Sunlight can be used to generate electricity using solar panels but it also can add heat to storage media like water, liquid metals, and molten salts. The problem for scientists is that those phase changes occur quite rapidly. If there were a way to slow them down and control how and when the transitions take place more precisely, a new heat storage battery could result.
Researchers at MIT think they have discovered a way to do exactly that. Writing in the journal Nature Communications, they say they have come up with a way to embed “molecular switches” into the phase change materials that absorb heat. Those switches can be operated by pulses of light to control when and how fast phase changes occur. They say it is possible to allow the release of heat to continue even past the temperature where a change in phase normally occurs.
“The trouble with thermal energy is, it’s hard to hold onto it,” says Jeffrey Grossman, one of the authors of the report. The researchers use “little molecules that undergo a structural change when light shines on them.” The result is phase change materials that release heat on demand. The ramifications are enormous. “There are so many applications where it would be useful to store thermal energy in a way that lets you trigger it when needed,” he says. “By integrating a light-activated molecule into the traditional picture of latent heat, we add a new kind of control knob for properties such as melting, solidification, and supercooling.”
According to MIT, “The researchers accomplished this by combining the fatty acids with an organic compound that responds to a pulse of light. With this arrangement, the light-sensitive component alters the thermal properties of the other component, which stores and releases its energy. The hybrid material melts when heated, and after being exposed to ultraviolet light, it stays melted even when cooled back down. Next, when triggered by another pulse of light, the material resolidifies and gives back the thermal phase-change energy.”
Co-author Grace Han adds, “The availability of waste heat is widespread, from industrial processes, to solar heat, and even the heat coming out of vehicles, and it’s usually just wasted. What we are doing technically is installing a new energy barrier, so the stored heat cannot be released immediately.”
In the lab, phase changes that normally occur in a matter of minutes can be deferred for up to 10 hours. “There’s no fundamental reason why it can’t be tuned to go higher,” Han says. “The energy density is quite significant, even though we’re using a conventional phase-change material.” The heat storage material the scientists used in the lab can store as much as 200 joules per gram, which is “very good for any organic phase change material.”
So, what is this discovery good for? The researchers say they have already gotten interest from people in rural India, where the heat storage technology could permit the capture of heat from the sun during the day and used for cooking or space heating after dark. “Our interest in this work was to show a proof of concept,” Grossman says, “but we believe there is a lot of potential for using light-activated materials to hijack the thermal storage properties of phase change materials.”
No one is talking about what this could mean for concentrated solar power systems just yet, but the implications are obvious. CSP is all about storing and releasing heat during phase changes. Better control of when and how that happens could make CSP more competitive as a source of zero emissions electrical power.