ORCID Identifier(s)

0000-0002-7941-3048

Graduation Semester and Year

2016

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Dereje Agonafer

Abstract

Computer cooling system design evolved over time with goals of increasing efficiency and decreasing cost. Early computers were essentially hand-built and very expensive. Reliable operation required aggressive cooling to maintain acceptable component temperatures and this was achieved with relatively low ventilation air temperatures. With time, the scale of operations increased to the point that operating cost began to strongly influence design decisions. Computer room air conditioners consumed substantial amounts of electrical power, in some situations almost as much power as the computer equipment. One cost saving idea used outside air when the ambient temperature fell below the normal cooling supply air from the computer room air conditioner. This modification acquired the term “free-cooling”. Substantial cost savings from free-cooling led to the desire to expand its use to higher temperatures. Continuing to expand on this approach, some facilities ventured into evaporative cooling which proved highly successful in locations with an amenable climate. Water's latent heat of evaporation cooled the air using very little electrical power. While evaporative coolers use much less power than direct expansion units of computer room air conditioners, they have more-restrictive limitations on the allowable climate conditions of temperatures and humidity. Also, by their nature, evaporative systems use considerable quantities of water. Cooling system designers continue seeking improvements in the on-going efforts to reduce operating costs. Temperature/humidity limits for evaporative coolers are a consequence of the upper temperature limit for data center cooling supply air and thermodynamic limits of water evaporation to cool the air. Evaporative systems' cooling capacity reach a minimum during the hottest part of a 24-hour cycle. Water consumption reaches a peak at this condition as well. Designed with the necessary cooling capacity at this hot condition, the systems have excess capacity during the cooler portions of the day. Thermal energy storage offers potential to address the two negatives of evaporative cooling, restrictive limitations and high water consumption, by time-shifting cooling capacity. Thermal energy storage enables time-shifting cooling capacity from coolest portion of the 24-hour cycle when the evaporative cooler has excess capacity. Stored cooling can augment the evaporative cooler's performance at times of challenging cooling demand during the hottest portion of the 24-hour cycle. With additional cooling from thermal energy storage the data center cooling supply air temperature can be maintained in hotter environments. Cooling from a thermal energy storage system also enables the reduction of water consumption. Thermal energy storage with free cooling, when no water is used, can provide cooling later to offset water consumption. For thermal energy storage, phase change materials offer economic and performance advantages. The latent heat of phase change can store energy using much less material than sensible heat storage. The near-constant temperature energy exchange of phase change can improve the system thermal performance relative to energy storage with changing temperature. A commercially available thermal energy storage medium comes in the form of a water-based slurry of micro-encapsulated organic wax. The small, micron-size capsules in water overcome one of the major engineering challenges with many phase change materials, low heat transfer during the liquid to solid phase transition with low thermal conductivity material. While conductivity may be low, the maximum conduction distance is the capsule radius which is also small. This study investigates the benefits of thermal energy storage (TES) integrated with an indirect/direct evaporative cooler in a data center application. Concepts for integration of the TES with the cooler are developed, evaluated, and compared. Performance of the most promising candidate concept is evaluated for extended temperature operation and water conservation potential at three representative geographic locations. Capital costs for TES to be integrated with an indirect/direct evaporative cooler are estimated. Finally, operating benefits in the form of reduced operating costs are combined to determine an overall cost benefit.

Keywords

Data center, Cooling, Thermal energy storage, Phase change, Evaporative cooling

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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