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Research Papers

The Effect of a Thermotropic Material on the Optical Efficiency and Stagnation Temperature of a Polymer Flat Plate Solar Collector

[+] Author and Article Information
Adam C. Gladen

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

Jane H. Davidson

Fellow ASME
Department of Mechanical Engineering,
University of Minnesota,
111 Church Street SE,
Minneapolis, MN 55455
e-mail: jhd@me.umn.edu

Susan C. Mantell

Mem. ASME
Department of Mechanical Engineering,
University of Minnesota,
111 Church Street SE,
Minneapolis, MN 55455
e-mail: smantell@me.umn.edu

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: Including Wind Energy and Building Energy Conservation. Manuscript received March 18, 2014; final manuscript received August 13, 2014; published online September 10, 2014. Assoc. Editor: Werner Platzer.

J. Sol. Energy Eng 137(2), 021003 (Sep 10, 2014) (6 pages) Paper No: SOL-14-1094; doi: 10.1115/1.4028366 History: Received March 18, 2014; Revised August 13, 2014

Solar hot water and space heating systems constructed of commodity polymers have the potential to reduce the initial cost of solar thermal systems. However, a polymer absorber must be prevented from exceeding its maximum service temperature during stagnation. Here, the addition of a thermotropic material to the surface of the absorber is considered. The thermotropic layer provides passive overheat protection by switching from high transmittance during normal operation to high reflectance if the temperature of the absorber becomes too high. A one dimensional model of a glazed, flat-plate collector with a polymer absorber and thermotropic material is used to determine the effects of the optical properties of the thermotropic material on the optical efficiency and the stagnation temperature of a collector. A key result is identification of the reflectance in the translucent state required to provide overheat protection for potential polymer absorber materials. For example, a thermotropic material in its translucent state should have a solar-weighted reflectance greater than or equal to 52% to protect a polypropylene absorber which has a maximum service temperature of 115 °C.

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References

Figures

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Fig. 1

Temperature dependency of the optical properties of thermotropic materials

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Fig. 2

Representative efficiency curves for a collector with- and without a thermotropic material. Dashed vertical lines represent a step change from clear to translucent state. Abscissa location of change depends on ambient conditions and Tswitch.

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Fig. 3

Schematic cross-sectional view of the modeled collector

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Fig. 4

Resistance network used to calculate the overall heat transfer coefficient

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Fig. 5

Collector optical efficiency versus the transmittance of a thermotropic material in its clear state

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Fig. 6

Predicted stagnation temperature as a function of the reflectance of the thermotropic material in its translucent state. Solid line represents nominal collector parameters.

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Fig. 7

Schematic drawing of radiosities and irradiances used to determine the overall reflectance of the thermotropic material-absorber laminate

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