WHAT’S HOT DC Alliance
Designing for demand to reduce opex
Correct system design methodology is essential in order to reduce operating costs, says Airedale UK sales director Paul Oliver.
There is no one ideal solution for data centre cooling. The actual demands of a data centre fluctuate on a daily basis and are nearly always lower than the maximum design figures. Some never reach their maximum design parameters.
Measuring efficiency
The accepted measure of efficiency for a chiller is ESEER (European Seasonal Energy Efficiency Ratio). The average chiller loading works out to be 54% of its full load and the temperatures under which it is measured only represents 7% of the typical UK ambient hours. Whilst this loading profile may match comfort cooling applications, few if any data centres will have these loading characteristics. Because the demands of the data centre are now higher densities at higher operating temperatures, it means that the traditional chilled water temperatures of 7°/12°C are no longer appropriate and we are more likely, to use chilled water at around 14°/19°C.
Selecting a chiller based on its ESEER rating only, without further considering its actual annual overall energy efficiency including its ability (or not) to meet the cooling demand with concurrent free cooling, is not appropriate. In the following example, two chillers are run at 7°-12°C: £ 400kW conventional chiller, ESEER of 4.1, cost £35k, energy consumption £53k p.a.
£ 400kW free-cooling chiller, ESEER of 4.0, cost £48k, energy consumption 28k p.a.
£ The increased cost of the free-cooling chiller, £13k, is paid off in less than two years.
When run at 14°/19°C, the free- cooling chiller is repaid in six months. Efficient design
Resistance of air movement in data centres is a demon. More air means higher velocities, more resistance and an increase in the power requirements of the system fans – which are a significant consumer of power in the data centre. EC fans consume power in relation to the cube of their speed. So a fan running at 80% speed uses 51% of the power of a fan running at 100%. A good quality EC fan fitted to an 80kW chilled water CRAC unit will cost approximately £3.4k p.a. to run. A poor quality fan badly selected and applied would increase the running cost by over 75% to around £6k p.a. It is usual to supply CRAC units on an N+1 basis but historically, the standby unit has remained dormant for long periods. In a system comprising four run units and one standby unit, it is much more efficient to run all five units at 80% airflow, than four units at 100%. This scenario alone
Reduced operating costs of a bespoke solution vs. conventional CRAC/Chiller system
would reduce the fan power consumption by around 49%. Integrated free-cooling
Rising temperatures in the data centre give greater opportunities for free-cooling. The opportunity for free-cooling is present when the ambient temperature is below the room operating temperature, i.e. 98% of the UK ambient year (London, UK). The best free-cooling chiller systems bring together concurrent free-cooling and mechanical cooling, enabling free-cooling to be captured whenever the ambient is below the return water temperature. Non-concurrent free-cooling chillers require a very low ambient temperature to operate and whenever free-cooling cannot deliver 100% of the required capacity, free-cooling is sacrificed and completely replaced by mechanical cooling. Some free cooling systems will only operate for less than 2.5% of the UK year and will actually cost more to run than a conventional chiller system.
Bespoke solution
When increased temperatures are combined with medium/high density loads; concurrent free-cooling; variable speed fans and clever, dynamic controls to create an intelligent, bespoke system, real end user benefits can be achieved in reduced power usage in the region of 56% and payback of the increased capital costs of the system retrieved in less than a year.
Resistance of air movement in data centres is a demon. More air means higher velocities, more resistance and an increase in the power requirements of the system fans – which are a significant consumer of power in the data centre 12
www.dcseurope.info I June 2012
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