DIETZ@slb-test.CSNET ("Paul F. Dietz") (10/21/86)
Gary Allen recently disparaged the powersat idea. Taking Gary's word that microwave power-beaming powersats are infeasible, I wondered if other schemes for orbital solar power are possible. If so, Gary's objections to microwave powersats are not arguments against space settlements. There's an obvious solar power scheme that is, in many ways, superior to conventional powersats. It's the Soletta idea: place large numbers of mirrors in orbit to focus light onto ground based collectors. The drawback to the scheme is that the finite angular size of the sun makes the reflected footprint on the Earth rather large. The sun is about .5 degrees across, as seen from Earth. A mirror at an altitude of 5000 kilometers will reflect a spot 43 kilometers across. The spot will in fact be somewhat larger, since the slant range will often be higher as the mirrors move, and the ground will be tilted relative to the beam, causing the spot to be elliptical. Let's say the spot is 60 km across. To get 1 sun of intensity in the spot will require 2800 km**2 of mirrors in space. If those mirrors are made of 1 micron aluminum foil the total mass (of the reflecting foil) is about 8000 tons. Since the mirrors will often be below the horizon from the receiver, this figure should be divided by some duty factor (say .05); however, mirrors can be timeshared between geographically dispersed receivers. (Interesting problem: how best to space the receivers, given that they should be far apart, yet should also be in dry areas?) I said 1 micron aluminum; it may be possible to build even thinner foil. Aluminum remains fairly reflective down to a few tens of nanometers. Some coating may be needed to protect against the solar wind, though. Also, considerable mass will be needed for a supporting framework, control electronics, attitude control flywheels and communications gear. How much energy can we get from these mirrors? Over three terawatts of light passes through a 60 km circle at Earth. If I assume 10% efficiency for converting this to electricity, we get 300 gigawatts of electricity. Often the mirrors will not be head-on to the sun, so some energy is lost, but the collectors will also get direct sunlight during the day, which should help compensate. At midnight mirrors directly overhead will be eclipsed, but at that time power load should be reduced anyway. The scheme has some big advantages: inefficiency and complexity of microwave transmission goes away, mass that must be placed in orbit per unit power output is much lower (perhaps 10 to 100x times less mass per unit power than conventional powersats, assuming many receivng sites), solar cells are on the ground where they're cheaper and safer. Disadvantages: does not scale down well, light pressure must be compensated for, *lots* of mirrors are needed, atmospheric absorption (build in deserts), astronomy is ruined, local heating. Also, I assume solar cells are cheap enough to make this fly; they currently are not (quite). One may want to focus more than 1 sun of light on the collector; perhaps mirrors that reflect in only some wavelength bands could reduce local heating.
Cate3.PA@XEROX.COM (10/31/86)
Paul Dietz <DIETZ%slb-test.csnet@relay.cs.net> writes: >There's an obvious solar power scheme that is, in many ways, superior to >conventional powersats. It's the Soletta idea: place large numbers of >mirrors in orbit to focus light onto ground based collectors. . . >Disadvantages: does not scale down well, light pressure must be >compensated for, *lots* of mirrors are needed, atmospheric absorption >(build in deserts), astronomy is ruined, local heating. Another option might be to build up in mountains, or on the plains in Wyoming or South Dakota. This would help make the winters lots more reasonable. It may even be possible to grow crops year round on land near the ground based collectors. Henry III cate3.pa@xerox.com