INTERNATIONAL JOURNAL OF RESEARCH IN AERONAUTICAL AND MECHANICAL ENGINEERING
Vol.1 Issue.8,
December 2013.
Pgs: 1-9
ISSN (ONLINE): 2321-3051
INTERNATIONAL JOURNAL OF RESEARCH IN
AERONAUTICAL AND MECHANICAL ENGINEERING
A Review of Thermoelectric Generator for Waste Heat Recovery from
Engine Exhaust
Dipak Patil1, Dr. R. R. Arakerimath2
1
G.H.Raisoni College of Engineering and Management, Wagholi, Pune, India,
dipak.patil@raisoni.net
2
G.H.Raisoni College of Engineering and Management, Wagholi, Pune, India
rachayya.arakerimath@raisoni.net
Abstract
Thermal management and energy crisis have been two major problems in this 21st century. Engine exhaust has
tremendous amount of energy which can be recovered by waste heat recovery systems. The thermoelectric
concept is seen as a perfect solution for recovering waste heat from engine exhaust and converts in to electric
energy. Since the use of semiconductor materials for thermoelectric applications, there has been a huge quest
for improving its figure of merits (ZT) to make it commercially viable. This review starts with thermoelectric
concepts and explains briefly the challenges in enhancing the figure of merits. The present study focus on
various operating condition i.e flow rate, temperatures of fluids and position of thermoelectric module. The
configurations of thermoelectric generator play a vital role for increasing effectiveness of the heat recovery
system. Thermoelectric generator technology can be incorporated with other technologies such as PV,
turbocharger or even Rankine bottoming cycle technique to maximize energy efficiency, reduce fuel
consumption and greenhouse gas (GHG) emissions.
Keywords: Thermoelectric Generator (TEG); Thermoelectric material (TEM); Heat exchanger (HE); Waste
Heat Recovery (WHR); Seebeck effect.
1. INTRODUCTION
In recent years the scientific and public awareness on environmental and energy issues has brought in major
interests to the research of advanced technologies particularly in highly efficient internal combustion engines.
Viewing from the socio-economic perspective, as the level of energy consumption is directly proportional to
the economic development and total number of population in a country, the growing rate of population in the
world today indicates that the energy demand is likely to increase [1].Substantial thermal energy is available
from the exhaust gas in modern automotive engines. Two-thirds of the energy from combustion in a vehicle is
lost as waste heat, of which 40% is in the form of hot exhaust gas. The latest developments and technologies
on waste heat recovery of exhaust gas from internal combustion engines (ICE). These include thermoelectric
generators (TEG), Organic Rankine cycle (ORC), six-stroke cycle IC engine and new developments on
turbocharger technology [2].
Being one of the promising new devices for an automotive waste heat recovery, thermoelectric generators
(TEG) will become one of the most important and outstanding devices in the future. A thermoelectric power
generator is a solid state device that provides direct energy conversion from thermal energy (heat) due to a
temperature gradient into electrical energy based on “Seebeck effect”. The thermoelectric power cycle, with
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charge carriers (electrons) serving as the working fluid, follows the fundamental laws of thermodynamics and
intimately resembles the power cycle of a conventional heat engine[3].
1.1 Basic Theory of a thermoelectric power Generator
The basic theory and operation of thermoelectric based systems have been developed for many years.
Thermoelectric power generation is based on a phenomenon called “Seebeck effect” discovered by Thomas
Seebeck in 1821. When a temperature difference is established between the hot and cold junctions of two
dissimilar materials (metals or semiconductors) a voltage is generated, i.e., Seebeck voltage. In fact, this
phenomenon is applied to thermocouples that are extensively used for temperature measurements. Based on
this Seebeck effect, thermoelectric devices can act as electrical power generators.
A schematic diagram of a simple thermoelectric power generator operating based on Seebeck effect is
shown in Figure 1. Heat is transferred at a rate of QH from a high-temperature heat source maintained at TH to
the hot junction, and it is rejected at a rate of QL to a low-temperature sink maintained at TL from the cold
junction. Based on Seebeck effect, the heat supplied at the hot junction causes an electric current to flow in the
circuit and electrical power is produced. Using the first-law of thermodynamics (energy conservation
principle) the difference between QH and QL is the electrical power output We. It should be noted that this
power cycle intimately resembles the power cycle of a heat engine (Carnot engine), thus in this respect a
thermoelectric power generator can be considered as a unique heat engine [1].
Figure 1: Schematic diagram showing the basic concept of a simple thermoelectric power generator operating
based on Seebeck effect [1].
1.2 Composition of a Thermoelectric Power Generator
Figure 2 shows a schematic diagram illustrating components and arrangement of a conventional single-stage
thermoelectric power generator. As shown in Figure 2, it is composed of two ceramic plates (substrates) that
serve as a foundation, providing mechanical integrity, and electrical insulation for n-type (heavily doped to
create excess electrons) and p-type (heavily doped to create excess holes) semiconductor thermoelements. In
thermoelectric materials, electrons and holes operate as both charge carriers and energy carriers. The ceramic
plates are commonly made from alumina (Al2O3), but when large lateral heat transfer is required, materials
with higher thermal conductivity (e.g. beryllia and aluminum nitride) are desired.
The semiconductor thermoelements (e.g. silicon-germanium SiGe, lead-telluride PbTe based alloys) that
are sandwiched between the ceramic plates are connected thermally in parallel and electrically in series to form
a thermoelectric device (module). More than one pair of semiconductors are normally assembled together to
form a thermoelectric module and within the module a pair of thermoelements is called a thermocouple. The
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junctions connecting the thermoelements between the hot and cold plates are interconnected using highly
conducting metal (e.g. copper) strips as shown in figure 2 [1].
The potential of a material for thermoelectric applications is determined in large part to a measure of the
material’s dimensionless figure of merit (ZT), where, α is the Seebeck coefficient, σ the electrical conductivity,
ρ the electrical resistivity, and κ the total thermal conductivity (κ = κ L + κ E; the lattice and electronic
contributions, respectively).
The power factor, α2σT, (or α2T / ρ) is typically optimized as a function of carrier concentration (typically
around 1019 carriers cm-3), through doping, to give the largest ZT. High mobility carriers are most desirable in
order to have the highest electrical conductivity. Semiconductors have been primarily the materials of choice
for thermoelectric applications.
Figure 2: Schematic diagram showing components and arrangement of a typical single-stage thermoelectric
power generator [1].
There are challenges in choosing suitable materials with sufficiently higher ZT for the applications. The
best thermoelectric materials defined as ‘‘phonon-glass electron-crystal’’, which means that the materials
should have a low lattice thermal conductivity as in a glass, and a high electrical conductivity as in a crystal.
1.3 Waste Heat from Exhaust Gases Generated from Automotive Application
Since 1914 the possibility of using thermoelectric power generation to recover some of waste heat energy from
reciprocating engines has been explored and patented. A schematic diagram (figure 3 b ) showing this patent of
converting waste heat into electrical power applied to an internal combustion engine using a thermoelectric
power generator is shown in Figure 3. In this invention, the exhaust gases in the pipe provide the heat source to
the thermoelectric power generator, whereas the heat sink (cold side) is suggested to be provided by circulation
of cooling water [1].
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Figure 3 (a, b): Converting WH into electric power applied to an IC engine using a TEG [1].
Thermoelectric power generators offer several distinct advantages over other technologies:
1. They are extremely and silent in operation since they have no mechanical moving parts and require
considerably less maintenance;
2. They are simple, compact and safe;
3. They have very small size and virtually weightless;
4. They are capable of operating at elevated temperatures;
5. They are suited for small-scale and remote applications
6. Typical of rural power supply, where there is limited or no electricity;
7. They are environmentally friendly;
8. They are not position-dependent; and
9. They are flexible power sources.
The major drawback of thermoelectric power generator is their relatively low conversion efficiency
(typically ~5%). This has been a major cause in restricting their use in electrical power generation to
specialized fields with extensive applications where reliability is a major consideration and cost is not.
Applications over the past decade included industrial instruments, military, medical and aerospace, and
applications for portable or remote power generation. However, in recent years, an increasing concern of
environmental issues of emissions, in particular global warming has resulted in extensive research into
nonconventional technologies of generating electrical power and thermoelectric power generation has emerged
as a promising alternative green technology.
2. Thermoelectric Material
2.1 Conventional Thermoelectric Materials
Thermoelectric materials (those which are employed in commercial applications) can be conveniently divided
into three groupings based on the temperature range of operation, as shown in Figure 3. Alloys based on
Bismuth (Bi) in combinations with Antimony (An), Tellurium (Te) or Selenium (Se) are referred to as low
temperature materials and can be used at temperatures up to around 450K. The intermediate temperature range
- up to around 850K is the regime of materials based on alloys of Lead (Pb) while thermoelements employed at
the highest temperatures are fabricated from SiGe alloys and operate up to 1300K. Although the above
mentioned materials still remain the cornerstone for commercial and practical applications in thermoelectric
power generation, significant advances have been made in synthesising new materials and fabricating material
structures with improved thermoelectric performance. Efforts have focused primarily on improving the
material’s figure-of-merit, and hence the conversion efficiency, by reducing the lattice thermal conductivity
[1].
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Figure 3: Figure of merit ZT shown as a function of temperature for several bulk thermoelectric materials [1].
2.2 Thermoelectric Material for Power Generation
It is essential to develop novel system with improved thermoelectric performance. Semiconducting glasses
have emerged as a new thermoelectric material. Key example of tellurium-based glasses, with high Seebeck
coefficients, very low thermal conductivities and tunable electrical conductivities, are presented. ZT values as
high as 0.2 is obtained at room temperature for several tellurium-based glasses with high copper
concentrations, confirming chalcogenide semiconducting glasses as good candidates for high-performance
thermo- electric materials. However, the temperature stability and electrical conductivity of the reported
glasses are still not good enough for practical applications and further studies are still needed to enhance them
[4]. For Polyethylenedioxithipene (PEDOT) based materials, a figure of merit (ZT) of 0.1 can be easily
obtained nowadays. Considering the advanced techniques for bulk material processing and intensive concerns
on PEDOT, a ZT∼100 (1) may be possible for PEDOT-based TE materials in the near future [5].
Graded n-type β-FeSi2/Bi2Te3 thermoelectric materials have been prepared by dip coating procedure using
Sn95Ag5 as bridge material. It is found that an apparent Seebeck coefficient is in between those of β -FeSi2 and
Bi2Te3 below 1500C and higher than both of them between 150 and 3000C. A measurement shown that the
maximum power output of the functionally graded material (FGM) is approximately 2.5-3 times that of
monolithic material β -FeSi2 at the same temperature difference, which suggests that not only do the graded
material benefit the superior performance of Bi2Te3 from low temperature side, but also make fully use of the
properties of β-FeSi2 at high one. Thermal stability for the functionally graded thermoelectric material β-FeSi2
/ Bi2Te3 could be significantly improved by sandwiching a Ni layer [6].
Thermoelectric materials fabricated by explosive (shockwave) consolidation have lower seebeck voltages
and thermoelectric figures of merits (ZT) than the conventional melt-grown alloys; although the melt-grown
alloys exhibited thermoelectric figures of merit less than half contemporary values. The fact that homogeneous
semiconductor systems are probably not achieved in melt grown thermodynamic materials based on
appropriately doped Bi2Te3. Novel mixture fabrications such as shock consolidation, involving the production
of a high density of interfaces, including sub-micron grain sizes, are also found to be inadequate. Rapid
prototyping/controlled layer rapid manufacturing, ultrasonic and microwave consolidation or sol–gel/aerogel
technologies may offer more efficient fabrication alternatives if inefficient scattering of electrons and phonons
can be avoided. [7].
Thermoelectric microgenerators and microcoolers are becoming technologically important for microelectro
mechanical systems (MEMS), but the conventional cutting and assembling techniques have limitation in
miniaturizing the dimensions of thermoelectric devices to the micrometer order. We have combined MEMS
technology and materials processing into a novel process to manufacture thermoelectric micro-modules with
densely aligned fine-scale and high-aspect-ratio P–N elements. Our process consists of the following major
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steps: (1) micromachining a silicon mold; (2) filling the mold with thermoelectric materials; (3) connecting Pand N-type elements and assembling the whole module. By using the present process, Bi–Sb–Te system
thermoelectric elements of 300m height and 40m cross-sectional width can be fabricated successfully. Microfabrication of TE materials can be possible by using silicon molding process [8].
Embedded interfaces can enhance the electronic and thermal transport properties of TE materials. A focus
on emerging routes to engineer the nanoscale grain and interfacial structures in bulk thermoelectric materials,
address in particular (i) control of crystallographic texture, (ii) reduction of grain size to nanocrystalline
dimensions, and (iii) formation of nanocomposite structures. While these approaches are beginning to yield
promising improvements in performance, continued progress will require an improved fundamental
understanding of the mechanisms governing the formation, stability, and properties of thermoelectric interfaces
[9]. Small grain size materials show significantly low thermal conductivity which are prepared by spark
plasma sintering method [10]. Polycrystalline samples are prepared by complex sol–gel method. The transport
properties of the samples are modified by changing La atoms for strontium atoms [12].
Technically, high-performing thermoelectric properties appear to depend sensitively on the nanostructure,
synthesis approach and device assembly. The synthetic approaches of thermoelectric nanomaterials, should be:
(1) scalable, high-quality and low cost, with tuneable thermoelectric properties, (2) the nanostructured
materials must be able to form dense compacts for machining/device integration (device-controllable), (3) the
nanostructured material should demonstrate an enhanced ZT over the bulk material and finally (4) the
compacted nanoscale features should be with high thermal stability for extended periods of time [9]. Cryogenic
grinding method is used for preparing thermoelectric material nanopowders. And this method may be used to
fabricate other metal nanopowders with a similar manner [13].
The complex structures helped enhance ZT with various approaches mainly by clathrates, skutterudites and
zintl phase. The substructure approach and incorporating nanostructures opened up new doors. Researchers
could achieve ZT up to 2.5 at room temperature in superlattices. However the fabrication of superlattices is
expensive and not suitable for scaling up for mass production. By proper architectural design, the efficiency of
thermoelectric system can be increased [14]. A Finite Element Model is used to develop and manufacture a
thermoelectric structure operating in Peltier mode with non-standard materials. Thus, both the reliability of the
Finite Element Model and the correct development and design of the new thermoelectric structure are
ascertained [15]. Improving a thermoelectric device’s (TED) power output and also minimizing the usage of
rare-earth materials (n or p-type) in waste heat recovery applications, a novel composite thermoelectric device
(CTED) has been proposed without compromising its thermal conversion efficiency. It is proposed that
building state-of-the-art novel CTEDs with a minimum amount of rare-earth metals reduces cost without
compromising thermal conversion efficiencies and performance [16].
3. Thermoelectric Generator
Thermoelectric power generators have emerged as a promising alternative green technology due to their
distinct advantages. The application of this alternative green technology in converting waste-heat energy
directly into electrical power can also improve the overall efficiencies of energy conversion systems [1].
3.1 Operating Parameters
The electrical power generation is observed to be a strong function of flow rate and inlet exhaust temperature.
The implications of varying inlet conditions could be very severe if proper conditioning of output power is not
carried out. The ZT value of high-temperature skutterudites decreases considerably along the flow direction
due to decreasing ∆T and temperatures at the hot-side junction. The thermoelectric modules close to the inlet
are exposed to much higher gas temperatures and hence generate higher electrical power output per unit area.
By optimizing the fin spacing and thickness, the heat transfer rate can be enhanced considerably [2].
The temperature difference between the hot and cold junctions of TEG increased as the engine speed or the
coolant temperature increase. The output voltage, according to the Seebeck effect, also increased as the
temperature difference increase. Therefore, the output power and thermal efficiency can be improved. The
temperature of cooling water is much lower than the exhaust gas, which caused a smaller hot temperature on
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the TEG hence induced a poor performance. Different materials suitable for lower temperature operation are
needed for the radiator module [17]. For low-temperature waste heat power recovery, results show that both
the maximum power output and the corresponding conversion efficiency are greatly affected by the operating
conditions, especially the hot fluid inlet temperature and flow rate [18].
The theoretical results meet well with the experimental values for low flow rates but not for high flow rates.
This is because for high flow rates the parallel heat flux takes a major importance within the global heat
transfer produced in the generator resulting in a decrease of the temperatures [22]. The generated voltage is
proportional to the temperature different when multiple thermoelements are connected together to form a
Thermoelectric Module (TEM). The proposed water based cooling system has advantages in term of stability
as well as capability to maintain the temperature different between hot and cool side of the TEM [19].
3.2 Thermoelectric Module Configuration
It is demonstrated that the proper size of a heat spreader can decrease the thermal resistance and that the
maximum power P max with a spreader can be significantly increased (up to 50%) as compared to TEG
without spreader. The optimum TEG module spacing (S) and its spreader thickness (Hsp) are found to be
strongly dependent on the waste gas heat transfer coefficient, hh; while they are weakly dependent on the
temperature difference between the waste gas and the cooling water, ∆T [20].
The parametric evaluation of the longitudinal model indicates that TEG performance improves for
configurations that have minimum TEG height and maximum TEG width. This result stems from the fact that
the skutterudites perform best at high temperature. The transverse design is found to be an improvement over
traditional, longitudinal designs. A transverse TEG with a single module length is found to be the most
favorable design among all the designs studied, with the highest electrical power generation and lower pressure
drops [21]. It is found the heat sinks attached to the downstream TEGs possibly cause the downstream wall
cooler which thus loots heat from the upstream hotter wall, resulting in a degradation of the performance of the
upstream TEGs. Consequently implementing more TE couples may not necessarily be worthwhile
economically. Leaving some downstream part of the heat exchanger uncovered with the TE couples on the
other hand results in a hotter downstream wall, more heat transferred to the upstream TE couples, and
consequently a larger power generation rate [22].
3.3 Heat Exchanger
The power generation of exhaust TEG (thermoelectric generator) depends on heat energy and thermoelectric
conversion efficiency. High efficiency heat exchanger is necessary to increase the amount of heat energy
extracted from exhaust gas. 1-Inlet 2-outlet heat exchanger increased hydraulic disturbance and enhanced heat
transfer, resulting in the more uniform flow distribution and higher surface temperature than the other [23][24].
A sensitivity analysis is carried out on the system model. It is found that exhaust gas parameters and heat
exchanger structure have a significant effect on the system power output and the pressure drop. Gas properties
should be precisely described. The configuration of the system should be carefully optimized to maximize its
performance [25]. The validation studies confirm that the effectiveness of an integrated TE heat exchanger in a
cross flow configuration is extremely sensitive to the operating temp. range of the TE device [26].
3.4 Maximize potential energy efficiency of the vehicles
The study also identified the potentials of the technologies when incorporated with other devices to maximize
potential energy efficiency of the vehicles. It should be noted that TEG technology can be incorporated with
other technologies such as PV, turbocharger or even Rankine bottoming cycle technique to maximize energy
efficiency, reduce fuel consumption and GHG emissions [3]. Although combined cycle systems are not widely
used, these will receive more attention for their significant development potential to achieve higher thermal
efficiency and to alleviate the atmosphere pollution problem. It will be a main research orientation of WHR
technology. [27]. Significant increases of system performance when TEG and internal heat exchanger are
combined with Organic bottoming cycle. It is also suggested that TEG-ORC system is suitable to recovering
waste heat from engines [28].
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4. Conclusion
Thermoelectric (TE) material should have a low lattice thermal conductivity and a high electrical conductivity
as in a crystal. Thermoelectric materials (those which are employed in commercial applications) can be
conveniently divided into three groupings based on the temperature range of operation. Alloys based on
Bismuth (Bi) in combinations with Antimony (An), Tellurium (Te) or Selenium (Se) are referred to as low
temperature materials and can be used at temperatures up to around 450K. The intermediate temperature range
- up to around 850K is the regime of materials based on alloys of Lead (Pb) while thermoelements employed at
the highest temperatures are fabricated from SiGe alloys and operate up to 1300K. The complex structures
helped enhance ZT with various approaches mainly by clathrates, skutterudites and zintl phase. The
substructure approach and incorporating nanostructures opened up new doors. Researchers could achieve ZT
up to 2.5 at room temperature in superlattices. However the fabrication of superlattices is expensive and not
suitable for scaling up for mass production. Improving a thermoelectric device’s (TED) power output and also
minimizing the usage of rare-earth materials (n or p-type) in waste heat recovery applications, a novel
composite thermoelectric device (CTED) has been proposed without compromising its thermal conversion
efficiency.
The electrical power generation of thermoelectric generator is observed to be a strong function of flow rate
and inlet exhaust temperature. The temperature difference between the hot and cold junctions of TEG
increased as the engine speed or the coolant temperature increase. The output voltage, according to the
Seebeck effect, also increased as the temperature difference increase. Therefore, the output power and thermal
efficiency can be improved. The parametric evaluation of the longitudinal model indicates that TEG
performance improves for configurations that have minimum TEG height and maximum TEG width. High
efficiency heat exchanger is necessary to increase the amount of heat energy extracted from exhaust gas. It is
found that exhaust gas parameters and heat exchanger structure have a significant effect on the system power
output and the pressure drop. The study also identified the potentials of the technologies when incorporated
with other devices to maximize potential energy efficiency of the vehicles.
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