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Lighting a Garden

(Edited series of posts from the site forum originally posted there between Feb 5th and March 10th, 2016.)

So i was looking into the power needs to light a garden through the night enough to keep the plants healthy. And i went to the NASA Technical Reports Server looking for papers on the topic. And sure enough, there were a number, going back decades. But they used odd specialized units for power - PAR, Photosynthetically Active Radiation, was measured in μmol/m2*s, being the number of photons received by each meter each second. They didn’t actually say how many watts the lights were drawing. But after a while i found a paper that stated that. That paper was using BloomBoss UFO LED Growlights.

At which point i did a facepalm. Of course. What was i thinking, not going to the small marijuana grow-op scene for detailed and accurate information on how to grow plants in artificial light. And a wide range of products designed for exactly that. That NASA researcher was a practical guy.

Yes, he actually has dreadlocks.

Anyhow, the light unit above is conveniently designed to illuminate 9 square feet for 90 W, so that’s 10 W per ft2, for easy scaling calculations for your… indoor garden. At 11 ft2 per m2, that’s 11000 ft2 for the 1000 m2 garden i’d proposed, and thus 110 kW to illuminate the garden - if it was all illuminated at the same time. If it is divided into 2 halves and each is illuminated 12 hours a day, that is 55 kW, or 36 kW if divided in thirds and illuminated for 8 hours. What a deal!

The paper referenced illuminated its plants 18 hours a day for some reason. Given that half the time the garden will be illuminated by good old sunlight, i bet 8 hours is fine. Now, to get these results, the lights were no more than 24 inches from the leaves (60 cm). To have an interesting garden with plants of many different heights, including bushes and trees, it would probably be best to put the lights on strings and carefully weave them among the plants. The distribution of the different LEDs would need to be precise to get the best results. The format is 8 red LEDS plus a blue LED. Overall the effect is of pink light - quite relaxing. A reflective tent of reinforced mylar will need to be extended over the whole garden when night falls to keep the light where it can do its work.

So, a garden is going to be rather easier than i’d feared. You do need the nuclear power plant to make it possible in the early days, but we already knew that makes a ton of sense. Once you bring a couple of payloads of water from the poles, and have the first sunken hall completely pressurized and balanced at a comfortable temperature, you can start right in on it. Once growing nicely, this garden would greatly reduce the payload mass of food needed from Earth (pro tip - eat bugs!) and be a great boon to the hall’s ambiance.

Reply from Robert Walker:

Right, I got a similar figure for my article about astronauts getting all their oxygen from growing food. 20 watts to illuminate 0.2 square meters for a modern high efficiency LED grow light. It’s the LEDs that reduce the power requirements so much compared with earlier experiments.

At the same time - if they grow all their own food, then they also automatically must produce all their own oxygen too. Because we turn nearly all the oxygen in the food into the CO2 that we exhale. Feces are just a tiny part of the total mass there, especially once you remove the water from them, most of the mass of our food is either exhaled as CO2 or its water, far easier to recycle.

So to complete the cycle, if we grow plants and they take up the CO2 that we exhale, they must also automatically produce enough oxygen for us to breath in to turn the food back into CO2 again next time we harvest it.

Only 50% of the plant mass is used as food, but that doesn’t matter. Even with burning up all the plant wastes, the excess oxygen after taking account of that still must exactly balance the amount of oxygenthe astronauts need to breath to eat the food.

Also the amount of growing area needed is far less than most would realize. If you use hydroponics and rapidly growing crops, then you can grow nearly all the food from a tiny area.

With the BIOS-3 experiments, it was 13 square meters per person, for 78% of their dry food requirements and nearly all their oxygen.

See my article here for more details and links to some of the research: Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?

Alright, now i’ve read the article properly, and i have a few thoughts. There is a lot of useful information and i imagine i’ll need to review the article once a garden space is being designed. And a few of the linked articles.

That’s all i can think of for now. Thanks for giving me a lot of ideas and a sound basis to start from. If you have input as to how to design the gardens, please hold forth!