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Google’s Project Suncatcher to put ML infrastructure in space

233 points · 553 comments · xnx

zactato · 1790267254 · focus · HN ↗
How are they solving the heat dissipation issues?
1. lopsotronic · 1790268994 · focus · HN ↗
  For real. It's an enormous problem solved only with 1) sheer scale, and 2) Science Fiction.
  Both of those are expensive as hell, by the way.
  Cooling via radiation follows Stefan–Boltzmann: P = εσAT⁴. Let's assume a good surface (emissivity ~0.9) at 300 K (27 °C) at 400 W per square meter per side. A flat panel radiating from both faces into deep space gets 800 W/m, not including the losses from, say, the Sun, or from IR coming off the Earth. Now, input power. Sunlight in orbit ~1,360 W/m², assume ~22% cell efficiency, we got 300 W/m². So each 1 MW compute, 3,300 m² of solar panel and minimum 1,200–1,500 m² of radiator.
  In case ya didn't know - 1 MW is tiny from a present-day-datacenter perspective. It's like 8 racks. So we're talking orbital megastructures here, many many many square kilometers, and this is with all the best case assumptions, and magic radiator panels that never see the sun, or the earth, or the moon.
  This is just the basic numbers here, by the way. There's a garbage truck full of other unsolvable problems if you poke your head in there.
  Aside from the "Avoid Regulations" aspect, and the "Everything That Burns Deorbiting is Depreciation" aka "The Starlink Trick", I'm not sure what the hell the draw is.
  1. mike_ivanov · 1790273664 · focus · HN ↗
    Radiating 1MW at 500K (227C) with a 0.4MW heat pump takes about 200 m^2 flat sheet surface. Inputs - solar+nuclear for double fun. So - quite feasible.
    ChickeNES · 1790275036 · focus · HN ↗
    And if you look at SpaceX's Starmind sats, they will have a 160 m^2 liquid radiator for 175kw/250kw peak compute.
    cyberax · 1790281339 · focus · HN ↗
    Do we even _have_ semiconductors that can work at 220C? And if you're thinking about using some kind of refrigeration cycle, its efficiency is going to be bad.
    tristanj · 1790282855 · focus · HN ↗
    1) The chips don't reach 220C. The 220C is the temperature at the hot end of the heat pump. The chips are on the cold end of the heat pump.
    2) The International Space Station has used a dual-loop ammonia/water-based heat pump to cool the station temperatures. It's been in place for several decades. Heat pumps are a proven technology.
    Other satellites have also used heat pumps, such as SES-17 in geostationary orbit <a href="https://www.esa.int/Applications/Connectivity_and_Secure_Communications/Very_High_Throughput_Satellite_ready_to_pump_heat" rel="nofollow">https://www.esa.int/Applications/Connectivity_and_Secure_Com...
    lopsotronic · 1790283609 · focus · HN ↗
    ISS's two external cooling loops hold about 540 kg of ammonia combined, together they dump 70 kW.
    andruby · 1790285312 · focus · HN ↗
    > The 220C is the temperature at the hot end of the heat pump. The chips are on the cold end of the heat pump.
    If we want the heat pump's cold end at about 40–65°C, then for each 1MW of GPU heat, we need another 1MW of heat pump power. Now you need 2MW of solar power.
    Good news is that the radiator at 227C (500K) can emit about 5× more heat per square meter than at 57C (330K)
    cyberax · 1790290763 · focus · HN ↗
    Of course. But their hot ends are nowhere near 220C. I don't think such heat pumps even exist right now except in labs.
    Looks like some experimental pumps within this region have CoP around 30%: <a href="https://www.sciencedirect.com/science/article/abs/pii/S0360544225039672" rel="nofollow">https://www.sciencedirect.com/science/article/abs/pii/S03605...
    So you'll need a lot of additional energy to run the pumps. Which will require additional radiator area.
    lopsotronic · 1790284655 · focus · HN ↗
    Moves 1 MW of heat with 0.4 MW of work? I.e. 2.5 COP {coefficient of performance). That's insane, and I mean that in a good way. Could you dig me up a cite for that?
    That's thumping the Carnot limit: [[T_cold / (T_hot − T_cold)]].
    2.5, while rejecting at 500 K, cold side's at least 357 K (eeehhhhhhh 84 °C) . . . and that's an absolutely perfect Carnot machine. At 50% Carnot -- a pretty good heat pump, real world performance is 40-60 -- cold side's at 417 K (144 °C). 417k, feeding your GPU coolant loops.
    hex4def6 · 1790295741 · focus · HN ↗
    Think you have an error -- it's t_hot / (t_hot - t_cold)
    With those numbers, ideal carnot would be 500/(500-357) = 3.5.
    Multi-stage could potentially get you to a COP of 2 or so. So 0.5MW.
    lopsotronic · 1790303431 · focus · HN ↗
    I believe that's Carnot COP for a heat pump used for heat+. I used the refrigeration version, T_cold / (T_hot - T_cold), which I'm 80 percent sure is the right one here.
    Depends on which heat you want
    Heat adding to hot side: COP_heat = Q_hot / W = T_hot / (T_hot − T_cold).
    Heat leaving the cold side: COP_cool = Q_cold / W = T_cold / (T_hot − T_cold).
    Another one (more common in the day to day, for me at least): heat-engine efficiency, η = 1 - T_cold / T_hot. Cycle forward to make work from heat.

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