In certain climates, cooling systems can save over 70% in annual energy costs by operating in economizer mode, corresponding to a reduction of over 15% in annualized power usage effectiveness (PUE). http://www.climate-charts.com/World-Climate-Maps.html
Operating in economizer mode saves energy by utilizing outdoor air during colder months of the year allowing refrigerant-based cooling components like chillers and compressors to be shut off or operated at a reduced capacity. In some climates cooling systems can operate primarily in economizer mode allowing the refrigerant-based modes (i.e. mechanical cooling) to serve as the secondary mode of operation or backup. The American Society of Heating, Refrigerating and Air-conditioning (ASHRAE) Standard 90.1 specifies the requirements for airside economizers. http://www.ashrae.org/standards-research–technology/advanced-energy-design-guides
It is this concept of a bypass of the compressor function that is central to all economizer modes. Historically, building an economizer mode into a data center cooling system was only justified in situations with extremely favourable weather conditions, such as high latitudes. However, economizer modes are now considered advantageous in almost all locations because data centers spend considerable time operating at light load.
An economizer mode must take advantage of a wide range of outdoor conditions in order to maximize the number of economizer mode hours and save energy. However, in periods of extreme hot outdoor conditions, it is necessary to at least partially depend on a refrigerant-based mode (i.e. mechanical cooling) to reliably maintain data center environmental conditions while saving energy.
The transition between economizer mode and refrigerant-based mode can be very complicated and could result in a temporary loss of cooling during the transition. Ultimately, the reliability of this transition rests on the controls. The control system for standardized pre-engineered cooling systems with integrated economizer mode is developed in conjunction with the hardware. This makes the control system inherently more reliable than customized control systems developed for unique cooling system installations in the field.
Air conditioner bypass via air heat exchanger or via heat wheel economizer modes have the simplest control systems. The most complex economizer mode control system is the chiller bypass via heat exchanger due to the dead band between the low condenser water temperature required by the plate-and-frame heat exchanger and the higher condenser water temperature required by the chiller.