6. Understanding energy efficiency within the data centre
In order to design for high energy efficiency, there is a need to match the IT load requirements to the design capacity of the M and E infrastructure. This is due to fact that most electrical and mechanical systems operate at maximum efficiency when at full load. Dependent on the type of device, the load to power function can vary significantly, but most M and E devices are designed to achieve their maximum energy efficiency near maximum load.
The same is true at a system level, meaning the overall energy efficiency of the data centre M and E infrastructure will occur when the system is close to its maximum rated capacity, or in other words when the IT electrical load is at 100%.
Much like today’s IT devices, the system wide capability for the data centre M and E infrastructure to ‘turn down’ from 100% load and remain efficient is very limited. Many of the devices within the more modern designs simply do not work properly without a suitable base load. This is particularly true of components such as absorptive chillers in CCHP plant.
Figure 1 illustrates a typical energy efficiency surface plot for a data centre, the vertical axis depicts the DCiE (Data Centre infrastructure Efficiency) where a DCiE of 1 represents 100% of the energy supplied from the utility being delivered to the IT equipment within the data centre. The illustration shows the influence of IT load on the overall efficiency of the data centre and thus the importance of ensuring a close match between data centre provisioned capacity and the average utilisation of this capacity at any point in time. This has a direct bearing on the optimal CMR design from an energy and carbon efficiency perspective.
If we look at an IT device, a server for example illustrated in Figure 2, we see a typical commodity x86 server’s load to power transfer function. It can be seen that the server’s electrical load varies between 200 watts with no load applied and 300 watts when it is working flat out. The yellow diagonal line shows how a load-to-power linear server would behave. That is, when it is doing no work, it is drawing little or no power.
There has been much effort within the data centre industry to develop metrics that span the entire data centre and map energy consumption to the useful work of the facility. The development of metrics that described the basic energy transfer of the physical data centre are now relatively stable and well understood. There is not yet any useful metric for the IT work or application work of the data centre. As such this section will address the metrics for the physical facility.
Data Centre Infrastructure (DCiE) or Power Usage Effectiveness (PUE) are two metrics in common use within the data centre industry for reporting the energy efficiency of a data centre. They are calculated as follows:
The total facility power is defined as the power measured at the incoming utility meter. The IT equipment power is defined as the power consumed by the IT equipment supported by the data centre as opposed to the power delivery and cooling components and other miscellaneous loads. For a full description of DCiE see the Green Grid paper on DCiE and PUE.
While both DCiE and PUE are simple metrics to understand and measure, their usefulness in predicting and managing energy efficiency are limited as both are influenced by how full the data centre is from an IT equipment power draw perspective (due to the fact that they are simple ratios). Therefore in terms of measuring and understanding the achieved vs. design efficiency of a data centre, they are of little use.
Figure 3 illustrates that on day one, if very little IT equipment is installed into the CMR the achieved efficiency shown by the DCiE metric would be close to zero. While such a figure may well be accurate it gives little indication as to how cost or carbon efficient the CMR will be under load in absolute terms. It is the absolute utility load (irrespective of howmuch IT equipment is installed) that will influence a data centre’s carbon footprint and any claims to be ‘low carbon’.
A data centre has a fixed base load, which would be drawn even if all of the IT equipment were to be unplugged, as such we can represent the facility power draw in two components, the fixed and variable power draw. This is represented by the fixed and proportional overheads. Whilst there is some non-linearity from the square law losses these are dominated by the fixed and proportional losses, allowing this representation to be an effective approximation as shown in Figure 4.
- Facility Power(zero): The power drawn at the Utility feed at zero IT electrical load
- Facility Power(full): The power drawn at the Utility feed at full IT electrical load
- Rated IT Load: The rated IT electrical load of the facility
- Fixed overhead = Facility Power(zero) / Rates IT Load
The ability to lower the fixed overhead of a data centre through modular provisioning will have a material impact on the facilities ability to be more energy responsive to IT load and thus have a greater measurable impact on energy consumed from the utility feed.