Smart windows

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Nanomaterials are enabling self-powered smart window systems for UV-protection, thermal comfort and solar conversion.

Nanomaterials have already greatly impacted the window and glass market, with titanium dioxide nanoparticles (nano-TiO2) applied in photocatalytic self-cleaning glass films. With the assistance of little UV light from fluorescence sources or sunlight, nano-TiO2 offers two unique properties: (a) strong oxidation power, and (b) super-hydrophilicity.

Figure 1: Self-cleaning window film.

There is already a great number of buildings worldwide that have been treated with such coatings that greatly benefit building maintenance, especially for skyscrapers, as they reduce the need for costly surface cleaning. There are a number of nanocoated glass products available, with Asia and Germany in particular proving to be strong markets, with some estimates placing the market at over $1 billion in Japan alone.

The smart windows market
The market for smart glass and smart windows is relatively small at present. However, recent advances in graphene and other nanostructured materials in transparent conductive films, electrodes and coatings have improved the viability of next-generation smart windows for energy saving and comfort in buildings and vehicles. Applications include:

  • Reduction of solar glare in windows and mirrors.
  • Improved heat and light control in buildings.
  • Energy conversion windows.
  • Transparent and flexible electrically switchable smart windows.
  • For use in privacy windows in internal glass partitions in offices.

 

The U.S. Department of Energy estimates that 30% of the energy used to heat and cool all buildings in the U.S is lost through inefficient windows at a cost of $42 billion per year. 1 2 3 Therefore, there is a significant opportunity for new energy efficient window systems, and nanomaterials are enabling new ‘chromogenic’ window technology.

Thermochromic windows
Thermochromic materials are able to change their optical properties in response to temperature change. Thermochromic windows use adhesive coatings to adjust tinting passively with window surface temperature.
Vanadium oxide thermochromic nanocoatings are being developed for energy efficient glass applications. The coatings are applied to smart windows that either absorb heat or permit its reflection, depending on their temperature. They self-control the solar radiation and heat transfer for energy saving and comfort in houses and automobiles.4 At temperatures above 30° Celsius (about 86 °F), the coating is transparent and the metal underneath reflects heat.

Figure 2: Metal strip coated with thermochromic nanoparticles. Image credit: Fraunhofer.

Electrochromic windows
Electrochromic windows utilize operable switches or automated building control systems to actively tint the window via electric current to improve energy-efficiency in buildings. Use of nanomaterials can provide electrochromic devices with improved colouration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. In order to develop electrochromic devices that go beyond the capabilities of commonly used electrochromic materials, researchers are utilizing transparent conductive nanomaterials such as graphene, new 2D materials and indium tin oxide nanoparticles.

Figure 3: Electrochromic material incorporating tin-doped indium oxide nanocrystals. Image credit: Lawrence Berkeley National Laboratory.

The huge global development of graphene and 2D materials is enabling new developments in electrodes for flexible smart windows, although most current focus in this area is on light emitting devices (LED), liquid crystal displays (LCD), flexible organic LEDs and touch screens.
Electrochromic cycling results for graphene have demonstrated that it improves electrochromic performance in terms of:

  • switching kinetics
  • activation period
  • coloration efficiency
  • bleached-state transparency, while maintaining ~100% charge reversibility.

 

Under development are glass-backed graphene electrolytes for voltage-tuneable transparency and opacity in electrochromic windows. The graphene surface can be switched from full transparency to a translucent or opaque darkened state. Applications are for reducing solar glare in windows and mirrors; improving temperature and light control in buildings and adjusting window opaqueness for privacy.
Researchers at the University of Exeter have developed electrochromic smart glass incorporating graphene. The graphene surface can be switched from full transparency to a translucent or opaque darkened state within five seconds. The system is fully reversible to transparency in two seconds.
Other 2D nanomaterials such as monolayer TMDs (such as MoS2 and WS2) and black phosphorus also display promise for application in optoelectronic components for smart windows. 5 6 7
If production of large-scale and high-quality 2D materials is fully successful then they could create a completely new market to address ongoing challenges and emerging applications in smart windows and glass. 8

References
1. Arasteh, D., S. Selkowitz, J. Apte, Zero Energy Windows, Proceedings of the 2006 ACEEE Summer Study on Energy Efficiency in Buildings, August 13-18, 2006, Pacific Grove, CA, http://gaia.lbl. gov/btech/papers/60049.pdf
2. 2010 Buildings Energy Data Book. US Department of Energy, Building Technologies Program, Energy Efficiency and Renewable Energy. Tables 1.1.1 and 1.2.3.
3. http://www.sciencedirect.com/science/article/pii/S0927024809000798
4. http://www.c3p.org/Workshop%202007%20Documents/VTeixeira_Solar%20energy%20coatings.pdf
5. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: a new direct-Gap
semiconductor. Phys. Rev. Lett. 105, 136805 (2010).
6. Splendiani, A. et al. Emerging photoluminescence in monolayer MoS2. Nano Lett. 10, 1271-1275 (2010).
7. Xia, F., Wang, H. & Jia, Y. Rediscovering black phosphorus as an anisotropic layered material
for optoelectronics and electronics. Nat. Comm. 5, 54458 (2014).
8. Ferrari, A. C. et al. Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale 7, 4598-4810 (2015).

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