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Microwave Cooling

Mind the satellites though …
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Cover a large portion of the Sahara (or any other large desert expanse) with solar cells. Use these to power a microwave emitter to beam energy OUT of the atmosphere. The frequencies used by GPS satellites are not absorbed by atmospheric gasses (including greenhouse gasses), so the net effect would be to cool the atmosphere. Same idea as putting big reflectors outside of the atmosphere, but without having to boost them into orbit.

The emitter would have to be powered off (or carefully aimed away) when GPS or other communication / military satellite are in its path, to prevent frying them.

Primary purpose would be to offset global warming, but if the transceiver that could be aimed (like Arecibo), could also serve for satellite and deep space communication, as well as radio astronomy.

a1, Aug 15 2021

Greenhouse effect, explained with pictures http://www.ces.fau....se-effect-works.php
Look closely at the graphs about 2/3 down the page, absorption spectrums [a1, Aug 16 2021]

A patent from 2002… https://patents.goo...tent/WO2002098996A1
… A surface coating composition which exhibits low solar absorption and preferential emission at wavelengths corresponding to atmospheric windows in the infra-red regions … [a1, Aug 16 2021]

Bill Gates will block out the sun maybe https://www.popular...periment-postponed/
[xandram, Aug 16 2021]

Why the L band is used for GPS https://ascelibrary.../9780784411506.ap02
L-band waves penetrate clouds, fog, rain, storms, and vegetation [a1, Aug 16 2021]

Effects of a large Sahara solar farm https://www.greenbi...limate-consequences
[scad mientist, Aug 16 2021]

Supplementary material for above study. https://science.sci...1/aar5629_Li_SM.pdf
Top of page 3 has calculations for effective albedo with solar panels [scad mientist, Aug 17 2021]

[link]






       Umm, I feel this would be a bit like opening your fridge door to cool your house...
An energy-using system like this will have some losses, that generally become heat.
Incoming sunlight has already passed through (and therefore heated, if only a little bit) the atmosphere.
neutrinos_shadow, Aug 16 2021
  

       [neutrinos_shadow], The atmospheric greenhouse effect isn’t just about how incoming sunlight heats things up, it’s about the balance of incoming energy, which wavelengths get absorbed and re- emited, and how much energy gets trapped versus escaping back into space. Water vapor, methane, and carbon dioxide are very good at trapping most IR wavelengths, but are largely transparent to microwaves (which is why GPS and other satellite systems use them) - so it will escape without further heating the atmosphere.
a1, Aug 16 2021
  

       [a1]; fair enough. I'm still not convinced that the system can emit enough energy to offset the losses (solar panels get warm, that sort of thing). I suppose it only needs to nudge over 50% to have a positive effect.
neutrinos_shadow, Aug 16 2021
  

       So the idea is to make microwave emitters that work directly from solar panels and aim them at the sky?
Voice, Aug 16 2021
  

       [neutrinos_shadow], of course, this is still a less than half baked idea. I don’t know if there’s enough desert and if the conversion can be efficient enough. I’m not good enough to do the math properly, but at best I think it could send back less than 2% of what would ordinarily be trapped.   

       [Voice], yeah, pretty much. Idea evolved from reading recently about some ultra-white surface roof treatments that not only to reflect visible light but also re-radiant absorbed energy in the specific narrow IR band that will pass through the atmospheric window. But that band is pretty narrow and water vapor still traps some of it. Microwave band is better but you can’t get that by passive re-radiation.
a1, Aug 16 2021
  

       OK, so yes, you could collect solar energy by some means within the atmosphere, and "beam" it out again, reducing net solar gain. Yes, if you were going to do that, you'd want to choose an outgoing wavelength that wasn't going to be absorbed by the atmosphere again. Microwave probably isn't good from that point of view, because it's strongly absorbed by water (and there's lots of that) - see the graph for H2O in your first link. And also, fried seagulls.   

       And then there's the efficiency of generating and emitting microwaves - if it's not very efficient, you do (as [netrinos shadow] points out), just end up heating the atmosphere locally more than you reject to space.   

       And then, there's the "build more machines/energy systems to mitigate the problems caused by too many machines/energy systems" - kind of argument.   

       I had a discussion with the scientist/inventor of the "ultra-white" surface coating you link to - I was considering investing, because the way it was described had huge potential in multiple ways/for multiple technologies. Ultimately I felt there were some issues with the experimental method and the answers were a little vague. So exciting potential, but risk of over- hype/under-deliver.
Frankx, Aug 16 2021
  

       Yes, but what if a multi-billionaire on a space jolly got in the way of the lethal beam of microwave radiation...?
hippo, Aug 16 2021
  

       this doesn’t sound any worse than Bill Gates wanting to block out the sun.(see link)
xandram, Aug 16 2021
  

       //Microwave probably isn't good from that point of view, because it's strongly absorbed by water //   

       Not the entire range of microwave frequenciess. There's more than one choice but look specifically at the absorption spectrum of L-band used by GPS (link).   

       // you just end up heating the atmosphere locally more than you reject to space. //   

       Net zero in that case, as it's still energy that's come in from outside of the atmosphere. Even if you don't beam it back out, it's not like you have any EXTRA to deal with.
a1, Aug 16 2021
  

       //what if a multi-billionaire on a space jolly got in the way of the lethal beam of microwave radiation...? //   

       Well, the original idea was to carefully aim and power cycle it to avoid mishaps, but ... hmmm. "Nice private space yacht you got there mate. Be a shape if anything ... happened to it."   

       Could finance the whole program.
a1, Aug 16 2021
  

       I thought you could finance it by using the powerful microwave beam to burn the moon's surface in the shape of various corporate logos, for which you could charge big money
hippo, Aug 16 2021
  

       // finance it by using the powerful microwave beam to burn the moon's surface in the shape of various corporate logos //   

       There are a lot of possible ways to mark up the Moon - but its apparent diameter from Earth is too small for an effective billboard.
a1, Aug 16 2021
  

       Yet another reason to bring the moon closer
hippo, Aug 16 2021
  

       Do as you will, but try to stay above the Roche limit.
a1, Aug 16 2021
  

       Interesting idea for some future time. If we could magically fund this now, just build the solar panels and use the generated electricity to offset energy technologies such as nuclear and fossil fuel that generate heat, but maybe someday when all heat producing energy generation is replaced, if the earth is still too hot, I guess beaming the energy away is a theoretical solution.   

       As to covering the Sahara with solar panels to cool the earth, the desert has a fairly high albedo and reflects about 40% of the solar energy back into space. Solar panels are black and reflect very little energy back into space. Lets say they reflect 5% (probably much less) back to space and convert 20% to electricity. That means that 75% of the sunlight is being turned to heat instead of 60%. If you can beam the energy back out with 75% efficiency, then you can just break even.   

       I hit an interesting article about the side effects of turning the Sahara into a solar farm. [link]   

       Note that if you use solar panels to replace some heat producing electricity generation, you actually do come out ahead. Nuclear plants apparently have a thermal efficiency of 34-40%.   

       On the other hand, a combined cycle natural gas plant can apparently be as efficient as 60%. So, using the numbers I said before, if you put solar panels on a desert, 35% more of the incoming solar energy is absorbed and 20% is converted to electricity, giving it a thermal efficiency of 20/35 = 57%. So a best in class natural gas generator may produce less thermal pollution than a solar panel added to the desert (assuming my numbers are close). Of course that doesn't take into account the CO2 released by the generator, but it does highlight why we should pay attention to where we install the solar panels. Solar panels placed above existing asphalt roads for example would have great thermal efficiency.
scad mientist, Aug 16 2021
  

       Good article on the desert to solar farm consequences, thanks.
a1, Aug 16 2021
  

       // produce less thermal pollution than a solar panel //   

       But solar panels cannot "produce" thermal pollution (even if they might locally heat up an area that you rather wouldn't). Whether it heats up the air, ocean, desert sand - or charges your Tesla - incoming sunlight is already part of the Earth's thermal budget. And it stays until/unless it radiates back out into space as EM in some form that gets through the atmospheric window.   

       And if humanity ever taps hydrogen fusion or other kind of "limitless" power here on Earth (quantum vacuum fluctuations, anyone?), we'll be in even more hot water than we are now. All the current worries about greenhouse / global warming will seem trivial in comparison.
a1, Aug 16 2021
  

       [Frankx]; //over- hype/under-deliver//
There's a lot of that these days. One that sticks in my mind is super-sonic air travel. So many companies have popped up with promises & pretty graphics, never to be seen again.
neutrinos_shadow, Aug 16 2021
  

       //Do as you will, but try to stay above the Roche limit.// I don't think extra, extra high tides would be pleasant. Or earthquakes.
Voice, Aug 17 2021
  

       [a1] Except that total solar energy gain of the earth is 1.5 ZWh/year.   

       Total human energy use is 160 PWh/year. Incoming solar outweighs that by a factor of 10^4. So if the reduction in greenhouse gasses allows .01% more of incoming solar energy to escape, it's balanced.   

       And much of that current energy use is, for human time scales, producing energy just as much as any sort of matter conversion type generation would.
MechE, Aug 17 2021
  

       [scad] Sand reflects about 40% of incoming energy back outwards, at surface level. But some of that will be absorbed by the atmosphere on the way back out (yes, what hits ground is heavily weighted to wavelengths the atmosphere doesn't absorb well, but "well" is not the same as "at all". I can't actually find numbers for net energy balance of sand, round trip.
MechE, Aug 17 2021
  

       // Total human energy use is 160 PWh year //   

       For now. You just wait until everybody has a Mr. Fusion in their house and car.   

       If you want a good conspiracy theory, imagine TPTB already have this capability, and are withholding it from the masses. Why? Because of what would happen if "they" let every person on the planet have a few free gigawatts to play with. We'll be the frogs in the pot of boiling water, turning up the burner on our own.
a1, Aug 17 2021
  

       // Sand reflects about 40% of incoming energy back outwards, at surface level. But some of that will be absorbed by the atmosphere on the way back out //   

       I was under the impression that the definition of albedo was the fraction of of solar energy reflected back to space, so the 0.4 number quoted for deserts already accounted for atmospheric absorption. However I can't seem to find a link that says that definitively. Wikipedia attributed the 0.4 number to a 1983 paper, but I could only find the abstract online. The article I linked to links to a study. In the study they say a 15% efficient panel will decrease the albedo of the Sahara dessert and that a 30% efficient panel will not affect albedo, with a footnote (30) saying "Materials and methods are available as supplementary materials." I found a link on that page to the supplementary materials [link], and it appears that they are using calculations similar to mine (top of page 3) except they say the solar panel reflects 10% of the light compared to my guess of 5%, and they assume efficiency of 15%, not 20%. They use 0.4 for Sahara albedo. However they don't specify whether or not these number take into account atmospheric absorption.
scad mientist, Aug 17 2021
  

       // But solar panels cannot "produce" thermal pollution // Sorry I was using the term "produce" loosely. If you compare the air/ground temperature in the dessert before installing solar panels, to the temperature after installing them, and it shows an increase, I am saying that increase in temperature was produced by the panel. A nuclear reactor sitting somewhere will actually produce heat. Both solar and nuclear will also produce electricity that will in most cases also be turned into heat elsewhere, but I'm not counting that as heat because as discussed in this idea, maybe we're beaming that energy off-planet, or as I discussed, offsetting electricity produced some other way.
scad mientist, Aug 17 2021
  

       How much pollution to make a solar panel these days?   

       If you're going to beam the energy away, use it to lift a satellite or something to orbit.   

       Or target some space junk with it.
RayfordSteele, Aug 17 2021
  

       // target some space junk with it //   

       Someone already suggested billionaire "accidentally" bumping off some spoiled Bozos personal space yacht - would that count?   

       For now I'm only suggesting this to re-radiate incoming solar energy to balance the atmosphere's thermal budget. If/when non-fossil, non-solar power sources account for enough of that thermal load, we can look at that.   

       I wouldn't worry about it until the lizard people let us have access to that technology though. Which they probably won't, unless we can show how we would solve little problems like this one.
a1, Aug 17 2021
  

       //imagine TPTB already have this capability, and are withholding it from the masses. Why? Because of what would happen if "they" let every person on the planet have a few free gigawatts to play with//   

       With enormous amounts of cheap energy comes the ability to build all sorts of thing that can reduce the total energy in our atmosphere and water. We could bake calcium and dump it into the water. We could paint all the roofs white. We could build a giant refrigerator for the ocean and send the heat upwards. We could stop burning oil for energy. We could extract CO2 from the atmosphere, break it apart into C and O and use the C to make cool space ships and swords.
Voice, Aug 17 2021
  

       Ever the optimist.
a1, Aug 17 2021
  

       //the air/ground temperature in the dessert before installing solar panels//   

       By the time you have finished the installation, your pudding will be cold, so it's a moot point anyway.
pocmloc, Aug 18 2021
  
      
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