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Research Papers: Electronic Cooling

Exergy Analysis of Data Center Thermal Management Systems

[+] Author and Article Information
Amip J. Shah1

Department of Mechanical Engineering, University of California, Berkeley, CA 94720-1740amipshah@engineeralum.berkeley.edu

Van P. Carey

Department of Mechanical Engineering, University of California, Berkeley, CA 94720-1740vcarey@me.berkeley.edu

Cullen E. Bash

 Hewlett Packard Laboratories, 1501 Page Mill Road, M/S 1183, Palo Alto, CA 94304-1126cullen.bash@hp.com

Chandrakant D. Patel

 Hewlett Packard Laboratories, 1501 Page Mill Road, M/S 1183, Palo Alto, CA 94304-1126chandrakant.patel@hp.com

1

Now at Hewlett Packard Laboratories, 1501 Page Mill Road, M/S 1183, Palo Alto, California 94304-1126.

J. Heat Transfer 130(2), 021401 (Feb 04, 2008) (10 pages) doi:10.1115/1.2787024 History: Received July 10, 2006; Revised May 25, 2007; Published February 04, 2008

The modeling of recirculation patterns in air-cooled data centers is of interest to ensure adequate thermal management of computer racks at increased heat densities. Most metrics that describe recirculation are based exclusively on temperature inside the data center, and therefore fail to provide adequate information regarding the energy efficiency of the thermal infrastructure. This paper addresses this limitation through an exergy analysis of the data center thermal management system. The approach recognizes that the mixing of hot and cold streams in the data center airspace is an irreversible process and must therefore lead to a loss of exergy. Experimental validation in a test data center confirms that such an exergy-based characterization in the cold aisle reflects the same recirculation trends as suggested by traditional temperature-based metrics. Further, by extending the exergy-based model to include irreversibilities from other components of the thermal architecture, it becomes possible to quantify the amount of available energy supplied to the cooling system that is being utilized for thermal management purposes. The energy efficiency of the entire data center cooling system can then be collapsed into the single metric of net exergy loss. When evaluated against a ground state of the external ambience, this metric enables an estimate of how much of the energy emitted into the environment could potentially be harnessed in the form of useful work. Thus, this paper successfully demonstrates that the proposed exergy-based approach can provide a foundation upon which the data center cooling system can be simultaneously evaluated for thermal manageability and energy efficiency.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Typical thermal architecture in a raised-floor data center. Best practice dictates dividing the room into hot aisles and cold aisles on either side of a row of racks.

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Figure 2

System layout and rack loading for test data center (top view). Each floor tile has an area of 0.6×0.6m2(2×2ft2). The floor-to-ceiling height is 2.7m(9ft), while the depth of the underfloor plenum is 0.6m(2ft) and the height of the ceiling return plenum is 1.2m(4ft).

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Figure 3

Results from numerical model (top view). Each map is a top view of the data center airspace at heights of 0.9m(3ft), 1.5m(5ft), 2.1m(7ft), 2.7m(9ft). (a) Temperature predictions (in °C) from model, and (b) map of exergy loss (in W) throughout data center. The locations of exergy destruction correspond well to hotspots or areas of recirculation estimated from the temperature map.

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Figure 4

Experimental validation of model. (a) Comparison of predicted and measured values. (b) Comparison of rack-level estimates given by different metrics.

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Figure 5

Exergy-based assessment of the entire data center. The majority of exergy loss occurs in the rack and CRAC units, although significant amount of exergy is also lost in the airspace. Nearly 21% of the supplied exergy is vented to the exhaust.

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