Who cares how much water a data centre uses, I’ve got a low PUE

PUE has become the de facto standard for measuring energy efficiency and by extension environmental impact  of modern data centres. But it’s not flawless or entirely complete. PUE is just one small measure of the environmental impact of data centres.

Firstly, let’s look at some issues with PUE itself. The more loaded a data centre (towards 100% capacity), typically the lower the PUE therefore seen as energy efficient data halls. On the other hand, lightly loaded data halls (towards 0% capacity) can have a dreadful PUE. This is because in most data centres (not our Colt Ftec data centre) technical infrastructure losses tend to be similar irrespective of the IT load they are supporting. Hence if you have little IT load, you have a very poor PUE, equally if you have an inefficient data centre, running it full will flatter your PUE! To make things more complicated, some data centres use chilled racks which appear to be IT load, but are in fact the technical infrastructure masquerading as IT load! Needless to say it produces wonderful PUE figures, but actually the efficiency is not as healthy as it looks! Or look at it another way – it costs more money to run the actual compute.

Typical of a developing industry, the data centre industry is notorious for measurements which seemed good at the time, but moved out of fashion as the real issues surfaced. Years ago the ‘square feet’ of a computer room was the only thing that mattered, then it was realised that resilience was probably more important and what we now know as Tier levels became key. Then the cost of energy rose and the power of a data centre was the key factor, rapidly followed by the ability of the data centre to distribute the power and the cooling – enter power density. PUE is one of those measurements, used as a loose guideline to measure the cost to run a data centre and the environmental impact of it.

However, the next big issue is on the horizon. Water, why? because some data centres use huge amounts of water in the cooling systems.  Market leading PUE figures of 1.2 or lower are often touted by data centre operators.  But be warned, it’s important to understand exactly what makes them so efficient. What it means is that the cooling is being undertaken by something other than the electrical power input. It’s important to investigate exactly what that is. One way is to use airside free cooling, another is to spray water into the air and use the evaporation of that water as the method of providing cool air for the data centre.

So what’s the problem.  In those using water, the water usage figures of these data centres can be truly colossal.  The NSA has just been cited as using 1.7 million gallons of water per day to cool its datacentre and it is by no means the only one. Does this matter, other than a bit of bad PR? Well, yes it does. There are very few sources of water – less than 1% of the earth’s fresh water is available for human needs. Thousands of acres of land are put under water at great cost to provide this critical resource for us all, and it is not unusual for drought warnings to appear even in western countries, supposedly with plentiful supplies of water.

And while we’re on the subject of water…

If you evaporate water into the air you provide a moist, warm environment which is perfect    for the growth of bacteria and in some instances very dangerous pathogens indeed – legionella is one of those – so you need to add a biocide which adds to your costs and of course needs to be human friendly (after all you are going to be breathing it in when you are working in the data centre).

Another option is fresh air cooling or free air cooling. It uses the outside air temperature to cool the data centre. Once the outside air is below a certain set temperature no other secondary methods of cooling are used. With ftec, we only use about 1 bath full of water to provide humidification, a trivial amount of this valuable resource and we can still achieve a low PUE figure of 1.21.

If water becomes scarce due to an extended dry spell, take a wild guess what takes priority – human drinking water or a data centre ? Just as availability and cost of power has become increasingly critical to data centres, water is about to take the same path.

What is important as an operator or consumer of data centre capacity is that we understand the different cooling methods available. Each method will have its advantages and disadvantages. But making an informed decision is key.

For more on this topic, watch our video here