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A new frontier in air conditioning efficiency for data centre cooling

A new frontier in air conditioning efficiency for data centre cooling

The data centre industry is continuing in its efforts to find innovative solutions to help reduce its carbon footprint, particularly when it comes to power and cooling. Here, Tim Mitchell, from Klima-Therm, tells us how the emergence of compact centrifugal  compressors has marked a new era for air conditioning efficiency and is helping to unlock environmental and business benefits.

One of the key areas of innovation in the data centre industry over the past 10 years has been improvement in energy efficiency. It is central to both improving profitability through reducing running costs and enhancing Power Usage Effectiveness (PUE) and other energy-related metrics to meet the sustainability requirements of corporate clients.

Despite the emergence of more temperature tolerant chips, one of the biggest components of data centre power usage remains cooling. There have been attempts to manage this with fresh-air-only ventilation systems, but issues with latent system requirements, space constraints and concerns around reliability and resilience mean that mechanical cooling ­– of one sort or another – remains the default choice.

Mechanical cooling relies on refrigerant compressors, an area of technology that had remained more or less static for decades until the emergence of compact centrifugal compressors around 10 years ago. Their appearance marked the start of a new era for air conditioning efficiency. In those early days, few recognised the impact this rather esoteric new technology would have on the market and the wider industry.

As one of the first adopters in the world of this new approach, I will admit we were slightly mesmerised by the idea of harnessing magnetic levitation bearings in a compact centrifugal design. It was a compelling proposition, as it overcame the need for oil in the compressor, thereby avoiding at a stroke all the problems that accompany conventional compressor lubrication, spanning operation (especially low-load operation as data halls are populated) and the requirements of ongoing service and maintenance.

That advance alone would have been highly significant and a major advantage for both data centre operators and service companies. However, when you add the exceptional efficiency gains – generally 50% better than traditional systems – smaller chiller foot-print, low start-up current, low noise operation, long-term reliability and overall low maintenance requirements, it is not hard to see why compact centrifugal technology has become such a game-changer.

In a nutshell, it enabled more cooling from less energy in a more compact space and required less power to start and fewer service visits to maintain. The fact that it is compatible with efficient and stable low Global Warming Potential (GWP) HFO refrigerants is a further major plus for data centre operators looking to future-proof themselves against legislative changes.

It is a classic example of a disruptive technology rewriting the rules. The early sceptics have been proved decisively wrong. With the technology well-proven, multi-million-pound investments are constantly being made in expanding production of Turbocor compressors and further refining the technology.

With take-up growing across the world, manufacturers of traditional cooling compressors are now looking to develop their own magnetic levitation-based compact centrifugal systems. However, as often with disruptive technology, it is a steep research and development curve and requires very substantial investment.

Early adopters are already working on the next generation of systems, which take the gains delivered by Turbocor-based systems to the next level. A stand-alone compact centrifugal compressor is already highly efficient and the opportunities for wringing further efficiency gains are inherently limited. However, there are significant opportunities for improving efficiency in terms of the overall chiller design and the performance of other key system components.

Heat exchangers are a key area of interest. For example, the recently introduced, multiple award-winning Althermo CMR is a new design of condenser heat exchangers. Coupled with the already outstanding efficiency of Turbocor-based chillers, it can be selected to deliver the multiple benefits of energy saving, lower noise, or smaller footprint in the proportions dictated by the project specific requirements. If the system is fully optimised for energy efficiency, it can create the most efficient dry air-cooled chiller in the world.

This has been made possible by replacing the traditional flat coils with cylindrical condensers. By packing more active heat exchange surface into a given space, the heat exchange capacity of the Althermo CMR cylindrical microchannel condensers is increased by 45% compared with traditional condensers. Combine this with the use of flooded, falling-film or spray type evaporators in a cascade system and there is currently no more efficient way to mechanically cool while using a low-risk, high-resilience dry system.

Importantly, this improvement in performance is achieved without increasing the chiller footprint, enabling more cooling capacity in a given space, while significantly reducing energy consumption.

With rooftop and plant room space often at a premium in new and refurbishment projects, this offers a major advantage over both conventional chillers and standard Turbocor-based machines.

The use of a cascade system with flooded, falling-film or spray type evaporators helps reduce the temperature difference between evaporation temperature and outlet temperature of the chilled water. This increase in evaporation temperature further reduces energy consumption.

Comparing the like-for-like performance of such a system with standard air-cooled Turbocor-based machines (at AHRI/EUROVENT conditions, with the same number and model of compressors) shows an increase in EER of up to +9.5% with one compressor and up to +15% with multiple compressors.

The highest EER value achieved is 4.35, but the most staggering comparison comes at part load or full load / less than peak ambient temperature conditions, where gains of +25% are typical compared to even the most efficient existing Turbocor-based machines.

This gives equivalent efficiency to Turbocor-based chillers equipped with adiabatic evaporative systems (at 50% relative humidity), but without the additional cost and complication of installation and maintenance associated with adiabatic systems.

It lends itself to use in refurbishment projects, where data centre cooling loads may have increased over time and the existing chiller is now under-sized, but where plant space is restricted and unable to accommodate a larger replacement. This situation arises often, particularly in city centres.

While manufacturers of conventional compressor technology seek to catch up with the compact centrifugal revolution, those who pioneered the technology are already several laps ahead – on both the refinement of the base technology itself and its application in ever-more efficient systems. The early adopters bore the risks but are now harvesting the fruits of embracing this game-changing new approach.

For data centre operators wrestling with the often conflicting demands of rising cooling loads, limited power head-room and site space restrictions, these latest developments in heat exchange and compressor technology offer potential solutions not hitherto possible with conventional mechanical cooling.

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