In a two-part article, David Barker, founder and technical director at green colocation and connectivity supplier, 4D Data Centres, discusses proven methods for decreasing the carbon footprint of data centres.
For some time data centres have been labelled as big energy users. Estimates put the usage of the data centre industry at 31GW per year or 1.3 percent of the world’s total energy consumption. Not only does this make data centres ‘carbon intensive’ (around 0.2 percent of the world’s carbon emissions or about half of Argentina’s emissions) but it means that traditional data centres are expensive to run.
Data centre CO2 emissions in 2009
In a traditional data centre there are three main areas where energy consumption and hence carbon footprint can be reduced:
1. Air-conditioning systems
2. Electrical infrastructure
3. The optimisation of server hardware and virtualisation.
This is the main ‘big win’ that will net the largest savings in energy usage, especially if replacing a legacy air-conditioning system that is poorly designed or over five years old.
The overall efficiency of a data centre is measured using the Power Usage Efficiency (PUE) metric that looks at the ratio between total building power versus IT load power. Most data centres come in around 1.8 - 2 with a very efficient data centre coming in around 1.1 - 1.4.
Historically, the energy usage of cooling systems was almost the same as servers and other IT equipment, but there are some improvements that can be made. Each air-conditioning manufacturer has their own take on the method but they largely fall into one of the following camps:
- Moving cooling closer to the heat load
- Liquid-based cooling
- Open/closed cooling architecture
- Increasing the cold/hot aisle delta ‘T’
- Economisation and adiabatic cooling.
For the purpose of this article we’ll focus on the two that have the biggest impact on energy consumption, which are moving the cooling closer to the heat load and economisation.
Moving cooling closer to the heat load
Traditional raised-floor, room-cooling systems with CRACs around the perimeter have proven to be an effective way to control data centre temperatures. However, with increased power densities in racks (over 6KW per rack) and greater variation in load across a data floor there are other ways to cool the data floor which may be more efficient at supporting this type of heat load.
One of these is to move the cooling closer to the IT equipment itself, by using in-row coolers for example. The fans consume a significant proportion of the energy in an air-conditioning system, so by shortening the path that the air has to take to and from the cooler you can save up to 30 percent with smaller fans that move less air.
For ultra-high density environments, such as those requiring 10KW and above, you can go one step further by mounting the cooling directly onto the rack which can provide energy savings of up to 70 percent when compared with cooling a similar load with perimeter cooling.
Economisation and adiabatic (or evaporative) cooling
The ‘holy grail’ of efficient cooling is the combination of airside economisation and adiabatic cooling that should deliver a PUE of at least 1.1. Both techniques are generally only applicable in new build data centres, but some aspects can be retro-fitted to existing buildings, as we see here.
Retrospectively fitted evaporative cooling units fitted at 4D Surrey
Airside economisation is the process whereby hot return air, which exits the servers, is vented directly outside the building, while cooler ‘fresh’ air is drawn into the system, filtered and then delivered to the servers on the data floor as cool supply air. Airside economisation provides energy savings when the outside ambient air temperature is below that of the hot return air being exhausted. The ambient air cross-over point is generally around 16 - 20 degrees. Once the cross-over point has been reached some systems will change over to a traditional cooling system (which doesn’t have the same energy savings).
A more efficient method is to use adiabatic cooling whereby the outside ‘fresh’ air is drawn through a wetted filter, or fine mist, using the process of evaporation to reduce the air temperature. A key advantage of an adiabatic system is that hot outside temperatures make the adiabatic evaporation process more efficient.
Airside economisation is best suited to climates with a stable, moderate temperature and humidity. Airside economised projects in colder, drier climates can also be very efficient but work best at sites that can tolerate wider humidity ranges for their technology equipment than is typical for many enterprise users.
In part 2, David Barker will discuss the other two areas of energy efficiency within a data centre: the electrical infrastructure and the optimisation of server infrastructure through virtualisation.
Posted by David Barker, founder and technical director at green colocation and connectivity supplier, 4D Data Centres
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