Storing solar energy as heat in synthetic molecules for future energy storage systems

Scientists at Germany’s Goethe University are investigating the potential of molecular solar thermal energy storage, a high-capacity method of storing solar energy.

Molecular solar thermal energy storage involves a large number of synthetically created molecules that together store energy from the sun – functioning as a ‘heat battery’.

The molecules contain mostly carbon, oxygen, nitrogen, and hydrogen. They do not require rare earths, heavy metals, or plastic.

Although still in the very early phases of development, scientists believe the method could be more efficient than conventional thermal heat storage systems which have used materials like volcanic rock, sand, molten salts, and aluminum. Direct comparison tests to these materials have shown positive early results.

According to Josef Wachtveitl, one of the Goethe University scientists involved in the research group, “All the steps – conversion, storage and release of energy – are united in a single molecule,” so the process is quite simple.

When the molecules come into contact with sunlight their structure changes. This is called a photo-induced reaction. The light causes the molecules to absorb energy, which they release later as they return to their original state away from sunlight.

The researchers are currently looking at three different molecules, which they refer to as photoswitches. These are norbornadienes, azaborines, and azobenzenes – all created in a lab. When exposed to light, all three switch from their ground state to a higher-energy storage state, changing their molecular structure and in some cases their color.

As the heat is stored directly, there are no conversion processes like turning electricity into heat. This means that the losses that otherwise occur during any energy conversion are absent, making the process more efficient than the average solar thermal system, say the scientists.

The new method allows “much more flexibility for storing solar heat,” said Wachtveitl. He emphasized its ability to store heat for longer and release it on demand – like a switch – compared to regular thermal systems.

Scientists are still tweaking some of the characteristics of the molecules and there are several German universities involved in fine tuning the process so it can eventually yield more quantifiable results. The project has a few years left to develop. It began in 2023 and will conclude in 2027, and is receiving a total of around €4.8 million ($5 million) in funding from the German Research Foundation.

Currently, however, the method is beginning to be tested in a real-world setting. For example, the molecules are placed on glass surfaces intended to be made into windows later on. The molecules on the glass can store energy during the day and release it again at night. Molecules that can be pumped within a closed cycle are also already in development. They could be installed as panels on roofs, where the sun will induce them to switch to their storage state.

Storage systems using the molecules can be used in cars, houses, factories, and even in sheds for animals, say the scientists. As they only use molecules, the systems are decentralized and big production plants are not required.

The research consortium behind the project has a YouTube channel, FOR MOST, which has short videos showing their work.