The next big renewable
energy source could be at our feet
By Will Cushman, University
of Wisconsin-Madison
Flooring can be made from any number of sustainable materials,
making it, generally, an eco-friendly feature in homes and businesses alike.
Now, however, flooring could be even more “green,” thanks to an
inexpensive, simple method developed by University of Wisconsin–Madison
materials engineers that allows them to convert footsteps into usable
electricity.
Xudong Wang, an associate professor of materials science and
engineering at UW–Madison, his graduate student Chunhua Yao, and their
collaborators published
details of the advance Sept. 24 in
the journal Nano Energy.
The method puts to good use a common waste material: wood pulp.
The pulp, which is already a common component of flooring, is partly made of
cellulose nanofibers. They’re tiny fibers that, when chemically treated,
produce an electrical charge when they come into contact with untreated
nanofibers.
When the nanofibers are embedded within flooring, they’re able
to produce electricity that can be harnessed to power lights or charge
batteries. And because wood pulp is a cheap, abundant and renewable waste product
of several industries, flooring that incorporates the new technology could be
as affordable as conventional materials.
While there are existing similar materials for harnessing footstep energy, they’re costly, nonrecyclable, and impractical at a large scale.
Wang’s research centers around using vibration to generate
electricity. For years, he has been testing different materials in an effort to
maximize the merits of a technology called a triboelectric nanogenerator
(TENG).
Triboelectricity is the same phenomenon that produces static
electricity on clothing. Chemically treated cellulose nanofibers are a simple,
low-cost and effective alternative for harnessing this broadly existing
mechanical energy source, Wang says.
Because wood pulp is cheap, abundant and renewable, flooring
that incorporates the new technology could be as affordable as conventional
materials.
The UW–Madison team’s advance is the latest in a green energy
research field called “roadside energy harvesting” that could, in some settings,
rival solar power — and it doesn’t depend on fair weather. Researchers like
Wang who study roadside energy harvesting methods see the ground as holding
great renewable energy potential well beyond its limited fossil fuel reserves.
“Roadside energy harvesting requires thinking about the places
where there is abundant energy we could be harvesting,” Wang says. “We’ve been
working a lot on harvesting energy from human activities. One way is to build
something to put on people, and another way is to build something that has
constant access to people. The ground is the most-used place.”
Heavy traffic floors in hallways and places like stadiums and
malls that incorporate the technology could produce significant amounts of
energy, Wang says.
Each functional portion inside such flooring has two
differently charged materials — including the cellulose nanofibers, and would
be a millimeter or less thick. The floor could include several layers of the
functional unit for higher energy output.
“So once we put these two materials together, electrons move
from one to another based on their different electron affinity,” Wang says.
The electron transfer creates a charge imbalance that naturally
wants to right itself but as the electrons return, they pass through an
external circuit. The energy that process creates is the end result of TENGs.
Roadside energy harvesting could, in some settings, rival solar
power — and it doesn’t depend on fair weather.
Wang says the TENG technology could be easily incorporated into
all kinds of flooring once it’s ready for the market. Wang is now optimizing
the technology, and he hopes to build an educational prototype in a
high-profile spot on the UW–Madison campus where he can demonstrate the
concept. He already knows it would be cheap and durable.
“Our initial test in our lab shows that it works for millions of
cycles without any problem,” Wang says. “We haven’t converted those numbers
into year of life for a floor yet, but I think with appropriate design it can
definitely outlast the floor itself.”
The Wisconsin Alumni Research Foundation holds the patent to the technology. Other authors
on the paper include Zhiyong Cai of the Forest Products Laboratory and
UW–Madison graduate students Alberto Hernandez and Yanhao Yu. The Forest
Products Laboratory and National Science Foundation provided funding for the
research.
