Passive cooling refers to "passive radiative cooling"[0]. This is a well established technique, but I have doubts on how well it will scale with the heat generated by computation.
Radiative cooling works by exploiting the fact that hot objects emit electromagnetic radiation (glow), and hot means everything above absolute zero. The glow carries away energy which cools down the object. One complication is that each glowy object is also going to be absorbing glow from other objects. While the sun, earth, and moon all emit large amounts of glow (again, heat radiation), empty space is around 2.7 Kelvin, which is very cold and has little glow. So the radiative coolers typically need to have line of sight to empty space, which allows them to emit more energy than they absorb.
This is exactly right, and an important fact is that there is a limited bandwidth for heat radiation. So essentially they need to create a giant lightbulb...
> Additionally, deep space is cold, which is accurate in that the "effective" ambient temperature is around -270°C, corresponding to the temperature of the cosmic microwave background.
There's a lot of bad information in their document too. This -270C temperature is ambient space, i.e. deep space. You may experience this when you're in the shadow of Earth or on the dark side of the moon but you're going to switch that negative sign to a positive when you're facing the sun... Which is clearly something they want to do considering that they are talking about solar power. Which means they have to deal with HEATING as well! I don't see any information about this in the document.
> he mass of radiation shielding scales linearly with the container surface area, whereas the compute per container scales with the volume
This is also a weird statement designed to be deceptive. Your radiation shielding is a shell enclosing some volume.
> Therefore the mass of shielding needed per compute unit decreases linearly with container size.
They clearly do not understand the mass volume relationship here. Density (ρ) is mass (m) divided by volume (V).
m = ρV.
Let's simplify and assume we're using a sphere since this is the most efficient, giving V = 4/3r^3. Your shield is going to be approximately constant density since you need to shield from all directions (can optimize by using other things in your system).
m ∝ ρr^3
I'm not sure what here is decreasing nor what is a linear relationship. To adjust this to a shell you just need to consider the thickness so you can do Δr = r_outer - r_inner and that doesn't take away the cubic relationship.
FWIW, i think their description for the radiation shielding is fine. Your analysis is off. If we assume the spherical case, the mass of the shielding is proportional to surface area, not the volume[0]. You might be confusing general radiation shielding and thermal shielding. Thermal shielding is easier because you can point things towards the sun, earth, and moon.
I am more concerned about heat dissipation, which should scale with surface area, but heat generation scales with compute volume.
[0]:
shell thickness, t
compute radius, r
shell volume is (r+t)^3 - r^3 = 3 r^2 t + 3 r t^2 + t^3 = O(r^2)
shielding/compute is O(r^2)/O(r^3) = O(1/r), ie their linear decrease
Massive radiators. The ISS has radiators that have a dissipation capacity of about 3m^2/kW. If we use that number, we'd need a 3000m^2 radiator per megawatt, which is the scale they're talking about. This could theoretically be brought down, but not even by an order of magnitude.
I wonder how much cooling the solar panels alone would need, when operating at that scale.
You cool the fluid by flowing it through the radiator. The radiator emits heat radiation into space and cools down the fluid. As long as the fluid is hotter than the equilibrium temperature of the radiator (determined by radiator, space and sun radiation), it will emit more energy than it receives and cool down the fluid.
Followup question, wouldn't nearly any cooling solution that works in space also work on the ground? Radiative cooling is the most basic/common cooling solution on the ground, the main challenge is just figuring out how to to move heat from the component to the radiator, which I don't think is solved by simply putting it in space?
> Radiative cooling is the most basic/common cooling solution on the ground
Thats tricky. I know the heat exchange components are called radiators but most of the heat they give off is by convection not radiation. (At least here on the ground.) I heard 80%-20% rule of thumb.
But you are right in the broad strokes. Cooling is not easier in space. Mostly because you have no convective heat transfer.
Oh right, that makes sense. So the argument is that comparing a 50C GPU+radiator in a 20C room vs a 50C GPU+radiator in 0K space, the one in space will dissipate more heat via radiation than the one on the ground? As you say, I'd expect that air cooling is much better than EM radiation, but I guess there is some basis for claiming the possibility that cooling in space is somehow better than on the ground, however unlikely.
I think other have already corrected you, but radiative cooling is probably the least common on the ground and the only viable option in space.
I can help explain why. On earth, we are surrounded by stuff. Radiative cooling relies on thermal radiation leaving an object. Crucially, it also requires the object to absorb less thermal radiation than it emits. On earth we are surrounded by stuff, including air, that emits thermal radiation. There is a window of wavelengths, called the atmospheric window[0], that will allow parts of the thermal radiation out into space, rather than returned back. Imagine shining a flashlight on tinted glass, the light will get through depending on the color. If the light gets through, it has escaped. If not, the light is returned and heats up your surroundings again.
Also on earth the other methods (conduction, convection, and phase changes) are more effective. The earth can be used as a very big heat sink. On a spaceship or satellite, you don't have the extra mass to store the energy, so radiative is the only option.
Is radiative cooling the most common on Earth? I don't think so. Most terrestial "radiators" actually work with convection, ie moving relatively cold air across hot metal fins, which doesn't work in space.
