Materials

Cooling/heating window film captures and releases solar energy

The MOST window film keeps rooms from heating up during the day, but warms them at night
The MOST window film keeps rooms from heating up during the day, but warms them at night
View 1 Image
The MOST window film keeps rooms from heating up during the day, but warms them at night
1/1
The MOST window film keeps rooms from heating up during the day, but warms them at night

A couple of years ago we heard about the MOlecular Solar Thermal (MOST) system, in which solar energy is stored in a liquid medium, then later released as heat. Now, the technology has been applied to a clear film that could be applied to the inside of windows in energy-efficient buildings.

Developed at Sweden's Chalmers University, the MOST film incorporates a norbornadiene–quadricyclane molecule. This causes the transparent polymer film to take on an orangey-yellow color when not being directly exposed to sunlight.

Once the sun rises in the morning and its rays strike the material, however, much of the sunlight's solar energy is absorbed by the molecule. More specifically, the molecule captures some of the incoming photons, causing it to isomerize – this means that it temporarily becomes another type of molecule, with exactly the same atoms but in a different arrangement.

As a result, the film not only turns completely colorless, but it also keeps much of the solar heat from getting into the room. The interior of the building thus stays cooler than it would otherwise, reducing the need to run the air conditioning.

In the evening, though – once the sun's rays are no longer hitting the film – the molecule reverts to its previous form, releasing the stored energy into the room as heat for up to eight hours. This reduces the need for the building's heating system to kick in.

The scientists are now working on both bringing down the price of the molecule, and increasing its concentration within the film. It is believed that these goals should be achieved in relatively short order, with a commercially-available window-retrofitting product soon to follow.

The research, which is being led by Prof. Kasper Moth-Poulsen, is described in a paper that was recently published in the journal Advanced Science.

Source: Chalmers University

3 comments
Mr T
I don't care what it's made of, a thin window film is going to be able to store buggerall energy. You need a lot of thermal mass to store a lot of energy, and a window film will have almost no thermal mass, regardless of what it is made of, because it is so thin, therefore it's volume is tiny. This is just another iffy article from a university trying to get funding, it's normal MO for most unis to release statements like this saying "we have made this amazing thing, now we just need more money". I've seen it countless times, and in almost all cases the "amazing thing" was not capable of doing what it claimed and simply disappeared once funding dried up. That's not to say that it doesn't do what they say it does re changing colour and blocking light to some degree, but low-e films and coatings already do this, at low cost, and are used extensively already. There is simply no way a coating on a window can store enough energy to make any detectable difference to the thermal comfort of a room. Block radiant energy, yes, store energy, no.
jareal333
This application doesn't seem to be very useful in cold climates. If it's hot outside during the day then we don't need additional heat at night so I don't see a practical application for this product unless it can be hooked up to a battery to store electricity like a solar panel.
Simon Redford
Mr T is of course quite correct and a more considered article about this system gives the energy storage capacity as 'up to' 0.133 kWh/kg (0.48MJ/kg) which suggests very little heat is collected and little will be returned to the room as presumably more is likely to be lost on the outer surface of the window to the cold outside. Far more benefit is likely through the sensible heating of high mass objects in the room by solar gain during the day. Put the grant money somewhere sensible!