Lighting our Space Habitats

In previous posts (Designing a Space Habitat and Farming in Space) I’ve argued that we do not want to use “natural sunlight” to illuminate our habitat and grow our crops. There are three primary reasons:

• Simplicity: We must have radiation (and meteor) shielding of about 10 tons per square meter of surface. A complex chevron array of mirrors would be needed to reflect light around this shield, plus mirrors would need active positioning to track the sun. Joints between windows, shield, and structure would be subject to thermal cycling, introducing failure points.
• Room: We don’t have enough surface area to position our farms on the inner surface of our habitat where reflected sunlight could be most easily directed. Artificial lighting allows growing crops in rooms with very low ceilings, supporting 5 to 10 times the population in the same size structure.
• Thermal Efficiency: Natural sunlight is largely heat, and heat dissipation is the primary limiting factor in the size and population density of a large space habitat. Every watt admitted or generated in the interior must be radiated away. The interior will be warmer than the blackbody radiating temperature of the exterior.

There are other considerations as well. Much of natural sunlight is not photo-synthetically active radiation (PAR). Plants appear green because they reflect this wavelength of light and do not utilize it (chlorophyll has absorption peaks in the red and blue regions of the spectrum). From an energy efficiency standpoint, natural sunlight is relatively poor (worse when considering infrared and ultraviolet which comprise 55% of the sun’s energy flux). Only 1%-2% of solar energy is converted into biomass by plants, compared to 8%-16% of the energy of optimized LED light sources (C4 plants - including crops such as wheat, corn, rice, barley, oats, and sugarcane -  have higher efficiency than C3 plants).

Sources for this information offer a wide variety of opinions. For LED lighting, see the research articles at LED Grow Lights Outlet. For sulfur microwave lights, see MacLennan et al. For a discussion of photosynthetic efficiency, see R.J.Bradbury.

Using available information on LED grow lights, optimal plant growth is achieved using approximately 72 PAR watts per square meter. As indicated in my earlier post Farming in Space, we should conservatively allocate 64 square meters per person to maintain a good (largely vegetarian) diet. This equates to 4600 watts per person to grow crops.

We’ll need additional illumination. According to The Engineering Toolbox, direct sunlight provides over 100,000 lux, and full daylight 10,000 lux. An overcast day (1000 lux) is equivalent to the needed light level in a store or a detail-intensive workspace. A normal work (office) environment may only require 500 lux, and home illumination and classrooms only 250 lux. Hallways are even less, perhaps 100 lux. A reasonable average is about 500 lux, which requires 250 watts of white light for my proposed habitat space of 50 square meters per person (ignoring the farms and central park).

The central park area requires much more light – enough to grow plants, and the park will need a white light spectrum. In addition to recreation, the park will grow fruit and nut trees. We have a lot of space to be brightly illuminated, about 10 square meters per person at 10,000-20,000 lux (half of the time), requiring an average of 350-700 watts per person (assuming doped sulfur microwave lamps).

One last point: contrary to current public opinion, I predict that the lights in a permanent space colony will provide moderate (non-zero) levels of ultra-violet light. During exposure to sunlight (containing UV light), human skin produces large amounts of vitamin D, a nutrient vital to health. See the non-profit Vitamin D Council for additional information about the value of this vitamin. “Current research has implicated vitamin D deficiency as a major factor in the pathology of at least 17 varieties of cancer as well as heart disease, stroke, hypertension, autoimmune diseases, diabetes, depression, chronic pain, osteoarthritis, osteoporosis, muscle weakness, muscle wasting, birth defects, periodontal disease, and more.” Vitamin D deficiency may only be the most obvious result of a lack of full-spectrum light in our lives.

Humans evolved to require gravity, and sunlight, and a varied diet. Until we learn otherwise, we need to replicate the conditions of life on Earth very closely in our new homes in space.

Farming in Space

In previous posts I’ve described plans for space habitats which include allowances and techniques for the closed system recycling we will need to establish self-sustaining life in space.

This post describes in some detail just what is needed, and where I found the information. Fundamentally, we will need to provide recycling for nearly everything in space.

Humans breathe oxygen which is plentiful in the form of oxides and silicates, but rare as the free element in space. We each need a bit less than a kilogram of oxygen per day (0.83kg on average, more under high work loads). Note that processes such as smelting metallic ores use heat and a reducing agents such as carbon to turn metal oxides into the metal plus oxides such as CO2. Green plants turn the CO2 into carbon (or carbohydrates), freeing the oxygen, and the carbon can be fed as raw material into the smelter. The only net products of smelting are metal (such as iron) and oxygen.

Humans consume water for drinking, food growth and preparation, cleaning, even entertainment. Since hydrogen is in short supply (until we can gather and use the resources of a comet, in which case we’ll have more than enough to throw away), we’ll need to carefully recycle water. Each human drinks or eats about 2.6 kg of water daily; more is needed for hygiene and growing crops, a lot more.

Working humans need to eat, on average, about 2500 Calories per day (some estimates suggest 2000, my wife’s diet suggests 1500). This can be provided by about 540 grams (dry weight) of food (50g protein, 70g fats, 420 carbohydrates).

The total consumption of a human is about 4 kilograms of oxygen, water, and food per day, and it should come as no surprise that that the same human produces 4 kilograms of waste (in the form of carbon dioxide, exhaled water, sweat, urine, and feces) per day.

Also unsurprising is that green plants (such as algae) can take those 4 kg of wastes and produce the needed food and oxygen with only the addition of energy in the form of light.

The complications arise from the need for a balanced, nutritious, and tasty diet containing all of the essential amino acids and fatty acids, and from the fact that we do not digest all portions of plants. Cellulose (fiber) is undigestible,  and we don’t even attempt to eat most of the plant material of a crop (stems, roots, leaves, bark).

The ecosystem we create in space must be perfectly balanced. When plants produce food for us to eat, they simultaneously produce the exact amount of oxygen needed to metabolize that food. But they also produce those stems, roots, leaves, etc., and excess oxygen to match. All of that extra plant matter must be fed to some other animals (such as rabbits or goats), or to fungi, or to bacteria, or burned. The excess CO2 must be captured and fed back to the growing plants of the next crop, because we don’t exhale enough CO2 to feed the plants, only enough to grow the food we ate – a fraction of the total plant material.

Note that we can meet all of our dietary needs by growing a variety of algae such as blue-green algae including spirulina, and chlorella.

Somewhat surprising is how little water is needed to grow adequate volumes of algae – as little as 6 to 10 liters per person. This is due to the extremely high growth rates of alga under optimal conditions.

However, an algae diet is not only boring, it doesn’t taste good. It is likely to only be used for long space missions where space and payload is at a premium, and even then at least 4 varieties must be cultivated to meet our dietary requirements.

Whether we are growing alga or traditional crops, much of human waste is not readily usable as fertilizer. Portions are, and some bacteria excel at producing nitrates out of urea and ammonia. But much solid waste cannot be so easily processed. Luckily, a technique is available to solve the problem: a Supercritical Water Oxidizer applies high pressure, modest temperatures, and oxygen to burn the carbohydrates to water and CO2, freeing nitrates and mineral salts in the process. This is called the Zimmerman Process.

The process boils down to:

1. feed CO2 and light to growing plants
2. harvest human-edible feedstuffs
3. feed much of the rest to animals such as rabbits, goats, and chickens, as well as vegetarian fish such as tilapia.
4. burn the rest of the plant matter to produce CO2 and ash (which is fertilizer)
5. feed food byproducts (and table scraps) to animals such as chickens or pigs (which when harvested produce still more byproducts)
6. use that Supercritical Water Oxidizer on animal and human wastes to convert them back into CO2 and fertilizers for the plants.
7. Condense water out of the air for drinking, and recycle irrigation water (which holds excess fertilizers) for plants.

Remember, as we are growing crops we need to feed them extra CO2 (much more than humans exhale), and store the excess oxygen they produce; we’ll restore the balance when we burn the crop residues and wastes. The “burning” doesn’t have to be an open fire. Feeding crop residues to goats counts as burning, as does using the plant matter as a reducing agent in the production of iron – both produce CO2 and free the water in the carbohydrates.

What crops should we grow? In general, dwarf varieties of grains, beans, and vegetables will satisfy most of our needs. We’ll have bread and pasta from wheat, rice, soybeans, oatmeal, lettuce, tomatoes, melons, potatoes, sweet potatoes, onions, herbs, etc.. I’m sure we’ll grow strawberries and other fruits, and eventually our parks will also serve as a source for nuts and fruits such as apples. I’m also quite certain we’ll grow grapes for wine, barley and hops for beer, coffee, and tea. Some human appetites insist on being satisfied.

That does leave the question of space. Just how big must our farm be? According to T.A. Heppenheimer’s excellent book Colonies in Space, the answer is derived from existing studies and experiments in high-intensity farming. Using dwarf varieties that have also been selected for short planting-to-harvest times, using interplanting (sowing the next crop before the current one is harvested), and optimizing CO2, water, light, and humidity, Heppenheimer calculates that 60 acres of farmland will support ten thousand people. This is only about 25 square meters per person.

I propose to average less than half that efficiency and allocate 64 square meters per person to include space for crop tending, to support a greater variety of foods, and to allow some extras to feed goats (for milk, cheese, and meat), rabbits (for meat), and chickens (for eggs and meat). Note that crops don’t require high ceilings; a single meter is good enough (on average), yielding a volume requirement of 64 cubic meters per person. This is less than the 100 cubic meters per person of living space I recommend in Designing a Space Habitat, where I also recommend about 33 cubic meters of workspace volume and an equivalent amount of overhead.

In that previous post, I assumed that 3 levels of living space would be allocated for the farms, but that may not be the best option. Rather, the farms are the primary source of waste heat. All that light energy ends up as heat and must be dissipated. The end caps of our cylinder expose a great deal of surface, so it makes the most sense to place our primary heat sources – the farms - adjacent to them. Using 8 meters along both end caps as our farms provides 64 cubic meters per person, independent of the size of our habitat (as long as we use the Kalpana geometry). Many plants need little gravity, indeed aquaculture (raising algae and fish) may require none, and these may be placed near the center. I expect that we’ll place livestock near the outer rim, as their needs for gravity are likely to mirror our own.

A future post will describe the lighting needs of the crops, and the technologies we’ll use to provide it.