Power & Cooling
As such air side economisers with adiabatic cooling can significantly reduce the hours requiring mechanical cooling, particularly for warm climates.
The above chart illustrates the concept of air side economiser with assisted adiabatic cooling using ASHRAE recommended envelope and 23°C / 60% RH target supply condition. The area in blue represents free cooling from the air side economiser. The green area represents the additional benefit using direct evaporative cooling (DEC) and the yellow area represents the benefit from indirect evaporative cooling (IEC). The conditions requiring purely mechanical cooling are reduced to the red area.
Typical energy savings of 80 to 95% are available when DEC is active and 30 – 93% when IEC is active vs. mechanical cooling (assuming high pressure spray vs. mechanical system).
Of course we must also consider that water is a valuable resource and commodity. But we must not forget the cost and impact to produce the electrical power. Let us consider the carbon footprint required to produce 1KWh in the UK is approximately 500g CO2 (source EDF energy) in comparison the average CO2 per litre of water is 0.5g (source
Carbonfootprint). As stated earlier, we get 680 Watts cooling per litre; therefore we need approximately 1.5 litres for 1KW of cooling. Considering a COP of 3.0 for mechanical cooling, we get 167g CO2 per KWh. Similarly the DEC Cooler with a typical COP of 156 will have only 0.0032g CO2 per KWh = 99.9% reduction in Carbon Foot Print for every KW.
These figures will vary depending on specific conditions such as design, location, equipment, COP etc;
February/March 2011
Fig 3
however adiabatic cooling offers substantial reductions over mechanical cooling. Rain water harvesting can help offer alternative solutions to further reduce the Carbon footprint.
The adiabatic cooling system has one further advantage. During the winter when cold external air is mixed with the warm return air, the humidity (RH) level can drop below the recommended envelope for data centres. ASHRAE recommends minimum RH levels in order to reduce the risk of static charge potential build up which significantly increases below 35% RH.
In this instance the adiabatic system can be utilised as a humidifier to increase RH levels to an acceptable level without the requirement for a steam humidifier which operates at 750 Watts per litre of water compared to 5 Watts typical for a high pressure spray system. Steam humidifiers can significantly increase a data centres power usage and should only be considered if adiabatic systems cannot possibly be used.
Today’s adiabatic systems in-
corporate sophisticated controls to manage washing, cleaning, emptying and purging cycles to guarantee the highest hygiene standards without the need for biocide treatments.
High pressure spray units present the latest in adiabatic technology and hygiene standards with many systems certified for use in critical applications such as hospitals, clean rooms and of course data centres.
Not all applications will be able to take advantage of air side economisers with assisted adiabatic cooling. Retrofits can be difficult or impossible to alter but new build sites have the opportunity to really take advantage of the possible savings.
Sites like HP Wynyard Park near Middlesbrough which uses an air side economiser design with high pressure adiabatic humidifiers is already setting new standards with a PUE of only 1.2.
Finally it is important to note that some locations with high humidity or a very cool climate may not benefit significantly from adiabatic cooling due to high RH levels or limited benefit.
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