AC/DC is not the name of the rock band, nor an allusion to anything regarding sexual preferences.

As I said in my last post, electricity consists of electrons in motion. However, there are two basic modes of motion: coming and going, or plain going: Alternating Current (AC) and Direct Current (DC). There’s pretty comprehensive wiki article on AC here.

Why should this matter? Simple. Data centres put electricity to two fundamentally different types of work. The first type of work is IT. All IT runs on DC. Just because it’s got a 110V/230V AC socket at the back doesn’t mean it consumes AC. The first thing the computer does is convert the AC to DC.

The second work to which electricity is put is cooling. There are quite a few technologies for this, and the optimal technology depends on the size of the data centre and the climate, but they all boil down to the same three types of components: heat exchanges, pumps and fans.

Why does this matter? After all, we’ve been converting AC to DC on an industrial scale for over a century, and, since the advent of the switch mode power supply, the conversion loss from AC to DC is tiny.

Here are a few reasons:

1. Photovoltaics generate DC, not AC. Converting DC to AC has never been as efficient as the other way around.
2. Batteries produce DC.
3. Industrial scale pumps and fans are much more efficient with AC (3-phase) than DC.
4. In order to get really green, we need to get really efficient. All the easy wins have been won; we need to look at the margins to find those remaining gains.
5. Engineering excellence.

So, in the next part(s), I’ll look at the DC part. I’ll move on to the AC part when I’ve finished with that.

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