One such material is gadolinium, which is magnetic
at around room temperature, but loses its magnetism
as it warms up.
In Priya’s harvester, a gadolinium soft magnet is
attached to a flexible plastic cantilever. A hard, or
permanent, magnet rests against the heat source.
Magnetic attraction pulls the two magnets togeth-
er, allowing heat to flow from the hard to the soft
magnet. This heats the soft magnet above its transi-
tion temperature, and it becomes nonmagnetic and
is pulled away by the cantilever. Separated from the
heat, the soft magnet cools; the drop in temperature
reactivates its magnetic properties, starting the pro-
cess over again.
While the soft magnet temperature cycles up and
down, it absorbs heat from the hot surface
via the hard magnet, cooling it and dispers-
ing the thermal energy into the environ-
ment.
But the energy contained in the heat is
recycled, because the piezoelectric cantile-
ver converts the soft magnet’s motion into
electricity that can be used or stored.
The device effectively harvests energy from
heat sources not dramatically hotter than
the ambient air — the kind most common
in homes and industrial plants.
“If you look around, you see mostly
low-temperature gradients,” Priya said.
“But at low temperatures, there aren’t many
promising solutions.”
In tests, researchers used the device to
harvest thermal energy from heat sources
around 70-80 degrees Celsius. The device operating
temperature is mainly governed by the heat required
to demagnetize the soft magnet; with currently avail-
able materials, it can be as low as 50 to 60 degrees
Celsius.
The energy harvester’s ability to operate at moder-
ate temperatures — along with its compact size and
mechanical simplicity — could make it practical for
household use. Priya envisions the heat emanating
from everyday appliances could be used to power
networks of sensors in data-driven “smart” homes.
The researchers are scaling up the thermal energy
harvester to increase its electrical output, refining
the materials and design, and adapting it for specific
applications.