White paper - 5 Points Relating to PUE and Other Metrics

 

Overview

In recent times the amount of power being adsorbed by datacenters has escalated dramatically. To put this expansion into context, it is estimated that data share of world power consumption is approximately 2%. At first site this may appear small but in reality it is enormous. A figure which is not surprising when you consider some of the 1800+ data centres in the world have individual electricity bills running into millions.

 

For data centre owners this can represent 40%+ of their Total Cost of Ownership – a figure that needed careful scrutiny. Recognising this and other topics all sides of the industry have joined together to form the Green Grid. This is a non profit making organization, enabling collaboration, which seeks to advance energy efficiency in data centres. One of the outcomes has been the development of two metrics referred to as Power Efficiency Utilisation – PUE and Data Center Infrastructure Efficiency - DCiE.

 

1. The calculations

PUE is calculated by dividing all the power entering the centre by the power used to run the IT load. Older data centres had a PUE of 2.5+, equivalent to 80% of the power being used for services such as cooling, ups and lighting. More recently new centres are claiming to have PUEs of less than 1.2, equivalent to 80% of the power now being used for the IT load - a major role reversal.

 

DCiE is a reciprocal of PUE and calculated by dividing IT equipment power by total facility power.  It is expressed as a percentage.

 

2. The measurements

Now that all sounds very good but unfortunately measurements have been imprecise with individual data centres taking views on what should or should not be included in their calculations. Typical anomalies being emergency lighting, backup generator heaters, server fans and power used in service areas. If this was not enough there were further problems:

• First, the time of the year when the measurements were taken, summer, winter or an average for the year. These seasons materially effect the power needed for cooling particularly if from time to time the data centre use “free cooling”.
• Second, the IT load itself – usually taken at full capacity, which could be way above reality.

 

The result has been some data centres making claims for PUEs of less than 1.1, which on the surface appear too close to perfection.

 

3. New PUE protocols

The Green Grid has recognised these short comings for some years and has working to established guidelines that should lead to greater global and inter company consistency. They published in February 2011 “Harmonizing Global Metrics for Data Center Energy Efficiency. Their goals were stated as

• Identify an initial set of metrics, define each metric, define the process for measurement and establish on-going dialog for development of additional metrics.

 

Hopefully this new imitative will both level the playing field and create more confidence in competing data centers claims.

 

4. Carbon Usage Efficiency

In addition to PUE the Green Grid has also worked on a new metric introduced in 2010: CUE. This measure is all about sustainability and will be important to business particularly those operating Corporate Social Responsibility Programmes.

 

One of the methods for determining CUE for those data centres that obtain all their energy from the grid and generate no CO2 themselves is calculated by dividing Total CO2 emissions by the Total Data Center energy – the latter being the same value as used in the PUE calculation, by the IT equipment energy. Other formulas are offered for other scenarios.

 

The Green Grids see their white paper #29 on this subject as the start of a process and one they hope will foster discussions within the industry on the subject.

 

 

5. Energy Reuse Effectiveness

Energy converted into warm water during the cooling process, for example, can be and are being exported outside the parameter of data centres to heat businesses or local communities. A good example, from as far back as 2006, is the NTS Bern data centre, which started to re-cycle heat to a local school. In its first winter of operation this saved 25,000 litres of heating oil. A valuable contribution to both the environment and society.

 

PUE does not take these types of energy saving into account. So once again the Green Grid has stepped in to set up a new metric ERE.

 

Conclusion

The data centre industry has woken up to the importance of energy sustainability and efficiency in all it forms. Early work created some use full, if a little crude, metrics but these have now been defined, refined and will be further developed in the best interest of the planet.