Cooling our data centre


In a traditional data centre cooling facilities consume significant energy thus impacting on the overall operating cost and carbon foot print. In fact up to 45% of data centre operating costs can be attributed to cooling (1). 

Also, according to (2), cooling infrastructure can represent 42% of a data centre’s energy usage. It is also recognised that cooling inefficiency is a key issue in data centres but one which can be improved through alternative cooling methods.

As a result data centre cooling is one of the areas where energy efficiency schemes can possibly yield significant energy savings. This section reviews some key aspects of data centre cooling designs and recommendations.

Cooling Infrastructure

The cooling infrastructure found in data centres is pretty much similar to that in buildings except that in data centres, the cooling system is expected to meet stringent requirements of availability, reliability and robustness.

Depending on the size of the data centre, the cooling system can vary from a small split computer room air conditioning (CRAC) unit in small computer rooms to large chilled water systems in enterprise data centres.

In general cooling systems are equipped with a mechanical refrigeration plant acting as the source of cooling, to an air flow stream that is passed through the hot servers to accomplish the necessary heat transfer.

In more modern cooling systems, the refrigeration system provides cooling to a liquid (commonly water) that circulates through a heat exchanger mounted in front or at the base of the server containment cabinet across which air is blown before it proceeds through the servers to accomplish useful heat transfer.

Daniel Kennedy (2) in his white paper “Understanding Data Centre Cooling Energy Usage & Reduction Methods” describes in detail various cooling systems used in data centres today and presents a general analysis of the relative operational cost and carbon emission savings for each system.

“Free cooling” versus chilling systems

“Free cooling” is a phrase that has developed to describe a data centre cooling method that utilises freely available means of accomplishing cooling. This could be through using free cold ambient air or free cold water or ice.

However “free cooling” in itself is not entirely free since some energy consuming components like fans and pumps are needed to make the design work. The benefit however is that the energy used by these components can be a lot less than the energy used by other mechanical cooling methods.

On the other hand chilling systems may be used to provide the cooling demand of the information technology (IT) equipment.  Chilling systems in general consist of mechanical refrigeration systems acting as the source of cooling for the data centre.

Data centres always need cooling whether outside ambient air temperature is lower than inside air temperature or not. A chilled water based cooling system can be modified by providing an additional parallel redundant cooling system sharing the same heat exchanger. When the outdoor temperature is lower than indoor temperature, compressors can then be switched off, allowing the parallel redundant system to provide the necessary cooling effect using ambient air. Such a system is commonly referred to as a water-side economiser.

Economisers are a form of “free cooling” since the only energy consuming component would be a pump and a few fans which significantly offset the high energy requirements of compressors.

The other form of “free cooling” uses an air-side economiser. Here, when out door temperature is below the indoor temperature, cold ambient air is drawn inside the data centre to accomplish cooling while the conventional mechanical chilling system is off. As only fans, humidifiers or dehumidifiers and controls are the only power consuming components, air-side economisers present a great opportunity for energy saving.

Therefore with air-side economising, vast amounts of cold outside air is brought into the data centre space for cooling whereas with water-side economising cold outside air passed through a heat exchanger from where it absorbs the heat carried by the cooling liquid circulating between then data centre space and the heat exchange.

Because of the heat exchanger, the cooling effect obtained by using water-side economising is less compared to air-side economising but in both cases attractive energy savings are realised (3).

The choice of cooling methodology depends however on many factors including:

Server temperature and humidity requirements

Of particular importance is maintaining the temperature and humidity of the air inside the data centre within acceptable limits. Too low humidity causes electrostatic discharge (4) and too high humidity causes condensation, rusting and hence IT equipment failures.

The American Society of Heating Ventilation and Air Conditioning Engineers (ASHRAE) specify the following guidelines for humidity and temperature in data centres.

This provides clear guidance for data centre managers and suppliers ensuring a high level of IT equipment availability and operating life. Clearly the deployment of air side economisers for ambient air cooling depends on the ability to control the data centre conditions e.g. moisture content and temperature. These determine where in the world air side economisers can be used and where they can not.

Conclusion

About half of the energy consumption in data centres is attributed to conventional cooling infrastructure i.e. chillers. If these systems were coupled with air-side or water side economisers to form an economiser-chiller hybrid, significant energy savings would be realised depending on the location of the data centre.

ASHRAE provided guidelines for sever temperature and humidity requirements that need to be adhered to. As such air-side economising would require proper monitoring and control of air temperature and humidity as well as any particulates that may exist in the air before bringing it into contact with the servers.

 

References used in this page:

(1) PriceWaterhouse Coopers 2007: Benchmarking Study on Iceland as a Location for Data Centre Activity, May
(2) Kennedy, D 2009: Rittal Corporation, White Paper 507: Understanding Data Center Cooling Energy Usage & Reduction Methods, accessed 12 May 2010 http://criticalpowergroup.com/downloads/Rittal%20Data%20Center%20Cooling%20Efficiency%20White%20Paper%20%20507.pdf
(3) Air-side and water-side economizers in the data center, 13 August 2009, online video, accessed 11 May 2010, http://link.brightcove.com/services/player/bcpid33551242001?bclid=33547488001&bctid=33576827001
(4) ASHRAE, 2008 ASHRAE Environmental Guidelines for Datacom Equipment, accessed 12 April 2010, http://tc99.ashraetcs.org/documents/ASHRAE_Extended_Environmental_Envelope_Final_Aug_1_2008.pdf