COLLEGE PARK, Md. -- Researchers at the University of Maryland and University of Colorado Boulder show in new research that the same tiny internal structures that trees use to carry water and nutrients can be used to create super-strong wood building materials able to passively take heat out of homes or offices.

Published today in the journal Science, the research is the latest from UMD materials science and engineering Associate Professor Liangbing Hu and his team to exploit the natural properties of wood to create an amazing range of wood-based technologies. Previously, Hu and collaborators have processed wood to be stronger than most metals, turned it translucent for an energy-saving window material, given it super thermal insulating properties, made low cost wood batteries, demonstrated how it could be used for low-cost water purification and desalination, and turned it into a flexible membrane that can convert heat into electricity.

“This [latest work] is another major advancement in wood nanotechnologies that the Hu group at the University of Maryland achieved: cooling wood that is made of solely wood—that is, no other component such as polymers—can cool your house as a green building material,” said Hu, one of the paper’s co-authors.

In their current work Hu, co-first authors Tian Li and Shuaiming He and four other UMD colleagues were joined by University of Colorado researchers led by Prof. Xiaobo Yin and including co-first author Yao Zhai, as well as by researchers from the University of California Merced and China’s Huazhong University of Science and Technology.

“When applied to building, this game-changing structural material cools without the input of electricity or water,” Zhai said.

The researchers tested their cooling wood on a farm in Arizona in warm, sunny, and low wind conditions. There, they tested the cooling wood and found that it stayed, on average, five or six degrees F cooler than the ambient air temperature – even at the hottest part of the day. It stayed on average 12 degrees cooler than natural wood, which warms up more in the presence of sunlight.

“The processed wood uses the cold universe as heat sink and release thermal energy into it via atmospheric transparency window. It is a sustainable material for sustainable energy to combat global warming” said Li.

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.

To see how much energy the wood saves, they calculated how much heat is used by typical apartment buildings in cities across the US in all climate zones. Hot cities like Phoenix and Honolulu would save the most energy, especially if older buildings had their siding and roofs replaced with cooling wood. Buildings across the US that were built after 2004, or now, would save on average 20% of cooling costs.

"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," said Orlando Rojas, a professor in the department of Bioproducts and Biosystems at Aalto University, Finland. Rojas was not involved in the research.

The research was funded by the University of Maryland.

A company co-founded by Hu, Inventwood, is focusing on the commercialization of these advanced wood nanotechnologies (www.inventwood.com).

A Radiative Cooling Structural Material Science , May 24, 2019