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Research Papers

Numerical Evaluation of the Optical Properties of Encapsulated Phase Change Particles for Thermotropic Materials

[+] Author and Article Information
Adam Gladen

Department of Mechanical Engineering,
University of Minnesota,
111 Church Street S.E.,
Minneapolis, MN 55455
e-mail: glad0092@umn.edu

Susan Mantell

Department of Mechanical Engineering,
University of Minnesota,
111 Church Street S.E.,
Minneapolis, MN 55455
e-mail: smantell@umn.edu

Jane Davidson

Department of Mechanical Engineering,
University of Minnesota,
111 Church Street S.E.,
Minneapolis, MN 55455
e-mail: jhd@umn.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received October 3, 2014; final manuscript received January 26, 2015; published online March 31, 2015. Assoc. Editor: Mohamed S. El-Genk.

J. Thermal Sci. Eng. Appl 7(3), 031002 (Sep 01, 2015) (8 pages) Paper No: TSEA-14-1232; doi: 10.1115/1.4029952 History: Received October 03, 2014; Revised January 26, 2015; Online March 31, 2015

Phase change thermotropic materials have been proposed as a low cost method to provide passive overheat protection for polymer solar thermal absorbers. One challenge to their development is control of the size of the phase change particles dispersed within the matrix. Here we explore encapsulation as a means to resolve this challenge with a focus on the selection of materials, including the encapsulating shell, to achieve desirable optical behavior. Hydroxystearic acid (HSA) particles in a matrix of poly(methyl methacrylate) (PMMA) is down selected from candidate materials based on its optical properties and the melt temperature of the dispersed phase. The optical properties (normal-hemispherical transmittance, reflectance, and absorptance) as a function of the properties of the encapsulation shell and the particle volume fraction are predicted at a wavelength of 589 nm using a Monte Carlo ray tracing model. A range of shell relative refractive indices, from 0.95 to 1, and thicknesses, up to 35 nm, can be employed to achieve greater than 80% transmittance in the clear state and greater than 50% reflectance in the translucent state.

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Figures

Grahic Jump Location
Fig. 1

The modeling domain

Grahic Jump Location
Fig. 4

(a) Transmittance in the clear state, mc−m,CS = 1.0054 at T = 29 °C and (b) reflectance in the translucent state, mc−m,TS = 0.9734 at T > 78 °C for HSA in PMMA at a volume fraction of 16% and thickness of 3 mm. The lightly shaded area indicates the region of acceptable transmittance (a) and reflectance (b). The darker shaded area in (b) is the solution space.

Grahic Jump Location
Fig. 3

(a) Transmittance in the clear state, mc−m,CS = 1.0054 at T = 29 °C, and (b) reflectance in the translucent state, mc−m,TS = 0.9734 at T > 78 °C for HSA in PMMA at a volume fraction of 20% and thickness of 3 mm. The lightly shaded area indicates the region of acceptable transmittance (a) and reflectance (b). The darker shaded area in (b) is the solution space.

Grahic Jump Location
Fig. 2

(a) Transmittance in the clear state, mc−m,CS = 1.0054 at T = 29 °C and (b) reflectance in the translucent state, mc−m,TS = 0.9734 at T > 78 °C for HSA in PMMA at a volume fraction of 18% and thickness of 3 mm. The lightly shaded area indicates the region of acceptable transmittance (a) and reflectance (b). The darker shaded area in (b) is the solution space.

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