Monday, August 10, 2015

Life support - growing plants for fun and profit

In the first life support post I discussed removal of CO2 and water vapor from residential air. Now we look at what happens to them downstream.

 Plants require water, carbon dioxide, light, a set of 14 essential minerals and reasonable temperatures. Many plants can handle microgravity and low pressures without difficulty. Some have useful properties like extracting pollutants from water, growing edible food or producing medically useful compounds. Their two main uses in space are to convert CO2 to oxygen for breathing and to produce food.



 The essential macronutrients are nitrogen, phosphorus and potassium. None of these are common off-planet; phosphorus in particular is likely to be the rate limiting factor for adding more plants. In addition to these three, calcium, sulfur and magnesium are required in moderate amounts. All three are common. The micronutrients are boron, chlorine, manganese, iron, zinc, copper, molybdenum and nickel. They are required in such small amounts that they are common enough in space. Plants also appreciate some silica.
 For hydroponic growing, one example of a complete nutrient source is Hoagland solution. There are alternatives, and individual plant species may prefer a different balance of nutrients.

 I should take a step back. Plants on Earth are commonly grown in soil outdoors. Most people have seen houseplants, which are grown indoors in soil. Plants use soil for two purposes: nutrients and physical support. If we provide these two things to plants another way then we won't need soil. This is good for space since soil is heavy and inefficient, plus it can promote the growth of undesirable microbes.

 There are three main methods of soil-less cultivation. The oldest is hydroponics, where a nutrient solution in water is pumped through a channel and the roots of the plants are covered with it. This method is used for commercial growing on Earth, particularly for plants that grow quickly and benefit from the clean environment (like lettuce and herbs). Alternatives are to grow the plants on floating rafts in ponds or in smaller containers. Many plants do well with this method, but some do not (especially storage plants like tubers and carrots). After harvest, the equipment must be thoroughly cleaned. Pumps need to be running at all times, so a pump failure can be dangerous.
 Next is aeroponics, which is spraying the nutrient solution onto the plant roots in high humidity. This avoids the flowing water of hydroponics and provides more root area for gas exchange. This method requires careful control of nutrients to avoid salt buildup on the roots, but can be used to grow nearly any plant and can be used in microgravity. After harvest, the equipment must be thoroughly cleaned.
 The third type is to use a sterile medium instead of soil. Washed sand or gravel, puffed clay, rice hulls, shredded coconut hull and sawdust have been used. This method is popular in commercial greenhouses that grow tomatoes, cucumbers, peppers and other vegetables. Nutrient solution is pumped into the medium periodically, which provides a more natural environment around the roots. Storage plants do much better with this method. Depending on the medium, it is either disposable or can be reused 2-3 times.
 A related technique is called aquaculture, or the raising of fish and hydroponic crops together. Fish eat food pellets and turn them into waste, rich in essential nutrients for plant growth. Plants extract the nutrients, cleaning the water and keeping the fish healthy. These systems tend to be more stable than hydroponics alone since there is a larger volume of water to buffer changes and a lower concentration of nutrients.

 While plants can tolerate much more radiation than humans (up to 1000 mSv per year in some cases), they still require some shielding so that solar storms don't kill them all. Transparent shielding is not feasible, so the plants cannot have direct sunlight. (Motorized shutters would be possible, heavy, expensive and terribly risky.) Another concern is that there is only around 40% as much sunlight near Mars as there is near Earth, which is not enough for many plants. All this adds up to mean either artificial light or concentrated sunlight passed through fiber optics.
 The amount of light a plant needs is complex. Plants can only use certain wavelengths of light to perform photosynthesis (to convert CO2 and water to oxygen and sugar). This is called photosynthetically-active radiation or PAR. PAR is measured in terms of the actual number of photons adjusted by their usefulness factor, in units of moles. Plants prefer a mix of blue and deep red light; the balance between the two can be used to control how much energy goes to leaves vs. roots, among other things. Summer plants do well with 26 mol PAR per m² per day, while spring and fall plants might prefer a bit less. Some plants are capable of using much more light; experiments with wheat have shown increased yields up to 140 mol PAR per m² per day. Some plants require a specific number of hours of light every day, while others can handle more than would be seen on Earth. Wheat can be grown under 20-24 hour lighting, for example.
 Lights are not normally measured in mol PAR, but rather in lumens or in watts. Lumens are useless for plants; they measure light with an adjustment factor based on how humans see, which gives more weight to yellow light and less to red. Plants require a lot of light, which means a lot of power, which means a lot of heat. The most efficient method of producing light right now is LEDs, light-emitting diodes. These produce very little waste heat and can be placed close to the leaves of plants. LEDs can produce about 1.7 ┬Ámol PAR per watt-second, or 6.12 millimol per hour per watt. Our 26 mol PAR can be provided over 12 hours, requiring about 600 ┬Ámol per second or about 354 watts. At 16 hours of light that's 34.7 mol PAR; at 20 hours it's 43.3 mol PAR and at 24 hours it's 52 mol PAR. These values are per square meter, and I'm using PAR to mean DLI or daily light integral in this case.

 Different species of crops have different yields, time to harvest, light and temperature requirements, etc., etc. It is difficult to generalize any of these values. The one case I can make is that NASA's study on biologically-assisted life support suggested that the amount of plants needed to handle one person's O2/CO2 would produce about half of that person's food. That means sourcing all of a person's calories from plants will produce too much O2 / consume too much CO2. If the CO2 levels drop too low in the growing areas then the plants will not produce and could die. What this means is we need to recover the carbon trapped in non-food parts of the plants, and we need to add carbon to the system in order to grow.
 There are a couple ways we can approach this. The easiest is probably to dry all the inedible plant matter and burn it. This will liberate the carbon as CO2 and recover the other nutrients as ash. One could use a wet thermal gasification process as well; it's basically the same thing with less mess and can handle sewage and trash at the same time. One could also compost the material into soil, allowing microbes to break down the contents. That would require switching to a soil-based process. A third option is to break down the plant matter into sugars and ferment them into alcohol; that can then be burned or used for plastic production (or recreation). A fourth option is to feed the plant waste to animals, then feed the leftovers of that to insects and feed the insects to animals, then gasify anything that's left.
 I prefer a mix of 3 and 4. Animals (primarily fish and chicken) would bolster the protein in the residents' diet while making use of a waste byproduct; the number of animals would be based on the system's waste production. However, oxygen is an export commodity for the station. Selling O2 means bringing more CO2 and H2O into the system, which leaves some extra C and H around for making plastic. In addition, aquaculture systems are used commercially on Earth; extensive research is available on the subject.

 In terms of area, I've seen values ranging from 10 to 40 square meters and perhaps 40-160 cubic meters per person. My own spreadsheet based on densely stacked racks comes to 11.5 m² floor space, 22.14 m² growing area and 46.2 m³ volume for one person (2.5 kg per day), but we want to allow for safety and variety. People don't want to eat tomato and potato for three meals a day simply because they are excellent producers. I also want to leave volume for animal production and associated equipment, so let's start with 30 m² floor space and 120 m³ per person. About 75% of that will be lit, or 43.3 m² of grow area. We need half of 354 watts per m² or 7,667 watts per person for lighting. Additional power will be needed for pumps and fans.
 The extra area provides reserve supplies of storable food like rice and beans, indulgence crops like coffee (1.62g/day*m²), tea (1.92g/day*m²) or cocoa (0.42g/day*m²) {all require several years to establish}, animal feed, plastic feedstock and extra O2 for export. One cup of coffee requires about 10 grams of grounds; a cup a day would use 6.17m² of floor space.

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