I was riding through the coastal Peruvian desert this afternoon. It was unaccountably cool outside, but utterly desolate.
It seems that, even in this desert, perhaps even especially here.the temperature of a building should be a matter of how much coupling it chooses to have to the radiant objects in its environment --- within the limits of their temperatures, of course. If the outdoors ever reaches 15 degrees (or 10, or 5), you can increase the coupling of your cool reservoir to the 15-degree objects until the cool reservoir is arbitrarily close to that temperature, and there is no fundamental limit on how little energy you can use or how large a cool reservoir you can cool. If the sun ever comes out, you can use that to heat your heat reservoir arbitrarily hot, up to the temperature of the photosphere, and there is also no fundamental limit on how little energy that uses, how quickly you can do that, or how large a heat reservoir you can heat. I'd realized this part before. What I hadn't realized before is that this also applies to water. If you limit your leakage of air and water, you only have to let out water when you want to, so you can raise crops using only the water that is incorporated into the finished product. You will likely have to vent to bring in oxygen or CO2 or get rid of radon, but you can cool the exhaust air arbitrarily close to the temperature of your cool reservoir in the process, distilling out most of the water you'd otherwise lose. (The dewpoint of the exhaust air will be just above the temperature of your cool reservoir.) So that limits water loss, subject to a minimum loss rate that depends on your air use rate and the humidity content of air at your exhaust temperature. How about water gain? Well, if the dewpoint of the outside air ever rises above the temperature of the cool reservoir, you can distill water out of the outside air, again in arbitrary quantities. So how to limit water and air leakage over a wide area, for example for growing crops? Put a roof over it, plastic a few decimeters under the dirt (or deeper if you want to grow trees with taproots) and vapor-barrier walls. If the structure is inflatable, it will be a lot lighter. And tearing down your inflatable plastic bubble every so often to prevent UV-induced cracking might still be a small leak, especially if you throw a blanket over the crops first.
