Ever since Benjamin Franklin’s lightning experiment in the 18th century, civilization has been intrigued by the idea of capturing lightning in a bottle. Over time, however, the idea evolved from literal to figurative. Today, we may be seeing a return to the literal. Scientists at the University of California, Santa Barbara (UCSB), may have figured out a way to bottle the sun, or at least its energy, using a Dewar pyrimidone solution.
One of the major challenges in solar energy utilization has been what happens when the sun goes down. During the day, solar harvesters capture energy across the solar spectrum and convert it into usable energy. A portion of this energy is stored for later consumption, using various storage solutions, most commonly batteries.
However, the majority of these solutions are bulky, complex, expensive, or inefficient in storing and converting energy. Chemical batteries, for example, are quite bulky. Also, the conversion from electrical to chemical energy during charging and back to electrical energy during discharge is not always efficient, often resulting in energy losses.
The researchers, Associate Professor Grace Han and her team at UCSB, have developed a new material that eliminates the need for batteries by storing solar energy directly as heat. It’s a new form of molecular solar-thermal (MOST) energy storage, an emerging class of solar technology that stores sunlight directly in chemical bonds on a molecular level and releases it later as heat.
The researchers’ approach uses a specially engineered liquid containing photoresponsive modified pyrimidone molecules. When exposed to sunlight, each molecule undergoes a reversible structural change, shifting from a low-energy configuration into a strained, high-energy form.
You can think of each molecule as a tiny spring. Sunlight “winds” the spring by forcing the molecule into a twisted, energy-rich Dewar configuration. The molecule then remains locked in that state, sometimes for months or years, without losing the stored energy. When a catalyst, such as heat or acid, is applied, the molecule snaps back to its original shape, releasing the stored energy as heat.
This process is repeatable, a desirable characteristic of the reusable, recyclable technology.
“Think of photochromic sunglasses. When you’re inside, they’re just clear lenses. You walk out into the sun, and they darken on their own. Come back inside, and the lenses become clear again,” says Han Nguyen, one of the researchers and the paper’s lead author. “That kind of reversible change is what we’re interested in. Only instead of changing color, we want to use the same idea to store energy, release it when we need it, and then reuse the material over and over.”
Jeff Liang, UC Santa Barbara
As is often the case in research, the researchers drew inspiration from nature. The synthesized molecule is based on the structure of a component found in DNA, which can reversibly undergo structural changes under ultraviolet light.
Now, as is highlighted in this 2017 research paper, other Molecular Solar Thermal storage systems do exist, with the main difference being the type of reversible molecular reactions, such as Azobenzene-based molecules and Dihydroazulene/vinylheptafulvene systems. However, most of the technology is still in the research and early pilot stages.
The Dewar pyrimidone system is the first in the field to achieve practical usability, marking a significant advancement. In the research, the Dewar isomer released enough heat to boil ~0.5 mL of water.
“Boiling water is an energy-intensive process,” Nguyen said. “The fact that we can boil water under ambient conditions is a big achievement.”
We’ve established that the Dewar pyrimidone system is great … compared to other MOST storage systems. But how does it hold up against other storage solutions? The answer is the same, “great.” This is one of the reasons why the researchers consider the system to be a breakthrough. For starters, unlike batteries that convert sunlight into electricity and then into chemical energy, this system stores energy directly, albeit as heat.
Furthermore, in addition to being able to charge and discharge (twist and untwist) repeatedly without losing its structure, the molecule can store energy for months. The Dewar isomer is extremely stable, with a calculated half-life of up to 481 days at room temperature.
Because the system is a solution of dissolved molecules, it is highly scalable and easy to integrate into existing systems. Increasing energy storage capacity is a simple matter of using a larger quantity of the solution. The solution can also be pumped, transported, and stored using conventional plumbing. This characteristic is behind the “bottled sun” description.
Lastly, in one of the ultimate tests of energy storage, the UCSB pyrimidone system achieves an energy density of about 1.6 MJ per kilogram, approximately double that of a standard lithium-ion battery at around 0.9 MJ/kg!
These characteristics make the technology applicable to many use cases. Here’s a sample scenario: A solar collector on a roof or structure circulates the liquid MOST material. During the day, sunlight converts the molecules into their energy-rich form. The “charged” liquid is stored in an insulated tank. When heat is needed, for example, for hot water, cooking, or space heating, the liquid is passed through a reactor where a trigger causes it to release its stored energy. The liquid returns to its original state and is ready to be recharged the next day.
Another use case is seasonal energy storage: the solution can be charged during summer, stored, and used for heating during the winter. The Dewar pyrimidone also has the potential to generate electricity via integration with thermoelectric generators and thermodynamic cycles (turbines).
A paper detailing this research was recently published in the journal Science.
