Why doesnt mankind "collect" thermal energy (there has to be some way: thermal couplings, detour over chemical energy, whatever) and after it has been concentrated at one point, turn it to electrical energy (like steam turbines do) or at least radiate it to space?
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This is due to thermodynamics, the three laws of which can be summarized as 1) You can't win; 2) You can't even break even; 3) You can't leave the game.
The crucial point here is that heat engines don't actually work on heat, they work on temperature differences. So you can't really "collect" heat and turn it into other forms of energy, because you need a colder place to transfer the heat to in order to convert the heat to say electricity. Which is why power plants are usually situated by oceans, lakes, or rivers, in order to use the water as the cold side of the generator. (And ones that aren't have large cooling towers, in order to use the air.)
When you do move heat around, say with the heat pumps used for home heating and cooling, you're always using some extra energy to "pump" the heat from one place or another. If you're heating, you move some heat from the ground outside to your house, but the net result is that the system of ground+house gets a bit warmer, because the electricity used for the pump becomes heat.
WRT sending the heat back into space, that's actually the cause of global warming. Atmospheric CO2 acts as an insulating blanket, preventing some of the sun's heat from being radiated back out into space. By increasing the amount of CO2 in the atmosphere, we've increased the thickness of the blanket, so the Earth gets warmer.
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So yours and Michaels first point is "collecting" heat does not work. Thats what I imagined :) As you said TEG´s work on temperature flux. But would it not be maybe possible via a reversible endothermic/exothermic chemical reaction? Maybe something we have in masses like water to hydrogen and back for example? – Martin Eckleben Jan 09 '20 at 19:39
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There is a way of converting water, not to hydrogen but to hydrocarbons, which are compounds of hydrogen.Plants can do it. The hydrocarbons can indeed be converted back to water, which happens when we burn them. Unfortunately this also produces carbon dioxide. Plants use sunlight to make hydrogen compounds in a process called photosynthesis. – Michael Walsby Jan 09 '20 at 22:23
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3@MartinEckleben: The good news is: we have a device which extracts CO2 from the atmosphere, and another which combines it with hydrogen to form synthetic fuel. The bad news is that it costs around $600 per ton of CO2 processed, which is orders of magnitude more expensive than planting a few hundred trees. See https://www.technologyreview.com/s/612928/one-mans-two-decade-quest-to-suck-greenhouse-gas-out-of-the-sky/ – Eric Lippert Jan 10 '20 at 01:11
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4@Martin Eckleben: Chemical reactions are subject to the same thermodynamic laws. (It's just more complicated to explain.) Endothermic chemical reactions have to be driven by some energy source. They might, as with photosynthesis using sunlight to turn CO2 & H2O into hydrocarbons & carbohydrates, store energy in the end products of the reaction. But when you go to extract that energy, say by burning firewood, you never get back 100% of the energy that went in. – jamesqf Jan 10 '20 at 05:18
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1@EricLipper Well, yeah, the main problem is that there's just so incredibly tiny amounts CO2 in the atmosphere to begin with. Trees suck at it too (other plants do much better), but they mostly reproduce on their own. There have been other proposals, like fitting vast algae tanks to e.g. coal power plants that consume the carbon dioxide emitted from the plant and sunlight, and produce fuel. Of course, taken as a whole this is far less efficient than something like a concentrated solar power, but at least it can be reasonably well fitted to existing plants and doesn't need continuous sunlight. – Luaan Jan 10 '20 at 08:00
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18@EricLippert When you said that we have a device which extracts CO2 from the atmosphere and makes fuel with it, my first thought is that the device you were referring to was a tree. - haha – reirab Jan 10 '20 at 09:00
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Just a tiny addition to the good answer: actually, that's what the atmosphere does with the excess heat energy - it makes more mechanical energy (wind) and more extreme weather in general, then radiates it back to space. (The atmosphere acts as a number of interconnected heat engines, the hot source being the earth surface and the cold one generally another area of the surface or the space). – fraxinus Jan 10 '20 at 12:43
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This answer is mostly right. The way you use the word "heat", though, gives the impression that heat is a state function, which isn't correct. Feel free to replace it with "internal energy" or "thermal energy". Objects and systems don't have heat. – Eric Duminil Jan 10 '20 at 14:24
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@Eric Duminil: I'm sure someone (though I'm not that person :-) could write a better and/or more technically accurate answer. I'm just trying to keep to high school level. Feel free to edit if you think it can be improved. – jamesqf Jan 10 '20 at 20:13
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"So you can't really "collect" heat and turn it into other forms of energy, because you need a colder place to transfer the heat to". Let's leave electricity (which you discussed in the rest of that sentence) aside for a second. This general sentence is unequivocally true, but is not an argument against the OP'S suggestion -- on the contrary, it is supporting the idea: The OP wants to radiate excess heat to space which is is 2.75K cold. – Peter - Reinstate Monica Jan 12 '20 at 22:31
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@EricLippert The good news is that you don't have to use the CO2 from the general atmosphere -- it would be a big step towards sustainability to turn CO2 from fossil or plant fuel exhaust into fuel again. CO2 is much more concentrated there, obviously. Trying to extract it from the atmosphere just needs to fight so much entropy. We could as well start extracting gold from the oceans... – Peter - Reinstate Monica Jan 12 '20 at 22:34
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@Peter - Reinstate Monica: Well, sure. And what's the (conceptually) simplest way to radiate more heat to space? Reduce the amount of CO2 in the atmosphere :-) (Though covering large amounts of the planet with reflective paint would also work.) Converting CO2 back into fuel is a non-starter, because to do that conversion, you need at least the same amount of energy that you got from burning the fuel in the first place. (In fact, you need more: see Rule #2.) So unless you use some external energy source to drive the conversion, as with photosynthesis, you're SOL. – jamesqf Jan 13 '20 at 00:57
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@jamesqf Obviously you would use solar energy for the process, otherwise it's nonsense, correct. But I think solar, and in general, renewable energy will be abundant in the future and during peaks it would be good to have something to do with the excess capacity. Synthetic fuel is extremely charming because in contrast to electricity the global infrastructure for transport, storage and use is already in place -- it's a drop-in replacement for fossil fuel. (Whether that's a good thing from a more progressive standpoint is another question.) But it hinges on copious solar energy. – Peter - Reinstate Monica Jan 13 '20 at 06:45
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@Peter - Reinstate Monica: It's obvious to you and me, but apparently not to a lot of people, as for instance when suggesting using the CO2 from a fossil fuel plant to grow algae for fuel. The problem is not getting CO2 out of the atmosphere - in fact, too high a CO2 concentration can be toxic to many plants: https://biology.stackexchange.com/questions/71312/can-plants-suffer-from-co2-poisoning – jamesqf Jan 14 '20 at 17:37
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@jamesqf I am not familiar with the algae proposal but if they grow under natural light it's a biological form of synthetic fuel production, if that oxymoron makes any sense ;-), powered by solar energy. – Peter - Reinstate Monica Jan 15 '20 at 07:16
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@Peter - Reinstate Monica: Sure, the problem is first one of scale - you'd need a LOT of area to consume the CO2 generated by a power plant, and second that the power plant is not needed at all for biofuel production. – jamesqf Jan 15 '20 at 20:33
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Collecting thermal energy is really hard. As others have said, things like heat pumps exist for moving heat around, but the laws of thermodynamics (which are fairly fundemantal in physics) require that moving heat around will always generate more heat.
Now, the amount of extra heat generated can be less than the amount of heat that's being moved - so if we had a way to build a heat pump that moved heat out of the earth system and into space, then it might be worthwhile. There are three problems with that,
We don't know how to do that. A system of pipes going into orbit probably isn't practical!
Having got the heat into orbit, getting it away from the equipment and into space is hard. You've probably learned about heat moving by conduction, convection, and radiation? The first two are not available in space, because there's no material to conduct into and no atmosphere to convect. So radiation is all that's left. On top of the hypothetical heat pump to orbit, we'd need a massive radiator system in orbit as well.
The scale at which it would have to be done is gigantic. Even if we knew how to do it, we might not be able to do it.
So it probably isn't theoretically impossible - at least so far as we know - but it's not something that's possible at the moment.
It's also worth noting that like all ideas that are about getting rid of heat rather than reducing the greenhouse effect, it doesn't actually solve the problem - it just mitigates the symptom. If we carried on emitting greenhouse gases while using this system then we'd have to keep using this system, more and more, for ever.
(theoreticians: am I missing something? Is there actually a theoretical, rather than practical, blocker here?)
Semidiurnal Simon
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1Thank you for the explanation it made a lot clearer and I agree on every point made. I actually thought (very shortly :) ) about every point you made before I asked. Just for the lulz: for 2. I thought about heating stone to magma with as hot as one can heat it and then discarding it to space - but immediately threw the idea as sending millions of tons of stone to space as payload is probably the heart attack of every space project financial officer :D – Martin Eckleben Jan 10 '20 at 15:18
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1: well some people do know how to directly radiate heat into space, though it hasn't gone mainstream yet, https://www.youtube.com/watch?v=7a5NyUITbyk or https://advances.sciencemag.org/content/5/10/eaat9480.full and similar 'radiative cooling' techniques exist. Though there isn't a heat pump involved, as you don't need one. This can reduce the greenhouse as it means rather than using power for cooling the systems are passive, so less fossil based energy is required. (there probably is something to transfer the heat to the radiative cooler, but there isn't a tube all the way to space) – Pete Kirkham Jan 10 '20 at 16:11
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3@Martin Eckleben The SpaceX BFR is projected to lift 150 tons of payload into low Earth orbit. Let's assume all of the energy of the magma is radiated into space (some will be reabsorbed by Earth). 150 tons of rock near its melting point (1200°C) has ~90 gigajoules of thermal energy. Incident solar energy on the Earth is ~180,000,000 gigajoules each second! Your hot-rock rocket would be a drop of water in a rainstorm. Removing CO2 from the atmosphere, or not adding it in the first place, reduces heating over many decades by altering the amount of solar energy trapped by the Earth. – WaterMolecule Jan 10 '20 at 17:03
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2WRT radiating heat to space, it might be instructive to look at the cooling system for the ISS: https://en.wikipedia.org/wiki/External_Active_Thermal_Control_System Not exactly a small project, and that's for 3-6 people and equipment, with no fossil fuel burning involved. And a mostly white habitat module, so it probably doesn't absorb much solar radiation. – jamesqf Jan 10 '20 at 20:20
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4@MartinEckleben: you would do well to start habitually computing orders of magnitude as in WaterMolecule's example; this is a valuable skill that many middle school students, and indeed many adults lack. People are not accustomed to thinking about heat on a planetary scale. The amount of sunlight falling on a single square meter of land at the equator at noon is an energy equivalent of 14 hundred watt light bulbs. Multiply that by all the square meters being illuminated, and that is rather a lot. – Eric Lippert Jan 10 '20 at 20:38
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@PeteKirkham interesting (and presumably the same as in Rupert Morrish's answer?). Thanks. – Semidiurnal Simon Jan 10 '20 at 21:07
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@EricLippert: 1360W/m² is the solar constant, measured above the clouds. Global irradiance is never this high on Earth, even at the Equator. You're right though, it's still a lot of power over the whole Earth : "1000W/m² * pi * R_earth²", which is around 130 Petawatt. – Eric Duminil Jan 10 '20 at 22:06
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6Futurama (https://theinfosphere.org/Transcript:Crimes_of_the_Hot) covered this scenario: "Fortunately, our handsomest politicians came up with a cheap, last-minute way to combat global warming. Ever since 2063 we simply drop a giant ice cube into the ocean every now and then...Of course, since the greenhouse gases are still building up, it takes more and more ice each time. Thus solving the problem once and for all." :) – CCJ Jan 10 '20 at 22:53
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"On top of the hypothetical heat pump to orbit, we'd need a massive radiator system in orbit as well." - Not necessarily; in theory at least if you had a way of upconverting thermal energy into, say light then you could radiate the bulk of it out into space from the ground (given suitable atmospheric conditions). Maybe physics disallows this, however black-body radiation can produce visible wavelengths, so perhaps not. – aroth Jan 11 '20 at 11:02
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@aroth: The needed temperature (and temperature difference) would be so high that the maximum efficiency would be very close to 0%, according to Carnot. As far as I know, the best option to radiate energy away from the Earth is to fire a laser to space. The most powerful continous laser can radiate 1MW, which is strictly peanuts compared to the 130PW the Earth gets from the sun. – Eric Duminil Jan 11 '20 at 14:17
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@aroth I am fairly sure that, even if we had a sufficiently low-energy way to do it, we would not want to be making significant patches of our planet's surface hot enough to radiate strongly in visible wavelengths. I mean, we'd get the most visible light at roughly the surface temperature of the sun... – Semidiurnal Simon Jan 11 '20 at 15:22
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FWIW, you (Eric, Aroth) have basically just created what David Brin did in his book, Sundiver. A refrigeration laser. Which is not scientifically feasible. – Draco18s no longer trusts SE Jan 12 '20 at 06:49
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Like passive radiative cooling?
The new materials reflect a broad spectrum of light, in much the same way as mirrors or white paint do. In the crucial 8–13-µm part of the infrared spectrum, however, they strongly absorb and then emit radiation. When the materials point at the sky, the infrared rays can pass straight through the atmosphere and into space. That effectively links the materials to an inexhaustible heat sink, into which they can keep dumping heat without it coming back. As a result, they can radiate away enough heat to consistently stay a few degrees cooler than surrounding air; research suggests that temperature differences could exceed 10°C in hot, dry places.
Rupert Morrish
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1That pretty neatly avoids the absorption due to carbon dioxide, too. Though I suspect that even with a concentrated effort, you'll never get anywhere close to the amount of radiation needed for any appreciable global cooling, not to mention the likely disruption of the weather patterns if you could. :P – Luaan Jan 10 '20 at 08:04
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5Key to this is that the sun is small in the sky, while the radiators emit over a hemisphere. The absolute heat flux is rather small, so they're mainly proposed for keeping solar panels cool (efficiency drops with increasing temperature) or to reduce the power consumption of refrigeration and air conditioning, i.e. very much local. It would be interesting to model whether this can be scaled up to the city level, to reduce urban heat island effects (also @Luaan) – Chris H Jan 10 '20 at 08:38
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3@ChrisH given that many urban areas don't even make use of the low hanging fruit which is trees, I don't see wide scale adoption of construction standards utilizing this material any time soon. – Turksarama Jan 10 '20 at 14:05
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@Turksarama of course you're right, but passive radiators can go on rooftops (of course so could solar panels, and adoption of those is pretty poor too). – Chris H Jan 10 '20 at 15:13
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I fail to see how a panel which is cooler than the environment will help cool the planet by radiating more than its surroundings into space. The opposite is needed: One which is hotter because one has concentrated dispersed heat in it, by which means ever. – Peter - Reinstate Monica Jan 12 '20 at 21:31
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As others have pointed out, if you use the energy, it turns right back into heat.
Radiating it back to space is at least theoretically possible. But there is a problem with scale.
The Earth receives about a 100 petawatts of energy from the sun. And it radiates almost exactly the same amount back out.
Everything humanity do with energy is about 0.02 petawatts. Even if we radiated all that energy into space it still wouldn't matter much.
Still, it is barely possible to do something like this, by bouncing the sun's light off the Earth surface as it arrives. While mirrors would be ideal for this job, they are expensive. Fortunately, any really bright white object does almost as good a job. Look up Albedo for more information.
It would take a very large area of land to make any difference. And land is expensive. And we would need a lot of white stuff to do the job. Even if it is cheap by the square meter (sq feet), it will be expensive by the million square kilometer (million sq mile).
So, this is on the list of things we could do if we only found the money for it.
Stig Hemmer
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This! The equilibrum you mention in your third paragraph will always hold - what we suffer from is that the the state of our athmosphere controls to what (ground) temperatures the radiation equilibrum corresponds. Adding greenhouse gases leverages a lot more temperature change than any "industrial" form of active radiation can counter – Hagen von Eitzen Jan 10 '20 at 20:59
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There is a severe misconception here: reflective surfaces do not cool via albedo. It is too late to rise albedo once solar radiation has reached the ground. It is trapped, mirror or not. They can help reducing energy consumption of air conditioning and avoid the heat island effect of urban areas (unproven). But better use the area for solar panels (or green roofs) than mirrors or white paint. – Jan 11 '20 at 21:19
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@EricDuminil (I compressed my reamrks) This is how greehouse gases work: en.wikipedia.org/wiki/Greenhouse_gas. For albedo to be effective (like in ice ages and ignoring other conditions), atmopsheric greenhouse gases must be low. You can't paint a continent white or cover it with mirrors. Covering all roofs with reflected surfaces would lower energy demand in hot areas, but could raise the overall atmopsheric temperature and have the contrary effect. web.stanford.edu/group/efmh/jacobson/Articles/Others/…. Albedo alone is not a cure, in our case of global warming. – Jan 12 '20 at 20:54
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@ebv: I give up. At least one thing you wrote is correct : "there is a severe misconception here". – Eric Duminil Jan 12 '20 at 21:05
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@EricDuminil: sorry, maybe i should have written "does not cool the atmosphere but prevent heating of the ground directly underneath". Unfortinately, there is more to it than just raising albedo, radiation must be able to esacpe to space. Peace :-) – Jan 12 '20 at 21:18
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Just to hammer it down: the amount of energy transferred to Earth by the sun every single day is colossal. In comparison, the heat generated by all human activities is negligible.
For instance, burning every single tree on Earth would release less than 1% of the heat the sun sends our way every day!
Taking human activities as an intuitive reference point is utterly misleading. Heat exchanges in the atmosphere involve massive amounts of energy that are far beyond anything human industrial capabilities can handle.
Greenhouse gases act like a kind of space blanket: a very thin and light insulating layer that can still trap quite a lot of heat.
Though the excess of gases we release only capture a tiny extra fraction of it (well below 1%), the energy the sun sends our way is so tremendous that this little extra heat is enough to mess up our climate badly.
Our problem is not to make any use of this extra solar energy. We could move around a minute fraction of it to our advantage (for instance concentrating it inside a house in winter and pumping it out in summer), but as a global system our habitat (the surface of the Earth and a bit of breathable atmosphere above) is receiving far, far too much heat, on a scale that dwarves all human uses of energy.
Our problem is to get rid of it, that means sending it some place where it can't wreak havoc with our climate. At this scale, that leaves only outer space or deep soil as potential dumps.
So far our atmosphere did quite a nice job of sending solar energy back into space, but we fouled it up. What a bummer...
We sure could think of just moving all that heat out of the way ourselves. But alas, as the 2nd law of thermodynamics and its dreaded entropy states, any kind of mechanical work requires additional energy and generates more heat. Even a refrigerator adds more heat into the environment than it pumps out of its internal compartment.
All we can hope to achieve is transfer heat from place to place. At the cost of more energy, raw materials, time, industrial capacity and, naturally, money.
That is dictated by fundamental laws of physics that no amount of wishful thinking can sway.
Painting deserts white or drilling deep underground geothermal wells or putting fancy space radiators into orbit are just sci-fi fantasies.
We simply don't even come close to having the energy, materials and technology to implement any of these on a scale that would solve the problem.
Picture yourself being wrapped in a space blanket, getting a bit too hot for comfort. Trying to move the hot air away from your body will only make you warmer, while removing the blanket might cool you down fast, with a lot less efforts.
Indeed, the only thing we could hope to act upon is the blanket itself.
Wisdom would dictate to start by not making it any thicker. Stop burning fossil fuels and breeding farting cattle for a couple of centuries, waiting for all the junk we spewed so far to dissipate.
There is also the crackpot scientist alternative: spewing more junk (like sulphur) into the atmosphere to try and mitigate the greenhouse effect. Frankly I hope I'll be dead before I see the idiots try.
kuroi neko
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1We're in for a rough ride, but maybe we can still strive to go down with style? – kuroi neko Jan 11 '20 at 21:57
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@EricDuminil Correct except for the disdain towards geo-engineering. We are geo-engineering already and should better get it right real soon. – Peter - Reinstate Monica Jan 12 '20 at 21:36
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Also, I don't think the answer is correct. 1. "All we can hope to achieve is transfer heat from place to place." This is exactly what the OP suggests, yet you seem to dismiss it. (2) "Even a refrigerator adds more heat into the environment than it pumps out of its internal compartment." Well, if the environment is space this is again exactly the OP's suggestion. We couldn't care less how much we radiate out as long as we cool our planet. Earth is by no means a closed system, and the OP proposes to exploit that fact. – Peter - Reinstate Monica Jan 12 '20 at 22:25
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I dismiss the wishful thinking about miraculous science and technology. There is not a single practical solution in sight to get rid of the heat, nor is there any practical replacement for oil, nor an infinite supply of raw materials. As for geo-engineering, it's a particularly dangerous pipe dream that allows to continue business as usual, basking in the illusion that technology we will allow us to tweak atmospheric composition at the turn a knob. A desperate solution that would most likely cause problems we can't even imagine yet. – kuroi neko Jan 13 '20 at 16:03
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@kuroineko I just would like to remark that business as usual is what's de facto happening. All past goals have been missed by a wide margin, and there is little indication of the fundamental global change which would need to happen quite quickly. A solution which "allows to continue business as usual" is dearly needed; the industrial, agricultural and transport infrastructure is inert and the material oriented lifestyle is attractive to both the ones who have it as well as to the ones who don't, yet. Hoping against all facts for an unprecedented global change is wishful thinking. – Peter - Reinstate Monica Jan 16 '20 at 07:57
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I am quite convinced no change will come and we will only try to react far too late, but geo-engineering would only make things worse. We can't even implement a decent pest control (another well known plague in Australia, for instance). It takes all the human hubris to think we could control our atmosphere. We are talking decades of inertia and highly unstable phenomena here. No amount of computing power could predict the true consequences of spewing sulphur or whatnot into the air. We might eventually do it as a desperate measure, but only blind luck would have that help us. – kuroi neko Jan 16 '20 at 16:50
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@kuroineko Your objections simply do not apply to stratospheric aerosol injection: The effects are well known from volcanic eruptions and are, like those, entirely and short-term reversible by simply stopping the injections. The oceans will still suffer from acidification but at least the level wouldn't rise as much, and the tropics would stay inhabitable. – Peter - Reinstate Monica Jan 16 '20 at 18:42
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Sure. Why don't we start releasing volcanic ash right away? Not profitable enough maybe? – kuroi neko Jan 17 '20 at 12:40
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Your idea: concentrating the heat, using the energy for something useful and then dissipating it into space could actually be accomplished by a space mirror.
There is a special orbit called L1 that's in between the Earth and the Sun and is just the right distance so that something in that orbit will track and orbit in sync with the Earth. It could both shade the Earth a bit and be a massive power plant.
You might check out this article from live science on space mirrors. It goes over the pros and cons of this idea.
Wikipedia also has an entry on this idea.
David Elm
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Hmm even this orbit is probably full of junk? Also gathering and sending up an amount of materials as needed for such a giant space mirror would probably not work due to scale? But very interesting idea :) – Martin Eckleben Jan 11 '20 at 09:55
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1No, Earth-Sun L1 is not full of junk yet. But the mirror or whatever shady device or devices one puts there must be gigantic to have a noticeable shadow on earth. Mass and technology far beyond our capabilities. We have already said it needs station keeping and steering because L1 is unstable (things fall out if not kept in place). Such a thing would be pushed away by the solar wind and solar radiation anyway. It won't work. The only thing that works is cutting down greenhouse gas emissions. – Jan 11 '20 at 20:55
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1It currently costs in the range of $1000 a pound to launch something into space. Given that you would need millions of square miles of mirror, a space mirror sounds very expensive. Putting the mirrors on the ground is a bit cheaper. – user4574 Jan 11 '20 at 22:53
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@user4574 The mirror would only(?!?) need to be about 600,000 sq miles (155,399,287 hectare) in order to provide a 1% reduction in solar energy reaching Earth. That is about the same size as the US state of Alaska. Or about the size of France, Italy, Spain, and Portugal combined, for our European members. – krb Jan 12 '20 at 04:47
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@krb The cross section of the earth that is facing the sun at any given moment is 49 million square miles. Taking into account that the mirrors will change angle as the earth rotates, and also that its night half the time you probably need more like 1.6 million square miles to reflect one percent of the light into space. – user4574 Jan 12 '20 at 05:56
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@user4574 The 600,000 square mile number comes from Lowell Wood of Lawrence-Livermore National Laboratory, who spent 10+ years studying this concept. Argue the math with him if you disagree. https://www.livescience.com/22202-space-mirrors-global-warming.html – krb Jan 12 '20 at 06:17
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@krb The math is pretty straight forward. The earth has a radius of 3958 miles. Therefore its cross section presented to the sun is pi * 3895^2 = 49,215,449 square miles. 1% of that is 492,154 square miles. The cross section of the mirror visible to the sun is proportional to the sine of the angle of the earths rotation. Therefore the mirror size must be increased by a factor of (pi /2) and then doubled again to account for night time. 492,154 * pi / 2 * 2 = 1,546,148 square miles. What is not taken into account is that different areas of the earth absorb different amounts of sunlight. – user4574 Jan 14 '20 at 04:27
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Of all the types of energy there is, heat is the "waste" of the energies. See, energy is only useful if organized, and heat is the least organized of them all. I.e, the energy must be able to push car wheel in that particular direction, not to every direction at random, like molecules of a hot gas would.
In order to reorganize the energy, according to the laws of physics (the second law of thermodynamics, in particular), you must have a cold place to where the heat could flow. Most (all?) heat engines relies on the environment being colder than the heat source in order for it to work: a car or a airplane engine only works because the ambient temperature is smaller than the temperature inside the engine, and a coal plant relies on the ambient being cooler than the steam turning the turbines.
By being immersed in a hot ambient, you can't use that very ambient as the cold side in order to extract energy from it, so you can't possibly build a device that is powered solely on the heat of the environment it is immersed in.
That is one half of the problem. The other is: if we could extract energy from the environmental temperature, we would certainly not radiate it away to space: we would use it as electricity, power our cars, planes, and would never need fossil fuels again.
lvella
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Fossil fuel based energy is very inefficient; the amount of heat wasted exceeds that which gets used. In addition global heating from enhanced greenhouse effect is adding heat at rates estimated at around 100 times that from total waste heat. In order to get zero global heating (whilst continuing to burn fossil fuels) by collecting heat and sending it to space requires collecting in excess of a hundred times more energy than human economies are currently using as well as developing and building and operating the technology to send all that heat somewhere else, ie to space.
That is effectively impossible, but developing a means to utilise low grade heat for energy generation could significantly aid the displacement fossil fuel burning, that would reduce global heating by that factor of a hundred. Technology for turning low grade heat into higher grade energy exists - eg Stirling engines - but they are not cost effective. Some other tantalising possibilities do exist, such as Nantennas aka Optical Rectennas.
However, effective low emissions energy options that reduce fossil fuel use and the enhanced greenhouse warming and that are cost effective already exist; I think those should be the primary focus of our current response to global warming.
Ken Fabian
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The problem with turning many forms of waste heat into other forms of energy by conventional (heat engine) methods is that significant temperature differences are needed - and that the temperatures making up the differences are counted from absolute zero (0 Kelvin). Only if that ratio is large, you get an efficient conversion. For example, if you look at the waste heat from a computer CPU, it might be at 340 Kelvin, 40 degrees Kelvin (Kelvins are absolute-zero-referenced centigrades for you Fahrenheit types) over an ambient temperature of, 300 Kelvin. You can't force the heat to build up to much higher levels since you will damage the CPU. The efficiency of a heat engine trying to make anything useful of this situation would be 40K/340K*100% = drumroll ... 11%. Hardly WORTH trying to recover.
This is because Carnot, one of the patron bastards of physics, made that the law in 1824, and no one repealed it yet.
This does not apply to heat radiation, which isn't heat per se but heat that has already been converted into long wave infrared light, which gets converted back into heat if it hits something. It can indeed be relayed at will by lenses, mirrors and prisms made of the appropriate materials (not: run of the mill window or optical glass!). Unfortunately, creating significant heat radiation is also something that takes high temperatures....
rackandboneman
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Optical rectennas appear capable of turning radiant heat (IR) into electricity, but currently only very inefficiently. Ultrafast diodes needed to get higher efficiencies. – Ken Fabian Jul 22 '23 at 01:42
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You mean, you literally capture IR radiation with a tiny dipole and rectify it? Wow! – rackandboneman Jul 23 '23 at 17:47
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Well the most straight forward way to radiate sunlight into space is with a mirror.
But creating millions of square miles worth of mirrors sounds very expensive. Probably in the range of a 100s of Trillions of US dollars. If we got rid of all government spending we might be able to pay for it over the course of a 100 years or so.
user4574
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Yes! Increase the albedo! I quite like this answer because it's a shortcut which avoids heat, in other words, high entropy, which would need to locally be reduced again (at the expense of more energy and entropy) in order to create high temperatures. A mirror should work well because the atmosphere is transparent for the wavelengths reaching the ground which implies that they can readily leave without heating the atmosphere much. Since you seem to be politically conservative I suggest to call your idea to send the radiation right back at the border the Trump principle. – Peter - Reinstate Monica Jan 12 '20 at 22:47
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@KenFabian My wording was a little off. My intent was to say that we would have to use the entire government budget (probably forgoing nearly all other spending) for many decades to execute a project that large. – user4574 Jul 31 '23 at 00:20
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Mankind does collect natural sources of heat and turn it into useful energy: heat pumps, solar arrays, geothermal etc, but it would be impossible to reduce global warming in this way. Most of the heat collected for useful purposes is in any case given back to the atmosphere when the energy is used. If you think about it, even wind farms are collecting solar energy, because it is the sun's heat which drives the winds. When the energy is used, it is re-converted into heat. As a physics student you should know this.
Michael Walsby
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1Yes, it is well possible to use regenerative energies like solar and geothermal to reduce greenhouse gas emissions and thus global warming. It is the greenhouse gases' trapping of IR radiation that causes warming. – Jan 09 '20 at 18:06
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Oh maybe I expressed myself wrong - I dont study physics - I only had it as a subject in school :) Lets leave that factor of energy creation completely aside (which would be only even more beneficial but is not the point of my question).
Would it work to dissipate more heat into space (by any means at all) to solve global warming?
– Martin Eckleben Jan 09 '20 at 19:16 -
2All the methods yet suggested of reflecting solar energy back into space, such as painting the deserts white, are environmentally unacceptable, as well as being prohibitively expensive.. – Michael Walsby Jan 09 '20 at 19:21
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1True, but no serious source suggests that. @MartinEckleben: energy can not be created, only transformed and converted. And yes, it would be trivially possible, by reducing greenhouse gas emissions to remove the atmospheric trap. – Jan 09 '20 at 19:29
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@ebv "energy can not be created" Yup the three laws of thermodynamics I understand (at least their symptoms). OK removing the atmospheric trap would be awesome but I dont see the world being able to do that fast enough. If we could "actively" take heat (energy) away from the atmosphere and send it "actively" to space I feel humanity would maybe be more committed as no one would have to lose comfort or life quality. – Martin Eckleben Jan 09 '20 at 19:47
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Eh; what about heat tiles? Tiles that take heat and turn it into electricity?https://en.wikipedia.org/wiki/Thermoelectric_generator I wanted to build an e-bike powered by these and cross the Arizona desert into Las Vegas (just a dumb dream). But; do not these tiles fulfil the fundamental conversion required (on a much larger scale) to solve this problem? I'm not a scientist. Don't beat up on people for having a desire to save our planet. Like the planet needs that; eh?
Mr. J. ward
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Looks like the diagram/details in 2.4 note on https://www.intechopen.com/chapters/65239 show you still need a cold source/temperature gradient to create power, which matches the thermodynamics reality of jamesqpf's answer. Not knowing the device, I wouldn't think they'd power a bike in the desert (theoretically could generate very limited power between sun-facing and shade-facing sides... but would think the gradient is tiny and a solar panel would be much more efficient as it's directly using the radiation) – JeopardyTempest Jul 28 '23 at 21:35
of the plant - and interesting method to turn the carbon from the natural gas back into rocks! https://www.youtube.com/watch?v=1zDZmIDbDO0 – CrossRoads Jan 09 '20 at 17:42