The rapid growth of data centres has highlighted the need for tailored cooling solutions that meet the demands of the hardware and the room constraints while optimising energy efficiency. Guy Hutchins of Trox looks at why carbon dioxide is gaining favour
Comfort air conditioning in offices has evolved significantly over the last decade by moving away from all-air systems to either chilled water (CHW) or refrigerant-based technologies.
However, this progress is not reflected in data centre cooling. Ironically, high-tech applications tend to employ outdated all-air systems that struggle to cool the ever-increasing heat loads produced by today’s cutting edge blade servers and state-of-the-art computers.
We can only assume that this is a result of the conservative attitude that prevails in the cooling of these facilities. This attitude is probably driven by the mission-critical nature of data centres.
The consequences of a failure in a data centre’s cooling system is unthinkable – it could even wipe out the business. That means new systems that are perceived to be untested are often viewed with suspicion.
However, the advent of blade servers and the growth in high density (HD) data centres means that conventional computer room air conditioning units (CRACs) can no longer cope with the high heat outputs generated, which can reach a staggering 15,600 W/m² or 30kW per cabinet.
In fact, even above 5kW per cabinet, air struggles to deal with modern computer technology. In order to preserve the use of CRAC units, more space must be allocated to the cabinets or they must be populated less densely so that the heat load is kept to around 5/8kW per cabinet.
In the table below we give a guide to the various media that could be chosen for rack cooling together with a simple classification of rack density.
|Rack Density||kW per Rack||Medium|
|Low Density (LD)||1 – 7 kW||Conventional Air|
|Medium Density (MD)||8 – 14 kW||Fan Hood Powered Air / CHW / CO2|
|High Density (HD)||15 – 24kW||CHW / CO2|
|Super High Density (SHD)||25+kW||CO2|
Managing the heat load
With the advent of blade servers and higher rack densities, heat loads are increasing and the IT design engineer has to progressively look for an alternative to conventional all-air systems.
There is an inevitability that as server processing power continues to increase, space, power and cooling constraints will ultimately drive data centres to become HD facilities. The conundrum is to decide how and when to move from a low to high density strategy in a cost effective and manageable manner.
Solutions that are scaleable and expandable and which can harness every last drop of available conventional cooling, and directly deliver it to the servers, must be the first option.
In doing so the low density suite is upgraded to a medium density facility, staving off the expense of jumping prematurely to HD. And then, only when HD is truly required and not merely an aspiration, should these scaleable medium density systems be upgraded with ‘drop-in’ liquid cooling coils. This philosophy holds true for small scale server rooms and large data centres alike.
The initial choice for liquid cooling is often water and indeed it is certainly a more efficient cooling medium for servers than air. However, it conducts electricity and water damage will have a disastrous effect on the electrical equipment and cabling with which it comes into contact. The risk of water leaks often deters deployment within a data centre environment.
The perfect medium
Carbon dioxide, on the other hand, is a perfect cooling medium for IT cooling applications because of its key properties.
For example, a carbon dioxide-based system:
- Saves up to 30 per cent in energy through a combination of its integrated R134a/CO2 chiller and CO2’s lower viscosity and pumping duties when compared to CHW CRACs. For example, a 250kW system reduces CO2 emissions by an impressive 290 tonnes a year
- Has zero ozone-depleting potential combined with low toxicity.
- Poses no danger to electrical equipment or cabling because CO2 is electrically benign
- Has seven times the cooling capacity of water per kg, reducing volume flows with smaller diameter distribution pipework
And, because CO2 cooling is more efficient than air and water alternatives, the footprint of IT equipment rooms can be reduced by 50 to 70 per cent, saving valuable space, as equipment densities can be increased.
The energy saving potential of CO2 is another compelling argument in its favour. For example, our carbon dioxide-based blade server cooling system CO2OLrac offers substantial savings in power consumption: for a 1MW installation, energy costs can be reduced by up to £155,000 a year.
The system, which is based on circulating CO2, is designed to absorb the heat rejected by blade servers at the rear of the cabinet. It represents a positive use of CO2, a bi-product of industrial processes, which could otherwise threaten the environment.
The pursuit of low energy cooling has led to further system developments; initially to include a free cooling option and more latterly to include thermosyphon.
The thermosyphon principal has been used widely in primary refrigerant circuits in data centres, but now a CO2 version is available in a scaleable form to provide HD cooling to a single cabinet if necessary.
The CO2 circuit is pump-less and energy efficient, particularly when combined with a free cooling primary circuit.
The thermosyphon cycle is driven by the cabinet cooler harnessing the server heat. As CO2 changes state, the lower density vaporised CO2 naturally returns to the high level CHW heat exchanger to be condensed back to liquid.
This simple cycle will continue all the while the server is producing heat and there is a CHW supply to condense the vaporised CO2.
We believe that all major organisations, particularly those with an environmental agenda, will want to use CO2 not only because of its green credentials but because it’s non-water and guarantees high capacity performance, not only to meet today’s demands but also for future proofing.
Guy Hutchins is sales director of Trox Advanced IT Cooling Systems