light reaction produces twice as much hydrogen as required for glucose; the excess is combined with the oxygen released from CO2 to form water.
This system could be used for a mission to Mars, but it relies entirely on food transported from Earth. Colonisation will require creating food on Mars itself. Depending on greenhouse farms could be tricky, and it would be nice to have a food recycling system for the interplanetary trip. The obvious candidate is to replicate nature's photosynthesis cycle to create sugar along with oxygen. Sugar could be eaten directly, or fed to yeast cultures for a more complex food supplement. The question then is how to do it?