Trees are cities’ natural air conditioners. They provide shade to block the sun and help to cool the city by transpiration. Through the transpiration process, groundwater is absorbed by the roots and transported to the leaves, where over 95% eventually evaporates into the atmosphere. This helps cool the plant as well as the neighbouring environment. Transpiration is also an efficient means to purify the groundwater, which benefits regions where freshwater is scarce.
In the Sustainable Functional Biomaterials (SBF) Lab at the University of British Columbia’s Faculty of Forestry, the team has been working on developing artificial trees for water treatment, including purification and desalination.
Trees contain different levels of pores within their three-dimensional trunks. These pores include millimeter to centimeter-sized lumen, micrometer-ranged stomata on the leaves, all the way down to those nanometer-sized porosities on the pit membranes. These pores are responsible for water and nutrient transport, as well as balancing the pressure within the cells. Water is constantly conducted through the cell lumen from roots to leaves, eventually evaporating into air, through the transpiration process.
Inspired by this transpiration process, the SFB lab utilizes nanotechnology and additive manufacturing techniques to develop layered structures that can mimic tree transpiration for water treatment. Very thin cellulose nanofibrils (3-5 nanometers in diameter; the diameter of a human hair is about 100,000 nanometers) can be isolated from forest and/or agricultural wastes – such as sawdust, shavings, hemp fibers and straws – and used as the building blocks of the artificial trees.
The CNFs can be further processed into three-dimensional structures using 3D printing technologies, which can create similar hierarchically porous structures (from millimeter to nanometer) as observed from the trees. The 3D printed monolith is covered with a black layer (which can also be derived from biomass by carbonization) to serve as a solar energy-absorbing layer. The bilayer structure contains pores for water absorption and transportation and the coated solar-absorbing layer converts solar energy to thermal energy, which can effectively evaporate the absorbed water. The SFB lab is equipped with a solar simulator that can simulate solar illumination, and fresh water can be generated from either wastewater or seawater.
Recently, the SFB lab, together with other UBC researchers, was awarded funding through the federal New Frontiers in Research Fund (NFRF) 2020 Exploration Stream, to explore developing artificial trees for extreme weather-resilient cities. The project aims to address issues that rapid urbanization is causing in already densely populated cities, which are home to half of the world’s population.
This interdisciplinary team is developing a triple-layered film that will mimic the ability of real trees to provide cooling and flood control. By attaching itself to building walls and roofs, this film is expected to transform cities into virtual forests of giant artificial trees by mimicking the natural transpiration process. Turning city buildings into giant tree-simulating evaporators can help to better control floods, by rapidly removing stormwater, and provide evaporative cooling effects for extreme weather conditions. The film will be constructed out of bio-based polymers, such as nanocellulose, from trees to minimize the environmental impact.
Dr Feng Jiang is an Assistant Professor at the Faculty Department of Wood Science, and a Canada Research Chair in Sustainable Functional Biomaterials. He also leads the Sustainable Functional Biomaterials lab at UBC. He can be reached at email@example.com.