Wednesday, November 18, 2009

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.


michael Hanlon said...

Stephen, Why dost thou only consider direct solar and wired electric LED as lighting sources? May I suggest two other methods which upon consideration may prove more beneficial?
1) {Chemical] Are there any steps identifiable in your recycling process which may involve the generation of photons?
2) [Radiation] Low level microwaves can cause fluorescent tubes to glow, no wiring needed. Wouldn't this be a low energy equivalent application similar to your solar satellite generation system?

Stephen D. Covey said...

I did not consider chemical / biological light sources at all. Since we MUST burn (in some way) all of the plant matter back to CO2 and H2O, those sources should be considered for secondary lighting. I'm not aware of any relatively bright chemical/biological sources (excluding combustion), and in any case these are not useful as the primary lighting for plants (simple energy considerations: no perpetual motion machines allowed).

Combustion itself I discount because the majority of the energy is emitted as heat which would have to be dissipated. I do see some steampunk fans liking the possibility of a space station illuminated with gas lamps or torches. But I digress....

Total system efficiency will likely rule out broadcast energy, since only a fraction of the microwave energy gets converted into light.

I had discounted fluorescent lighting because LED lighting is slightly more efficient, and quite a bit more environmentally friendly.

I considered sulfur microwave lamps, and they are quite efficient for nearly white light, but they use a local (non-broadcast) microwave source. For plants, they are not as efficient as LEDs since much of their light emission is not absorbed by chlorophyl.

Having said all that, I would not claim to be a lighting expert, and others may have worthwhile suggestions. View my post as a starting point. How would YOU feed the plants?

protothad said...

Hi Stephen.

I've been reading your articles regarding capturing Apophis and space colonization in general. Great stuff. I'm just curious if you've put any similar thought or research toward colonization of the moon? It has the disadvantage of a gravity well, but the advantage of not needing initial capture. Perhaps it could be a bootstrap toward eventual asteroid capture? The feasibility would hinge largely on what sort of resourced could be easily mined (recent evidence of water ice is encouraging) and how much effort would be involved in lunar launch facilities (likely a magnetic linear accelerator). Any thoughts?

michael Hanlon said...

ProtoThad, according to Heinlein, the Moon Is a Harsh Mistress.

How to repeatedly travel there economically? The cost of and availability of fuel required to make the first 100 trips to the moon for development would be (pardon the pun) astronomical. It would take 100 trips to have a viable footprint on that body. If you go to Scientific American and search "Planetary Bombardment" and then follow George Musser's report three of the society meeting, you will see that I have outlined a course of action that would meet our needs in the near term. We do not have to wait for 99942 Apophis. There are hundreds of objects that travel along with E&M in a similar orbit just beyond the moon. So, first step is to pass by the moon and grab one of those free locomotives that are out there, bring it back, place it in a orbit around both the E and the M, start Stephen's Colony there too, and use it for free(almost) travel a couple of times a month. that should get that bootstrap laced up. Thanks for burning grey cells on the issue. All inputs are relevant. (Attached are pix of a crude model of the orbit from two angles: Equatorial and 45 deg above the equatorial)

Michael Hanlon

michael Hanlon said...

Sorry protothad, I couldn't get pix loaded here nor through here and onto your email. Perhaps Stephen can help. The pix were files PICT0087.jpg and PICT0092.jpg.

protothad said...


If you want to email the pics to me directly, my address is tdphette (AT) gmail (DOT) com.

I agree there is a penalty to be paid in setting up operations on the moon. The question is, how does that compare to resources needed to capture and begin developing an asteroid? Could resources on the moon (i.e. water/fuel/reaction-mass) be lifted from the moon at lower cost than from Earths gravity well? Perhaps a small moon base would be a reasonable stepping stone toward capturing an asteroid. I'm at least interested in studying the issue to see if that could be the case.

michael Hanlon said...

A possible application for a rail launcher could be up above the steep part of Earth's gravity well. Slow Accelerative forces there would allow for shooting things to the moom (Where you have to catch them) or.......
Use rail launchers to boost to the {xox} velocityies when the asteroid locomotive passes the E & M. Chemicals up out of the wells to the launchers then solar electric for magnetic impulsion and free ride on the rock. One more piece of the puzzle solved (hopefully) Yeah! At the jump off (a scary bit I hadn't addressed) Use the rail to do catching!! Those things do by their nature have a forward and backward direction which is reversible by simply reversing the current and sequencing. The launcher becomes a catcher. So, we need the design for a sledge that will act as a bullet at launch and a baseball into the catch. Even if the aim is off on the catch end, the mag field could act as a directing force, causing correct alignment.
Thanks for bringing up the rail launcher again protothad.

michael Hanlon said...

I recognize that quote off of a friends Profile page at Fun Trivia where I play as Mehaul on the team, Flakes. In the profile area we are allowed to put comments and a very nice woman from the UK wrote that very sentence and only that sentence. Her name is Carol and shee goes by the moniker 'Bucknallbabe' It would be quite an enjoyable coinky dink if its her. and the gobledy comes from a bad internation link.

michael Hanlon said...

Still off topic on lighting (candles?)::

I just watched "Those Magnificent Men in Their Flying Machines; or, how I flew from London to Paris in 25 hours and 11 minutes" (Phew!)
Readers here may know I've liked to equate the similarilties of the opening of America's West to development as it relates to the colonization of space. The Movie is about those years just after Orville and Wilbur and shows how all the nations of the world jumped on the bandwagon and tried to get the edge in aviation. So, now I can add a new simile to my repetoire: the Wright Brothers were like USA and USSR showing spaceflight can be achieved. Now everyone else wants to get involved. And, just as back then, there are dozens of ways of accomplishing that on the drawing boards of people with foresight.

One cute clip shows one gentleman's smarts in trying to get off the ground: He dropped a weight from a height and through pullies started off his craft at one g! He translated falling forces to hrizontal pulling much like Aircraft carrier catapults of today, gaining some v's without any other mechanical means. That is similar to my vision of taking advantage of available bouyant forces.
If for no other reason, watch the movie for three reason: 1) Benny Hill chase scenes and beach changing booths; 2) Red Skelton's vignets of early man's desire to fly; and. 3) Gert Frobe (Auric from Goldfinger) playing the trumpet without a trumpet, hilarious.

Jennifer said...
This comment has been removed by the author.
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Stephen D. Covey said...

A recent conversation prompted me to add a couple of points to this conversation:
1) PAR (Photosynthetically Active Radiation) is not measured precisely, but as a range. Plants absorb a still narrower range of blue light and a narrow range of red light quite efficiently, but neither band absorbs anything of a longer wavelength, and the further short of the optimal wavelengths, the less efficient the energy absorption. IE, yellow light may be absorbed, but any energy in a photon above the red is lost as heat. This means that 72 watts of PAR can actually be produced using less than 72 watts of LEDs. Optimized wavelengths (not white light) can produce that intensity of useful light from just 35 or 40 watts of electrical power.
2) The intensity / power is while the plants are illuminated, and for most plants that is NOT 24 hours per day but (except when flowering) is optimally longer than 12 hours per day. If the AVERAGE illumination is 16 hours per day, the average power consumption (over 24 hours) is still less, and it makes a great deal of sense to have several different farm areas with the light/power rotating between them.
3) Any light that doesn't get absorbed by a leaf is wasted. Thus it is best to have tall plants grown among ground covers such as wheat, or young seedlings grown between larger, more mature plants. And white or mirrored ceilings, walls, and soil!

The bottom line is that it MAY be possible to get by with much less power per person devoted to lighting the farms.