Power Usage Efficiency (PUE) is a poor data center metric

Problems with PUE

Power Usage Effectiveness (PUE) is a widely used metric which is supposed to measure how efficient data centers are. It is the unit of data center efficiency regularly quoted by all the industry players (Facebook, Google, Microsoft, etc.).
However, despite it’s widespread usage, it is a very poor measure of data center energy efficiency or of a data center’s Green credentials.

Consider the example above (which I first saw espoused here) – in the first row, a typical data center has a total draw of 2MW of electricity for the entire facility. Of which 1MW goes to the IT equipment (servers, storage and networking equipment). This results in a PUE of 2.0.

If the data center owner then goes on an efficiency drive and reduces the IT equipment energy draw by 0.25MW (by turning off old servers, virtualising, etc.), then the total draw drops to 1.75MW (ignoring any reduced requirement for cooling from the lower IT draw). This causes the PUE to increase to 2.33.

When lower PUE’s are considered better (1.0 is the theoretical max), this is a ludicrous situation.

Then, consider that not alone is PUE a poor indicator of an data center’s energy efficiency, it is also a terrible indicator of how Green a data center is as Romonet’s Liam Newcombe points out.

Problems with PUE

Consider the example above – in the first row, a typical data center with a PUE of 1.5 uses an average energy supplier with a carbon intensity of 0.5kg CO2/kWh resulting in carbon emissions of 0.75kg CO2/kWh for the IT equipment.

Now look at the situation with a data center with a low PUE of 1.2 but sourcing energy from a supplier who burns a lot of coal, for example. Their carbon intensity of supply is 0.8kg CO2/kWh resulting in an IT equipment carbon intensity of 0.96kg CO2/kWh.

On the other hand look at the situation with a data center with a poor PUE of 3.0. If their energy supplier uses a lot of renewables (and/or nuclear) in their generation mix they could easily have a carbon intensity of 0.2kg CO2/kWh or lower. With 0.2 the IT equipment’s carbon emissions are 0.6kg CO2/kWh.

So, the data center with the lowest PUE by a long shot has the highest carbon footprint. While the data center with the ridiculously high PUE of 3.0 has by far the lowest carbon footprint. And that takes no consideration of the water footprint of the data center (nuclear power has an enormous water footprint) or its energy supplier.

The Green Grid is doing its best to address these deficiencies coming up with other useful metrics such as, Carbon Usage Effectiveness (CUE) and Water Usage Effectiveness (WUE).

Now, how to make these the standard measures for all data centers?

The images above are from the slides I used in the recent talk I gave on Cloud Computing’s Green Potential at a Green IT conference in Athens.


Data Center War Stories talks to the Green Grid EMEA Tech Chair Harqs (aka Harkeeret Singh)

And we’re back this week with another instalment in our Data Center War Stories series (sponsored by Sentilla).

In this episode I talked to the Green Grid’s EMEA Tech Chair, Harqs (also known as Harkeeret Singh).

The Green Grid recently published a study on the RoI of energy efficiency upgrades for data center cooling systems [PDF warning]. I asked Harqs to come on the show to discuss the practical implications of this study for data center practitioners.

Here’s the transcript of our conversation:

Tom Raftery: Hi everyone, welcome to GreenMonk TV, this is the Data Centers War Stories series sponsored by Sentilla. The guest on the show today is Harkeeret Singh aka Harqs. And Harqs is the EMEA Tech Chair of Green Grid. Harqs welcome to the show.

Harqs: Thank you Tom.

Tom Raftery: So Harqs as the Tech Chair of the Green Grid in the EMEA region, you get a larger overview of what goes on in data centers than some of the previous interviewees we?ve had on the show.

We have been talking about kind of the war stories, the issues that data centers have come across and ways they have resolved them. I imagine you would have some interesting stories to tell about what can go on in data centers globally. Do you want to talk a bit about some of that stuff?

Harqs: The Green Grid undertook a study which implements a lot of good practices that we all talk about in terms of improving air flow management and increasing temperature, putting in variable frequency drives. And what we did is after each initiative, we measure the benefit of — in terms of — from an energy consumption perspective.

And Kerry Hazelrigg from the Walt Disney Company led the study in a data center in the southeast of the US. And we believe that is representative of the most data centers, probably pre-2007, so we think there is a lot of good knowledge in here which others can learn from.

So I am going to take you through some of the changes that were made and some of the expectations but also some of the findings some of which we weren?t expecting. So starting with — there was five different initiatives, the first initiative was implementing the variable speed drives. And what we found was, they installed new CRAC units and CRAH units which they put in variable frequency drives in the standard, there is 14 of them, and then they retrofitted 24 existing CRAHs. And took out the fixed speed drives and put in the variable speed drives.

The expectation was we would find a reduction in energy consumption and fan horsepower. And also there was a potential of maybe looking always the providing of coolant to the right place in the data center. And once we put those in, we found out that they didn?t actually introduce any, any hotspots, which was a positive thing. But some of the things that were a little different from what we expected and the PUE didn?t reflect the savings. That was because there was external factors, things like the external weather which impacted the PUE figure as well. So you need to bear that in mind as you make changes. You need to look at the average across the year.

The other issue that we found was by putting in variable speed drives they found it introduced harmonics to the power systems. And that came through the monitoring tools and so they are putting — they put in filtering to help resolve those harmonics.

The last issue was also around skills, so they had to train the data center staff on using variable frequency drives and actually maintain them. This was the biggest power saving, it was a third of the overall saving and the saving in total was 9.1% of energy consumption and that?s saved some thing in the order of $300,000 in terms of real cash and the PUE went down from 1.87 down to 1.64 by doing these five initiatives.

The second issue was actually putting in the air flow management. So things like the blanking panels and the floor grommets, putting in the cold tiles where they are supposed to be, and that was for around 7 inch cabinets and the findings were that that reduced the cold aisle temperature because you have less mixing, and also increase the temperate on the hot isle in terms of the temperatures going back to the CRAH. So that was interesting.

We saw that being a key enabler to actually increase in temperature, so you have cold to cold aisles and hot to hot aisles because of those mixing. There wasn?t any energy savings for this piece in itself, but it was in this — airflow management activity is an enabler in that it allows you to then do some optimization and also to increase temperature without risk.

The third activity was relocating the sensors that the CRAHs worked off from, way from the return to CRAC and return to CRAH which is what most data centers use today to actually aligning that to sensors on the front of the cabinets. So actually moving from return air to supply and that?s the specification that ASHRAE provides, that?s what we should be controlling, the temperature and the humidity of the air going into the servers. They themselves say they don?t really care about what the temperature is coming out of back of the servers. Well the rest of us do from a — making sure that it?s not too hot for our data center operators.

So what we did was move those sensors to the front of cabinets and what that did was that optimized the fan speeds and actually started to raise the temperature and the cold air that was required by the servers. It did take them a little awhile getting the locations right, so making sure that they have them moving them around as much as possible, looking the CFD to make sure they are optimizing and putting it in the right place eventually. And that was a small improvement, but it was the — again another enabler for increasing temperate. So there was only a few percent improvement by doing that, but what it does is when you start look at increasing temperature you are increasing temperature at the right point.

Tom Raftery: So how much did it increase temperature by — was it like from 20 to 25 or… —

Harqs: That?s a good question as the next initiative they did was, they were increasing the temperature, so I was just about to — so they went from 18? C, which was what their original set point was and they took it up to 22? C. Now obviously that?s still in the middle of the ASHRAE standard. So there is still more scope there to become better. But it wasn?t just increasing the temperature in the room but it was actually increasing the temperature of the chiller plant which — where the biggest savings were, so if you increase the temperature of the room that then allows you to increase the temperature of your chiller plant.

And that?s — they increase the set point of their chiller plant from 6.7? C to just under 8. And what they found was, there was significant savings due to the reduction in compressor and condensor fan power. And what they found was for each and I’m going to do this in degree F because they calculate degree F. So they went from 44 to 46? F. For every degree F they increased the set point of the chiller, they found out that reduced just over 50 kilowatts of chiller energy consumption.

Now in terms of other people?s data centers, they are also — your mileage may vary depending on the configuration and where you are, but that?s what their significant saving was. By doing that what they found was — by doing it this way, where they put the air flow management in place and then they increased temperature in the room, increased the set points of the chiller plant they found that actually there was — that made no significant impact on the data center in terms of hot spots or anything like that. So there is no detrimental impact to the data center by doing this. Obviously the saving of the energy was a positive and saved real money.

Tom Raftery: Alright, Harqs that was great, thanks a million for coming on the show.


IBM reckons Green is where economic and ecological concerns converge

I love this ad. It demonstrates that not only has IBM a sense of humour but also that they have the right story – today, with soaring energy prices, Green is where economic and ecological concerns converge.

Last year IBM announced Project Big Green. This was a commitment by IBM to re-direct $1 billion USD per annum across its businesses to increase energy efficiency! Serious money by anyone’s standards.

This isn’t just some philanthropic gesture on IBM’s part. By making this investment the company expects to save more than five billion kilowatt hours per year. IBM anticipates it will double the computing capacity in the eight million square feet of data center space which IBM operates within the next three years without increasing power consumption or its carbon footprint. In other words they expect to double their compute power, without adding data centers, nor increasing their carbon footprint!

This year, IBM have gone even further! As an extension of their project Big Green they have announced ‘modular data centers’ similar to Sun’s S20 product. They come in three sizes and IBM claims they are

designed to achieve the world’s highest ratings for energy leadership, as determined by the Green Grid, an industry group focused on advancing energy efficiency for data centers and business compute ecosystems.

I’d love to see comparable metrics between the S20 and IBMs modular data centers.

However, the take home message today is that IBM is committing serious resources to its Green project. Not because they care deeply for the planet (I’m sure they do) but because they care deeply about the bottom line and with increasing energy costs, there is now a sweet convergence between doing the right thing for the planet and for the shareholder!