The location of a data center is just above the Arctic Circle (North Pole). The outside air temperature is never above 21°C/70°F. Taking energy efficiency as the highest priority, which cooling system should you recommend?

An airside economizer system uses cool outside air to cool the data center, a method often called "free cooling." Given the location is in the Arctic Circle where the ambient temperature never exceeds 21°C (70°F), the external air is always within the acceptable thermal envelope for IT equipment. This allows the data center to be cooled year-round using only fans to circulate the filtered outside air, completely avoiding the high energy consumption of mechanical refrigeration (compressors and chillers). This makes the airside economizer the most energy-efficient solution for this specific climate, directly aligning with the stated highest priority.

A. Standard air conditioning is the least energy-efficient option because it relies on a power-intensive refrigeration cycle, which is unnecessary in this cold climate.
B. This statement is fundamentally incorrect. The outside air temperature is the single most critical factor enabling the use of highly efficient economizer cooling modes.
C. A waterside economizer is also highly efficient but involves an indirect heat exchange process (air-to-water, then water-to-air), making it slightly less efficient than a direct airside economizer.

1. ASHRAE TC 9.9. (2021). Thermal Guidelines for Data Processing Environments (5th ed.). Section 2.2, "IT Equipment Inlet Air," defines the recommended and allowable temperature ranges for IT equipment. The scenario's maximum temperature of 21°C falls comfortably within the recommended range, validating the continuous use of untreated outside air for cooling.

2. U.S. Department of Energy, Federal Energy Management Program. (2012). Best Practices Guide for Energy-Efficient Data Center Design. Page 21, Section "Air-Side Economizers," states, "Air-side economizers offer the greatest potential for energy savings in appropriate climates... They reduce or eliminate the need for mechanical cooling by using cool outside air."

3. Lawrence Berkeley National Laboratory. (2012). A Glimpse into the Future of Data Center Cooling. LBNL-5882E. Page 4 discusses how data centers in cool climates can achieve PUE values approaching 1.0 by relying almost exclusively on free cooling with air-side economizers, highlighting it as the pinnacle of efficiency.

4. Sharma, R. K., et al. (2015). Energy efficient cooling of data centers. Renewable and Sustainable Energy Reviews, 43, 1195-1209. (https://doi.org/10.1016/j.rser.2014.10.083). Section 3.1.1, "Air-side economizer," describes this method as the most common and energy-efficient form of free cooling when climatic conditions are favorable, as described in the question.