Energy saving thermoactive coatings – THERMOCOAT
Energy saving thermoactive building coatings
Because of rapid population growth and disappearance of green spaces in urban areas, solar energy is mainly absorbed by paved surfaces, causing the surface temperature of urban structures to become several degrees higher than ambient air temperatures. As surfaces become warmer, overall temperature increases. This phenomenon called “urban heat island” leads to an increasing demand of energy, accelerating also the production of harmful smog and causing human thermal discomfort and health problems.
Why near infrared thermoactive materials are interesting for our purposes
This project seeks to cool cities in summer. Our main approach is to reduce absorption of sunlight by buildings and pavements. The simplest way to reduce solar absorption is to replace dark surfaces that strongly absorb sunlight, such as black or gray roofs, with light-colored surfaces that strongly reflect sunlight, such as white roofs. Interestingly, slightly less than half of sunlight is visible to the human eye. Visible and near infrared wavelengths count for around 95% of the electromagnetic radiation coming from the sun. Light-colored surfaces stay coolest because they strongly reflect both visible and invisible sunlight. Moreover, studies show that white roofs reduce air-conditioning costs by 20 percent or more in hot, sunny weather. White roofs are highly capable for energy savings in hot and dry locations and are becoming popular for large commercial buildings with extensive rooftop surface areas. However in chilly regions, especially during winter, white roofs are not recommended since they reflect the thermal heat that could be used to heat the building, thus making the building even colder. Moreover, when dark surfaces are needed for aesthetics or to reduce brightness, one can use special «cool-colored« materials that stay moderately warm by reflecting only the invisible component of sunlight. Thermochromic coatings can achieve both heating and cooling by changing their optical properties (in particular in visible and/or near infrared range) under temperature changes. Nevertheless, it is worth noticing that roofs changing color in function of the season or the weather might not be accepted by all consumers.
The coatings developed in this project will focus on the near infrared part of the sun light spectrum ([0.8 µm - 3 µm]), which corresponds to about 50% of the total energy brought by sun radiation. The main applications would concern roofs, walls but could also be extended to outside pavements. Our purpose is to add Vanadium Dioxide (VO2) inside a polymer-based coating to modulate the reflectance of near infrared radiations according to the coating’s temperature. VO2 is a thermochromic compound and is well known to undergo an Insulator-Metal Transition leading to a high dielectric constant change, in particular in the near infrared. Above this transition temperature, VO2 behaves like a metal and reflects the sun light. Below this temperature, near infrared wavelengths are transmitted by the material. The dielectric constant change of VO2 is low is the visible spectrum range (particularly for its imaginary part) so the color of the coating will not be affected a lot. To increase the reflectance variations of the coating, a photonic effect will be added through a structural arrangement of the VO2 particles inside the polymer matrix. A numerical model will be developed to calculate the reflectance of nanostructured VO2 coatings. This model will be used to optimize the arrangement of VO2 particles needed to maximize the reflectance of the coatings.
As a physical demonstrator, nanostructured coatings will be fabricated through self-arrangement using colloidal polymer suspension. This synthetic route is well-known to produce ordered opal structure. This approach will take advantage of the thermo-activity of VO2, the film-forming ability of latex, deformability of the dried polymer and the photonic behavior of the ordered arrangement. This structure might not be the most effective, as may demonstrate the numerical study. However this structure will serve as a physical demonstrator of the concept and as validation model material for the numerical study.
A software interface to simulate the optical properties (reflectance / transmittance) of three-dimensional ordered structures such as opals has been realized. The results obtained in the case of a perfectly ordered opal are in very good agreement with the Bragg law predicting the position of the partial photonic bandgap (corresponding to a high increase of the reflectance in a narrow wavelength range) of such a structure. The results obtained are also in very good agreement with results previously published in the literature. The software also makes it possible to take into account defects in the structure (random variations of the spheres radius and/or distance between spheres).
Significant results have also been achieved in terms of material elaboration. Numerous methods for synthesizing VO2 nanoparticles have been explored and it has been possible to synthesize thermochromic particles of micrometric size. Nevertheless it remains difficult to obtain nanoparticles with a size of about 100 nm. Various techniques are under study to try to reduce the particle size, such as submitting the particles to ultrasounds or to an Ultra Turrax type mechanical grinding process.
Experiments were also carried out in terms of latex (colloidal suspensions of polymer particles) synthesis. The objective here is to obtain a latex with a low particle size dispersion, in order to generate a polymer opal presenting a photonic band gap. It has been shown in the literature that photonic effects disappear when the polydispersity index (PDI) is greater than 20%. Different latexes were generated, with average particle sizes ranging from 85 nm to 950 nm, and PDIs ranging from 0.3% to 25%.
The future actions of the project will be to compare the simulated optical reflectance/transmittance spectra on non-perfectly ordered structures to experimental data. As polymer nanocomposite samples containing VO2 nanoparticles of are not yet available, we plan to perform these comparisons on materials having an easier elaboration process such as binary colloidal crystals formed by an ordered polymer and nanoparticles. Two different types of nanoparticles are envisaged: dielectric (for example SiO2) and metallic (gold or silver) nanoparticles.
Once the results of the optical simulations confirmed by comparison with experimental data, the optical reflectance/transmittance spectra obtained for different material structures will be integrated in a simple thermal model and/or in a simulation software for building energy consumption in order to optimize the structure of the material directly according to the associated energy consumption.
The elaboration of structured opals from the previously synthetized latexes will go, and new experiments will be conducted in order to introduce VO2 nanoparticles in this opal. Since September 2018 some American nanoparticle manufacturers are starting to propose thermochromic VO2 nanoparticles. Thus, it was decided that final synthesis tests of VO2 nanoparticles will be carried out in September 2018 in order to improve our own knowledge about the nanoparticle elaboration process. In parallel, a purchase of commercial nanoparticles is underway in order to avoid delays in the project. In order to achieve an ordered nanocomposite material, the VO2 nanoparticules will have to be made compatible with the latex.
A review paper in the journal Advanced Engineering Materials is under evaluation : A review of Vanadium Dioxide as an actor of nanothermochromism: challenges and perspectives for polymer nanocomposites
Jenny Faucheu1, Elodie Bourgeat-Lami2, Vanessa Prévot3
Two oral communications have been accepted in the national conference Matériaux 2018 (19-23 novembre, Strasbourg, France)
Numerical simulations of the optical behavior of non-perfect photonic crystals obtained through colloidal sedimentation
Cindy Péralle*1, Jenny Faucheu1, Renée Charrière1, Jean-Marc Chenal4
Latex based nanocomposites containing vanadium dioxide nanoparticles for smart thermochromic coatings
Xavier Ingouf* 1, Jenny Faucheu1, Elodie Bourgeat-Lami2, Vanessa Prévot3
1Mines Saint-Etienne, Univ Lyon, CNRS, UMR 5307 LGF, Centre SMS, Saint-Etienne, France
2Université Claude Bernard Lyon 1, CPE Lyon, CNRS, UMR 5265, Villeurbanne, France
3Univ. Clermont Ferrand, CNRS, SIGMA-Clermont, UMR 6296, ICCF, Aubière, France
4Université de Lyon, INSA Lyon, Villeurbanne, France
Roofs and pavements comprise over 60% of urban surfaces in most large cities. On a sunny summer afternoon, these typically dark, dry surfaces get hot and in turn heat the air. The air in nearby rural areas tends to be cooler, because the rural surfaces are more reflective (absorbing less sunlight) and wetter (dissipating some solar heat gain by evaporating water). This elevation in urban air temperature is called the heat island effect.
The THERMOCOAT project seeks to cool cities in summer. The simplest way to reduce solar absorption is to replace dark surfaces that strongly absorb sunlight, such as black or gray roofs, with light-colored surfaces that strongly reflect sunlight, such as white roofs. Light-colored surfaces stay coolest because they strongly reflect both visible and invisible sunlight. White roofs are highly capable for energy savings in hot and dry locations and are becoming popular for large commercial buildings with extensive rooftop surface areas. However in chilly regions, especially during winter, white roofs are not recommended since they reflect the thermal heat that could be used to heat the building. Moreover, in historical districts and for aesthetical reasons, other colors might be more adequate. Thermochromic coatings can achieve both heating and cooling by changing their optical properties, in particular in visible and/or near infrared (IR) range, under temperature changes. By focusing on the IR wavelengths, the color of the coating will not be affected.
The outcomes of the THERMOCOAT project are dual. First, the project aims at optimizing the approach of thermoactive photonic coatings for better control of heat island effect through the development of a specific coating. During the project, an electromagnetic model will be developed in order to predict the optical properties of an ordered polymer coating loaded with thermochromic vanadium dioxide (VO2) nanoparticles. This electromagnetic model will predict the optical behavior of thermochromic nanocomposite depending on the structural parameters of the material, and in particular the resonant Bragg scattering effect. The purposes of this model will be both to provide a better understanding of this resonant scattering effect as well as optimizing the structural parameters of the coating in order to obtain the best energy saving impact. Nanostructured coatings (opal structure) will be fabricated through self-arrangement using colloidal polymer suspension. This approach will take advantage of the thermo-activity of VO2, the film-forming ability of latex, deformability of the dried polymer and the photonic behavior of the ordered arrangement. This model nanocomposite coating will exhibit significant difference between its optical properties in the near IR range between its cold and warm phases. This structure will serve as a physical demonstrator of the concept and as validation model material for the numerical study. Second, taking advantage of the scientific data generated during the project, this projects aims at increasing the awareness of communities and population on Heat Island Effect by developing a serious game about the influence of the choice of materials (type and colors) for buildings on the heat island effect and energy consumption.
The THERMOCOAT project takes advantage of complementary skills available in the Ecole des Mines de Saint-Etienne: expertise on optical simulations and characterizations combined to expertise in nanocomposites and polymers elaboration. In addition to dissemination towards the scientific community, the project intends to develop specific dissemination materials toward civil society to increase the awareness of communities and population on Heat Island Effect and its impact on global warming and thermal discomfort in the cities.
Madame Renée CHARRIERE (Ecole des Mines de Saint-Etienne)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
EMSE Ecole des Mines de Saint-Etienne
Help of the ANR 239,760 euros
Beginning and duration of the scientific project: September 2016 - 42 Months