(Source: LBL)
Windows can convert sunlight into IR heating on demand

"Smart windows" are a hot new field of research in the building industry.  According to [PDF] the U.S. Environmental Protection Agency 39 percent of all power use in the United States and 72 percent of electricity use.  

On average a household pays $2,000 USD a year in energy costs relating to their home, and of that over half goes to heating and cooling.  Thus smart windows -- which have the potential to save on energy cost by either generating electricity via transparent solar panels or by modifying the sunlight passing through them -- are being closely watched by the half trillion-dollar U.S. building industry.

One team actively pioneering smart window technology is Delia Milliron's nanocrystal research team at the Molecule Foundry, a research center within the U.S. Department of Energy's Lawrence Berkeley National Laboratory (LBNL).  Professor Milliron's research focuses on windows that "switch" their transparency, based on an applied electric charge.

smart window teamThe smart window dream team -- (from left:) Guillermo Garcia, Delia Milliron, and Anna Llordés

In a 2011 paper, published [PDF] in the ACS journal Nano Letters, Prof. Milliron and her team showed that windows containing special nanocrystals could be used to selectively control the light.  The windows exploited the electrochromic effect, which uses a burst of charge (2.5 volts) to rearrange particles, changing the color and/or transparency of the material.  In her latest work, published in the prestigious peer-reviewed journal Nature Letters, Prof. Milliron shows off a much more advanced version of this technology, which her team is in the early stages of commercialize.  

The window control layer is now made with a soupy "matrix" of niobium oxid (NbOx) and nanocrystals of indium tin oxide (ITO).  Together the materials act synergistically to allow the user to control both the amount of light passing through the window, and the amount of light that is converted to heat.

smart window material
Green -- the NbOx matrix; blue -- ITO nanocrystals

This dual functionality is critical, as past designs suffered from single-mode syndrome.  For example a material that could be switched on to solely increase heat (via near infrared (NIR) light production) would perform poorly during the summer months, as it would be unable to cool the building by blocking sunlight.  By contrast a material that blocked sunlight but did not produce heat would be useless in the winter months when solar heating was desired.

The new material packs both of those functionalities into a single package allowing the user accommodate all seasons.

Describes Prof. Milliron:

In the US, we spend about a quarter of our total energy on lighting, heating and cooling our buildings.  When used as a window coating, our new material can have a major impact on building energy efficiency.

From a materials-design perspective, we've shown that you can combine very dissimilar materials to create new properties that are not accessible in a homogeneous single phase material, either amorphous or crystalline, by taking nanocrystals and putting them in glass.

The most exciting part has been taking this project all the way from synthesizing a new material, to understanding it in great detail, and finally to realizing a completely new functionality that can have a big impact on technology.  Taking a materials development project all the way through that process is really quite remarkable. It really speaks to what we can do at Berkeley Lab, where you have access to not just the scientific facilities but also to people who can inform your perspective.

On a final note, one critical barrier to commercialization is the rarity of the metals used -- indium and niobium.  However, in a fortunate twist the two nanomaterials interact with each other synergistically.  While the interface between them is tiny, it manages to greatly amplify the behavior of each material, meaning you need to use much less material (i.e. you can use a thinner thin film).  This ultimately makes the "smart windows" much closer to commercial reality than past designs.

Sources: Nature Letters, LBL

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