111–120 of 137 posts
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You're off by about an order of magnitude there. The human body uses ~10MJ per day which is ~100W. This not only includes radiation losses but also convection and evaporation.
I am not. As I said this outgoing radiation is mostly balanced by incoming radiation. But it does illustrate the theoretical maximal capacity for radiative cooling, when stuck naked in interstellar space.
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Portable air conditioners are bad. All the hot components are in the room with you and it takes air from the room and blows it outside. Technology Connections has a good video about this: https://youtu.be/_-mBeYC2KGc?t=355
That's true of the ones that have only one hose to the outside. Those with two hoses are much better, especially if you insulate the hoses: https://news.ycombinator.com/item?id=24023979
An idea I was told about relatively recently that kind of blew my mind: - Space is cold (~3K) - The atmosphere is really quite transparent in the wavelegth 8-13 μm - If one can construct an optical filter (e.g. grating) which is highly reflective except for a bandpass at 8-13μm, it is possible to reflect most solar energy but still "see" the cold of deep space - It's possible to build a box with sufficiently low ther…
If I understand correctly, this isn't quite right. If it's in the shade, then whatever is shading it is radiating more heat onto it (because the shading object is hotter than space) and possibly reflecting its own radiated heat back. If it's under a clear sky then it's receiving less heat, so what it loses by radiating has more effect. Does that sound right?
An idea I was told about relatively recently that kind of blew my mind: - Space is cold (~3K) - The atmosphere is really quite transparent in the wavelegth 8-13 μm - If one can construct an optical filter (e.g. grating) which is highly reflective except for a bandpass at 8-13μm, it is possible to reflect most solar energy but still "see" the cold of deep space - It's possible to build a box with sufficiently low ther…
This feels like tricking Maxwell's daemon.
All this system is doing is providing a readily accessible "dark corner".
The author doesn't really explain it well because his explanation breaks some physics laws, so it doesn't "suck" your body heat away. Instead, it lowers the amount of heat radiating onto you from your environment by replacing hot walls with cold walls, which will lower the temperature of your skin. It's efficient specifically because it doesn't cool by convection (like every other cooler). This allows the designers t…
Thermal radiative coupling is bidirectional. Sucking away is appropriate and important as you do actually have to displace the energy somewhere. It works the same way when sucking air. A simpler non-radiating equivalent would be a reflective foil, or a polished metal - you could enclose yourself within it, get no radiating heat from environment, yet it wouldn't help cool you, because your radiation has no place to go…
This is a good analogy because (being pedantic) you can't suck air either. You can only make a region of lower pressure, into which the higher-pressure air pushes itself.
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I still feel like "sucking" is misleading because it doesn't change how much heat you radiate, only how much heat is radiated on to you. A better analogy would be like a curtains for heat.
I think that the analogy is used because people underestimate how much heat is lost to radiation. Human body radiates energy away at about 800W. It's not noticeable, because normally it is balanced by a similar flux of thermal radiation coming in from the environment. If the apparent radiative temperature of the environment is reduced it can indeed feel like it is sucking heat out.
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> New buildings are rather efficient in US. The problem is old buildings that use 5X the energy for HVAC. I was describing my home renovation project to someone who lives in Brazil. I don't how true this is for that entire country, but she said that it's very uncommon for Brazilian homes to have insulation.
This is the same for many homes in Spain and Portugal too. For most of the year it's warm enough outside and you can be comfortable without AC (especially if you are in an older building with thick stone walls), but then for the one or two months of winter it is so cold inside, even though it may only get down to 10C (50F) outside. Most houses rely on resistive electric heaters (if they have any) which end up being v…
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This event took place 15 times in the last century. How can this be yearly?
I agree it's hardly a yearly event, but this is definitely the even he is referring to. Theoretically yearly would be a better term.
The whole comment is hyperbole.
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Here's something I don't understand though, if we put a blackbody in deep space, would it eventually be cooled/heated to the temperature of the cosmic background radiation? The space around us is filled with photon coming from Sun at ~5500K, but it doesn't heat anything to anywhere near that. So given the current density of cosmic background radiation at 411 photon cm -3, would it be sufficient to place a blackbody t…
> if we put a blackbody in deep space, would it eventually be cooled/heated to the temperature of the cosmic background radiation? No. As you note, in space in our vicinity there is a hot object, the Sun, in the sky, so the "temperature" of space in our vicinity is not really the 3K of the CMB, because an object placed in space in our vicinity will not come to thermal equilibrium at that temperature, but at some high…
I still don't see if this answer my question, assuming CMB is the only source, but the photon density has diminished significantly, would there be enough CMB photons to keep the object from going lower than 3K from emitting blackbody radiation?
The gist of my question is: Will there ever be a time where there are not enough CMB to heat the blackbody? Just like solar photons at 5500K, there are not enough of them to heat stuff at Earth to the same temp.
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This is the same for many homes in Spain and Portugal too. For most of the year it's warm enough outside and you can be comfortable without AC (especially if you are in an older building with thick stone walls), but then for the one or two months of winter it is so cold inside, even though it may only get down to 10C (50F) outside. Most houses rely on resistive electric heaters (if they have any) which end up being v…
Completely true, many old homes in Madrid are freezing in the winter. New homes are more regulated in efficiency, however. I lived in a new A-rated flat with sophisticated central heating and thermosolar, paying 500€/mo for the actual home and about 10€/mo for heating.