Has Refrigeration Been Eliminated From Data Centre Cooling?

DATA CENTRES have moved a long way in a relatively short period of time in their designs. Initially they tended to utilise expensive and energy hungry cooling systems, all based on a design premise of 19°C air temperature and chilled water temperatures of 6°C, with refrigeration being the only source of cooling. In the next phase chilled water temperatures were slowly increased (where used) to 10°C to maximise energy and free cooling was introduced on systems that were water based.

These changes had one common goal – to reduce energy costs. We’d like to kid ourselves that the force driving these design changes was that we were being “green” but we all know the reality was a need to reduce our energy costs, in an era where they were identified as 33% of total operating costs. Along the path, refrigerant CRAC DX systems were left behind as somewhat expensive relics and have slowly been removed or left for standby basis.

Turbocor Compressors

The introduction of turbocor compressors was a real gold rush for the companies selling them. They had super-efficient compressors, flooded evaporators and oversized everything, which was great if chilled water was still required. In reality these had their own downsides, ranging from expensive specialist service requirements to super expensive replacement compressors. If the wonderful futuristic levitating bearings went wrong and the entire compressor required replacing you’d be looking at a figure of well over £20,000, which you can bet wasn’t mentioned in the sales pitch!

However, even turbocor chillers have now been left far behind in the race by server manufacturers to sell their equipment with higher and higher air on temperatures. This has driven the original 19°C 50% RH requirements to up to 27°C and beyond. ASRAE itself now deems that 27°C is a normal running condition, but 32°C can be acceptable for short periods of time. So, with these temperatures, can we now just use fresh air systems to cool our data centres rather than refrigeration?

The answer is - not entirely. Fresh air systems require large foot prints and won’t fit all foot prints - new data centres have to be designed with real estate in mind and where you are trying to build will probably more than dictate what you eventually turn to. As with all things, there are those that advocate the systems and those that reel off many disadvantages, whilst ignoring the benefits.

As our ambient temperature now regularly hits 35°C at some point during summer and the quantity of adiabatic water that has to be stored in case of mains water failures, many systems still have full dx or chilled water backups to provide their N+1 or N+N system. The other major down side is their ability to shift enough air in medium to high density solutions, this together with the fact that most servers now work with a 12°C rise across them at full tilt, if we have a design of 27°C that means that the hot aisle is close to 40°C and uncomfortable to work in.

So, is there an alternative that potentially gives a closed solution without having to duct large amounts of air in and out of a centre whilst still promising a refrigeration free, or chillerless data centre?

Well, nearly……. The rise of the water cooled rear door within centres, along with the return of cooling towers to industry thought processes, provides this perfectly. Or, as a cooling tower alternative, the use of a hybrid adiabatic cooler combined with a relatively cheap and cheerful chiller as the +1, is growing in its popularity due to efficiency cost and working environment.

Having already achieved a truly chillerless centre operating with the above and winning CEEDA gold award accreditation last year, we have clearly identified and proven that this can work (with a temperature neutral system that provides the front of the cabinets temperature or old cold aisle at the same temperature as the rear which used to be the hot aisle).

Many people do not like the idea of cooling towers because of legionella and their health and safety requirements. But, to those in the know, not one properly controlled tower within the UK has actually been the cause of legionella and, if properly maintained, will also not become at risk of it either.

However, as we have found, this does raise an interesting quandary - with the rise of water temperature and the possibility of using hybrid adiabatic coolers with a chiller mix, the actual energy saving with cooling loads below 500kW are in reality outweighed by water usage and water treatment costs and therefore the actual green credentials are somewhat false and the running costs overtake the energy savings.

Leak Prevention System (LPS)

One of the biggest sticking points that many people still have with water cooled systems is water being in such close proximity to the servers and data. Well, we can easily address this, by running the chilled water through a leak prevention system (LPS) which operates the cooling water under a negative pressure and therefore mitigates the risk of water escaping from the cooling system completely (whilst keeping it running - even with a leak). This British patented product has now been successfully employed in many countries across the world and can be designed for systems from 50 to 2MW.

So, in answer to the question of chiller less or refrigeration less data centres, I am convinced that not one size fits all and there are pros and cons for all systems. BUT it’s fair to say that the advantages of fitting super-efficient super expensive chillers has now been expelled and that good old natural sources of cooling such as bore holes, adiabatic hybrid cooling and cooling towers through evaporative basis, is the future for most retro fit and new systems. - Simon Davis, Sales Director

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