The challenge to reduce mechanical cooling and carbon footprint has led to further developments in free cooling and adiabatic cooling. This article specifi cally looks at high energy use data centres, and at the evolving recommendations to reduce their energy consumption both by the use of free cooling equipment on mechanical cooling systems and by the introduction of adiabatic cooling equipment. The article follows a previous CPD in the July 2010 Journal, titled Evaporative cooling enhancement on air cooled chillers. However, one of the driving forces to develop lower energy consumption cooling systems has been the white paper prepared by the ASHRAE Technical Committee 9.9, 2011 Thermal Guidelines for Data Processing Environments1. The principal elements of this paper will be considered, followed by the low energy cooling options that are currently available to meet the recommendations of the white paper.
ASHRAE TC 9.9, 2011 Thermal Guidelines for Data Processing Environments
The first edition in 2004 sought to bring some recommendations for data centre environmental conditions. It was upgraded in 2008, with the aim of producing an environmental envelope that would lead to high reliability and energy effi ciency, and then further upgraded in 2011. The main emphasis of the latest edition is to extend the recommended envelope to provide more options for the type and use of data centre server equipment, and to take into account the effects of different environmental conditions on server equipment. For example, the higher the selected room temperature, the greater power is required to operate the server equipment and the lower its reliability. This has to be balanced against the requirement for smaller size cooling equipment, free cooling and/or adiabatic cooling enhancement. Figure 1 shows the recommended envelope and the allowable envelopes for the different data centre classes developed.
Figure 1: ASHRAE environmental classes for data centres
The ‘allowable’ limits expressed in Figure 1 are described as being acceptable to operate at for short periods of time, without affecting the overall reliability and operation 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;
- 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;
- Low power usage effectiveness (PUE) (the ratio of power delivered to the site to the power used by the IT equipment);
- Adequate ventilation;
- Maximising the use of ambient conditions;
- 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, 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.
Figure 2: Free cooling options to meet the required environmental envelope
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 efficiency, and this suits the centrifugal, oilfree, magnetic bearing, low starting/running current range of compressors. Energy efficiency 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;
- Indirect cooling with an economiser;
- Adiabatic cooling;
- 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 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 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 waterto-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.
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 fine 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.
Figure 3: Hybrid air cooled chiller
Free cooling hybrid chillers
There are a number of free cooling and adiabatic chillers on the market today (including the one illustrated in Figure 3). Some have screw compressor technology and inverter drives with electronically controlled heat rejection fans, to try to reduce the high start up currents and high part load running currents, while others incorporate the centrifugal, oil-free, magnetic bearing compressor package already mentioned. These have a combination of both free cooling for low ambient operation and adiabatic cooling for high ambient scenarios (hence, ‘hybrid’). The compressors have particularly low starting currents of about 5 amps per 350 kW cooling capacity. Seasonal EERs greater than 12 are easily achievable with these chillers, plus the higher chilled water temperatures allowed with the environmental envelope recommended for data centres. Table 1 gives an indication of the performance for the different options in which the chiller can operate.
A further development to this hybrid chiller is the addition of an actively managed evaporative system. Water is absorbed by a porous natural-fibre honeycomb array in the direct air path of the coils. The adiabatic cooling effectively reduces ambient temperatures in the immediate vicinity of coils by up to 10C, lowering condensing temperatures and significantly improving the chiller’s energy performance. The adiabatic advantage also increases chiller capacity at peak load conditions, enabling it to cope with high ambients that might otherwise overwhelm a standard chiller. The system can be set to activate automatically at a pre-determined external temperature. Water consumption is low. In UK conditions, £600 worth of water consumed a year results in energy savings worth some £8,000. In summary, the energy savings and improved performance also provide the following benefits:
- Extended working life due to reduced compressor run-time;
- Reduced service and maintenance, due to reduced run-time;
- Payback time further reduced;
- Extended chiller capacity at peak load, enabling it to cope with extreme ambients that would defeat other chillers.
Floating head pressure
The system operates with a floating head pressure, providing opportunities for savings not available to conventional designs. Unlike standard chillers that have fixed head pressure, this centrifugal compressor package constantly selfregulates and optimises its performance in response to ambient conditions and load.
Figure 4: Water cooled Turbocor chiller with adiabatic cooler
Water cooled options
Where internal plant room space permits and/or noise design criteria is exceptional, water cooled versions of these low energy chillers can be used, matched with closed circuit cooling towers or adiabatic coolers. Figure 4 illustrates a typical system. Although doubling up on plant items, water cooled versions of the oil-free technology can achieve ESEERs (European Seasonal Energy Efficiency Ratio) in excess of 10, while using the higher allowable chilled water temperatures and capitalising on adiabatic cooling at higher ambient conditions. Centrifugal, oil-free chiller packages up to 2.4 megawatts (MW) are available. Lower condensing water temperatures can be achieved with this equipment and will produce a very efficient overall cycle. Lower starting and running currents for this type of equipment can also reduce infrastructure cabling cost and maximum demand charges, as well as running cost.
© Terry Welch and Steve Chaplin (Klima-Therm) 2012
References
- ASHRAE TC 9.9 white paper 2011 Thermal Guidelines for Data Processing Environments
- Fresh Approach, CIBSE Journal April 2011, Marcus O’Brien
- Klima-Therm 2011 Adiabatic Free Cooling Options for Data Centre