>San Diego has a mild climate and we opted for pure outside air cooling. This gives us less control of the temperature and humidity, but uses only a couple dozen kW. We have dual 48” intake fans and dual 48” exhaust fans to keep the air cool. To ensure low humidity (<45%) we use recirculating fans to mix hot exhaust air with the intake air. One server is connected to several sensors and runs a PID loop to control the fans to optimize the temperature and humidity.

Oh man, this is bad advice. Airborn humidity and contaminants will KILL your servers on a very short horizon in most places - even San Diego. I highly suggest enthalpy wheel coolers (kyotocooling is one vendor - switch datacenters runs very similar units on their massive datacenters in the Nevada desert) as they remove the heat from the indoor air using outdoor air (+can boost slightly with an integrated refrigeration unit to hit target intake temps) without switching the air from one side to the other. This has huge benefits for air control quality and outdoor air tolerance and a single 500KW heat rejection unit uses only 25KW of input power (when it needs to boost the AC unit's output). You can combine this with evaporative cooling on the exterior intakes to lower the temps even further at the expense of some water consumption (typically far cheaper than the extra electricity to boost the cooling through an hvac cycle).

Not knocking the achievement just speaking from experience that taking outdoor air (even filtered + mixed) into a datacenter is a recipe for hardware failure and the mean time to failure for that is highly dependant on your outdoor air conditions. I've run 3MW facilities with passive air cooling and taking outdoor air directly into servers requires a LOT more conditioning and consideration than is outlined in this article.