I know you've been talking about ducting the exhaust; have you considered sealing the room and running the intake through a household air filter?
Yes, have considered that. A household filter + box fan doesn't let much air past. But despite that minimal airflow, managed to turn the filter completely brown in 3 days (terrible smog those days). I actually have some clean room grade hepa filters (2' x 2' x 1') and now have fans with sufficient pressure and airflow that I could use those filters as well, but haven't built that out yet. Also, I most likely need a cheap prefilter for any large particles if I don't want to immediately ruin the main air filter. Either way, a useful solution is not in the immediate horizon, but suffice it to say I can see why most datacenter's don't simply use unfiltered outside air.
It's a toss up whether you're better off filtering the air, or avoid the entire problem by using a heat exchanger so you can recycle the same indoor air. Every heat exchanger costs you in efficiency so you want to minimize those. For every heat exchanger, you need to trade off surface area, airflow and/or water flow, and temperature rise. There are also diminishing returns: you need twice the cooling capacity to achieve a 5 degree temperature rise compared to a 10 degree rise. Beyond a certain amount of airflow, additional cooling trails off significantly as you spend more fan power to get very small cooling improvements. So using a free-air heat exchanger (to avoid using dirty and low humidity outside air), you'll probably need a very large amount of materials (copper, aluminum), and a lot of fan horsepower (both on the indoor side and the outdoor side) to achieve an indoor temperature 10F above outdoor ambient.
Right off the bat you're using twice as much power (at least) on fans and handicapping yourself by 10 degrees. Even if the rest of your facility is very efficient (good cold / hot aisle containment, air handlers close to the heat loads, etc), you'll still need a chiller 4-6 months of the year (there goes your PUE) -- evap cooling probably won't cut it.
If things are worse, say you've got a typical multi tenant datacenter with 1U servers (very high cpu temperature rise above ambient), sloppy hot / cold containment, inconsistent power density (a row of 10kw racks across from a row of 3kw racks), and so on, forget about efficiency. At this point you just pass the costs along like everyone else.
Using direct outside air, under full load at a moderate fan rpm, these CPUs run at around 30C above ambient. If you wanted to use noctua fans or water cooling, that could be reduced to a 15-20c temperature rise, at the expense of buying the coolers and using up space to put them somewhere (the servers would be 4-5u instead of 2u with either of those solutions, and at least $100 more each). Not doing that currently (except for some of my gpu servers which are 4-5u anyway), but if you had a facility using exclusively that kind of CPU cooling, you wouldn't have to do much to cool the air, even during the Phoenix summer. The capex savings on a datacenter filled with servers where evap was "overkill" is a pretty exciting idea.
Love what you're doing; I'm taking baby steps in the same direction, though my situation is complicated by an impending cross-country move.
Another question; apologies if you've covered this already (still trying to wrap my head around datacenter power config): Household 120 + 120 = 208v? Or did you get something special wired in?
Thanks, it's a lot to learn which I really enjoy.
As to the wiring, it depends how your home is wired and how the transformer servicing your home is wired. Typically, you have 2 "hot" wires, 1 Nuetral, and 1 ground wire. Wiring an outlet to Hot (either one) + Nuetral gives 120v. Wiring up Hot1 + Hot2 gives either 208 or 240v depending on the transformer used by the electric company.
Commercial power will normally be one of the following:
120/208 3-phase, or 277/480 3-phase.
Lets take 120/208. Here it's the same but you have 3 "hot" wires on 3 phases, not 2.
Connect any phase to nuetral, and you get 120v. Connect any phase to any other phase, and you get 208v. The aggregate amount of amps you can draw at 208v in this scenario is equal to 1.73 times the rating of any single phase, so long as you load all 3 phases equally.
So, for 200 amp 3-phase service (ignoring 80% de-rating), you could draw 200a of 120v on each of the 3 phases -- total of 72,000VA.
Or, you can draw 3 phases of 208v, but you can only draw a maximum of 1.73 times 200a total, if you balance it equally between each phase. Here again, 200 * 1.73 * 208 = (approximately) 72,000VA.
At home this is similar, on "200a service", you can draw 200a 120v from Hot1 + Nuetral, and 200a 120v Hot2 + Nuetral, or you can draw 200a from Hot1 + Hot2 (either 208v or 240v depending on your wiring) -- or some combination of the two as long as you don't exceed 200a on either phase.