Optical properties and field test results of spectrally-selective solar control window film that enables not increasing downward reflection

Abstract

Recently high reflective materials such as high reflectance paint for wall and solar control film or Low-E glass for window are utilized as a building façade aiming for reducing the cooling load of the buildings. From a view point of the direction of reflected rays, almost half of the energy goes upward due to diffuse reflection in case of paint, meanwhile most of energy goes down to the ground due to specular reflection by the glass. This leads to deterioration of thermal environment of surrounding buildings, and is one of causes of urban heat island. Retro-reflective façade materials are promising candidates for mitigation of urban heat island. The authors successfully developed a new type of transparent solar control window film Albeedo that selectively reflects near-infrared rays towards the sky. The film has a saw-tooth microstructure with a spectrally-selective reflection layer. This microstructure is able to reflect infra-red rays toward the sky. The upward reflectance of the film is measured by our custom made spectroscope. Firstly, the sample is placed in a large scale integrating sphere. Then, solar energy absorptive materials are arranged under the sample to absorb the transmitted component and positioned where the downward reflection goes to. Finally, a halogen lamp irradiates the sample at an incident angle of 60° as a light source. The result of the upward reflectance of near-infrared is about 33%. We also conducted field test measurements to evaluate the energy savings of the interior of the building and the degree of reflection of the exterior.