environment envelope. Figure 2 shows this as the blue area. This free cooling can be tempered with recirculated air from the space when the outside temperature is too low. Water-based economisers use the outside
Indirect adiabatic cooling
80% rh Mechanical cooling
Recommended envelope
Allowable envelope Class A1
Fresh air free cooling
Figure 2: Free cooling options to meet the required environmental envelope 18
-20 -15 -10 -5 0 5 10 15 20 DB temperature (C) 23 25 27
of the IT equipment. This presents a client with two options regarding reliability of IT equipment and energy savings, both in the IT equipment and the cooling plant: ● Option 1: use IT equipment that is optimised predominantly for reliability; and
l Option 2: use IT equipment that is optimised predominantly for energy saving and compressor-less cooling.
The ASHRAE white paper suggests that the industry requires both options, but also needs to avoid having option 2 inadvertently increase the purchasing cost of option 1 through mandatory requirements not desired or used by the end user. For example, expanding the temperature and humidity ranges can increase the physical size of the IT equipment and its required air fl ow (fan size). This can affect the embedded energy costs and power consumption, with the end result of increased cost of the IT equipment. The principal design goals to be considered
in the light of the recommendations from the ASHRAE 2011 paper2
are:
● Reliability of the IT equipment; l Low power usage effectiveness (PUE) (the ratio of power delivered to the site to the power used by the IT equipment);
l Adequate ventilation; l Maximising the use of ambient conditions; and
l Minimising the use of mechanical cooling.
With the recommended environmental envelope from the ASHRAE white paper, the possibilities are great for reducing mechanical cooling for large energy users such as data centres. Based on the psychrometric chart analysis in Figure 2,
52 CIBSE Journal April 2012 20% rh 30 32
Direct adiabatic cooling
35 40 45 50
air to cool a fl uid-based (water or glycol solution) system through a heat exchanger in the fresh air supply duct. The fl uid is pumped to a second heat exchanger in the space supply air to provide the required cooling. These are not as effi cient as air-side economisers, because they require a water- to-air temperature difference for the heat exchange at both ends, but they may be the preferred – or only – option where supply and extract ducts are not close together. There will be some energy consumption with both these systems, in the form of additional fan and pump power.
however, while the majority of cooling can be achieved without mechanical cooling, the need for reliability is one of the dominant factors, as the right weather and operating conditions cannot be completely guaranteed. Therefore, a system with mechanical cooling will normally be installed. The expansion of the possible environmental operating area for data centre cooling has spawned a revolution in the approach to cooling design and a new challenge to the equipment manufacturers to innovate and design suitable new solutions for low energy equipment. Where mechanical cooling is incorporated
into the system, it must also maximise its effi ciency, and this suits the centrifugal, oil- free, magnetic bearing, low starting/running current range of compressors. Energy effi ciency ratios (EERs) of these machines actually improve at part load, which is ideal for applications like data centres, since the compressors may run unloaded for long periods when adiabatic cooling and free cooling are meeting the majority of the cooling demands. The cooling options for data centres are:
● Direct cooling with an economiser; l Indirect cooling with an economiser; l Adiabatic cooling; and l Mechanical cooling. An air-side economiser simply uses the outside ambient air for cooling when its temperature is low enough to meet the space cooling demand within the recommended
Figure 3 Hybrid air cooled chiller
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Maximising effi ciency using adiabatic cooling3 The free cooling range of operation can be extended by using adiabatic cooling. This is the evaporation of water in either a fi ne spray or an air washer method. Every litre of water evaporated requires about 2,500 kilojoules (kJ) of energy, and this energy is taken directly from the air, cooling it to a temperature close to the air wet bulb temperature. This is most effective in high dry bulb temperature, low humidity conditions. Control limits are set to avoid the humidity rising above the recommended environmental envelope. In Figure 2, the green area indicates the extended area of free cooling possible. Additional energy savings can be made
by installing a secondary air- or water-based heat exchanger, with adiabatic cooling in the extract air from the conditioned space. The cooling of the extract air, in turn, provides more cooling to the incoming fresh air. Since the adiabatic cooling takes place in the extract air side of the heat exchanger, there is no restriction on the humidity level. This further free cooling appears in Figure 2 as the pink area.
Moisture content
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