Harnessing the Sun’s energy is one of the cleanest ways to generate electricity on Earth. It does, however, come with an obvious limitation. Once darkness falls, solar panels stop generating electricity, creating a gap between when energy is produced and when it’s often needed most. To bridge the divide, a team of scientists may have found a way to capture and store energy from the Sun, extending its use into the night.
Researchers from China redesigned the internal structure of wood, converting it to a porous sponge that can absorb sunlight and store it as heat. The engineered material can be used to generate electricity from solar energy even when the Sun is no longer shinning. The findings, published in Advanced Energy Materials, could help overcome solar power’s most glaring weakness.
Eternal sunshine
There is currently no way of generating solar power in darkness, but scientists have developed clever hacks to try and extend thermal energy after the Sun goes down. The most common solution is battery storage, whereby electricity produced during the day is stored in large-scale batteries and then released at night.
Researchers are also experimenting with stacking different materials on top of each other to absorb certain wavelengths of light to help reduce wasted energy. While stacking material can help extend the generation of electricity, there are issues with durability, cost, and scalability.
Instead of combining layers from different materials, the researchers behind the new study turned to one scalable and environmentally-friendly solution: balsa wood, a lightweight, soft type of wood with a naturally porous structure that can be modified. It’s also good at insulating heat, making it an ideal energy-storing system.
On its own, however, raw wood reflects sunlight and absorbs water. That’s why the researchers first had to strip it of its lignin, a complex molecule that helps trees stay upright by hardening their structure. Removing the lignin helps boost the wood’s porosity and makes it better at absorbing light.
The researchers then chemically engineered the wood’s inner surfaces, coating the walls of its channels with ultrathin sheets of black phosphorene. The phosphorene interacts strongly with a broad range of light and has high electrical conductivity. It does, however, degrade when exposed to oxygen.
To help overcome the phosphorene’s limitations, the researchers wrapped each nanosheet in a protective layer made from tannic acid and iron ions. This molecular shield helps prevent oxidation while also improving the absorption of light.
That’s not all. Next, the researchers added silver nanoparticles to the wood to amplify how the material interacts with sunlight. They then placed hydrocarbon chains, molecules made of hydrogen and carbon arranged in a chain-like structure, onto the surface of the wood. The bonds between the hydrogen and carbon can store large amounts of energy and release it later as heat. The hydrocarbon chains also made the wood extremely water repellent, with a contact angle of 153 degrees.
Finally, the team behind the new study filled the wood’s inner channels with stearic acid, which can also help in storing sunlight when heated and releasing it when it’s cooled.
Timber!
After the balsa wood got its makeover, the researchers put it to the test. When sunlight hits the material, it heats up the stearic acid. Once the light is removed, that stored heat is released gradually and produces electricity in the dark.
The material converted around 91.2% of sunlight to heat, conducting heat nearly 3.9 times more efficiently than using the wood’s natural state. Using a thermoelectric generator, the engineered wood produced up to 0.65 volts of electricity during one cycle of sunlight.
The wood also proved to be extremely durable. Its performance remained more or less the same throughout 100 heating-cooling cycles and self-extinguished within two minutes to avoid burning. “Our work presents a scalable and environmentally friendly wood-based platform for advanced solar thermal energy harvesting,” the researchers wrote in the study.
The results are still preliminary, as the researchers need to prove that their new design can be used on a wide scale to harvest energy from the Sun and convert it to electricity after dark. If successful, however, it could be an all-in-one efficient source of clean energy.
