Targeting data centre PUE

Targeting data centre PUE

Airedale International Air Conditioning Regional Manager Stuart Kay examines how integrated, high efficiency cooling solutions can reduce the data centre PUE.

The EC estimates that the 56TWh of power used by data centres in Western Europe in 2007 is likely to double to 104TWh by 2020. Given the constraints of power supply, the more energy efficient the data center, the greater the power capacity available for computing. The bi-product of this efficiency is that the facility user will pay for less energy.

Reducing the PUE
The Power Usage Effectiveness (PUE)1 has been broadly adopted as a benchmark to enable data centre operators to quickly estimate the energy efficiency of their data centers and make positive changes. The more efficient the data center, the closer the PUE is to 1, indicating that a greater proportion of the power required by the facility is used to drive the IT equipment.

Since cooling accounts for around 58% of a typical legacy data centre power usage, cooling is an area where the data centre manager can substantially drive down the PUE.

Reactive cooling
Traditional ‘on/off’ cooling is inefficient in power use, like boiling a whole kettle of water for one cup of tea. Power usage in the data center is not static and most servers are under-utilised. An intelligent, efficient cooling system can sensitively vary the amount of cooling to match changing heat loads over time.

Variable air volume
Fan efficiency is pivotal to reducing cooling power. In a 24/7 application, just one kW of power saved per hour over a year, is equivalent to approximately 4,020kg CO2. When heat exchangers, airside systems and/or outdoor units are engineered with very low air flow resistance to accommodate EC (electronically commutated) fan technology, there is greatly enhanced fan efficiency particularly at part load. With the additional application of intelligent control logic, an EC fan is up to 70% more efficient under part load conditions than a standard AC (alternating current) fan.

By reducing air volume in response to changes in room demand, the fan power utilised is significantly decreased. Typically a 50% drop in air volume will result in an 83% reduction in fan power input.

Variable speed control
The variable speed control of latest technology 30 - 90Hz inverter driven scroll compressors featured in selected high efficiency airside and liquid chiller units, allows fully modulated cooling from 25 -100% when operating at part load, to exactly match the cooling demand.

Revolutionary centrifugal compressors present near silent, oil-free operation and ultra efficient, infinitely variable speed control. Chillers incorporating this technology can offer significantly low energy mechanical cooling because they are able to match load requirements exactly. Typically the centrifugal compressor technology will give a reduction in running power of up to 20% compared with a high efficiency screw chiller.

Both these compressor technologies have low starting currents, removing power-hungry transient starting ‘spikes’.

Ambient air free-cooling
In a free-cooling system, cooler ambient air is moved across a large surface air to water heat exchanger to pre-cool the secondary cooling medium. Only if additional cooling is needed will a mechanical input be required to remove further heat at a reduced rate. Since free-cooling operates with as little as 1ºC differential between ambient and return fluid temperatures, the continuous system operation and the high temperatures of a server environment in relation to ambient air, mean that a free-cooling chiller or dry cooler will use very little mechanical cooling.

By dealing with a higher grade heat and subsequent higher processor air off temperatures, a larger proportion of the year can be spent in free-cooling mode. A chiller designed with integrated, simultaneous mechanical / free-cooling achieves 0-100% free-cooling and will always use free-cooling first before initiating mechanical cooling. Such a chiller typically saves more than 45%Sup>2 of the energy consumed by a conventional chiller.

Integrated solution
Taking all these technologies and by using advanced controls logic, multiple numbers of rack-based heat exchangers, airside units and chillers can be integrated across a whole data centre to create an intelligent, high efficiency cooling solution in which cooling products interact and communicate. Such a solution pays dividends in maximising plant capability whilst reducing power usage through combined efficiencies.

The whole solution is managed from a single, centralised controls platform specifically designed for data centres and which is fail safe, with two-way communication, alarm handling and an IP address that can sit on an existing Ethernet and be browsed remotely.

Managing air volume
Significant power can be saved by managing air volume to the space across multiple airside units by sharing airflow duty between n and standby units. The more units, the greater is the benefit. In the graph shown (Shared Air Volume), running three airside units instead of two, gives a 70% saving in power of 5.3kW. Over a year this represents a saving of 46,428kWhs of energy. Unit airflow is managed by either measuring the airflow across all units and sharing the total required for the space, or by measuring the floor void pressure and averaging between units. Either option relies on the utilisation of the standby unit.

Temperature control
By monitoring the room load and automatically adjusting chilled water setpoints to match this load, we see a significant increase in plant energy efficiency at the higher water temperatures and we also increase the free-cooling threshold. For a 1°C increase in fluid temperature we see an 8.5%3 increase in chiller energy efficiency, so by introducing the standby airside plant as a means of reducing fan power usage, we also see a secondary advantage by having additional heat exchanger surface which allows room heat removal at a closer temperature difference.

IPP power monitoring
The PUE itself gives a snapshot of data centre efficiency. IP addressable control technology enables an operator to measure, chart and log the PUE over time. Energy data is automatically fed into the control platform, allowing a live PUE to be calculated and displayed with historical trend logs, all assessable remotely via the IP interface. The operator can see not only what is being run, make changes and see the effects of these changes, but importantly, is back in control of reducing energy costs across the data centre.

1 Developed by the Green Grid™ consortium

2 Based on Met. Office average ambient figures for London, UK at 10/15ºC, 20% ethylene glycol

3 Based on Met. Office average ambient figures for London, UK at 6/12ºC on wet systems using free-cooling chillers

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