Energy-Efficient Smart Windows
Aerogel VO2-PMMA Thermochromic Film as Retrofit Material for Single-pane Windows
Design, modelling and energy analysis of thermally-insulating and optically-switchable aerogel VO2-PMMA thermochromic film for energy-efficient window application.
Year:
2020
Affiliation:
- University of Boulder Colorado
- Norwegian University of Science and Technology
- China Hauzhong University of Science and Technology
- SINTEF Building and Infrastructure
Schematic illustration of heat conduction and solar radiation heat transfer across a single-pane window adopting 3.0 mm aerogel and 4 µm VO2-PMMA thermochromic film coated on 50 μm BoPET film.
Commercial and residential buildings account for ~ 40% of the total energy consumption in the United States. Compared to other building components, because of the high thermal conductivity (~ 1.0 W/(mK)) and high infrared emissivity ~ 0.84 of the float the glass, windows remain the weakest thermal barrier between the indoor and outdoor environments, directly responsible for 40 ~ 60% of the total energy loss from buildings. According to the ASHRAE standard energy loss for single pane windows is in the range of 60 ~ 250 W/m2. Therefore, minimizing the heat loss through windows requires an increase in the thermal resistance of windowpanes and regulation of the radiative heat transfer between windowpanes and the environment. Based on a recent evaluation by the U.S. Department of Energy, energy-efficient window retrofits for the existing single-pane windows can potentially reduce 1.2 quads (1.266×1018J) of domestic energy consumption in the United States. The objective of our work is centered on development of smart window retrofitting materials that improve the energy efficiency measures, by decreasing
the heating and cooling loads, which consequently reduce the building energy consumption. To achieve that, based on the recent progress of recently synthesized flexible and transparent aerogels, and promising solar transmission regulation ability of the durability-enhanced VO2-PMMA thermochromic film, an optically-switchable and thermally-insulating film for energy-efficient windows retrofit materials was proposed. The developed model consist of 3.0 mm aerogel insulation layer and 4μm VO2-PMMA film coated on 50 μm biaxially-oriented polyethylene terephthalate (BoPET) sheet, which can both dynamically manipulate the solar transmission according to ambient temperature and minimize energy loss due to high thermal resistance of the aerogel without blocking out the useful energy and daylight from the sun. The AVP film achieved a thermal transmittance (U-value) of ~ 3.0 W/(m2K), which is approximately ~50% less than the value of single-pane windows, a luminous transmittance of 0.60 in cold climates, 0.50 in hot climates and a solar modulation ability of ~ 17%. A coupled heat conduction and solar radiation heat transfer model was developed to evaluate the energy performance of the film, suggesting that employing AVP film as a window retrofit could not only reduce the heat loss but also greatly improve the performance of single-pane windows by improving thermal comfort and avoiding moisture condensation in cold climates and overheating and sun glare in hot climates. The proposed model has effective application potential as a retrofit for increasing the energy efficiency of single- pane windows.