Thursday, May 4, 2017

Project Destiny: Habitats and Food, part 1

This is a topic post referring to Purdue University's project Destiny.
Here is my introductory post for the series.
The subject is section 7, Mars habitats. By necessity, section 8 (Food Production) is also discussed.

Due to the broad scope of this section, I've broken it into three separate posts. This post discusses life support.
Part 2 discusses the actual structures and habitable volume.

 Headline results:
 $265.52 billion in savings (74.5%) by using industrial equipment instead of legacy ISS hardware. By using an integrated biological life support system with advanced air composition management, nutrient cycles are almost completely closed and a straightforward route for makeup mass is available from Martian resources.

Details after the jump.

Life Support

 The baseline plan is to use advanced life support systems with projected cost, performance and other characteristics from NASA references. Significant amounts of resources like water and nitrogen are discarded due to process inefficiencies. The plan has a relatively high level of technical development already completed and thus is fairly low risk, but it also has a significant ongoing resupply requirement from Earth. To make this clear, habitation is estimated to run $177 billion over the project, and life support system costs are $176.3 billion of that (99.6%). Likewise, the oxygen removal system in hydroponics represents $107 billion out of the $179.6 billion for food production (59.6%).  

 Life support needs to maintain a few things with high reliability: air quality, air composition and water quality. Temperature and humidity are also important. The first two are handled by a two-part system while the third is handled through integration with hydroponics. Climate control has a few options.

 Air quality

 Individual dwellings will be equipped with air circulation and filtration, in the form of a fan and an activated carbon filter. This is to avoid 'hotspots' of high humidity, condensation, foul odors or CO2. The carbon filter traps most VOCs and odor molecules, and must be replaced periodically. The filters will be manufactured entirely on Mars: reusable steel frames with wire-mesh backing, then a layer of waste fibers (or rockwool if necessary) with trapped carbon particles.

 Industrial areas or other areas with high dust / particulate loads would use an electrostatic device (no moving parts, no consumables) as the primary filter, followed by the same carbon filter for VOCs.

 The primary Earth-sourced parts for this are electric motors. If a source of copper can be found on Mars then a wire plant and motor winding facility can close the gap.

 Air composition

 Humans and plants have quite different preferences when it comes to environmental controls. Plants prefer high humidity, high temperatures and high CO2 concentrations. Humans prefer the opposite of that, for the most part. The driving factor for the design is CO2; the best way to remove it is with a molecular sieve. Materials suitable for removing CO2 require dry air, so an additional molecular sieve step to extract the water is involved. An example system is NASA's four-bed molecular sieve (4BMS CDRA). I will assume that my existing design is used, more or less, but scaled up to industrial levels. The final unit will include three steps: water extraction, CO2 extraction and nitrogen extraction. Return air from any part of the habitat can be processed by the same unit, since the output streams are water, CO2 (>98%), nitrogen (>98%) and oxygen (95%)

 The extraction process involves a significant temperature swing, so a heat pump is used to condition the output air to the correct temperature. Water is reintroduced to the processed air to maintain proper humidity. The 'waste' output from this process is hot, humid CO2 and slightly carbonated liquid water, both of which would be sent to hydroponics.

 The basic design is an oxygen concentrator. (Here's a product list from one supplier.) These use pressure-swing adsorption to trap the nitrogen and larger molecules in the air and allow oxygen and smaller molecules to pass through. The oxygen step can be done with 68.2 tonnes of equipment in 202 m³ using nine units of the OS-48-94 product from the above link. Adding the two additional beds for dessication and CO2 stripping should no more than double that. Finding a price is a bit difficult, but let's assume the Mars-ready unit is no more than $10 million. That should be reasonable for a well-established industrial device with a competitive market. An alternative technology would be membrane diffusion; here's a look at that circa 2002 (pdf).

 The primary Earth-sourced parts for this are electric motors. If a source of copper can be found on Mars then a wire plant and motor winding facility can close the gap. The high-efficiency scroll compressors used in the pumps may also remain an Earth import, depending on Martian metallurgy development. Much of the rest will be straightforward to make locally, including the molecular sieves. Even so, the entire air composition system for a quarry can be shipped on a single ITS, margin included.

Water Quality

 Wastewater from all systems in the habitats follows the same path. I'll assume North American wastewater volumes of 0.3 m³ per person per day, or 7,500 m³ per day per quarry. (That's 2 million gallons per day.) Cost estimates are all over the map, but here is a source suggesting 17$ of capital cost per gpd. Applying the usual 2.5x factor, that should be around $85 million per quarry.

 First step is an impeller that homogenizes any solid bits, then a swirl separator to remove grit, then a holding tank.

 The water is then introduced to Spirulina cultures through a membrane that allows dissolved compounds to cross but blocks microbes. The microalgae extracts nutrients from the water, adds oxygen and accumulates biomass which is then used in aquaculture. This is done in coaxial tubes as a counterflow, with LED lighting around the outside and the wastewater feed in the center. As the algae add oxygen, microbes in the waste stream aerobically digest organics to return CO2.

 Cleaned water is UV and pressure-swing sterilized, further processed by a second algae stage (whose biomass is used as a nutritional supplement for humans), filtered by carbon and potentially by ion-exchange resins if necessary, then distributed as potable.

 Potable water intended for storage can be chlorinated, using the abundant perchlorate salts from certain Martian soils as a starting material. This is not ideal, as circulating chlorine is a risk for aquaculture and a corrosion factor for structures. Without chlorination, it may be necessary to purge supply lines periodically with a disinfectant.

 Wastewater is returned to the holding tank to keep the system dilute, carrying some oxygen to facilitate breakdown of organics in the tank and dissolving more nutrients.

 The sludge that remains after several cycles (and the grit from the vortex filter) is drawn off and processed through a supercritical water oxidation reactor. This is a process that chemically oxidizes any remaining particulate matter in the waste, producing a stream of water, CO2, mineral salts and oxides that is passed into the second-stage algae system. Insoluble solids can be further processed (chemically and/or electrically) for mineral recovery if necessary.

 Each of these flows must be monitored for heavy metal content and for the presence of pathogens in the 'clean' side of the system. If an accumulation of metals is detected, the sludge can be dehydrated and treated with EDTA to leach it out. Unlike Earth systems, the heavy-metal inputs to the system should be traceable and preventable.


 None of these steps involve mass loss. All available nutrients are biologically recycled, save for a small fraction of mineral oxides. Certainly all nitrogen is recovered, though some of it will return to gaseous N2. Fertilizers and mineral nutrients are only required to expand the hydroponic volume and biomass or to build up stores of food. Most of the equipment mass can be sourced on Mars, with components like motors, sensors, compressors, microprocessors and wiring coming from Earth at first.

 The primary source of loss will be leaks and airlock cycles. Makeup gas can be provided by compressing raw Martian atmosphere and sending it through the air composition system, with a secondary separation system to recover the argon. Excess CO2 would be vented, allowing the system to build up sufficient nitrogen as needed.

 By switching to an industrial design for air composition, vast savings can be realized. The oxygen removal assembly for hydroponics was baselined at $2.676 billion per quarry, while ECLSS for the residential system was estimated at $4.2 billion. Our new estimate is $90 million for both ORA and CDRA functions. Based on Earth heatpump prices and applying a 2.5x scaling factor, climate control and filtration should run about $63 million per quarry. Water processing should run about $85 million, though that's a pretty loose estimate. The baseline development costs were estimated at $12.5 billion; this seems rather high, but I'll take it as a conservative value for a complex system. All together that's $238 million, a savings of $6.638 billion per quarry or $265.52 billion for the project.

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