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Supercomputers, Super Efficiency


©2016 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 58, no. 1, January 2016

By Terry Autry, P.E., Member ASHRAE; Aaron Andersen

About the Author
Terry Autry, P.E., is a senior vice president at The RMH Group in Lakewood, Colo. Aaron Andersen is the deputy director operations and services, Computational & Information Systems Laboratory, at the National Center for Atmospheric Research in Boulder, Colo.

To predict the track of a hurricane or simulate 100 years of global-scale climate change takes an incredible amount of computing power. The National Center for Atmospheric Research (NCAR) has supercomputers that can run 1 quadrillion calculations per second. To put that into context, if all of the spectators in a large football stadium calculated a math problem every second, they would be at the game for nearly 1,000 years.

A data center made up of supercomputers needs a specialized approach to ensure it is energy efficient.

The University Corporation for Atmospheric Research (UCAR), which oversees NCAR facilities, was tasked in 2010 with installing the next generation of supercomputers at an NCAR site.
UCAR is a nonprofit consortium of more than 100 North American member colleges and universities focused on research and training in the atmospheric and related Earth sciences.

Having outgrown facility and utility resources at NCAR’s Mesa Laboratory in Boulder, Colo., a decision was made to build the new NCAR-Wyoming Supercomputing Center (NWSC) in Cheyenne, Wyo. The resulting $70 million industrial computing facility can accommodate supercomputing systems with power densities up to 1,000 W/ft2 (10 764 W/m2) at petascale performance (greater than 1 quadrillion floating point operations per second). The NWSC encompasses 170,982 gross ft2 (15 885 m2) and includes two 12,000 ft2 (1115 m2) raised-floor data rooms, a data storage facility, a networks operations center, a visitor center, administrative space, and a central utility plant.


Energy Efficiency

With initial plans for more than 4 MW of combined air- and liquid-cooled computer load, the owner challenged the design team to minimize energy consumption in the facility.

Power usage effectiveness (PUE) is a common metric for determining how effectively the data center infrastructure systems can deliver power to the computer systems. NWSC was designed with a target PUE of under 1.1, meaning 90% of energy for the entire complex is used for data center computing. With this low PUE, the NWSC is among the 1% most efficient data centers worldwide and four times more efficient than NCAR’s legacy facility in Boulder, Colo. After more than two years of operation well under the design IT load (50%), the facility’s PUE is trending toward 1.15, which compares favorably to the design PUE of less than 1.1.

Figure 1 shows the annual and daily PUE for August 2014 to September 2015. The typical summer increase in PUE is shown. Significant excursions in April, June, and July represent preventative maintenance windows for large portions of the IT load. Increased PUE in November and February represents use of energy recovery heat pumps, which are not as energy efficient as economizer-based cooling under certain outdoor conditions. A broader view of the energy use shows heat pumps are effective in spring and fall.

To achieve high levels of cooling system efficiency, the design engineers harnessed the cool, dry climate of Cheyenne by using cooling towers to deliver 65 °F (18°C) chilled water directly to NCAR’s liquid-cooled supercomputers for 98.6% of the year. That same 65 °F (18°C) chilled water is moved into a fan wall with cooling coils for the air-cooled computers. A cooling system schematic is shown in Figure 2 . The facility’s backup mechanical chiller is periodically exercised and tested, but is only required to operate for approximately 40 hours per year.


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