Solar power has a simple problem. When the sun goes down, the power stops.

Researchers in China say they may have found a way to stretch solar energy into the dark by redesigning wood so it can absorb sunlight, store it as heat and later help generate electricity when there is no sunshine.

The team converted balsa wood into a porous sponge-like material and reported the results in the journal Advanced Energy Materials. The engineered wood is designed to tackle one of solar power’s biggest limits, the gap between when energy is produced and when it is often needed.

There is currently no way of generating solar power in darkness, but scientists have tried several ways to keep solar energy available after sunset.

The most common approach is battery storage. Electricity produced during the day is stored in large-scale batteries and then released at night.

Researchers have also tested stacking different materials on top of each other so they absorb specific wavelengths of light and reduce wasted energy. But the study says that approach has problems with durability, cost and scalability.

Instead, the team behind the new study used balsa wood, a lightweight, soft wood with a naturally porous structure that can be modified. The researchers said it is also good at insulating heat, which makes it suitable for storing energy.

Raw wood, though, has drawbacks. It reflects sunlight and absorbs water.

To change that, the researchers first removed lignin from the wood. Lignin is a complex molecule that helps trees stay upright by hardening their structure. According to the study, removing lignin increases the wood’s porosity and improves its ability to absorb light.

The team then chemically engineered the inner surfaces of the wood by coating the walls of its channels with ultrathin sheets of black phosphorene.

The study says phosphorene interacts strongly with a broad range of light and has high electrical conductivity. But it also degrades when exposed to oxygen.

To address that, the researchers wrapped each nanosheet in a protective layer made from tannic acid and iron ions. They said this molecular shield helps prevent oxidation and also improves light absorption.

The team then added silver nanoparticles to the wood to boost the way the material interacts with sunlight.

After that, the researchers placed hydrocarbon chains onto the surface of the wood. These molecules are made of hydrogen and carbon arranged in chain-like structures. The bonds between hydrogen and carbon can store large amounts of energy and release it later as heat.

The hydrocarbon chains also made the wood highly water repellent. The study reported a contact angle of 153 degrees.

The final step was filling the wood’s inner channels with stearic acid. The researchers said stearic acid can also help store sunlight when heated and release it when cooled.

After modifying the balsa wood, the team tested how it performed.

When sunlight hits the material, it heats up the stearic acid. Once the light source is removed, the stored heat is released gradually and produces electricity in the dark.

The material converted about 91.2 percent of sunlight into heat, according to the study. It also conducted heat nearly 3.9 times more efficiently than the wood in its natural state.

Using a thermoelectric generator, the engineered wood produced up to 0.65 volts of electricity during one cycle of sunlight.

The researchers also tested its durability. They found the wood’s performance stayed more or less the same across 100 heating and cooling cycles.

The study said the material also 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 findings are still early. The researchers still need to show that the design can work on a wide scale to harvest solar energy and convert it into electricity after dark.

But the study points to a different way of dealing with the basic timing problem in solar power, storing the sun’s energy in a wood-based material as heat and releasing it later when light is gone.

The engineered wood produced up to 0.65 volts of electricity during one cycle of sunlight.