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.