Thermoelectric Generators: Recovering Your Lost Heat
By Neal Lewis
Nobody likes to lose things. We get frustrated when we lose our keys, coats, and even
thoughts. Yet, each day, we lose enormous amounts of energy in the form of heat, and nobody
gets frustrated. When you use your laptop, you’re left with a warm lap or desk. This heat, which
is expelled from many devices we use every day, is known as waste heat. It’s an inefficiency
which is accepted in our energy squandering society. A thermoelectric generator (TEG) takes
this pesky waste heat and converts it to usable electrical energy. This may sound too good to be
true, but work is underway to find the right combination of materials and efficiency to make the
use of TEGs a reality.
It is important to understand how these curious converters work. TEGs use heat by way
of the Seebeck effect, which is the conversion of a temperature difference to an electric potential,
or voltage. When heat is applied to one side of the TEG, a temperature gradient is created. In
response to the gradient, the charge carriers (particles in the material which are free to move and
carry an electrical charge such as an electron) move from the
warm side to the cool side. Modern devices use two types of
semiconductors which have different charge carriers to create an
electrical potential. The n-type semiconductors are dominated by
negative charge carriers, or electrons, and p-type semiconductors
are dominated by positive charge carriers, or holes (holes are
simply the absence of an electron which can move through a
Figure 1: The Seebeck effect shown
by the movement of charge carriers
from the hot side to the cool side,
creating a voltage. Source: S. Lee et
al.
material, carrying a positive charge). Figure 1 illustrates the two semiconductors connected.
When heat is applied to one side, the charge carriers move to the cool side. In the n-type, this
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means electrons move to the cool side, giving a negative charge. In the p-type, holes move to the
cool side, giving a positive charge. This charge separation creates a potential difference between
the n-type cool side and the p-type cool side, producing electrical energy. Obtaining electrical
energy from waste heat can be considered energy recycling, which has the potential to improve
the efficiency of processes plagued by waste heat. This potential creates a need for materials
which achieve the Seebeck effect efficiently.
But what makes a ‘good’ thermoelectric material? Like most things in life, thermoelectric
materials have a grading system, which is known as the figure of merit. A good figure of merit is
a value of one. This figure is influenced by three fundamental materials properties: the Seebeck
coefficient, thermal conductivity (ability to conduct heat), and electrical conductivity (ability to
conduct electricity). The Seebeck coefficient is a measure of a material’s ability to exhibit the
Seebeck effect, and so a high Seebeck coefficient is important. A TEG needs to transport charge
carriers easily, so it needs an electrical conductor. But at the same time, a TEG has to maintain
its temperature gradient to be efficient, so it needs a poor thermal conductor. Thermal
conductivity and electrical conductivity happen to have a linear relationship, which introduces
the main issue with thermoelectric materials; How do you minimize only one side of a linear
relationship?
When thinking about minimizing thermal conductivity, consider the mechanisms which
conduct heat. Heat is transported through a material by charge carriers and by vibrations of
atoms which are known as phonons. Since charge carriers need to move easily for high electrical
conductivity, the thermal conductivity must be limited by keeping phonons from transporting
heat. Think of these phonons as little, heat-carrying waves moving through a material. When
these waves run into an electron, hole, or another phonon, the wave’s direct path through the
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material will be deflected. This deflection is called a scattering event and makes it harder for the
phonon to move through the material, thus decreasing the thermal conductivity. Large levels of
phonon scattering are found in glass-like materials, making them ideal candidates for
thermoelectric applications. Are there glass-like materials which have high enough electrical
conductivity and Seebeck coefficient to give a figure of merit close to one?
The answer is yes, but with an asterisk. Bismuth telluride exhibits low thermal
conductivity, like glass, while maintaining high electrical conductivity. In 2008, Bed Poudel of
GMZ Energy, an industry leader in heat to electricity conversion, showed that bismuth telluride
alloys can exhibit a figure of merit around one, up to 250ºC. This compound is one of the best
thermoelectric materials used today in terms of efficiency, and it achieves nine percent
efficiency. The asterisk is the cost of bismuth telluride. Tellurium is a rare earth metal, and it is
very expensive to acquire, making bismuth telluride TEGs too expensive for commercial use,
especially for nine percent efficiency. Using bismuth telluride commercially would be like
paying fifty dollars to get ten more songs on your iPod. It just isn’t worth it. There are two ways
to make this technology “worth it.” To keep with the same analogy, we could either find
materials which add five hundred songs for fifty dollars, or find materials which make adding
those ten songs cost only one dollar.
To find thermoelectric materials which achieve efficiency similar to bismuth telluride at a
lower cost, we can consider oxide materials. Oxides are common materials and much cheaper
than rare earth metals such as tellurium. Relaxor ferroelectric oxides are particularly promising
for thermoelectric research. Ferroelectricity is a phenomenon in which a material has
spontaneous electric dipole moments that can be switched with an external electric field. Relaxor
ferroelectrics have very small electrical domains, or regions of uniform alignment of electric
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dipoles. These small domains inherently scatter phonon, giving relaxor ferroelectrics glass-like
characteristics, which means low thermal conductivity like bismuth telluride. But can they
conduct electricity?
Relaxor ferroelectrics need to be reduced to have high electrical conductivity. Reduction
involves heating the material under low oxygen partial pressure, which pulls oxygen atoms out
of the material. Since charge can’t be created or destroyed, the removal of oxygen atoms has to
be compensated, which leads to an increase in the electrical conductivity of the material. Recall
from above that a missing electron is called a hole and has a charge of positive one. Since
oxygen has two electrons that participate in its bonds, removing an oxygen atom is like having
two holes, but the material can’t have that positive two charge. To counteract the missing oxygen
(called an oxygen vacancy), two free electrons are introduced into the material. These electrons
are free because they aren’t contributing to any bonds, they are only being used for their charge.
These free electrons increase the number of charge carriers (electrons), and thus increase the
electrical conductivity.
The missing oxygen sites in the material also help further limit the thermal conduction of
an already glass-like material. Oxygen vacancies are phonon scattering sites, thwarting heat
transfer by phonons. This is a bonus effect of the reduction process that makes relaxor
ferroelectrics a logical choice. But the idea is only practical if there are relaxor ferroelectrics with
high Seebeck coefficients.
As it turns out, Clive Randall and Susan Trolier-McKinstry’s research group at Penn
State, a group renowned for ferroelectric research, found that strontium barium niobate (SBN)
has a Seebeck coefficient similar to bismuth telluride, giving it a competitive figure of merit
among current thermoelectric materials at a fraction of the cost. Although it is only one relaxor
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ferroelectric, SBN shows that oxides are a legitimate source for cheaper thermoelectric materials.
Even though SBN is comparable to bismuth
telluride for thermoelectric applications, don’t
expect to see SBN thermoelectric devices in
next year’s line of cars or latpops. SBN misses
the mark because it only exhibits these nice
properties in one crystallographic direction.
SBN’s atoms are arranged in repeating units
Figure 2: A model SBN crystal. The c-direction atom
arrangements give the desired properties, but the a and b
directions do not. Source: S. Lee et al.
called crystals, illustrated in Figure 2. The image on the right shows the atomic arrangement in
the c-direction, which gives SBN’s thermoelectric properties. Because the arrangements in the a
and b directions do not exhbit the desired properties, a random combination of the three
directions leads to poor performance. It is difficult and expensive to make a bulk material with
its crystals oriented in the same direction, so SBN is not yet a cheap substitute for bismuth
telluride.
This little snag in oxide thermoelectrics research is far from a defeat. SBN shows promise
to further pursue oxide thermoelectrics. It could be a stepping stone to a better thermoelectric
oxide or to a new idea in thermoelectric research. Dr. Harald Bӧttner of Fraunhofer Institute for
Physical Measurement Techniques believes that TEGs, with the efficiency of current bismuth
telluride systems, can cut carbon dioxide emissions and decrease gas consumption by up to seven
percent. Consider that a nine percent efficient device could lead to a seven percent more efficient
engine. That may mean only a few extra miles to the gallon, but if a TEG can be made cheaply,
those few extra miles will be worth it. With innovation, TEGs can take us from a wasteful to a
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more managed and efficient world. And perhaps, if we know we aren’t losing as much energy,
we would be less frustrated when we lose our keys.
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References
Lee, S., Bock, J., Trolier-McKinstry, S., & Randall, C. (2012). Ferroelectric-thermoelectricity
and Mott transition of ferroelectric oxides with high electronic conductivity. Journal of
the European Ceramic Society, 32(16), 3971-3988. Retrieved November 22, 2012, from
the ScienceDirect database.
Poudel, B., Hao, Q., Chen, X., Liu, J., Dresselhaus, M., Chen, G., et al. (2008). Highthermoelectric performance of nanostructured bismuth antimony telluride bulk alloys.
Science, 320, 634-638.
Wustenhagen, V. The promise and problems of thermoelectric generators. Advanced
Nanotechnology. Retrieved December 11, 2012, from http://www.oflexx.com/fileadmin/media/documents/chip_silver_edition__The_promise_and_problems_of_thermoelectric_generators.pdf