[habinero]

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).

[DoctorOetker]

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.

[mlyle]

> 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.

[DoctorOetker]

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?

[mlyle]

> 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 both. You have to spread a lot of heat very evenly over a very large surface area. This makes a big, high-mass structure.

> how is it solved on earth?

We pump fluids (including air) around to move large amounts of heat both on Earth and in space. The problem is, in space, you need to pump them much further and cover larger areas, because they only way the heat leaves the system is radiation. As a result, you end up proposing a system that is larger than the cooling tower for many nuclear power plants on Earth to move 1/5th of the energy.

The problem is, pumping fluids in space around has 3 ways it sucks compared to Earth:

1. Managing fluids in space is a pain.

2. We have to pump fluids much longer distances to cover the large area of radiators. So the systems tend to get orders of magnitude physically larger. In practice, this means we need to pump a lot more fluid, too, to keep a larger thing close to isothermal.

3. The mass of fluids and all their hardware matters more in space. Even if launch gets cheaper, this will still be true compared to Earth.

I explained this all to you 15 hours ago:

> If this wasn't a concern, you could fly a big inflated-and-then-rigidized structure and getting lots of area wouldn't be scary. But since you need to think about circulating fluids and actively conducting heat this is much less pleasant.

You may notice that the areas, etc, we come up with here to reject 70kW are similar to those of the ISS's EATCS, which rejects 70kW using white-colored radiators and ammonia loops. Despite the use of a lot of exotic and expensive techniques to reduce mass, the radiators mass about 10 tonnes-- and this doesn't count all the hardware to drive heat to them on the other end.

So, to reject 105W on Earth, I spend about 500g of mass; if I'm as efficient as EATCS, it would be about 15000g of mass.

[mlyle]

This should say "to move 1/50th the energy".

[DoctorOetker]

By saying that something is impossible to do cost-effectivey, one is implicitly claiming they have rigorously combed through the whole problem space, all possible configurations and materials, and exhaustively concluded it is not possible cost-effectively.

Imagine now instead of a pyramid, a cone. Imagine the cone is spinning along its symmetry axis. One now has a local radial pseudoforce, a fake gravitational force along the radial direction (away from the symmetry axis).

Now any fluid with a liquid-gas phase transition above the desired radiator temperature but below the intended maximum compute operating temperature (and there is a lot of room for play for fluid choice because the pressure is a free parameter) can be chosen to operate in heat-pipe fashion. Suppose you place the compute at a certain point along the outer rim of the cone, and fluid that condenses on the cone wall will flow to the circular rim at the base. the compute is inside a kind of "chimney" and the lower half of the chimney (and the compute in it) are submerged by the fluid. The fluid boils and vaporizes, and rises up the chimney and is piped to the central axis and flows out in a controlled distributed fashion. all of the pipes could be floppy aluminum foil (or mylar etc.) pipes, since they are all pressurized during normal operation.

Some of the liquid phase could be pumped up to the central axis at the base and cool the rear side of the solar panels as well. I don't see the problem. The power density of solar panel heating (and thus power density on the cone surface) are very similar and perfectly manageable with phase-transition cooling /condensing.

At some point you are just prodding until people hand you working designs on a silver platter.

[adrian_b]

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.

[nomel]

> 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".

[habinero]

Yeah, I was speaking imprecisely. I don't mean "efficiency" in the thermodynamics sense but in the "it is really slow" colloquial sense.