eder@ssc-vax.UUCP (Dani Eder) (04/05/88)
> >In article <35092@kestrel.ARPA> king@kestrel.ARPA (Dick King) writes: > >! ! and solar power satellites would be even bigger thermal > >! ! offenders than solar cells on the ground. > From this fragment it is not clear if you are talking about the universe as a whole, or just the Earth in regard to Solar Power Satellites (SPSs). Allow me to introduce myself. My name is Dani Eder. In 1984 I had the pleasure of working on a study entitled "Design of a Solar Power Satellite Built of Lunar Materials". The study was sponsored by the Space Studies Institute, headed by Dr. Gerard K. O'Neill. He has done the most towards spreading the idea of SPSs. One of our study advisors was Dr. Peter Glaser, who invented the idea of an SPS. Another was Gordon Woodcock, who helped found the L5 Society (now National Space Society), and was at that time head of the Space Station program at Boeing. I joined the Boeing Company in 1981, one year after Boeing completed a major Study of SPSs for NASA. This was one of two parallel studies, the other was performed by Rockwell International. The studies done in 1980 were the last to be funded by the US government. Since I hired into the organization that did the studies, I know about half the people who worked on them. I have about 18 inches of shelf filled with study reports and information related to SPSs. All of that said, I will try to address the subject of thermal balance brought up above. Heat Balance of the Universe A solar power satellite intercepts sunlight that is on it's way from the sun outward into the rest of the universe. Some of the light is reflected from various surfaces of the SPS. In the canonical silicon photovoltaic/2.45 GHz microwave version of the SPS, most of the reflection is from the first surface of the silicon cell, which being pointed normal to the sun, reflects back in the direction of the Sun. Heating the Earth Some of the light is absorbed by the cells, but not converted to electricity. This makes the cells warmer. They radiate from front and back in the infrared, a percentage of which is captured by the Earth. Actually, it would be 0.36% of the infra-red emission, which in turn is some fraction of the total sunlight intercepted. The purpose of the cells is to convert sunlight into electricity. During the 1980 studies it was considered daring to claim 15% conversion efficiency from sunlight to DC. Today, there are multi-band-gap (also known as tandem junction) cells that can achieve 20-30% efficiency in the laboratory. These work by having multiple layers made of different semiconductors. Each one has a band gap tuned to different parts of the solar spectrum. For example, the silicon layer in normal single layer cell actually works best in the near infra-red, corresponding to a temperature of 2000K. The electricity on the SPS is then converted to microwaves by an array of magnetrons. A magnetron is the device found in your microwave oven. By conrolling the relative phases of the microwaves from the magnetrons, you can steer the beam to wherever your receiving antenna is. The receiving antenna consists of numerous half-wavelength dipoles connected to diodes. The diodes rectify the current induced in the dipoles, producing a DC output. If I recall correctly, the conversion efficiencies are 85% for DC to microwaves in orbit, and 85% for microwaves back to DC on the ground. About 1% of the microwaves are absorbed by water vapor in the atmosphere on a normal day, rising to about 3% during a severe thunderstorm. Thus, for each kilowatt of DC delivered to the ground, 188 watts of waste heat is released at or above the receiving antenna. The worst case for the infra-red heating is an additional 33 Watts of waste heat spread evenly over the entire Earth, assuming all the sunlight not made into electricity in the SPS is radiated as heat, and all the heat is absorbed by the Earth (as in a black body). These figures compare to 1850 watts of waste heat produced by] a 35% efficient coal-burning power plant/kW of electricity. This is not all we can do. We typically deliver the power to the Earth's surface at 300 Watts per square meter. This means to produce 1 kilowatt, we need 3.96 square meters at the equator. Typically, the dipoles have a 50% full mesh behind them (about what your typical backyard satellite dish has) . This mesh intercepts, over 4 square meters, an average of 500 watts of sunlight, at a typical temperate latitude. If we increase the reflected component of that sunlight compared to what the original ground did (white paint vs. green plants), then we can claim a credit against the added waste heat. I do not know if you can make the net heat balance exactly the same, but you can make some improvement. The total result is that an SPS releases from 10% down to possibly zero percent as much waste heat as a coal-fired power plant. Dani Eder/Boeing/Space Station Program/ssc-vax!eder
jwm@stdc.jhuapl.edu (James W. Meritt) (04/06/88)
In article <1834@ssc-vax.UUCP> eder@ssc-vax.UUCP (Dani Eder) writes:
:Allow me to introduce myself. My name is Dani Eder. In 1984 I had the
:pleasure of working on a study entitled "Design of a Solar Power Satellite
:Built of Lunar Materials". The study was sponsored by the Space Studies
:Institute, headed by Dr. Gerard K. O'Neill. He has done the most towards
:spreading the idea of SPSs. One of our study advisors was Dr. Peter
:Glaser, who invented the idea of an SPS. Another was Gordon Woodcock,
:who helped found the L5 Society (now National Space Society), and was
:at that time head of the Space Station program at Boeing. I joined
:the Boeing Company in 1981, one year after Boeing completed a major
:Study of SPSs for NASA. This was one of two parallel studies, the other
:was performed by Rockwell International.
:
:The studies done in 1980 were the last to be funded by
:the US government. Since I hired into the organization that did the
:studies, I know about half the people who worked on them. I have about
:18 inches of shelf filled with study reports and information related
:to SPSs. All of that said, I will try to address the subject of
:thermal balance brought up above.
Whoops! Don't you know that interjecting facts from a qualified
person is the best way to mess up a good philosophical discussion!!?!?!?!?!?
;~)
Fortunately, we have at hand a number of people totally immune to reality,
so this one time you are forgiven!
Disclaimer: Individuals have opinions, organizations have policy.
Therefore, these opinions are mine and not any organizations!
Q.E.D.
jwm@aplvax.jhuapl.edu 128.244.65.5