Novel coating turns windows into radiators

MUCH of the heat loss from buildings is through the windows, but researchers in Sweden have developed a transparent coating that causes glass to heat up in sunlight.

Physics professor Alexandre Dmitriev at Gothenburg University and his team say that their discovery could help to save energy because if warmth is not escaping through windows less heating will be required. Glass acts as a heat sink. Even state-of-the-art low heat emission windows still lose heat. For example, if the indoor temperature is 20^oC and it is 0^oC outside, the inside glass temperature will be around 14-16^oC.

“If we look at the average temperature for walls, ceilings, floors and windows, the window temperature needs to be as close to indoor temperature as possible because we want to avoid windows stealing heat. If we can prevent the heat from disappearing to heat sinks such as glass surfaces, we will have a more comfortable indoor environment,” said Dmitriev.

The big challenge has been to create a coating that still allowed sufficient light through. Dmitriev and the team created “nano antennae” which absorb sunlight and generate high-energy electrons that heat up the glass.  Dmitriev and the team used a hole-mask colloidal lithography technique to coat glass surfaces, with layers of nickel and Al₂O₃ built up through thermal evaporation.

The nano antenna coating can heat up a regular glass surface by 8 K when it is illuminated by sunlight. The light passing through the coated glass has a colour rendering of 98.76%, meaning that visible colours are largely unchanged.

The nano antennae are easy and cheap to manufacture, and could therefore be used to cover large surfaces, for example on the kind of all-glass facades found on office buildings.

The coating only absorbs sunlight on one side, so coated windows could act to aid cooling in warmer weather, acting as a heat shield and saving energy on air conditioning.

“If the windows are mounted so that they can be turned over, they can be transformed into solar reflectors instead of a heating element,” said Dmitriev.

He and the team will now work to further optimise the antennae to produce more heat and absorb a broader spectrum of light.

Nano Letters doi.org/cfmm