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ADVANCED NANOTECHNOLOGY
ADVANCED NANOTECHNOLOGY
The Promise
and Problems
of Thermoelectric
Generators
BY DR. VOLKER
WÜSTENHAGEN,
HEAD OF
R&D SYSTEMS
Thermoelectric generators that use heat to generate electricity could
play an important role in achieving a large-scale, sustainable energy
solution. However, the conflicting material characteristics needed for
such devices pose a formidable challenge. Oerlikon is using recent
advances in thin film multi-layers and nanotechnology to develop a
mass production solution for a new generation of thermoelectric
generators.
T
hermoelectric generators (TEG) are solid-state
energy converters that combine thermal, electrical,
and typically, also semiconductor properties to
convert heat into electricity or electrical power directly
into cooling. Basically, a TEG creates voltage because
charge carriers in metals and semiconductors (in the
generator) are free to move much like gas molecules,
while carrying charge as well as heat. When a
temperature gradient is applied to a TEG, the mobile
charge carriers at the hot end tend to diffuse to the cold
end. The build-up of charge carriers results in a net
charge at the cold end, producing voltage.
Due to this functionality, thermoelectric generators have
moved to center stage in today’s “green technology”
debate because they can be used to recover waste
heat and convert it into electrical power. Especially the
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heightened global awareness of the environmental impact
of global climate change has contributed to a boom in
interest in TE technology.
Accessing a new energy source
There is clearly a great need for thermoelectric generators
in a very wide field of applications. Most of the world’s
energy is still supplied by the combustion of fossil
fuels, which are the main cause of the CO2 emissions
responsible for climate change. Because only a fraction
of the energy released by the burning of fossil fuels is
converted into mechanical energy or electricity and most
of the energy is released as heat, the newest generation
of TE generators (see diagram on the right) could
effectively access this largely untapped source of waste
heat for conversion into additional electricity.
The type of solid-state energy conversion represented by
a TE generator has great appeal in terms of its simplicity.
As these generators have no moving parts, they are
silent, reliable and scalable, making them ideal for small,
distributed power generation. For example, satellites and
spaceships have been using TE generators for power
for many years, such as on deep space probes such
as Voyager. Solid-state Peltier coolers, which reverse
the thermoelectric principles to create a cooling effect,
provide precise thermal management for opto-electronics
and passenger seat cooling in automobiles.
Small size, big impact:
This close-up of a relatively small TE
generator module features a scalable
technology.
Efforts are already underway to replace the alternator
in cars with a thermoelectric generator mounted on the
drivetrain to improve fuel-burning efficiency (see “The
Case for Thermoelectrics in Cars”). Another advantage
of thermoelectric generators is their scalability — waste
heat and co-generation sources can be as small as a
home water heater or as large as industrial or geothermal
sources. In a smaller version, the TEG can be used
for energy harvesting to build up independent power
supplies, which is important for the newest sensor
technology. Advances in TE designs could also enable
the replacement of compression-based refrigeration
systems with solid-state Peltier coolers.
Building the ideal TE generator
To maximize power-generation efficiency, the
temperature differential between the hot and cold sides
of a TE generator should be as large as possible. As
material properties vary with the temperature, they
exhibit optimum performance over a relatively narrow
temperature range. As a result, in order to maximize the
efficiency of power generation modules, individual TE
elements are usually formed from two and sometimes
three different TE materials laminated together in the
The TE principle
When heat is absorbed on one side of a TEG (red arrow)
the movable charge carriers begin to diffuse, resulting in
a uniform concentration distribution in the TEG along the
temperature gradient, and producing the difference in the
electrical potential on both sides of the TEG. To maximize
the power generation output, p-bars and n-bars (see circles)
are connected together in a cell. Due to the thermoelectric
effect, electrons flow through the n-type element to the colder
side while in the p-type elements, the positive charge carriers
flow to the cold side. This illustrates how connecting the
p-bar and the n-bar augments the voltage of each bar and
the voltage of each unit cell. These unit cells are assembled
in long sequences to eventually build a TEG. Interestingly
enough, the thermodynamic principle can be reversed;
by forcing voltage through a TEG a cooling effect (“Peltier
cooler”) is achieved. In the future, today’s compressionbased refrigeration systems could be replaced with such
higher efficiency, solid-state Peltier coolers.
Heat absorbed
Substrates
Thermoelectric
elements
+Current
Metal
interconnects
External
electrical
connection
Heat rejected
Heat absorption
h+
Heat flow
p
e–
n
Heat rejection
Small bit of the future:
Example of a low temperature
Bi2Te3 TE generator module
used for development
purposes; with a nominal
output of >500 watts.
(Illustration source: G. Jeffrey Snyder and Eric S. Toberer, “Complex
Thermoelectric Materials,” nature materials, Vol. 7, February 2008)
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ADVANCED NANOTECHNOLOGY
ADVANCED NANOTECHNOLOGY
direction of current flow to form segmented elements.
Each TE material in the laminate structure is chosen
for its superior performance over the range of its
temperature exposure. For effective waste heat recovery
from vehicle exhaust (an operating condition with about
a 350° C temperature differential – see sidebar) the
efficiency needs to be about 10%.
For well over 50 years, researchers have been unable
to produce a high-efficiency thermoelectric material.
Because of conflicting material characteristics, previous
TE generators have long been too inefficient to be costeffective in most applications. They need materials
that are both good electric conductors (else electron
scattering generates heat on both sides of the barrier and
throughout the materials) and poor thermal conductors
(or the temperature difference that must be maintained
between the hot and cold sides will produce a large heat
backflow). Similarly, the Seebeck effect (see “TECH
TALK” on the next page) should be maximized.
Efficiencies of different TE materials:
ZT
Room temperature
1
SiGe
PbTe
0.8
Bi2Te3
BiSb
0.6
0.4
0.2
0
0
200
400
600
800
1000
1200
Temperature in K
The efficiency values (ZT) of the most important TE materials as a
function of temperature.
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Nanotechnology shows the way
Optimizing these parameters is difficult because they are
all affected by the electronic properties of the materials
that have electrons conducting unwanted heat as well
as the electric current. It is necessary to optimize these
properties simultaneously. The best performance
is achieved with materials such as heavily doped
semiconductors, such as bismuth telluride or silicon
germanium. Finally, for semiconductors, it is desirable
to have a base material that can be both p- and n-typedoped, so that the same material system can be used on
both sides of the junctions.
In part, the recent resurgence of interest in
thermoelectrics gained momentum with the advances in
nano-structural engineering, which led to experimental
efforts to demonstrate high-efficiency materials. At
the same time, complex bulk materials (such as
skutterudites, clathrates, and Zintl phases) have been
explored and it was found that high efficiencies could
indeed be obtained. These complex high-efficiency
materials that manage to decouple the conflicting
properties have led to a renaissance in the field: a wide
array of new approaches, from complexity within the unit
cell to nano-structured bulk and materials combined in
thin-film multi-layer structures, have all led to better and
better solutions. Given the complexity of these systems,
the new approaches to higher-efficiency materials benefit
from collaborations between chemists, physicists and
materials scientists.
TECH TALK: The Seebeck Effect
The Seebeck effect is a basic principle of thermoelectronics and describes the conversion of temperature
differences directly into electricity. The effect is that a voltage, the thermoelectric electromotive force (EMF),
results from a temperature difference between two different metals or semiconductors. This causes a
continuous current in the conductors if they form a complete loop. For example, a thermoelectric generator
creates energy due to mobile charge carriers in metals and semiconductors that can carry a charge as well as
heat (See also “The TE Principle” on the previous page).
The Case for
Thermoelectrics in Cars
Many technical processes use less than one-third of the energy they employ. This
is especially true of automobiles, where even highly efficient combustion engines
use only 1/3 of the energy contained in the fuel (by converting it into mechanical
power) and 2/3 of the energy is wasted as exhaust heat.
Scientists all over the world are
A TE device can make good use of this
Thermoelectric solutions from Oerlikon
developing ways of harnessing the
huge differential. Driven by the flow of
Oerlikon is working on the development of substrates
and thin film multi-layers that effectively combine these
characteristics. However, the search for an efficient
substrate becomes more challenging because of the high
temperature range that also affects the contacts between
the different materials. Collaborating with the specialists
at O-Flexx Technologies, who have developed a
proprietary technology for transforming heat into energy,
Oerlikon is looking to reconcile the conflicting material
properties.
unused waste heat from machines,
heat between the hot exhaust fumes
power stations, and cars in order to
and the cold side of a coolant pipe, the
reduce fuel consumption and raise
charge carriers pass through special
efficiencies. For example, researchers
semiconductor layers, producing an
at BMW and at the Fraunhofer Institute
electric current.
The goals for the research efforts at Oerlikon are to allow
TE generators to take advantage of the lower production
costs and the greater yields made possible by the use
of the latest semiconductor manufacturing platforms –
such as the SOLARIS system from Oerlikon – and the
corresponding processes for fabricating TE materials and
devices. In one of the current R&D projects at Oerlikon,
the newest generation TE generators are now being
tested and optimized for mass production for several
important markets.
BMW in the lead: Close-up of a TE generator based on
Bi2Te3 material with Pmax of 200 watts.
are developing TE materials, modules
and systems to harvest the waste heat
in automobiles – that could potentially
improve fuel consumption.
TE on the inside: This cut-away drawing of a BMW
535i sedan shows the location of the TE generator in
the most recent tests. As the automotive leader in TE
research, BMW sees great promise in TE technology
to provide electrical power needs for the car of the
future. BMW estimates that the electricity needed
to power the growing number of processors in an
automobile will soon reach 1KW.
By implanting TE devices in the
drivetrain of a BMW 535i vehicle,
the researchers have shown that
thermoelectricity can be fed into the
“The temperatures in the exhaust
car’s electronic systems to help cut
pipe can reach 700° C. or more,”
fuel consumption and reduce CO2
says Dr. Harald Böttner, head of the
emissions. As a result, BMW eventually
Thermoelectric Systems department
eliminated the alternator in the test
at the Fraunhofer Institute for Physical
vehicle by supplying power from a TE
Measurement Techniques (IPM). “The
device instead.
temperature difference between the
exhaust pipe and a pipe carrying
engine cooling fluid can thus be several
hundred degrees Celsius.”
“If done correctly, we can cut gas
consumption by 5-7%,” adds Dr. Harald
Böttner.
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ADVANCED NANOTECHNOLOGY
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Bright Solution to
the Manufacturing
Efficiency Race
New SOLARIS system enables a range of
cutting edge applications in a small footprint.
Responding to the rapid shifts and
emerging technologies in the world
of high-tech manufacturing, the new
SOLARIS platform aims at a wide range
of new manufacturing processes with
unprecedented process flexibility.
BY GERHARD DOVIDS, BUSINESS DEVELOPMENT MANAGER
Compared to previous deposition systems,
Oerlikon has developed a radically different
platform – SOLARIS – for emerging
applications based on nanotechnology and
clean energy production. Similar in design to
high throughput production systems used in
the optical disc industry for DVD and Blu-ray
discs, this new system is a high-speed single
substrate sputtering system with ‘single
substrate’ handling and high throughput of
about 1,200 substrates per hour.
SOLARIS comes with six individual process
chambers. The multi chamber design
provides flexibility (up to six different layer
materials possible) and will allow various
configurations and processes as PVD (DC,
DC pulsed, RF), CVD, PECVD, etch, RTP
(Rapid Thermal Processing), etc. Process
conditions can be individually varied over
a wide range to optimize the deposition
process. The patented multi-source (MSQ)
cathode can hold up to four different
target materials, which can be sputtered
simultaneously (co-sputtering).
Due to a simple carrier system design,
different substrate sizes and materials can
be processed with minimal modification
costs. With PV applications, not only
the front side of a wafer can be coated;
backside passivation and metallization is a
further option for this system.
SOLARIS also adds some new skills: a
smaller size (3.3 x 2.0 m), unprecedented
flexibility to shift from one application to
another (for different products), different
substrates (wafers, glass, and foils), and
compatibility with current production lines
to ramp-up production quickly. The range
of applications underlines the system’s
versatility:
Photovoltaics: suitable for crystalline Silicon
solar cell processing, SOLARIS applies
silane free thin layers of SiN:H (silicon nitride
containing hydrogen) on the front and
aluminum on the back of the cells; hydrogen
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passivation is done by introducing ammonia
during deposition
Touch panels: SOLARIS applies a
transparent conductive layer and antireflective coatings on the touch screen
panels
They contribute noticeably to an increase
in solar cell quality and efficiency as well
as fab utilization, to help decrease overall
production costs.
The new system has had a successful
start in the market, with the first SOLARIS
systems delivered to key customers earlier
this year. Currently, our Application Lab is
busy processing customer wafer samples.
“One of the world’s largest solar cell
manufacturers was one of the first clients
to install SOLARIS,” adds Colm Connell,
Global Sales Manager at Oerlikon. “And
the feedback we’ve received is extremely
positive – they consider the new platform
revolutionary!”
A bright solution indeed.
Thermoelectrics: “green energy” devices
that generate electricity when heated;
the layers applied by SOLARIS conduct
electricity but not heat, keeping the device
running
Energy storage: a family of applications
that includes highly efficient (smaller and
lighter) thin film batteries and advanced
super capacitors for energy storage
Semiconductors: NEMs, microchips,
MEMS, LEDs, OLEDs, micro-sensors, micro
fluidic devices (MFD), etc.
SOLARIS
Key features
Quick substrate change: simplified
with a carrier system
Integrated substrate loading /
unloading
Small system footprint: 3.3 x 2.0 m
Sunny start for SOLARIS
Already an established supplier in the thin
film solar cell market with Oerlikon Solar,
SOLARIS will enable Oerlikon to also go after
the major part of the solar cell market based
on crystalline Si solar cells.
Flexible configurations:
independent process stations
For example, a crystalline solar cell requires
an anti-reflective (AR) layer on the front and
a contact layer on the backside. AR layers
are coated with PECVD processes and most
backside contacts are screen-printed. The
respective production equipment has a very
large footprint and must be cleaned and
maintained frequently. The consumables
are also expensive, adding to solar cell
production costs.
Multi-source sputtering: alloy
development with up to 4 different
materials
With its processing architecture, SOLARIS
will help the solar industry quickly implement
cleaner, more cost-efficient, and more
reliable production processes that are
superior to conventional batch processing
platforms. Experience and process knowhow gained from the high-speed production
solutions used in data storage production
lines are combined with a small footprint
and automated single substrate handling.
Multi layer capability: each
chamber can run different
processes and deposit different
materials
High throughput: up to 1,200
substrates / hour (dry cycle time
<3.0 sec)
Layer uniformity (+2%): with
substrate rotation during sputtering
Sputter chamber diameter: up to
240 mm
Rapid Thermal Processing: with
temperatures up to 400° C
Etch surface cleaning and
activation
Easy integration: in automated
inline production systems
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