Comment by habinero
18 hours ago
It's not as exciting as you think it is. "emissivity higher than 0.99 over a wide range of wavelengths" is basically code for "it's, like, super black"
The limiting factor isn't the emissivity, it's that you're having to rely on radiation as your only cooling mechanism. It's super slow and inefficient and it limits how much heat you can dissipate.
Like the other person said, you can't do any better than blackbody radiation (emissivity=1).
> and inefficient
Well acttshually, it's 100% efficient. If you put 1W in, you will get exactly one watt out, steady state. The resulting steady state temperature would be close to watts * steady state thermal resistance of the system. ;)
I don't think you could use "efficiency" here? The math would be based on thermal resistance. How do you get a percentage from that? If you have a maximum operating temperature, you end up with a maximum operating wattage. Using actual operating wattage/desired operating wattage doesn't seem right for "efficiency".
Lets assume an electrical consumption of 1 MW which turned into heat and a concommitant 3 MW which was a byproduct of acquiring 1 MW of electrical energy.
So the total heat load if 4 MW (of which 1 MW was temporarily electrical energy before it was used by the datacenter or whatever).
Let's assume a single planar radiator, with emissivity ~1 over the thermal infrared range.
Let's assume the target temperature of the radiator is 300 K (~27 deg C).
What size radiator did you need?
4 MW / (5.67 * 10 ^ -8 W / ( m ^2 K ^4 ) * 300 K ^4) = 8710 m ^2 = (94 m) ^2
so basically 100m x 100m. Thats not insanely large.
The solar panels would have to be about 3000 m ^2 = 55m x 55m
The radiator could be aluminum foil, and something amounting to a remote controlled toy car could drive around with a small roll of aluminum wire and locally weld shut small holes due to micrometeorites. the wheels are rubberized but have a magnetic rim, on the outside theres complementary steel spheres so the radiator foil is sandwiched between wheel and steel sphere. Then the wheels have traction. The radiator could easily weigh less than the solar panels, and expand to much larger areas. Better divide the entire radiator up into a few inflatable surfaces, so that you can activate a spare while a sever leak is being solved.
It may be more elegant to have rovers on both inside and outside of the radiator: the inner one can drop a heat resistant silicone rubber disc / sheet over the hole, while the outside rover could do the welding of the hole without obstruction of the hole by a stopgap measure.
> The radiator could be aluminum foil,
As I've pointed it out to you elsewhere -- how do you couple the 4MW of heat to the aluminum foil? You need to spread the power somewhat evenly over this massive surface area.
Low pressure gas doesn't convect heat well and heat doesn't conduct down the foil well.
It's just like how on Earth we can't cool datacenters by hoping that free convection will transfer heat to the outer walls.
Lets assume you truly believe the difficulty is the heat transport, then you correct me, but I never see you correct people who believe the thermal radiation step is the issue. It's a very selective form of correcting.
Lets assume you truly believe the difficulty is the heat transport to the radiator, how is it solved on earth?
2 replies →
Yes, graphene appears to offer a negligible improvement over other kinds of paints based on black carbon, e.g. Vantablack.
The research article linked above does not claim a better emissivity than Vantablack, but a resistance to higher temperatures, which is useful for high temperature sensors (used with pyrometers), but irrelevant for a satellite that will never be hotter than 100 Celsius degrees, in order to not damage the electronic equipment.