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Abstract
Window coatings with dynamic solar transmittance represent an excellent opportunity to reduce building heating and cooling loads, which account for >40% of energy consumed by the built environment. In particular, inorganic vanadium dioxide-based thermochromic coatings offer long lifetimes (>30 years) and can be passively integrated into a window system without additional electronics or power requirements. However, their limited solar modulation depth and wide phase-change hysteresis have traditionally restricted their ability to adapt to changing weather conditions. Here, we derive an optical performance limit for thin film vanadium dioxide coatings, which we find to be far beyond the current literature. Furthermore, we experimentally demonstrate a solution-processed multilayer thin film coating that uses temperature-dependent optical impedance matching to approach the optical performance limit. The thin film coating demonstrated has a record solar transmittance modulation of 21.8% while maintaining a high level of visible transparency (∼50%) and minimal hysteresis (∼10 °C). This work represents a step-change in thin film thermochromic window coatings and, as a result, establishes planar thin film vanadium dioxide as the most viable morphology for high-performance thermochromic windows.
Highlights
Heating and cooling loads account for >40% of energy consumed within the built environment,[3] and window coatings with dynamic solar transmittance represent an excellent opportunity to reduce this load.[4−6] In particular, inorganic vanadium dioxide-based thermochromic coatings offer long lifetimes (>30 years7) and can be passively integrated into a window system without additional electronics or power requirements.[8−10] The basic principle of a vanadium dioxide (VO2) dynamic window coating is that in its low-temperature monoclinic VO2(M) state, the coating is highly transparent to solar radiation, whereas in its high-temperature rutile VO2(R) state, solar transmittance is reduced significantly,[11] such that the dynamic window passively regulates solar heat gain in response to changing seasonal conditions.[12]
There are a number of fundamental material challenges associated with the performance of vanadium dioxide in dynamic window coatings, and progress in solving these challenges has most recently been reviewed by Faucheu et al.[13] and Chang et al.[9]
It is interesting to note that much of the early work investigating vanadium dioxide as a window coating material focused on using thin film deposition methods, such as sputtering[14,15] or chemical vapor deposition,[11,16−18] both of which can be implemented in large-scale manufacturing;[19,20] a significant drawback of thin film vanadium dioxide as a dynamic window coating material is that changes in solar absorption and reflectance typically oppose each other,[11,21−27] which dramatically reduces solar modulation ability
Summary
Reducing energy consumption is vital to reducing CO2 emissions and global warming.[1,2] Heating and cooling loads account for >40% of energy consumed within the built environment,[3] and window coatings with dynamic solar transmittance represent an excellent opportunity to reduce this load.[4−6] In particular, inorganic vanadium dioxide-based thermochromic coatings offer long lifetimes (>30 years7) and can be passively integrated into a window system without additional electronics or power requirements.[8−10] The basic principle of a vanadium dioxide (VO2) dynamic window coating is that in its low-temperature monoclinic VO2(M) state, the coating is highly transparent to solar radiation, whereas in its high-temperature rutile VO2(R) state, solar transmittance is reduced significantly,[11] such that the dynamic window passively regulates solar heat gain in response to changing seasonal conditions.[12]. There are a number of fundamental material challenges associated with the performance of vanadium dioxide in dynamic window coatings, and progress in solving these challenges has most recently been reviewed by Faucheu et al.[13] and Chang et al.[9]. These challenges can be split into two categories: transition based, that is, the temperature-dependent hysteretic response of the coating, and the optical properties, that is, visible transparency and solar modulation ability. We rigorously derive and examine the optical constants of vanadium dioxide and find that the opposing changes in reflectance and absorption of thin film vanadium dioxide are a direct result of the opposing changes in the refractive index and Received: October 19, 2019 Accepted: January 30, 2020 Published: January 30, 2020
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