Sol, C; Portnoi, M; Taylor, A; Poduval, R; Parkin, I; Papakonstantinou, I; (2016) A hybrid model for the design and optimisation of flexible vanadium dioxide nanoparticle thermochromic films. Presented at: 2016 E-MRS Spring Meeting, Lille, France.

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Abstract

As cities grow and become more densely populated, there is a increasing issue of heat management, with buildings expending significant energy resources to maintain comfortable living temperatures. In many parts of the world, this entails the use of both heating and cooling during daylight hours depending on ambient temperatures. Due to the variation in the desired temperature modulation classical solutions can become counter productive in their aim of maintaining comfortable temperatures. To avoid such counter productivity, it is important to employ adaptive intelligent solutions which are able to switch their functionality based on circumstance. Here, we present a model for the design and optimisation of thermochromic smart windows based on the switching properties of vanadium dioxide (VO2) nanoparticles. In recent years vanadium dioxide has generated a broad range of interest due to its heat-mediated transition between a monoclinic distorted-rutile phase and a tetragonal rutile phase, which occurs at a critical temperature tunable via doping with tungsten. The phase transition of vanadium dioxide significantly modulates its optical properties, with the high temperature tetragonal state absorbing considerably more infrared radiation than the lower temperature monoclinic state due to a reduction in band-gap energy. It is such that a window coated with a composite polymer-nanoparticle film may passively vary its transmission of heat based on the ambient temperature, in doing so reducing the variation in temperature from the critical temperature and thus reducing energy usage in temperature management. In comparison to vapour deposited VO2 thin films, VO2 polymer-nanoparticle films may be retrofitted to existing windows and have reduced processing costs. We quantify the smart window performance over the transition between hot and cold state with the two key metrics of this field, transmission modulation of solar irradiance ΔTsol and transmission modulation of solar luminosity ΔTlum; an ideal window design will maximise ΔTsol whilst minimising ΔTlum. The model presented combines both finite-difference-time-domain (FDTD) and Monte Carlo ray-tracing to create a hybrid model capable of modelling both micro- and macroscopic properties of a material composite. Specifically, FDTD can be used to model the optical response of nanoparticles of any geometry not otherwise available with analytical techniques, enabling the design and optimisation of novel nanoparticle geometries with improved modulation of infrared radiation. The results from our FDTD studies are then used in our C++ coded ray-tracer to generate probabilities for absorption, scattering and differential scattering during the transmission of single photons through the window; this process is then repeated many times to build up a macroscopic understanding of the properties of the window. Uniquely, the use of our hybrid model enables us to model both specular and diffuse transmission, from which we can investigate the level of haze resulting from the nanoparticles, a property that is often overlooked but very important in nano-particulate window design.

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