Billions
of dollars lost each year as waste heat from industrial processes can
be converted into electricity with a technology being developed at the
Department of Energy’s Oak Ridge National Laboratory.
The
high-efficiency thermal waste heat energy converter actively cools
electronic devices, photovoltaic cells, computers and large waste
heat-producing systems while generating electricity, according to Scott
Hunter, who leads the development team. The potential for energy savings
is enormous.
“In
the United States, more than 50 percent of the energy generated
annually from all sources is lost as waste heat,” Hunter said, “so this
actually presents us with a great opportunity to save industry money
through increased process efficiencies and reduced fuel costs while
reducing greenhouse gas emissions.”
Initially,
Hunter envisions the technology being used for cooling high-performance
computer chips, thereby helping to solve an enormous problem facing
manufacturers of petaflop-scale computers. These mega machines generate
massive amounts of heat that must be removed, and the more efficient the
process the better. Turning some of that heat into electricity is an
added bonus.
Hunter’s
technology uses cantilever structures that are about 1 millimeter
square in size. About 1,000 of these energy converters can be attached
to a 1-inch square surface such as a computer chip, concentrated
photovoltaic cell or other devices that generate heat. Although the
amount of electricity each device can generate is small – 1 to 10
milliwatts per device – many arrays of these devices can be used to
generate sizable amounts of electricity that can power remote sensor
systems or assist in the active cooling of the heat generating device,
reducing cooling demands.
The
underlying concept, pyroelectricity, is based on the use of
pyroelectric materials, some of which have been known for centuries.
First attempts to use this technology to generate electricity began
several decades ago, but these studies have been plagued by low thermal
to electricity conversion efficiencies – from about 1 to 5 percent.
This
is also the case for techniques using thermoelectric, piezoelectric and
conventional pyroelectric platforms. However, using arrays of
cantilevered energy converters that feature fast response and cycle
times, Hunter’s team expects to achieve efficiencies of 10 to 30 percent
– depending on the temperature of the waste heat generator – in an
inexpensive platform that can be fabricated using standard semiconductor
manufacturing technology.
“The
fast rate of exchange in the temperature across the pyroelectric
material is the key to the energy conversion efficiency and high
electrical power generation,” Hunter said, adding that ORNL’s energy
scavenger technology is able to generate electrical energy from thermal
waste streams with temperature gradients of just a few degrees up to
several hundred degrees.
The
device is based on an energy harvesting system that features a
micro-electro-mechanical, or MEMS, pyroelectric capacitor structure that
when heated and cooled causes current to flow in alternate directions,
which can be used to generate electricity. In this configuration,
cantilevers are attached to an anchor that is affixed to a waste heat
generator substrate. As this substrate becomes hot, the cantilever also
heats and bends because of the bi-material effect, similar in principle
to the bimetal switch used in room and oven thermostats.
“The
tip of the hot cantilever comes into contact with a cold surface, the
heat sink, where it rapidly loses its heat, causing the cantilever to
move back and make contact with the hot surface,” Hunter said. “The
cantilever then cools and cycles back to the cold heat sink.
“The
cantilever continues to oscillate between the heat source and heat sink
as long as the temperature difference is maintained between the hot and
cold surfaces.”
Other
developers of this technology, which is funded by the Laboratory
Directed Research and Development program, are Nickolay Lavrik,
Thirumalesh Bannuru, Salwa Mostafa, Slo Rajic and Panos Datskos.
UT-Battelle manages ORNL for DOE’s Office of Science.
