Thread

1. 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.

         1. 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.

            1. 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.

               1. 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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