On Sun, Jul 20, 2003 at 11:14:45PM -0400, Erik Reuter wrote: > If the walls are 1km thick and have an average thermal conductivity of > 80 W/m K, and if the asteroid has the same temperature/depth profile > at all points on the surface, then radiation and conduction must be > equal and we get (assuming emissivity of 1)
> (300K - T0) * 80 / 1000 = 5.67e-8 * (T0^4 - 3^4) > assuming the outside temperature is 3K. The solution is T0=125K, which > results in heat radiation of 14 W/m^2. The sunlight reaching the top > of the Earth's atmosphere is about 1400 W/m^2. Even if only about one > quarter of this is absorbed by the habitat (due to shadowed areas and > imperfect absorption), it still seems the habitat would have more of a > problem staying cool than staying warm, if it orbited at 1 A.U. from > the sun. The habitat would need either to be in a further orbit (about > 5 A.U.'s), or to have high reflectivity on its outer surface (99%), or > to have lots of radiative thermal panels that stick out only on the > shadowed side, enough to increase the effective surface area by about > 100 (but that might be difficult since it rotates they would have to > go in and out). I was thinking about this, and the above solutions are not very smart. The problem is that we want to bring sunlight into the habitat, so there is a large heat input (this can't be helped much by filtering out the UV and IR since sunlight peaks in the visible, maybe you could get a factor of 2 heat reduction this way, which is not enough). In equilibrium, the heat output must equal the heat input. So we have to get the heat out again. Since there is not matter outside the habitat, the only way to get rid of the heat is by radiation. How can radiation be increased? Only two ways really: increase the temperature of the emitting surfaces or increase the surface area (I don't think the Sundiver laser solution would work here, for at least 2 reasons: technology - we are far, far away from making such a laser, and efficiency - a laser is not a very good heat pump since it generally needs to have quite a bit more power input then it can pump out as light) Increasing "dark" (shadowed) surface area is difficult since the habitat rotates. So that leaves increasing the temperature of the radiating surfaces of the habitat. Which is actually not too difficult. In fact, we could probably solve both of the problems we have been discussing (atmospheric stability, habitat temperature regulation) in one fell swoop. Make the Ni/Fe walls of the habitat sufficiently thin (I will work on a calculation sometime later) to conduct more heat, which will raise the temperature of the outside. But this will also make the inside surface too cold. But on the rim that is fine since we will cover it with dirt and rock which has a lower thermal conductivity than Ni/Fe. But then we are back to not enough heat flow from the inside rim surface to the outside. But wait, the endcaps don't need to be warm for humans. In fact, we would like them to be quite cold at the center, around 40C to 50C colder than the rim, in order to give the air a decent lapse rate and keep the atmosphere stabile. So make the walls of the endcaps as thin as possible (there is a lower limit that provides structural integrity due to gas pressure), and vary the thermal conductivity of the endcaps radially (either by wall thickness or adding varying thickness insulation) so that the inside endcap temperature drops of radially at about 8-10C per km. Perhaps the windows on the endcaps could even be made of sapphire (or if we allow future technology, diamond) so that they are decent (or great) thermal conductors. The net result would be that a lot of heat would flow from the rim to the endcaps, raising the temperature of the outer surface of the endcaps, increasing the radiation heat dumping rate. If more heat dumping is needed, copper veins could be made running through the dirt and Ni/Fe of the rim. Then heat pumps could pump heat from the inside of the habitat to the copper veins, which would then conduct the heat out and radiate it away. Actually, to bring the Sundiver idea "down to earth" maybe a similar principle, without a laser, could be used. The temperature difference between inside and outside the habitat is quite extreme, so you could run quite an efficient heat engine to generate power. This heat engine could then power a heat pump (there is no thermodynamic limit on the efficiency, heat pumped over work input, of a heat pump that is pumping heat from a higher T region to a lower T region -- even pumping heat from lower T to higher T is allowed to have an efficiency greater than 100%, and the limit is infinite going the other way). I'm not sure how practical this type of heat engine/heat pump is, but I don't think it breaks any thermodynamic laws. But it may not even be necessary if the radiating endcaps idea I mentioned above is sufficient to cool the habitat. -- "Erik Reuter" <[EMAIL PROTECTED]> http://www.erikreuter.net/ _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
