The chemistry trail to store solar energy

With photovoltaic panels, "most of the time, solar energy is immediately used in the form of electricity or thermal energy," Rémi Métivier, CNRS researcher and co-supervisor of the study, reminds AFP. To store it, it is necessary to use batteries, for example.

The system he is working on - with the acronym MOST (MOlecular Solar Thermal) - is based on the chemistry of photochromic molecules.

"They have been used for a long time in sunglasses, for example," with lenses that darken or lighten depending on the ambient light, explains Keitaro Nakatani, professor of chemistry at the École Normale Supérieure (ENS) Paris-Saclay and co-supervisor of the study.

They have the property of changing color, but not only that. For more than ten years, researchers have been working on the ability of photochromic molecules to store energy from sunlight, and to conserve it before releasing it. All repeatedly.

"This higher energy molecule will be able to exist for periods of several days to several weeks," Léa Chocron, lead author of the study published in Chemical Science, the flagship journal of the prestigious British Royal Society of Chemistry, told AFP.

"The idea is to be able to restore this energy on demand, not at all in the form of electricity, as with a photovoltaic panel, but in the form of heat," continues the doctoral student in chemistry in the PPSM laboratory at ENS Paris-Saclay.

"Heat on demand"

An obvious application would be a "fluid circulating on the roof, for example, which would spontaneously charge with energy and then circulate in our radiators to heat at night," adds Mr. Métivier.

The principle is simple and is based on the irradiation of an organic molecule with ultraviolet rays, like those of the sun. The latter undergoes a chemical transformation changing its energy level.

The team used two molecules from the diarylethene family designed by chemists at the University of Paris-Saclay. "No more polluting or dangerous than a classic dye," according to the researcher, who is attached to the PPSM laboratory.

Once "charged," these molecules released their energy with the introduction of a small dose of an acid used in organic chemistry. In an extremely short time, ranging from five minutes to an hour, depending on the formula.

"To simplify, it's almost like a switch" which would allow "this heat to be released on demand", summarizes Mr. Métivier.

He specifies that if the principle applied was already known, the contribution of the study is to "completely explain and quantify it".

A detailed understanding of the mechanism at work and all the processes that accompany it therefore allows us "to go further, to develop and propose other systems that would be even more efficient," he adds. Whether in the capacity to conserve energy or in the number of times the molecule can be charged and discharged, like a battery.

A work of "light chemist" carried out with specialists in solar energy, in this case from the PROMES laboratory, of the CNRS in Perpignan, and whose industrial horizon will not appear for a good ten years.