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29-04-2011, 09:50 AM #1
Air and water side economisers in Data Centers
There is a common myth that recently pervaded the IT sector, namely the use of so called “Air side Economisers” or ASE later in the document. ASE consists in fresh air intake into the Data or Telecommunication Centres (Datacom as referred later on), in order to obtain a pre-cooling of the air. The recent publication of the technical specification of a big social network Datacom (available at http://www.opencompute.org/) further introduced elements of confusion.
Nonetheless at least one viable alternative is available, namely the so called “Water side Economising” or WSE later in the document, that is a heat exchange process, in which the heat transfer media is water.
I will try to discuss the pros and cons of both methods.
Multiple requirements are facing:
- Need to reduce the energy costs and carbon footprint for cooling the Datacom, by exploiting cool outdoor conditions
- Pressurisation of Datacom to avoid ingress of dust and other volatile organic and inorganic compounds (VOCs or VICs)
- Possible heat recovery for heating adjacent living spaces.
Installing air handlers (AHU) with multiple configurations of coils, and heat exchangers possibly satisfies two of three above requirements, while being quite energy hungry.
A different approach would be by directly blowing outdoor air into the Datacom, after a filtration and an evaporative cooling treatment. The energy efficiency of the latter solution is undisputed. It is also true that many US Datacom centres are progressively reducing the classic compressor cooling and favouring the evaporative cooling (EC), but it is my humble opinion that EC still shows signs of immaturity, while carrying an intrinsic risk towards the integrity of the electronic equipment.
Specifically, even whenever a good grade of filtration and pre-emptive maintenance are ensured, the presence of particulate (PM10 and PM2.5) may damage or create micro short circuits over the PCBs.
In the greater areas of metropolitan cities, specifically in SE-Asia, but also in Europe at or around chemicals and petrochemicals compound plants, the presence of elements and VOCs was noted. It is ascertained that Chlorine, SOx, and Hydrogen Sulphide are causing creep corrosion, via corrosion, and formation of Sulphur crystals that may originate short circuits.
Secondly, it must be noted that EC is consuming enormous quantities of water, which should avoided where historically water is scarce, and also for ethical reasons.
Thirdly, it can’t be ignored that the latent heat added to the Datacom by the EC process technically makes controlling humidity impossible or extremely costly.
Even where EC is not necessary, or “Air side Free Cooling” (ASFC) is used (that is plain admission of cold winter air), the pollution originated by the presence of chemical compounds in outside air, and the associated corrosion phenomena, are still critical points that can’t be ignored.
Last but not least, the Latitude of Datacom is fundamental when ASFC is planned, as the ratio between implementation costs and energy saving shall be carefully analysed from a financial point of view. In fact, statistics of savings based on historical and climatological data do not guarantee similar or comparable financial results in the future.
The requirements are the same than ASE, but in this case the heat vector fluid is water.
There are two universally accepted methods:
- Outdoor air to water chiller, with Free Cooling exchange section. In this case, the on board microprocessor automatically switches between compressor and free cooling based on the actual weather condition. The process is total transparent to Datacom close control units (CCUs)
- Water-cooled CCUs, connected to outdoors dry coolers, and built-in Free cooling coil. In this case the switching between compressor and free cooling is done via the CCU on-board microprocessor, by reading the return water/brine temperature from the dry cooler.
The main advantages of this approach are:
- Totally segregated Datacom volume. No possibility of VOC/VIC pollution ingress.
- Easier installation, lower first costs, and quicker set-up compared to ASE.
- Close circuit ensures no water consumption at the expense of a minimal cost of pumping.
Disadvantages can be summarised as follows:
- Again, historical and climatological data do not guarantee successful financial breakeven in the expected term.
- The use of a secondary fluid (water or brine) reduces the overall efficiency, due to an additional thermal exchange (air to water to air).