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.
The Effect of a Thermotropic Material on the Optical Efficiency and Stagnation Temperature of a Polymer Flat Plate Solar Collector
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.
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Gladen, A. C., Davidson, J. H., and Mantell, S. C. (September 10, 2014). "The Effect of a Thermotropic Material on the Optical Efficiency and Stagnation Temperature of a Polymer Flat Plate Solar Collector." ASME. J. Sol. Energy Eng. April 2015; 137(2): 021003. https://doi.org/10.1115/1.4028366
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