Editor's note: The following is based on "Reducing Data-Center Energy Use Without Compromising Uptime," presented by Don Beaty, PE, FASHRAE, during HPAC Engineering's seventh annual Engineering Green Buildings Conference, held Sept. 23 and 24 in Baltimore as part of HVACR Week 2010.

Many data centers are "mission-critical," meaning essential to the core function of an organization. As such, they must remain operational at all times. However, that does not mean significant improvements in energy efficiency and sustainability cannot be made.

The keys to conserving energy in data centers without impacting uptime are:

• Selecting optimum design conditions.
• Developing practical and effective load characterization.
• Treating power and cooling as a service, rather than physical infrastructure.

This article focuses on balancing energy efficiency with demands for uptime, as well as the gains possible with an American Society of Heating, Refrigerating and Air-Conditioning Engineers- (ASHRAE-) driven basis for design.

Selecting Optimum Design Conditions
Data centers commonly are multivendor information-technology (IT) environments, with each vendor having its own environmental (e.g., temperature and humidity) specifications. The net result is no standardization and arbitrary decisions about operating temperatures, such as, "Cold is better." Basically, temperatures are selected based on the worst-case scenario, which means overdesign.

Recognizing a lack of standardization, IT-equipment manufacturers selected ASHRAE to provide a platform for standardizing their environmental specifications. That initiated the creation of ASHRAE Technical Committee (TC) 9.9, Mission Critical Facilities, Technology Spaces and Electronic Equipment.

ASHRAE environmental specifications. In 2004, ASHRAE TC 9.9 published the book "Thermal Guidelines for Data Processing Environments."1 Critical to the effort was the inclusion of all major IT-equipment manufacturers and their agreeing on sets of "recommended" and "allowable" conditions. Prolonged exposure of operating equipment to conditions outside of the recommended range can result in decreased reliability and longevity. Occasional, short-term excursions into the allowable envelope may be acceptable.

It is important to note:

• The ASHRAE environmental specifications are measured at the air inlet of equipment.
• The ASHRAE environmental specifications are retroactive. In other words, they also apply to legacy equipment.
• The ASHRAE environmental specifications are within the range each major IT-equipment manufacturer used in the past.
• Operating within the recommended range produces a reliable environment.

The 2004 specifications were 68°F to 77°F and 40-percent to 55-percent relative humidity (RH).

In 2008, ASHRAE TC 9.9 revised the environmental specifications, 2 widening the range and enabling more hours for economizers. The dry-bulb lower limit was decreased to 64.4°F, while the upper limit was increased to 80.6°F. The lower moisture limit was decreased to a 41.9°F dew point, while the upper limit was increased to 60-percent RH and a 59°F dew point (Figure 1).

IT equipment is expected to function for short to moderate periods of time outside of the recommended range, but within the allowable range. This may be all that is necessary to accomplish a "compressorless design," which can have a significant impact on energy use, maintenance, and capital costs.

IT equipment typically has a much higher cost than power and cooling infrastructure. Operating this equipment outside of manufacturers' recommendations may be risky. The best approach is to achieve with stakeholders consensus on the frequency and duration of excursions into the allowable range.