Comparing me Energy Conservation Capabilities of Spectrally Selective and Conventional Applied Window Film

摘要

Compared to conventional applied window film, high light-transmitting spectrally selective film cost-effectively blocks unwanted solar energy from entering windows and reduces air conditioning costs without darkening building interiors, impeding the ability to see through existing glass, or changing the external appearance of a building. According to the California Energy Commission, 30 percent of a building's cooling requirements are a function of heat entering through existing glass. Stopping heat at the window is the most effective means of lowering temperatures and reducing HVAC operating costs. In new construction, reducing heat at the window can mean the need for smaller and less-expensive HVAC systems. The solution to overheating through windows is to specify solar control glass or applied window film, though even the best solar control glass performs no better than the best applied window film. Solar control glass can be selected for optimum energy performance in reference to the geographic orientation of any given building or section of a building. However, even in new construction, the cost of solar control glass often exceeds the cost of standard glass to which a solar control film is later applied. For existing buildings experiencing problems from heat through windows, the most expensive option is to replace existing glass and frames with a new window system designed to block heat and deal with a building's energy performance needs. Less expensive is keeping existing frames and replacing only the glass. In either case, building managers may be understandably reluctant to replace existing windows or glass whose performance is generally adequate, though not optimum, in the case of blocking unwanted heat. For all existing glass and in much new construction, applied window film is the least expensive and preferred solution to mitigate the impact of too much solar heat entering windows. Conventional dark and reflective applied window films successfully block a significant amount of solar heat, thereby reducing the use of HVAC systems. Unfortunately, these same films reduce a significant percentage of visible light through the glass. Most of these films are highly reflective in daylight, giving them a mirror-like appearance when viewed externally. In artificial light and at night, internally they appear mirrored. In the case of retail establishments, visible light is reduced inside the store and shoppers outside cannot see clearly inside. Most conventional window films transmit less than 35 percent of visible light, a good 35 percent less than the 70 percent necessary to be undetected by the naked eye. The result is that building interiors are correspondingly darkened, often requiring the use of increased illumination. This leads to higher electricity consumption that may increase inside temperatures requiring more air-conditioning. Increased utility costs defeat the major benefit of the film—cost savings.