What is free cooling and how can it reduce air conditioning operational costs?
Free cooling provides significant opportunities to generate energy savings. It can take effect when the difference between the outside supply and return temperatures is as little as 1°C. This means that, in a 24/7 data centre with a typical room temperature of 24°C, over 95% of the year can be spent with free-cooling active.
Concurrent free cooling
Airedale pioneered the concept of ‘concurrent’ free cooling in data centres over 15 years ago. In addition to energy savings through reducing the need for mechanical (DX) cooling, concurrent free-cooling also maximises the part-load efficiencies of components such as EC fans, inverter-driven pumps and centrifugal compressors. The variable speed control on such components allows load to be very precisely matched to cooling duty, reducing energy consumption and unnecessary wear. EC fans for example are up to 70% more efficient than AC fans at part-load.
The use of temperature sensors and sequencer controls allows for cooling to be staged, ensuring a smooth transition from mechanical cooling to air free cooling. Airedale’s ACIS™ controls logic and sequence management, for example, can configure between two and six chillers to run as master/slave and run/standby. On sites with an air cooled and free cooling chiller, the sequencer ensures that when the ambient is low, the free cooling chiller is the first to start up.
In terms of the costs of ownership, energy represents by far the biggest item of expenditure, often outweighing the initial capital costs over the course of a unit’s lifetime; in the case of a traditional DX computer room air conditioning (CRAC) system, estimated energy costs over the course of five years can add up to 5 or 6 times the initial cost of the unit. Free cooling units can easily pay for themselves within the first 1 to 2 years and, by the end of five years, total costs of ownership can equate to around half those of DX systems. Free cooling opportunities can therefore make a significant saving on operational budgets, generating PUEs of 1.1 or below.
Reduced fan speeds
Energy can be reduced further by setting fans to operate at less than full capacity without compromising performance at full load; under Fan Affinity Law, a fan working at up to 80% of capacity for example will reduce energy consumption by up to 50% for the same performance.
Elevated supply and return temperatures
Raising supply and return temperatures for example to 18°C and 24°C respectively can also generate free cooling opportunities without compromising computer room functionality, still well within the ASHRAE (Class 1 & 2) recommended data centre upper operating temperature of 27°C (80.6°F).
Adiabatic or evaporative technologies
Adiabatic cooling applied to air side direct or indirect cooling represents a highly effective energy saving alternative where there is sufficient external footprint. For the more compact systems, adiabatic sprays or wetted media add a significant enhancement to the operational efficiency of the latest generation of free cooling chillers. Adiabatic is a heat-free method of transferring energy between systems through a fine water spray or media that can significantly reduce air temperature, especially in warm, dry conditions. This lower air temperature is then used as a cooling medium and, based on London ambient temperatures, can easily achieve ASHRAE recommended conditions using 100% free cooling. In more challenging environments, an optional mechanical cooling module will ‘top-up’ the cooling capacity with a partial DX supplementary cooling section.
For data centres, indirect heat exchange air handling units (AHUs) provide essential water and air filtration to prevent ingress of potentially damaging contaminants and, with their substantial energy savings potential, AHUs can drastically cut energy costs compared with DX and chilled water systems over the course of their lifetime.
A comprehensive building energy management (BEMS) module like the ACIS™ system allows monitoring and management of HVAC, lighting and other power units across large sites. Live and historical capture of power quality, energy consumption, CO2 emissions and PUE calculations, provide for informed decision-making to eliminate energy wastage and identify energy saving opportunities across building services; in non-critical environments, systems can be further optimised by tailoring parameters to different zones and to meet peak and off-peak conditions, again reducing energy consumption.