solar energy

entropymaker · Nov 30, 2004

does anyone know what the actual energy output of the sun in kWhxm2 in outer space.. from Sol on the moon or near Earth

nexium · Nov 30, 2004

Only about 10% reaches the surface (as sunlight) on the average. A few rare locations, get about 300 watts per square meter, at 1 pm on the longest day of the year. These locations are practical for harvesting solar energy. Unfortunately few humans live close by, with rare exceptions and about 200 kilometers is about the farthest we can send electricity, before losses become prohibitive. We could harvest the energy and build model cities around the solar receiving sites. I suggest very large free flying, hot air and/or hydrogen balloons (we don't have enough helium for a million very large balloons) with a large stearable mirror to beam sun light through holes in the clouds to solar sites on the surface. These would serve all the northern hemisphere, except very close to the North pole. These would be lanched near the equator when winds will carry them North to be recovered months later near the Arctic Circle, late summer and early fall. At present recovery would be too costly in Antarctica, so this is a Northern Hemisphere solution. The balloons would fly at up to 30 kilometers, altitude late afternoon, sinking to perhaps 15 kilometers (hydrogen has negligible fire and explosion hazzard that high) about sun rise. This would allow them to beam sun light to solar sites on the surface, up to 200 kilometers away in some directions. SPS = solar power satellites are impractical, until we have cheap access to space, perhaps not then. Neil

siarad · Nov 30, 2004

In the 1950's there were all sorts of ideas about space-borne collectors beaming energy to Earth via MASER, forerunner of LASER not yet invented. However the problem of injury by entering the beam seemed to dissuade further study. Probably the same would apply to light mirrors beaming, although the cheapness would allow less powerful beams than the smaller number of expensive microwave above.

Saiph · Nov 30, 2004

really? Figuring out the solar constant (i.e. the energy per square meter, from the sun, at 1 au) is actually pretty easy. Now, pinning down how much (or rather little) it varies takes a bit of work, observation, and theory. But the value, is easy. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

nexium · Nov 30, 2004

The maximum allowable leakage for microwave ovens is 1/10 watt per square centimeter. The beam would be whole body, 1000 watts per square meter, which is likely more hazardous, than the small portion of a person likely to be irradiated by a microwave oven. This would be about 8 times higher energy density than average sunlight and about three times brighter than brightest sunlight on Earth's surface. On the optimistic side exposure would typically be about one millisecond as the beam panned from one solar site or rectenna to the next. Even if the aiming mechanism failed, exposure would typically be very brief for each person. and clothing typically reduces energy absorbsion of light by ten or more times. An allowable density of several kilowatts per square meter would be less costly as the site size is one million square meters = one square kilometer for a one gigawatt beam, with hot spots here and there in the beam, which would likely be several kilowatts per square meter at least briefly. For microwaves a tin foil hat would reduce brain exposure. I agree balloon mirors would likely deliver about 0.001 gigawatts, so we would need thousands to make a signifcant dent in the energy needs of the Northern Hemisphere. Neil

silylene old · Nov 30, 2004

The bird lobby would put a quick stop this concept. Imagine all the cooked goose every Spring and Autumn ! <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

bobvanx · Dec 1, 2004

<blockquote>In reply to:<hr />8 times higher energy density than average sunlight<hr /></blockquote>You don't need that high of an energy density to make the system effective. 1/4 the intensity of sunlight is enough to make it better than sunlight. All we have to do is find the least innocuous frequency to run at. The true limiters of terrestrial PV are the efficenecy of the cells and the time in sunlight. Even a 20% efficient cell is in darkness (or at the wrong angle to the rising/setting sun) so that its effective energy conversion is less than 10%. Rectennas (and their dipoles) convert better than 80% of the microwave beam into electricty. Efficiencies above 95% are considered likely by people who work on this sort of thing. But let's use 80%. Consider the plot of land that generates 200watts with terrestrial PV: 100% sunlight x 0.5 day x 20% cell efficiency = 10% overall While that same piece of land, converting a microwave beam at 1/4 sunlight power: 25% sunlight x 1 day x 80% dipole efficiency = 20% overall So... we really don't have to worry about exposure, and we still get a net doubling of the power output, and here comes the real kicker: It's delivered in the kind of power the electric companies LOVE: baseline load. This is the kind of electricity that our coal and nuclear plants currently deliver. And that wind power never will.

nexium · Dec 1, 2004

1/4 the intensity of average sunlight would require 32 square kilometers of rectenna for a one billion watt = one gigawatt beam. 1/4 of the solar constant = 1370 watts per square meter = 342 watts per square meter = about 3 square kilometers which would be available near some cities. ie Jacksonville, Florida has a swamp that large, 7 miles from the city center. If the microwave wavelength is two centimeters = 15 gigahertz, the dipoles are one centimeter long, so at least 7,000,000,000 dipoles are needed to cover 3 square kilometers, letting only 20% pass though the array and wasted. Admittedly a somewhat longer wave length is a likely choice. At one cent each the dipoles cost 70 million dollars. I presume diodes are used to convert the RF to dc at 80% efficiency, so we are down to 0.64 gigawatts and we still need 100,000 plus inverters to produce 60 hertz, 3phase, for the power grid. Do you think we can avoid 3 step up transformers by connecting a hundred plus inverter outputs in series? I think the 80% efficiency is extremely optimistic. A million free flying balloons would sometimes be several beaming to a solar site, and occasionally none, so this would not be the baseline power electric companies love. Neil

siarad · Dec 3, 2004

The amount of light falling on the Earth has declined over the last 20 years so global warming must've been via other frequencies. (er wavelengths colours to cut circulatory arguments.) <img src="/images/icons/laugh.gif" />

Saiph · Dec 3, 2004

not really. Astronomers have long known the absorption capabilities of the earths atmosphere. It is required for absolute photometry (where we determine what the apparent magnitude of a star is after taking into consideration atmospheric absorption). it's often expressed as a function of airmass and zenith angle. The sun radiates quite a bit like a blackbody, and so knowing a portion of the spectrum allows for us to know how the rest of it behaves. Bolometric corrections (what you describe correctly as considering the entire EM spectrum) have been practiced by astronomers for decades. Add in the additional factor that we can closely observe the sun's non-black body behavior easily (it's heavier on the IR than a pure black body), and we can get the solar constant to within any reasonable accuracy. Now within the last decade or two I will agree that we are now able to take direct measurements of the sun's bolometric magnitude. However we replace the uncertainty of the earth's absorbtion properties, with an uncertainty in our detectors quantum efficiencies. You really need to ease up a bit on your reliance on sheer empirical data. A lot of good solid data (accurate and precise data) can be achieved without direct measurement if you consider the appropriate factors. Direct measurement is done where possible, when possible, even if it is just to confirm previous approaches. But it doesn't mean we were unable to do so before. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

siarad · Dec 4, 2004

Whoa I said light not energy & pointed out other frquencies must account for the energy increase.

nexium · Feb 18, 2005

We have a sizeable group of alarmists who will oppose even a 1/4 sunlight energy beam, whether microwaves, monochromatic light or sunlight reflected from a mirror. Similar groups oppose most any change advocated by anyone. Abiding by the wishes of minorities has some advantages, but we need to realize that expediency is occasionally essentual to provide even marginally for the soon to be 7 billion humans on this planet. With cheap access to space, we can perhaps build a square 4 kilometers by 4 kilometers = 16 square kilometers covered with solar cells or a mirror surface orbiting at an altitude of about 20,000 kilometers. This is tethered to a transmitting assembly. The tether typically has some slack, so the two assemblies can be positioned separately. In the case of the photo voltaic cells, the giant panel is positioned to receive the sun's rays most efficently. This may allow 20% 24/7 except for twice per year when the Earth eclipses the Sun briefly on about five consectitive days. The moon will eclipse the sun less often, I think. There will be small losses in collecting the electricity from the solar cells and the power line in the tether. With premium solar cells we can get 274 megawatts per square kilometer = 4.38 gigawatts for 16 square kilometers. If the cells are conected in series parallel, we can put one million volts dc at 4380 amps into the tether power line and receive perhaps 980,000 volts at 4370 amps at the transmitter end. I suggest ten traveling wave tubes = klystrons in series (we will need many series strings to use 4370 amps) producing about 2.22 gigawatts of microwave RF. This will be fed through wave guides to three feed horns on a giant parabolic dish perhaps a kilometer in diameter. The feed horns allow three separate beams to three separate rectennas up to perhaps 1000 kilometers apart on Earth's surface. The beams will deliver about 0.6 gigawatts = 600 megawatts to each rectenna during the peak demand period early each evening. The

nexium · Feb 18, 2005

Magnetrons are more efficient than klysitons, but klysitrons can be modulated with broadband data, which can be received over about 1/2 of the solar system, plus about 1/2 of Earth's surface with a modest antenna due to the 180 megawatts plus that is scattered by the three beams, Earth's atmosphere and reflected by the three rectennas. This communication bonus may be worth more than the electricity delivered. Instead of solar cells a mirror the same size can send a beam of concentrated sunlight to the transmitter assembly, where a heat exchanger makes steam which turns a turbine which turns a dc generator which powers the klysitrons that produce the three beams etc. This system may be slightly more efficient than the solar cells, but it will require human oversite at the satellite, whereas the solar cell system can possibly be serviced by robots. The steam system can likely supply one of the beams during an eclipse. The three beams can possibly power three spacecraft going in approximately the same direction, be used to deflect slightly an asteroid or comet threatening Earth, power a colony at the moon or at L1 or other uses, while the satellite is over the Atlantic or Pacific ocean. The cities depending heavily on the satellite during peak demand may experience rolling blackouts, if their beam is pre-empted for other uses. Neil

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