Greening the data center: The fallacy of hot aisle

Today I’ve been reading IBM data center gets deep energy retrofit. In this post, I’m going to talk about the cooling techniques described here and the hot aisle/cold aisle technique in general.

One of the “green” cooling techniques commonly touted is hot aisle/cold aisle. In this technique, cold air is introduced at the front side of the cabinet, hot air is exhausted from the back side of the cabinet, and rows alternate back to back and front to front. This allows the hot aisle to be separated from the cold.

The IBM cooling mentioned in the article adds water cooling coils at the back door of the cabinet to cool the exhaust air directly.

Both these methods are certainly better than simply dumping air out the back of the cabinets, however, I consider them far from being green.

In both cases, energy has to be applied to move air against its natural tendency. Additionally, dumping the hot exhaust air out of the cabinet and then having to corral it again in the hot aisle containment is hardly efficient.

Finally, consider the energy required by the extensive active cooling of the IBM system and it becomes a very odd shade of green.

So, what are the keys to green cooling? Make use of the natural cooling of ambient air, keep as fine a control of airflow as possible, and take advantage of the natural air movement of the air to use as little energy moving it as possible.

So, how do we accomplish this?

1. “Chimney” airflow cabinets with the exhaust directly vented into a plenum.

2. All air handling above the cabinets (cold air falls to the cabinet intakes, hot air naturally rises through the exhaust plenum). Just another reason to skip the raised floor!

3. Free air cooling.

You have to put the energy into the air in the form of waste heat, let that energy make the air work for you!

Vern, SwiftWater Telecom
Data center, web hosting, Internet engineering


2 responses to “Greening the data center: The fallacy of hot aisle

  1. Hi Vern,

    IBM’s coolblue water filled back doors are actually stupid and wrong headed. Here’s why:

    Objective is to cool the components inside the cabinet, secondary objective is to get the hot air out and back to the CRACs a hot as possible (chimneys are good for this). CRACs run at peak efficiency with very hot air at the inlet.

    Coolblue cools the nice hot air inside the hot aisle and breaks the model. Stupid.

    Much, much better to put the cold door on the front of the rack!

    I hate thinking that is not systematic and joined up.



  2. Steve:
    Excellent point, I didn’t even consider that IBM’s rear door cooling is in the wrong place, no matter which cooling philosophy you subscribe to. Well, at least it looks impressive.

    Thanks for the comment!


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