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Phase change materials (PCMs) have tremendous capacity as passive components to recover and repurpose thermal energy from transient power systems. However, PCMs are only effective if the time scale of the thermal energy storage and retrieval rates match those required for a particular system. We develop a framework to assess the efficiency of pulsed thermal energy storage based on the concept of “thermal impedance,” drawing upon an analogous approach from electrical energy storage. We experimentally characterize a 1 cm thick paraffin-infiltrated copper foam composite PCM subject to pulsed heat boundary conditions up to 1 W cm−2 and demonstrate a decrease in thermal impedance by up to a factor of 2.5× in the regime in which melting occurs (τon = 10−1 to >102 s) relative to a reference case in which melting does not occur. This represents both a signature of the ability to extract or retrieve thermal energy via latent heat, as well as an experimentally accessible measure that provides insight into the internal dynamics of a composite PCM volume. These principles can serve to design the internal structure of composite PCM elements for pulsed thermal systems.
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CHARACTERIZATION AND MODELING OF DENSITY AS A FUNCTION OF TEMPERATURE FOR PARAFFIN WAX PHASE CHANGE MATERIALS (PCMs)
This paper presents a study on the characterization of density as a function of temperature for phase change materials (PCMs). More specifically, in this study we analyze organic alkane PCMs, often called paraffins. PCMs are materials that have the ability to absorb a substantial amount of heat during phase transition from solid to liquid, and therefore prove to be useful in thermal energy storage. The density of paraffin wax PCMs is largely dependent on temperature, and during the phase change process, the density decreases dramatically as the PCM transitions from solid to liquid. Consequently, the PCM experiences dramatic volumetric expansion during this transition. Besides the thermal energy storage uses of PCMs, this volumetric expansion that they exhibit is also used in thermal actuator applications, often referred to as wax motors. While density of PCMs does affect their thermal and mechanical performance, the property is not well-characterized within the literature. In this paper, we examine ten paraffin wax PCMs with varying meltingtemperatures and characterize their densities as a function of temperature. This characterization was done usinga piston and cylinder dilatometer test setup within a temperature-controlled thermal chamber that we designedand validated to the well-characterized density properties of water. The density and temperature relationships werefurther analyzed using piecewise linear regression analysis to develop mathematical models of density as it relates totemperature, which will be useful to those wishing to analyze designs in which PCMs are used, such as in PCM-filled heat sinks.
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- Award ID(s):
- 2209751
- PAR ID:
- 10537465
- Publisher / Repository:
- American Society of Mechanical Engineers
- Date Published:
- Journal Name:
- Heat Transfer Research
- Volume:
- 55
- Issue:
- 5
- ISSN:
- 1064-2285
- Page Range / eLocation ID:
- 59 to 79
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1615/HeatTransRes.2023049214
