Researchers have discovered a strong, sustainable solution for passive cooling. 

According to research by the University of Maryland and the University of Colorado, wood could be a simple solution. 

Using tiny structures found in wood—cellulose nanofibers and the natural chambers that grow to pass water and nutrients up and down inside a living tree—this specially processed wood has optical properties that radiate heat away.

At the University of Maryland, Liangbing Hu, co-first authors Tian Li and Shuaiming He and others in the department of materials science have been working with wood for many years. Hu's team has invented a range of emerging wood nanotechnologies, including a transparent wood, low cost wood batteries, super strong wood, super thermal insulating wood, and a wood-based water purifier.

"This is another major advancement in wood nanotechnologies that [the] group at University of Maryland achieved: cooling wood that is made of solely wood—that is, no any other component such as polymers—can cool your house as a green building material," says Hu.

The team at Colorado have been working on materials for radiative cooling, including thin films and paints.

By removing the lignin, the part of the wood that makes it brown and strong, the UMD researchers created a very pale wood made of cellulose nanofibers. They then compressed the wood to restore its strength. To make it water repellent, they added a super hydrophobic compound that helps protect the wood. The result: a bright white building material that could be used for roofs to push away heat from inside the building.

They took the cooling wood out into the ideal testing condition of a farm in Arizona where the weather is always warm and sunny, with low winds. There, they tested the cooling wood and found that it stayed, on average, around six degrees F (16 degrees C) cooler than the ambient air temperature—even at the hottest part of the day, the cooling wood was chillier than air. It stayed on average 12 degrees (-11 degrees C) cooler than natural wood, which warms up more in the presence of sunlight.

"The processed wood uses the cold universe as a heat sink and releases thermal energy into it via atmospheric transparency window. It is a sustainable material for sustainable energy to combat global warming" says Tian Li of the department of materials science at UMD.

The mechanical strength per weight of this wood is also stronger than steel, which makes it a great choice for building materials. It is ten times stronger than natural wood and beats steel's strength, reaching 334 MPa·cm3/g (compared to 110 MPa·cm3/g for steel). It also damages less easily (scratch test) and can bear more weight (compression test) than natural wood.

"Prof. Hu and collaborators show yet another use of wood that is not only structurally strong but useful as active component for energy management. It is interesting that the same material that releases heat upon combustion can be used for cooling, offering new opportunities in green buildings," says Orlando Rojas, a professor in the department of Bioproducts and Biosystems at Aalto University, Finland.