Swedish chemists develop molecular technology that can store solar heat and release it during cooler periods of the day, evening out indoor temperatures to a comfortable level
A window film with a specially designed molecule could be capable of taking the edge off the worst midday heat and instead distributing it evenly from morning to evening. The molecule has the unique ability to capture energy from the sun’s rays and release it later as heat. Image: Yen Strandqvist/Chalmers University of Technology
Nobody wants to live in a greenhouse. While large windows are pleasant to have in a house and vital in a car, on a sunny day the intensifying effect of all that glass can raise temperatures to an uncomfortable – or even dangerous – level. This can, of course, be alleviated with air-conditioning, but this consumes energy. Simply closing the blinds is another option, but that blocks the view.
The Swedish team, from Chalmers University of Technology, is trying to turn this disadvantage into an advantage. “The aim is to create a pleasant indoor environment even when the sun is at its hottest, without consuming any energy or having to shut ourselves behind blinds. Why not make the most of the energy that we get free of charge instead of trying to fight it,” said chemist Kasper Moth-Poulsen, who is leading the research.
In a paper in the journal Advanced Science, Moth-Poulsen and his colleagues explain the concept they call MOST (MOlecular Solar Thermal Storage). They previously published early research on this concept in the journal Energy and Environmental Science in October last year.
The concept is based around the properties of a molecule called norbornadiene (NBD)–quadricyclane (QC). Composed of carbon, oxygen and nitrogen, this molecule contains two benzene rings and a six-membered hexane ring with two double bonds. This ring is capable of absorbing energy, which makes it change form or isomerise into a saturated form. This is a reversible process, so when it changes back into its unsaturated state it releases the energy.
Moth-Poulsen’s team has developed a method for immobilising this molecule into a polymer film without affecting its energy-related properties. If coated onto a window, this film has a yellow-orange tint if it is not exposed to sunlight. As it captures solar energy, the tint vanishes and the film becomes colourless, and as long as the sun is shining on it, it remains in this state, continuing to capture energy and preventing the heat from penetrating the glass. At dusk, the material would start to release its stored energy, warming up and acting as a heating panel as the outside temperature drops. It’s gradually recovers its tint, and in the morning will begin to absorb energy again.
In their previous research, the team had suggested using the molecule in an energy capture system that could store the solar energy for an extended period, such as from summer to winter, and then release it gradually to heat a building throughout the colder season. Using the material as a window film would be compatible with this system, they state, and it would also have advantages even if it were used without the additional energy storage.
The team’s research is now focusing on increasing the concentration of NBD-QC in the film and finding ways to reduce the cost of the technology. “The step to applying our film is so short that it could happen very soon. We are at a very exciting stage with MOST,” said Moth-Poulsen.
Unless the thermal capacity of the molecule is incredibly high you will need a lot of it to store a useful amount of heat. Let’s have some numbers.
You’ll find some in the linked papers.
They quote about 50kJ/kg. Let’s say we want to store 1kWh in a large domestic window. That gives 72kg. That’s not the thin film they are talking about but getting towards the weight of the glass. Is that practical?
This article reminded me of a truly disappointing experience I had back in 2004, when we tested what appeared to be a very promising product that ended up being a scam, in a way.
In a nutshell: Forget about using films to control the inside temperature of a CLOSED space, like the interior of a car or small room… There was an “advanced” product that used what was called “Very High Tech” to control the temperature by using a window film based on Fabry-Perot interferometry to reject the Infrared part of the solar spectrum. That product even won a “Coveted Technology Of The Year Award in 1989”!
So far so good. Except that any film installed inside a window will reflect that IR power back, and eventually, that IR energy will heat the glass of the window enough to cause it to become itself very hot (even hotter than without the “solar control film”)… so that after less than two hours, the glass starts to re-radiate heat into the closed space. In carefully controlled tests at Instituto Mexicano del Petroleo in Mexico City, we tested two cars (white color sedans) with and without the very “advanced” film provided by a local distributor of the maker (“V-KOOL International”).
In the film maker simplified tests, they used a small panel of glass lined with their film, which is exposed on one side with a powerful IR incandescent lamp. In that simple test anybody can feel that the V-KOOL film lined glass is indeed filtering a huge amount of heat… BUT, the hand of the person checking the effect is held behind the lined glass and it is placed in an OPEN SPACE. So, the “demonstration” is very convincing, but when the film is applied to the inside of all windows in a CLOSED car parked under the sun, the damn expensive film only ‘kind of worked’ for the first hour or hour and a half… soon after that initial time, the car interior reached the same temperature than without the film! The film made the glass of the windshield and all other windows of the car, initially cool, to become even hotter than without the film, Fabry-Perot Technology, Spectral-selective, Award-winning, Sputtered coating not withstanding!
To be effective, the heat rejecting coating MUST be placed OUTSIDE the glass, otherwise, the glass itself will be eventually heated by the IR energy traveling
twice across the glass (or even more times, since the glass-air surfaces reflect a large part of the beams), and this causes them to become heated and re-radiate that heat into the closed space of the car or room; so, the “solar control effect” is temporary at best. Great if you plan to park your car less than an hour under the sun, but forget to gain ANY advantage after the first 60 to 90 minutes, claims of 94% IR rejection simply are misleading!. Probably, a solar control film placed OUTSIDE the glass could work, but weather would attack it soon, so the elusive great solar control film for our cars is still in our imagination.