Cooling of electronics systems with phase change materials under periodic temperature conditions

What is it about?

Many engineering systems, notably electronics products, (i) are designed as closed enclosures, (ii) operate in a variable temperature environment, such as the outdoors, and (iii) present a need for refrigeration (cooling) as to maintain their normal operation. Passive techniques — that do not demand energy (work) consumption — are a cost-effective and reliable way of attaining the desired cooling. The use of phase change materials (PCM) — that absorb heat by melting during the hottest times and later one releases the heat by freezing during the coldest times — is a common passive cooling technique that can reduce temperature peaks and fluctuations inside the enclosures. This work presents a fundamental heat transfer theory of this kind of systems and exactly derives the solution of the describing equations. Moreover, this work presents the best possible performance of a passive cooling system, based on the second law of thermodynamics.

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Why is it important?

Most problems that fall within the scope of this work are solved numerically — which solves a single particular configuration under a particular set of conditions — and thus do not provide a general theory that can be applied to many cases. This work takes a theoretical approach and develops a mathematical model — the theory — which can be applied to all cases that fit the model description. The modeling is simple enough as to have wide application, and is complex enough so that most basic design parameters — that the practicing engineer need to specify for each case — are included. Moreover, the usage of the second law of thermodynamics to determine the performance limit of a heat-only system is an unusual application of the second law of thermodynamics that is traditionally employed as to derive performance limits for heat-into-work systems — such as Carnot engines and refrigerators — or as to determine best configurations, as in entropy generation minimization techniques and as in the Constructal Theory; therefore this work presents yet another niche of application for the second law of thermodynamics. Furthermore, this work is the first to illustrate an application of zero-phase modeling of a coupled, Stefan-like problem, which originates from "The Zero-Phase Stefan Problem" of the first author, and that models the melting and freezing problem inside the closed enclosure.

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