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#1 2005-04-13 18:08:13

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,168
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Re: Closed-loop LSS for ISS & Mars - existing chemical/mechanical technology

I've already said I would like to see the U.S. Habitation module for ISS launched to test the lift support system in space. I still want to see this. Prototypes of flight equipment have already been tested on the ground at the Johnson Space Center. However, the Sabatier reactor has only been proposed, it hasn't been tested yet. I would like to propose a couple further modifications.

The Advanced Life Support project at Johnson also tested a system that included an incinerating toilet to extract moisture from solid human waste. Including that, they achieved 97% water recycling closure. One member from this message board reported Purdue is working on an advanced LSS that uses biological treatment systems to achieve 96.5% closure. I would like to recommend that we stick with the flight hardware already built by Hamilton Sundstrand and simply add the Sabatier reactor, but replace the toilet with an incinerating one. The current toilet design for ISS is a trash compactor; it squeezes out liquid to be filtered for water.

The big idea I had today is another existing technology: direct CO2 electrolysis. Oxygen generation by proton transport membrane electrolysis means integrating water and oxygen recycling. Oxygen recycling inefficiency will be made up by consuming water. To avoid gradual loss of water add direct CO2 electrolysis as a separate oxygen generation system. NASA had already looked at this, but not in combination with water electrolysis. It requires significantly more energy but could be used as a supplement to control water loss. This uses thin tubes of zirconia filled with CO2 gas, the gas is heated to 900°C and electrolysis across the tube wall draws oxygen through. The zirconia acts as a molecular sieve, keeping CO2 and CO in but letting O2 pass through. Much but not all of the CO2 is decomposed into CO and O2. Since this doesn't use water at all, it permits generation of oxygen without consuming water. Human cellular respiration will combine oxygen from the air with dry carbohydrates from food to form CO2 and water. If the electrolysis/Sabatier system is mostly closed, extra water generation from human metabolism will replace the rest. In fact, dehydrated food still has some water, often 50% by weight. This would permit residual water in dry and dehydrated food together with water generated by human respiration to replace any losses. The two oxygen generation systems (water electrolysis/Sabatier vs. CO2 electrolysis) to be varied to control water replenishment.

Chemical formulae
human respiration: 6 O2 + C6H12O6 -> 6 CO2 + 6 H2O
water electrolysis: 2 H2O -> 2 H2 + O2
Sabatier reactor: 4 H2 + CO2 -> CH4 + 2 H2O
CO2 electrolysis: 2 CO2 -> 2 CO + O2

Some of the energy from hot CO2/CO waste gas could be recovered with a Peltier effect device. Heat once side of the Peltier device with the hot gas, cool the other side with a heat sink that is allowed to radiate in space. Heat flow across the Peltier device will generate electricity without any moving parts. I believe RTGs use this, but they have a radioactive source for the heat.

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#2 2005-04-13 21:33:01

RobS
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From: South Bend, IN
Registered: 2002-01-15
Posts: 1,701
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Re: Closed-loop LSS for ISS & Mars - existing chemical/mechanical technology

Robert, how much energy are we talking about? I am under the impression that CO2 electrolysis will generate a kilogram of O2 for maybe 2 or 3 kilowatt-hours of electricity, but equipment is usually less efficient than the theoretical energy of a chemical reaction. Even so, I would think a person's oxygen could be supplied by a continuous 1 kilowatt of electricity (24 kilowatt-hours per day).  Do you know?

                -- RobS

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#3 2005-04-14 00:07:57

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,168
Website

Re: Closed-loop LSS for ISS & Mars - existing chemical/mechanical technology

According to one NASA web site regarding the ISPP precursor that would have been on the 2001 Mars Lander, it takes 900°C using a zirconia catalyst and takes 50% more energy than theoretically required. I think that's pretty good. It says it uses 1.6 volts but doesn't say how much power per kg of oxygen. It also says molten carbonate could reduce that to 500-700°C but there are practical things that need research to make it a working system. Another web page has conversion rate vs. power per square centimeter: 30% @ 1.6 volts and 0.33 amps/cm^2, highest is 70% @ 1.9 volts and 0.5 amps/cm^2 but it doesn't say how much time or how much oxygen produced. Another web page says 1 tonne per month at 12kW, which works out to 8.76kWh per kg of O2. Electrolysis of water consumes 4.1164kWh per kg of O2. A Sabatier reactor that combines H2 with CO2 to form CH4 and H2O is exothermic; it generates heat. That means direct electrolysis of CO2 takes a little more than twice as much electricity as electrolysis of water, in addition to significant heat. That's why I say direct CO2 electrolysis would suppliment water electrolysis, not replace it. It would only be used to control water loss. You could produce oxygen via direct CO2 electrolysis and let water accumulate from sweat, breath, and urine; collected with the dehumidifier and toilet. Water recycling would take less energy than oxygen production via CO2 electrolysis, but nothing is 100%. This is a high energy method to increase available water.

For those who like crunching numbers, NASA's Advanced Life Support project states that each person consumes 0.84kg of oxygen per day.

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