The sun gives life to plants and microorganisms, provides us with warmth and daylight, and is an endless source of renewable energy. By all accounts, solar energy should be our first choice for heating our water and homes and powering our cars, but there’s just one problem that has stumped the solar industry for quite some time – the issue of storage. Scientists have struggled to find a cheap and efficient way to store the energy generated by solar power for the long-term.
Until now, the best method we have come up with to store solar energy is in the form of a battery. Tesla has been leading the charge in developing this technology, and while it is very effective, it’s expensive. To have the company’s new Powerwall system installed in your home, it’ll cost approximately 20 thousand dollars . Not surprisingly, this cost is prohibitive for most people who want to power their homes with renewable energy.
The team in Sweden is hoping that this new technology will give more people the opportunity to power their homes with sunlight.”A solar thermal fuel is like a rechargeable battery, but instead of electricity, you put sunlight in and get heat out, triggered on demand,” says Jeffrey Grossman, who leads a lab at MIT that works on such materials .
Photoswitches For Solar Energy Storage
Before going any further, let’s get a few things straight:
Isomers are two molecules that are made up of the same atoms (let’s say, they have the same “ingredients”), but these atoms are arranged differently in space . Think of isomers like a pair of gloves: both gloves have four fingers and a thumb, but a right-hand glove will only fit the right hand, and a left-hand glove will only the left hand. In this case, our two isomers are norbornadiene (NBD) and quadricyclane (QC) both are made up of hydrogen, carbon, and nitrogen, but these atoms are in different sequences.
Photoisomerization occurs when the molecules in one isomer (NBD) absorb sunlight and become excited. This causes them to rearrange themselves to become a new isomer, in this case QC. When NBD is converted to its isomer QC, energy gets trapped within the molecule. This new energized molecule is stable and has strong chemical bonds, which is why it can store that energy for such a long time . The important aspect of this process is that it can be reversed .
Releasing the Energy
The ability to store that energy is incredibly important, but once it’s stored, what’s the point of having it if you can’t get it back out? It’s like locking something in a safe and then losing the key.
In order to get the energy that was trapped in the QC molecule, you have to convert it back to NBD. To do this, the team at Chalmers passed the QC molecule through a catalyst to rearrange the molecules back to their original state .
This process causes the molecule to release energy in the form of heat. The scientists working on the project found that when the molecule was passed through a catalyst, it heated up the fuel by 63 degrees Celsius, or 113 degrees Fahrenheit .
This liquid heat could be used to heat homes, offices, public spaces, and much more.
Renewable and Cost-Effective Energy
This solar fuel can be stored in uninsulated tanks inside houses or factories, or could even be trucked or piped between cities and solar farms. Kasper Moth-Poulson, one of the team members working on the project, explains that both the fuel and the catalyst experience very little damage during the process, which allows them to run the system in a closed-loop, picking up sunlight and dropping off heat over and over again.
“We’ve run it through 125 cycles without any significant degradation,” Moth-Poulsen says .
Per kilogram, this fuel can store up to 250-watt hours of energy, which is twice the capacity of the Tesla Powerwall battery .
The Future of Solar Power
While this work is very exciting, it is not quite ready to be commercialized just yet.
“We’ve made a lot of progress,” Moth-Poulsen says, “but there is still a lot to figure out.” 
So far, the team has developed multiple fuel variants, so the next step is to create a single fuel that has a long shelf-life, high energy density, and good recyclability .
Right now, the fuel’s efficiency is quite low, since it only responds to the shortest (ultraviolet and blue) wavelengths of the sun, which is only five percent of the solar energy available to us. The team is working on increasing the fuel’s sensitivity to respond to a broader range of wavelengths .
When it comes to electricity production, hotter is always better, and Moth-Poulson is aiming to up his temperature increase to 80 degrees Celsius (176 degrees Fahrenheit) or more .
The hope is that this new technology will be available in the next ten years .
“When I started, there was really only one research group working on these kinds of systems,” Moth-Poulsen recalls. But progress has drawn others to the challenge. “Now there are teams in the U.S., in China, in Germany — about 15 around the world,” he says .
With so many groups working on the project, we may be heating our homes with sunlight by 2030.
- ‘Scientists Create Liquid Fuel That Can Store The Sun’s Energy For Up to 18 Years’ Science Alert Carly Casella. Published January 25, 2019.
- ‘Liquid Norbornadiene Photoswitches for Solar Energy Storage’ Wiley Online Library Ambra Dreos et al. Published March 25, 2018.
- ‘Powerwall’ Tesla
- ‘Molecular solar thermal energy storage in photoswitch oligomers increases energy densities and storage times’ Nature Mads Mansø et al. Published May 16, 2018.
- ‘Structural Isomerism in Organic Molecules’ Chemistry Jim Clark. Published September 12, 2020.
- ‘Scientists Reveal Strange Molecule That Can Store Sun’s Energy For 18 Years’ Forbes Trevor Nace.
- ‘Macroscopic heat release in a molecular solar thermal energy storage system’ Royal Society of Chemistry Zhihang Wang et al. Published 2019.
- ‘Scientists are trying to bottle solar energy and turn it into liquid fuel’ NBC News