Mitigation of hysteresis due to a pseudo-photochromic effect in thermochromic smart window coatings

Abstract

The aim of thermochromic window coatings is to reduce the energy consumption in the built environment by passively switching between a high solar transmitting state at low temperatures and low solar transmitting state at high temperatures. Previous studies have highlighted the negative impact of phase transition hysteresis on the performance of reflection based thermochromic films. However in the literature, the best reported results have depended on vanadium dioxide nanoparticle composites and anti-reflective structures that modulate light via changes in absorption rather than reflection. In light of these factors, this work aims to demonstrate theoretically, how the effects of phase transition hysteresis and gradient differ between absorbing and non-absorbing thermochromic films. To quantify and compare the performance of films with different transition characteristics, we define a metric based on the varying net energy flux through the window over the course of a year, including thermal energy re-radiated into the building from the film. Specifically, and importantly for the field, we demonstrate that a pseudo-photochromic effect in absorbing thermochromic films mitigates the detrimental effects of phase transition hysteresis and gradient that have been reported for reflection based thermochromic films. We find that for moderate hysteresis widths of 15 °C, the performance of the non-absorbing case drops to ~60% of its initial value whilst the performance of the absorbing film only drops to ~95%. As a result we find that the absorbing case outperforms the non-absorbing case when hysteresis widths are greater than 8 °C.

Atomic layer deposition of vanadium oxides: process and application review

“…We mention in passing that the transmittance based metrics presented above are not the only parameters that smart window researchers need to consider when developing next generation coatings. Equally important, albeit broadly overlooked in the literature, are the hysteresis width and gradient of the coatings, which need to be tightly controlled or otherwise the performance of the windows may be inexorably compromised [187].…”

Thermochromic VO2−SiO2 nanocomposite smart window coatings with narrow phase transition hysteresis and transition gradient width

“…Several theoretical works [25,26,27] have highlighted the importance of reducing the hysteresis width of thermochromic films to improve their overall energy saving performance, as wide hysteresis loops mean that coatings are less responsive to changing climate conditions. Several previous works have shown that the hysteresis width can be reduced significantly by doping vanadium dioxide with metal ions [5,18], however this generally results in an increase in transition gradient [5] which has also been shown to be detrimental to the overall energy saving performance [25,28,27]. Table 1 shows a summary of literature for composite thermochromic coatings, from which we can see that improvements in solar modulation and visible transmission for composite coatings generally result in increased hysteresis width.…”

“…Table 1 shows a summary of literature for composite thermochromic coatings, from which we can see that improvements in solar modulation and visible transmission for composite coatings generally result in increased hysteresis width. It must also be noted how infrequently the transition gradient is discussed in the literature, despite having been shown to be critical to the overall energy saving performance [25,28,27]. In the only study found with a high optical performance (T lum = 48.5% & T sol = 15.7%) that reports both the transition hysteresis and gradient width, both hysteresis and gradient widths were found to be wide (17.4 & 11.3 • C respectively) [29].…”

3D mesoporous structure assembled from monoclinic M-phase VO2 nanoflakes with enhanced thermochromic performance

“…The demand for energy-efficient smart windows utilizing thermochromic materials is constantly increasing due to their potential to dramatically reduce the energy consumption of the buildings by modulating the transmission of light and heat. [3][4][5][6][7] Vanadium dioxide (VO 2 ) has been widely investigated for smart windows because of its excellent thermochromic properties. [8][9][10] In particular, monoclinic M-phase VO 2 undergoes a metal-toinsulator transition (MIT) at a critical temperature of $68 C. [12][13][14] The monoclinic crystal structure of VO 2 displays infrared transparent below the critical temperature while in a rutile crystal structure it is infrared-reective above the critical temperature accompanied by abrupt changes in the electrical, magnetic, and optical properties.…”