On 05/11/2015 06:21 PM, Randy Carpenter wrote:
That is .8-1.6A at 5v DC. A far cry from 120V AC. We're talking ~5W versus
~120W each.
Granted there is some conversion overhead, but worst case you are probably
talking about 1/20th the power you describe.
His estimates seem to consider that it's only 5V, though. He has 825
Pis per rack at ~5-10W each is call it ~8kW on the high end. 8kW is
2.25 tons of refrigeration at first cut, plus any power conversion
losses, losses in ducting/chilled water distribution, etc. Calling for
at least 3 tons of raw cooling capacity for this rack seems reasonable.
8kW/rack is something it seems many a typical computing oriented
datacenter would be used to dealing with, no? Formfactor within the
rack is just a little different which may complicate how you can deliver
the cooling - might need unusually forceful forced air or a water/oil
type heat exchanger for the oil immersion method being discussed
elsewhere in the thread.
You still need giant wires and busses to move 800A worth of current. It
almost seems like you'd have to rig up some sort of 5VDC bus bar system
along the sides of the cabinet and tap into it for each shelf or
(probably the approach I'd look at first, instead) give up some space on
each shelf or so for point-of-load power conversion (120 or 240VAC to
5VDC using industrial "brick" style supplies or similar) and
conventional AC or "high voltage" (in this context, 48 or 380V is
"high") DC distribution to each shelf. Getting 800A at 5V to the rack
with reasonable losses is going to need humongous wires, too. Looks
like NEC calls for something on the order of 800kcmil under rosy
circumstances just to move it "safely" (which, at only 5V, is not
necessarily "effectively") - yikes that's a big wire.
--
Brandon Martin
