VO2-based thermochromic smart window: From energy savings to generation

Abstract

Approximately 60% of all energy consumed in building sector is used for space heating, cooling, lighting and ventilation; therefore, energy-conserving measures should be implemented to decrease energy losses. Compared with other elements of the building envelope that separate the indoors from the outdoors (such as walls, roofs and fenestrations), windows are often considered less energy-efficient building components with high maintenance requirements. A large amount of energy is lost through windows, which are the weak links in buildings from an energy perspective, but these features must nevertheless be retained for indoor comfort, permeability and harmony between humans and nature. Smart window is an assembly of a substrate (glass or other transparent materials) and a light- adjusting material, which can change its transmittance via absorption or reflection of solar light with specific wavelength ranges under some physical or chemical stimuli such as light, irradiation, electricity, gas, temperature, etc. The smart window aims to reduce the heating/cooling energy consumption of buildings and CO2 emissions. According to stimulation mode, the smart window can be divided into electrochromic window (ECW), gasochromic window and thermochromic window (TCW). Vanadium dioxide (VO2) smart window is the main kind of TCW, which can absorb over 90% ultraviolet radiation (up to 99% by combination with other absorbents), and can adjust solar heat transmittance by response to environment temperature changes with almost constant visible light transmittance. In recent years, important breakthroughs in both fundamental studies and mass manufacture were reported successively on VO2 TCW. From historic viewpoint, the development of VO2 for the practical use was hindered seriously by several shortcomings, including unpopular color, high phase-transition temperature, weak thermal shield performance, low luminous transmittance and solar energy modulation ability. Among these issues, the low optical performance is the most concerned problem, and it is hard to improve synchronously. This issue has been addressed through structural design, including single layered and multi-layered structures, nanoporosity along with periodic porosity, and nano-composite. The nano-composite VO2 film has made an important breakthrough on the optical performance and also mass production. Efforts are still being made on the improvement of optical performance. Making use of the visible waveband of solar light could further improve the utilization of solar energy. In this aspect, two trends should be paid attention,combination of visible light regulation materals (such as hydrogel hybrid) with VO2 nanoparticals and combination of VO2 with solar cells. The former can improve the solar energy modulation ability, and the latter combines energy savings and generation, which was reported by our team and helpful to utilize solar energy more efficiently. This paper reviews the recent research progress of VO2-based thermochromic/hybrid films with specific emphases on the structure-performance relations and makes perspectives in further development.