CSvax:Physics:piner (09/18/82)
There has been some debate on solar power going on, and I would like to add my two cents worth. There are several methods of getting useful energy from sun light on the moon. I will discuss only two. 1) Solar cells. These cells produce electrical power directly when exposed to light. Very nice, but there are problems. Well, on the moon those problems are much easier to overcome. First, directing the cells towards the sun. The sun moves much more slowly across the "sky" than on earth. The mass of the cells is the same, but the weight is much less. So their support system can be built much lighter than would be required on earth. Hence, incredibly small motors can do the task. Indeed, motors may not be needed at all. Since there is no atmosphere very large temperature gradients are possible due to the solar radiation. Mechanical systems using bimetal strips can be designed to point the cells at any source of radiant heat. Thus no energy is used and the alignment is automatic. A side note, some cars use bimetal strips on the engines. On my car, a bimetal strip is linked to the choke and mounted on the manifold. When the manifold gets hot, the strip changes shape and pulls the choke off. Such systems are simple, cheap, and require no logic (other than that used by the designer). Furthermore, since there is no atmosphere, you get the full power of the sun as soon as it comes over the horizon. On earth, the best you can do is about 25 percent and that is only at noon. The other problem is storage. I did some simple calculations and found that the moon has a circumference of 6800 miles. If we are talking about putting a lot of people on the moon, say 100 million or so, one could justify building a superconducting power line around the moon. Bury the cables deep, and they could be kept cold for a fraction of the cost required on earth, and you only lose power during elcipses. Such a project is a large one, but no bigger than current earth bound projects such as the Siberian pipe line. This brings me to my second proposal. 2) Thermoelectric power. The temperature difference from one side of the moon to the other is huge. If you build a thermalcouple around it's circumference electrical power could be generated directly on a continuous basis. Such a system is incredibly simple. A lot of wire, and not much else. Such systems can be used on a local basis too, one side in light, the other side in shade. But then you only have power during light. An only for a fixed system. In any case, if we are talking about a large number of peole on the moon, then solar power is the way to go. The first colony however will probably have to depend on nuclear power, because you have to start somewhere, and nuclear power plants have fairly high energy densities, in other words, if you have to ship fuel to the moon, nuclear is the cheapest. If you want to generate power from what the moon has to offer, solar is the best bet.
REM@MIT-MC@sri-unix (09/20/82)
From: Robert Elton Maas <REM at MIT-MC> Regarding bimetal strips in chokes in cars: Those have been a source of malfunction in every car I've ever had. Because they have no smarts, when they jam they aren't aware of it, take no corrective action, report nothing to the operator (driver). For the moon I hope we use something with some smarts. I propose an omni-direction solar array (inefficient but gives some power whenever the sun is in the sky) for bootstrapping the computer logic; alternately a radioactive-decay heat source. Then use that energy to run a smart aiming device for the main batch of solar cells or other solar energy collection/conversion devices. Then use that energy to run the mining experiment station.
REM@MIT-MC@sri-unix (09/20/82)
From: Robert Elton Maas <REM at MIT-MC>
Date: 18 Sep 1982 1825-EDT
From: Margot Flowers <Flowers at YALE>
If solar energy sufficient for the needs of the dark half could be
generated at the poles (which would always recieve sunlight that
is not greatly diminshed by atmosphere as it is on the earth),
That's wrong. The Moon tips north and south with respect to the sun,
just like the Earth does (Summer and Winter), although not the same
amount. The effect is similar. Half the year the north pole is in
darkness and half the year the south pole is in darkness, with some
grazing lighting during the boundary region (Spring and Autumn
equinox). A high tower might get light a little more than half the
time at either pole, whereas a ground-based station might get light a
little less than half the time. Two high towers are need for coverage
all the time (except during eclipses).
then
the farthest transmission lines would have to reach would be to the
moon's equator, at most "only" one quarter the diameter of the moon
(still a somewhat long distance).
Your geometrical terminology is lacking. The distance around the moon
is called the "circumference", not the "diameter". You're off by a
factor of PI (3.1415926535...).
But because each pole gets light only half the time, there's an
additional factor of two, because worst case is supplying the south
pole from the north pole during southern Winter and vice versa during
northern Winter. Thus you're really off by a factor of 6.
I propose three stations on the equator separated by one third of the
circumference. That way each supplies energy for a little less than
half, and their less-than-halves overlap allowing smooth transition
from one to the next, avoiding power glitches as current in cables is
reversed gradually between the two sites. Except for the master trunk
that girdles the Moon at the equator, worst case is quarter circumference.
(That proposal is in the context of centralized production. Acutally I
prefer distributed production whereby each station has its own solar
energy, with computer making it track the sun, and decreasing level of
activity each night to conserve limited energy storage. At least in
the forseeable future, say 50 years.)REM@MIT-MC@sri-unix (09/22/82)
From: Robert Elton Maas <REM at MIT-MC> I think it's quite clear that in the long run we want to have habitat and industry etc. in space, probably a la Dyson sphere around each star we homestead. It's not so clear what method we want for powering planet-based habitat and industry, whether simply tap off the Dyson power grid, or have local solar energy with local storage, or have a ring of energy girdling the planet to avoid need for local storage, or have local nuclear fission power, or if we solve the problem local nuclear fusion power. I think it's too early to decide on one or the other, although we ought to keep all those possibilities in mind towards the end of the next 50 years as we approach the time they will be implemented. (My guess, none of the above, by 50 years from now we'll have a new and better idea.) My suggestions how to supply energy to lunar mining stations deals with the bootstrapping period, from when we first establish an experimental mining station on the Moon until we have enough industry on the Moon to begin to consider linking all of it together into large power grids or mass-tossing networks of manufacturing stations (imagine using a mass-driver to toss pellets of pure titanium from the titanium-extracting station to a place where it's needed to make titanium-iron alloy, this toss perhaps being between points hundreds of miles apart!). Our urgent problem now is that (1) the money-holders don't think space is worth money because they think it's too expensive or impossible or doesn't reap enough rewards, (2) the scientists haven't really worked out all the possibilities and created a proposal for action (some starts have been made here with Pournelle's space policy proposal), (3) because of 1 and 2 hardly anything is moving along and thus we simply aren't bootstrapping ourselves into space. Currently I stick mostly to things that will be useful for getting started. In addition to the currently-planned shuttle activities of chemical-manufacture experiments and large-space-telescope, these include: development of SEPS (Solar Electric Propulsion System = solar-powered ion rocket) and a general space-tug capability, development of a full-scale mass-driver, launching of a permanently-staffed LEO (Low Earth Orbit) station, surveying L-4 and L-5 for debris, surveying the polar regions of the Moon for water ice, surveying near-Earth asteroids and comets for minerals, experimenting with remote-control mechanisms and robotics to determine whether they are feasible, and actual starting of experimental robot mining on moon (in polar regions if water is found there, else in equatorial regions). It is in this context that I debate whether robotics is sufficiently developed for mining, whether solar or nuclear energy should be used, ... and dismiss power grids on the moon as being too far in the future but still worth discussing briefly to aid our long-range perspective.