Smart windows for passive control of solar radiation as a function of external ambient temperature

K. Golding, N. Gross, A. Donval, M. Oron, D. Nevo
KiloLambda Technologies, Ltd.,

Keywords: smart window, power reduction, passive, solar radiation


A well-known fact is that nearly half of the energy consumed throughout the developed world is used for heating, cooling and lighting of buildings. Buildings are huge energy consumers, since their functions like cooling, heating, lighting and ventilation require large amounts of energy, approximately 30–40% of consumed energy worldwide. Due to the continuing population growth, and as a consequence, of the consumed energy, mitigation measures of energy consumption are urgently needed. In this direction, a European Directive (2010/31/EU) for nearly zero-energy buildings after 2020 has been issued. Windows are a key factor in building energy consumption, because thermal energy is transferred through their transparent surface. During summer, windows allow heat to pass into the building and during winter, heat escapes. It is estimated that windows are responsible for 40% of the total building energy losses. Concerning heat transfer, it would be ideal for a window to adapt its thermal properties to the outdoors environmental conditions. Windows, like essential architectural elements, should provide the essential functionality to make the resident or the worker feel comfortable and reduce glare while permitting building’s lighting. Therefore, windows functionality in the modern architecture moves forward toward “smart windows”. The ecological goal of “smart windows” is to exploit solar energy resources to meet the energy needs of a building. “Smart” glazing can be divided into two major categories: passively switching (photochromic, thermochromic and thermotropic) and electrically activated types. For the electronically switchable technologies, costs are in the range of several hundred $US/m2. As to photochromic glass, it is not yet produced in sufficient quantities and sizes presently, neither its durability is much lower than needed today for glazing. Thermochromic and Thermotropic (passive) based fenestration is not yet commercially available, but its research and development is close to commercial implementation, and is promising lower installation and life-cycle costs, as presented in this paper. KiloLambda has developed a novel process, using inclusions of nano-crystals in a transparent polymeric matrix. At a pre-designed temperature this novel composition is transparent, whereas a temperature change creates a refractive index variation in the nano-crystals and the matrix, making it a scattering media, scattering off the excess solar light. This proposed technology – DHR – Dynamic Heat Reduction, a novel energy-saving technology, is a dynamic layer, which changes its transparency according to the outside temperature, using the sunlight benefits on a cold day, and reducing the sunlight on a hot day. The DHR is a passive coating/film triggered only by the solar radiation and ambient temperature. It reflects a desirable level of solar heat, while allowing enough solar lighting. Being a passive solution, it enables a cost/effective solution without the need for additional CapEx/Opex associated with the installation and operation of active technologies such as electrochromic systems. KiloLambda surpassed the traditional techniques of light power control by developing nano-technology based optical power control capabilities in a passive (no electrical power) manner and applying them to everyday applications.