Wow, It's Humid in WI!

During my presentation at the Wisconsin Focus On Energy event last week, I covered energy efficiency opportunities in data centers.  As usual, I made the case that everything depends on the environmental conditions you want to maintain - essentially, what is the temperature and humidity tolerance you have for the air going into IT equipment?

Many data center operators are supplying air that is overcooled, and within a very narrow humidity range, oftentimes 45 to 55 percent. This drives excessive energy use, because chilled water temperatures have to be low, and humidifiers and dehumidifiers have to operate often (and in some instances we find some CRAHs doing one and some the other!).

I pointed out that these tolerance levels are a hangover from the days of magnetic tape and punch cards, and that IT equipment manufacturers specify a much wider allowable temperature and humidity range. In fact, many industry-leading operators have done away with humidity control altogether, and are pushing supply air temperatures into the mid-70 degrees F range.

The layman's version of this part of the story is: turn up the temperature (and widen your humidity tolerance) on your air conditioning and you'll save energy and money.

The follow-on part of the story is that higher and wider tolerances allow for longer operation of air-side free cooling. I showed a slide that I obtained from Mark Hydeman at Taylor Engineering in Alameda CA that shows that as you tighten your tolerances on humidity, you decrease the time that you can run on outside air.

The chart showed this for four locations in the country, but none that has a climate like Wisconsin's, and I go a lot of push back from the audience saying that humidity is too high in WI to allow for air-side economizers. (I anticipated push back on low humidity tolerance, not high, but attendees reported condensation issues in their facilities.)

I asked Mark to run the numbers for Madison, and here are the results:

 

Air-side Economizer Annual Operating Hours Madison Wisconsin (Dane County Regional Airport)
	Dry Bulb Temperature <=70˚F	Dry Bulb Temperature <=80˚F
No upper or lower humidity limits	7,377	8,310
Lower humidity limit: Dew point temperature >= 42˚F	2,611	3,526
Both upper and lower humidity limits: Dew point temperature >= 42˚F and <= 59˚F	1,900	2,203
Both upper and lower limits as above, and relative humidity <= 60%	258	545

 

Note that setting a lower humidity limit alone drops the free cooling hours by about two-thirds, and setting both high and low limits drops free-cooling availability to around 2000 hours a year or about a quarter of the time.

Despite this analysis, I would recommend that anyone considering building a new data center in Wisconsin carefully consider air-side economizers, as the payback would likely be in the three or four year range. Further, water-side economizers, in new construction or as a retrofit, are likely to have an even better financial return.

As always, thanks to Mark, who is a leader in the discussions at ASHRAE on recommended environmental conditions for data centers.

Comments

[Ross Bennett]

Mark,

Thank you for the humidity information. Excellent presentation at the Energy Efficient Computing Summit.

Were the airside economizer annual operating calculated assuming 100% outside air?

Thank you,

Ross