Angle-selective reflection surface for energy efficiency

Solar heat reflective paint coated roof is known as cool roof. These cool roofs have an ability of saving energy consumption due to its high reflection against solar irradiation. However, it is suggested that these cool roofs should not be used at cold area because its reflection makes temperature of walls lower. These cool roofs should absorb a solar irradiation when cold situation to raise energy efficiency. Therefore, both reflection and absorption compatible roof is needed. This characteristic can be obtained from active or a passive characteristics-modulating technics. In this study, we focused on the geometrical relationship between the Sun and the Earth, and angular variation of reflection by microstructure. We calculated an angular solar irradiance distribution at the Earth’s arbitrary point from theoretical model and numerically analyzed an angular selective reflective microstructure at single wavelength using Rigorous coupled wave analysis (RCWA) method to verify possibility of angle-selectivity. A microstructure was optimized to its effective reflectivity and absorptivity by changing its geometrical parameters. The optimized microstructure works as the surface has 71% reflector during summer solstice while works as a 48% reflector during winter solstice, from our estimation.