New material can harvest sunlight by day and release heat at night

As solar power becomes a bigger part of our overall energy mix, scientists are working on more efficient ways of storing the power of the Sun for use during the night-time, or on particularly cloudy days. And now a new type of material has been developed that can do just that - store solar energy when it's in abundance, and release it as heat later on as required.

The transparent polymer film developed by a team from MIT can be applied to many surfaces, including glass and clothing. So imagine a warm jumper that goes with you from room to room, so there's no need to fiddle with your central heating controls. Or a windshield overlay system that can burn away the ice on your car first thing in the morning, thanks to energy it had built up from the previous day.

"This work presents an exciting avenue for simultaneous energy harvesting and storage within a single material," the University of Toronto's Ted Sargent, who wasn't involved in the research, told MIT News. "The approach is innovative and distinctive."

Many solar energy storage initiatives focus on converting the energy to electricity and then keeping the electricity saved for later use. This approach instead uses a chemical reaction that produces heat rather than power: in this way the energy can be retained indefinitely in "a stable molecular configuration", until it's ready to be deployed, the researchers explain.

The key to the process is a molecule that can remain stable in one of two configurations. Sunlight kicks it into a long-lasting 'charged' mode, and then another stimulus - such as light, heat, or electricity - is used to return it to its original, 'not charged' state, and a burst of heat given off as a result.

Such solar thermal fuels (STF) have been developed before, but this new method is the first based on a solid-state material (in this case a polymer) rather than a liquid, and that can make all the difference in terms of how it can be used. What's more, it's based on inexpensive materials and with widespread manufacturing in mind.

The researchers are continuing to tweak the existing formula: they want to remove the slight yellowish tinge that the polymer currently has, and boost the heat level increase from 10°C up to 20°C. One of the first practical uses could be in electric cars, which can suffer from reduced driving ranges in cold weather due to the extra energy required for heating.