Free data centre cooling – fresh air versus recycled air
28Oct
posted by: Rob Elder, Keysource
Data centres continue to be in demand and the trend is set to grow with more consumerisation of IT and the growth in cloud computing.
As a result, there is an increasing reliance on data centres to support critical systems ensuring maximum availability.
Another key industry focus is the reduction in energy consumption of the data centre. Whilst improvements in the power usage of IT equipment has become the main focus of the chip and server manufacturers, data centre cooling has become particularly important, yet we are seeing very different approaches.
The use of free cooling is widely recognised as an effective way to enhance data centre efficiency, reduce operating costs and improved power usage effectiveness (PUE). However, with the drive to be more efficient there are now many options available to data centre owners and operators. The challenge is to consider the choices affecting all operational aspects of the facility not just efficiency.
The free cooling approach is essentially referring to compressor free cooling, which is achieved by using low ambient air temperatures. However, there are two very different options available – “direct” using filtered fresh air directly into the data centre, or “in-direct” using a heat exchanger or similar, to achieve a fully re-circulating system.
At present most designs, in order to provide resilience in all conditions, have mechanical systems as back-up, so overall the capital expenditure and ongoing operating costs are broadly the same for direct and in-direct options, so there is little to choose between the two based on price. Efficiency levels are also largely comparable, with both able to operate in the sub 1.2 annualised PUE range.
Despite these similarities, there are major differences that should be considered by anyone exploring a free cooling solution for any data centre refurbishment or new-build project. Furthermore, there is no one-fit solution because there are simply too many parameters that need to be considered, such as location, temperature, humidity, resilience and risk factors.
Contamination
With direct solutions there are a number of operating challenges; one being the risk of contamination by introducing fresh air. The additional time and cost for necessary changes of filters needs to be considered, especially in built-up urban areas.
Another consideration is the effect to the availability of the facility. With incidents such as fire, smoke or other fumes, the fresh air systems often need to be turned off, so a secondary system with 100% of the cooling capacity is required. This backup system is rarely efficient and, when used, draws significant amounts of power. Typically this can be more than the IT systems and the impact of this peak facility power demand or peak PUE which could be well above two is in oversizing generators, switchgear and electrical systems and in having to keep this power in reserve.
Building restrictions
Another consideration for direct systems that need to be evaluated during the design is the impact of integrating the associated ductwork and plant to move sufficient air in and out of the building. In particular, for refurbishments or installations in an existing property this cannot be ignored, especially for landlocked rooms, because of the potential for additional building work, cost and security.
An example of an alternative approach that does not use fresh air but delivers high efficiency and performance is that specified for PGS in Weybridge, Surrey. This facility uses an in-direct, high efficiency system recycling the internal air and not introducing any fresh air. This overcomes all of the considerations above and, having operated for over two full years, is an excellent example of true annualised PUE figures. With a full 2N cooling system and an annualised PUE of 1.15L2,YC it is one of the most efficient facilities currently in operation. However with a peak PUE of 1.5 it provides significant indirect savings on electrical systems and more power available for IT.
A different approach
We are also seeing an increasing number of operators in the co-location marketplace utilising this kind of in-direct cooling solution as part of a wider modular data centre approach. This would enable the development of facilities in a number of phases, without impacting performance and whilst achieving high levels of availability and efficiency.
For example, business-class data centre provider, Datum, opened its flagship data centre earlier this year at the high-security QinetiQ Cody Technology Park in Farnborough, Hampshire. The first phase provides an initial 250 rack positions with an average and maximum IT load of 3.5 and 20kW per rack respectively, and has been developed to be easily scalable up to 1000 racks. It possesses high levels of availability and an annualised design PUE of 1.25 L2,YC.
The design uses an in-direct cooling solution that not only delivers world class levels of efficiency and performance, says the company, but also the infrastructure needed for high availability and flexibility. The ability to effectively scale the cooling allows the company to develop capacity in an economically manageable and highly sustainable way.
The next step
The next big step for data centre operators will be to consider removing the chillers altogether. For those with a direct fresh air approach, it is not only temperature rises that require mechanical cooling but other events outside of the operator’s control.
In contrast, with an in-direct solution, which is optimised for efficient operation in all situations, we can consider removing the chiller. For when temperatures rise above 24ºC, control systems can elevate the supply air temperatures within the data centre up from 22ºC. With most servers able to operate at much higher temperatures, and ASHRAE increasing the allowable limits, this strategy could not only reduce capex but also improve efficiency.
There are many benefits of free cooling, but there are key decisions to be made during the design process. Any data centre owner or operator should take a close look to ensure any planned solution meets their requirements in terms of risk, availability, efficiency, maintenance and construction.
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