2 IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION, 2018
Electrical Power Conditioning System for
Thermoelectric Waste Heat Recovery in
Commercial Vehicles
Arash Edvin Risseh, Student Member, IEEE, Hans-Peter Nee, Fellow, IEEE, and Christophe
Goupil, Member, IEEE
Abstract—A considerable part of the fuel energy in vehicles
never reaches the wheels and entirely converts to waste heat.
In a heavy duty vehicle (HDV) the heat power that escapes
from the exhaust system may reach 170 kW. The waste heat can
be converted into useful electrical power using thermoelectric
generator (TEG). During the last decades, many studies on
the electrical power conditioning system of TEGs have been
conducted. However, there is a lack of studies evaluating the
electrical instrumentation, the impact of the converter-efficiency,
and the TEG arrangement on a real large-scale TEG on-board
a drivable vehicle. In this study, the most important parameters
for designing electrical power conditioning systems for two TEGs,
developed for a real-scale HDV as well as experimental results
demonstrating the recovered electrical power, are presented.
Eight synchronous inter-leaved step-down converters with 98 %
efficiency with perturb and observe maximum power point
tracker was developed and tested for this purpose. The power
conditioning system was communicating with the on-board
computers through the controller area network and reported
the status of the TEGs and the recovered electrical power. The
maximum recovered electrical power from the TEGs reached
1 kW which was transmitted to the electrical system of the
vehicle, relieving the internal combustion engine.
Index Terms—Thermoelectricity, waste heat recovery, power
converter, power management, silicon carbide MOSFET,
maximum power point tracker, inter-leaved converter, internal
combustion engine, energy harvesting, heavy duty vehicle,
thermoelectric generator, renewable energy sources, exhaust
system.
ABBREVIATIONS
ABV Actual battery voltage
ATS After treatment system
CAN Controller area network
ECU Electronic control unit
EGR Exhaust gas recycling system
emf Electromotive force
EPA Environmental protection agency
HDV Heavy duty vehicle
HX Heat exchanger
ICE Internal combustion engine
LHC Long haulage driving cycle
MPPT Maximum power point tracker
PWM Pulse width modulation
RBV Requested battery voltage
The authors are with the School of Electrical Engineering, KTH,
Royal Institute of Technology, Osquldas vagen 10, 10044 Stockholm,
e-mail: (risseh@kth.se; hansi@kth.se). and Universit
`
e Paris Diderot e-mail:
(christophe.goupil@univ-paris-diderot.fr)
Si Silicon
SiC Silicon carbide
TEG Thermoelectric generator
TEM Thermoelectric module
I. INTRODUCTION
A
CCORDING to the European commission the transport
sector is responsible for 25 % of the emissions of the
greenhouse gases today. In fact, the low efficiency of the
internal combustion engine (ICE) in vehicles is responsible for
the considerable amount of greenhouse gas emissions from the
transport sector. For the same reason, a significant amount of
fuel energy converts to heat during the combustion process,
and goes directly through the exhaust system and out to the
ambient. According to J. Haidar et al. up to 35 % of fuel
energy converts to waste heat and escapes from the exhaust
system in a diesel engine [1]. The efficiency of ICEs in
that case may be 34 % i.e. 140 kW waste heat for a larger
400 kW diesel engine. However, the average efficiency of an
ICE over driving time may be as low as 15 % [2]. Taking
advantage of thermoelectricity the waste heat in the exhaust
gases can partly be recovered and converted to useful electrical
energy, which can be fed back into the electrical system of the
vehicle. This action will relieve the alternator and the ICE,
improving the overall efficiency and the fuel consumption.
Thermoelectric generation is based on the Seebeck effect
and can be implemented as a single thermoelectric module
(TEM), usually with large number of thermo-couples. TEM is
a compact solid-state device producing no pollutants and no
need for maintenance, which converts heat flow to electrical
power. Thermoelectric generators (TEG) have been used in
different applications and power ranges, as heat harvesting
systems or in cases where supplying electrical power in
commercial ways are impossible [3]. For instance, TEGs have
been used in medical applications to supply pacemakers [4],
[5], aerospace [6]–[9] and in military applications [10], [11].
Thermoelectricity has also been combined with photovoltaic
systems to harvest heat energy [12], [13].
Several studies on waste heat recovery (WHR) in
automotive applications have been performed in recent years
[14]–[16]. Srinivasan et al. found that TEGs in light- and
heavy duty vehicles have the capability to generate 2-3.5 kW
electrical power [17]. In another investigation Q.E. Hussain
et al. discovered that in a hybrid vehicle where the ICE
RISSEH et al.: ELECTRICAL POWER CONDITIONING SYSTEM FOR THERMOELECTRIC WASTE HEAT RECOVERY IN COMMERCIAL VEHICLES 3
does not run continuously, the mass of the TEG affecting
the time-constant is an important parameter to consider. They
proposed a TEG for a 2.5 L gas-electric hybrid vehicle
recovering 300-400 W under an EPA highway driving cycle
[18]. According to Arsie et al. 449 W could be obtained using
a TEG in a personal car equipped with a 70 kW diesel engine
[19]. D. M. Rowe et al. measured 315 W electric power from
a TEG in a family-size 1.5 L car [20] and also J. C. Bass et
al. reported about a 1-kW prototype-TEG, which was designed
for a Cummins NTC 350 diesel engine [21].
In order to completely utilize the advantages of a TEG, the
design of the electrical power conditioning system is extremely
important. N. A. Zarkadis et al. performed a study on the
electrical system of a TEG for marine applications, where a
high-ratio step-up converter was proposed and simulated [22].
In [23] and [24] extensive studies have been conducted on the
control system of a converter connected to an emulated TEG.
Moreover, Zhang et al. conducted a study where a prototype
of a thermoelectric-photovoltaic hybrid system and its power
conditioning system were designed. This experiment, using an
induction heater to heat up the hot side of a real TEG, was
performed in a laboratory [25].
Thermoelectric effects are complex phenomena where
different physical actions are affecting each other
simultaneously. Therefore, a TEG-system needs to be
investigated under real operating conditions considering the
thermal, mechanical, and electrical systems [26]. Although
many studies have been performed in this field, there is a lack
of investigations on electrical power management systems
with a large number of TEMs, operating under real conditions
where all physical effects are considered. In a well-designed
TEG the connection of TEMs, the converter topology and
the control system are important parts of the electrical power
management system. However, the impact they have on the
entire system from efficiency-, weight- and cost point-of-view
has never been studied for a large-scale TEG operating under
real conditions. The impact of the electrical arrangement of
a TEG is especially important in automotive applications, in
which there are restrictions regarding the volume and weight
and where the heat source and cooling capacity are limited.
It has to be mentioned that studying thermoelectricity using
temperature-controlled heater and/or cooler, emulates an
unlimited heat- source or sink, resulting in over-dimensioned
power management system. Therefore, research on suitable
power conditioning systems for TEGs operating on vehicles
under real conditions is on demand.
Recently the authors of this paper representing KTH
(Royal Institute of Technology) together with Scania, TitanX,
Eberspaecher and Swerea IVF designed, manufactured and
tested two TEGs on a drivable HDV. The TEGs consist
of 464 TEMs and were operated under real conditions,
with exhaust gases on the hot side and coolant from the
vehicle cooling system on the cold side. The TEGs were
communicating with the on-board computers reporting the
status and operating conditions. The entire system needed to
fulfill all requirements and safety regulations that on-board
components have to meet in a drivable vehicle. The aim of the
main study was to high-light the advantages and limitations
of TEGs in real-scale commercial vehicles [16]. Moreover,
the objective of the main study was to ascertain the amount
of the actual net power (=gross power−all hydraulic and
electrical losses) that can be obtained from a full-scale TEG in
a HDV. Generally, a TEG system consists of heat exchangers
(HXs), a cooling system, a number of TEMs and an electrical
power management system. In this paper, the last mentioned
subsystem i.e. the design of the electrical power management
system, which have to handle the gross power of the TEGs,
are discussed. The TEGs were developed to operate on a HDV
(DC13-10-440 Euro6-12.74 L-2300 Nm-440 kW), considering
power variations and the thermoelectric effects. Furthermore,
the obtained efficiencies of the power converters, using
two different types of semiconductor devices, are compared
and also an optimum connection strategy for the TEMs is
proposed. To the best knowledge of the authors this is the
first time an extensive study on the entire electrical power
management system, designed for a TEG on-board a real-scale
and drivable HDV, is presented.
In next Section a short overview of the complete system
and requirements for the power conditioning system will be
presented. The thermoelectric effects and their relations with
the electrical effects will be discussed briefly in Section III. In
Section IV the important parameters for designing a suitable
power management system, as well as efficiency maps for the
converters, based on experiments, are presented. In Section V
the experimental results on the gross electrical power of the
TEGs, collected from an HDV running under real conditions,
will be presented and lastly the conclusions are given in
Section VI.
II. ELECTRICAL SYSTEM OVERVIEW
During the pre-study Scania localized the possible heat
sources to be used for the TEG in the HDV. Due to the
amount of available heat power, temperatures and the added
hydraulic and pump losses, the most suitable heat sources
were determined to be the After treatment system (ATS) and
the Exhaust gas recycling system (EGR) [16]. Therefore, two
main TEGs were designed to recover the heat power from
the ATS and EGR. When the heat flux passes through a
TEG, an electromotive force is produced which can supply an
electrical load. Since the voltage and the current are functions
of the temperature difference (∆T ) across each module in a
TEG, the variation in temperature will change the internal
resistance, output voltage and thereby the output power of
the TEG. Therefore, connecting a number of TEMs in a
string with different ∆T will cause additional power loss. In
fact, it is beneficial to divide a large TEG into smaller units
where the local ∆T s are close to each other. Therefore in
this study, each TEG was divided into four sub-TEGs and in
order to control the power flow from the TEGs to the electrical
system of the vehicle, 2x4 identical high efficiency inter-leaved
DC/DC converters were designed, see Fig. 1. A more detailed
explanation about the local temperature differences in a TEG
is given in Section IV-A.
4 IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION, 2018
Figure 1. An overview of the TEG-system on the HDV. Due to the local
temperature differences each TEG was electrically divided into 4 sub-TEGs
and connected to individual inter-leaved sub-DC/DC converters.
According to Scania, the operation of an ICE in an HDV
can be described by the 9-steady-state-points (9-LHC) which
together create the long haulage cycle. In other words, 9-LHC
emulates the real driving conditions for a long haulage HDV.
The most important parameters in the 9-LHC, provided by
Scania, are presented in Table I.
The open circuit voltage (V
OC
) and internal resistance (R
in
)
of each sub-TEG are the two most important parameters for
designing the sub-converters. These are functions of the load
of the ICE described by 9-LHC and analytically calculated.
Since there are 8 sub-TEGs operating at 9 different LHCs,
only values for one sub-TEG, is presented in Table II later on.
Table I
TABLE SHOWS THE EXHAUST GAS TEMPERATURES IN THE ATS AND THE
EGR AS A FUNCTION OF THE SPEED AND LOAD OF THE ENGINE. THE
MEASUREMENTS PRESENTED IN THIS TABLE ARE PROVIDED BY SCANIA.
LHC 1 2 3 4 5 6 7 8 9
Engine
Speed[RPM]
1000 1000 1000 1150 1150 1300 1300 1300 1300
Relative
Load[%]
25 50 100 25 75 25 50 75 100
ATS
Temp.[
◦
C]
248 347 386 259 352 251 323 346 396
EGR
Temp.[
◦
C]
318 452 551 335 489 325 425 481 560
III. THERMOELECTRICITY
When the junctions of two dissimilar conductors (A and
B in Fig. (2)) are exposed to a temperature gradient, the
thermal energy will be transported from the hot junction to the
cold junction. The transportation of the heat causes movement
of charge carriers in the conductors, and creates an open
circuit voltage called relative Seebeck electromotive force.
This phenomenon is known as Seebeck effect. The temperature
difference generates the voltage, and the heat flow through
the thermoelectric elements drives the electrical current to an
external load [27], [28].
Figure 2. Due to the Seebeck effect the applied temperature gradient at the
junctions causes an open circuit voltage.
The resulting voltage V
oc
is a function of temperature
difference and material properties and can be described by
V
oc
=
Z
T
2
T
1
(α
A
− α
B
)dT = (α
A
− α
B
)(T
1
− T
2
), (1)
which can be rewritten as
V
oc
= α∆T, (2)
where T
1
and T
2
are the temperature at the cold and hot
junctions, α
A
and α
B
are the absolute Seebeck coefficient of
each conductor, respectively and α is the differential Seebeck
coefficient.
Typically, 127 to 254 pieces of highly doped n- and
p-type semiconductor elements, referred to as a thermocouple,
are sandwiched between two ceramic plates and form a
thermoelectric module (TEM), see Fig. 3. In order to provide
higher power a number of TEMs may be be connected
together, which create a thermoelectric generator (TEG).
Figure 3. The figure shows the leg-configuration, the direction of the heat-
and electrical current in a TEM (left), and a typical commercial TEM made
by a large number of n- and p-type semiconductor thermocouple (right).
During steady-state operation, a TEM can be modeled as
an ideal voltage source V
oc
with an internal resistance R
in
in
series. From the energy balance, the produced output power
of a TEM, P
el
can be derived by:
P
el
= Q
in
− Q
out
= αI∆T − R
in
I
2
, (3)
where Q
in
and Q
out
are the thermal power entering to the
hot side and releasing from the cold side respectively, I
is the electrical current, and R
in
is the internal resistance
of the TEM. It has to be mentioned that studying the
parameters in (3) alone, will not give complete information
about the performance of the thermoelectric device. The other
parameters that need to be considered in order to determine the
RISSEH et al.: ELECTRICAL POWER CONDITIONING SYSTEM FOR THERMOELECTRIC WASTE HEAT RECOVERY IN COMMERCIAL VEHICLES 5
performance of a TEM are dependent on the device material.
An ideal thermoelectric device would have an infinite large
electrical conductivity and Seebeck coefficient, and would
be able to keep a large temperature difference, which is
achieved by a low thermal conductance. The dimensionless
figure-of-merit, ZT is defined in (4) and gives the rate of
performance of a thermoelectric device.
ZT =
α
2
σ
κ
T, (4)
where σ and κ are the electrical- and the thermal conductivity
respectively, and T is the absolute temperature [26], [29].
Since the thermal and electrical conductivities are correlated
and dependent on the charge carrier concentration, a large ZT
is obtained by optimizing these parameters through the carrier
concentration [30].
IV. ELECTRICAL POWER MANAGEMENT OF THE TEG
A TEG is made of a number of TEMs, and in a vehicle there
are some restrictions that influence that number and the design
of the TEG. The most important factors are the available heat
power and the space dedicated for the TEG in the vehicle. The
heat power is varying with the engine load and the gas mass
flow, which are dependent on the type and size of the engine.
In this study two different TEGs were designed and placed in
the exhaust system of a Scania HDV, one downstream ATS
and the other upstream EGR [16]. The design of the TEGs
was based on 9-LHC where the speed of the ICE and the
load is varying in a wide range, which results in variations in
the exhaust temperatures and the gas mass flow. According to
the 9-LHC, the gas temperatures in the ATS change between
248 and 396
◦
C, and between 318 and 560
◦
C in the EGR
depending on the load of the ICE.
The temperature variations in the exhaust gases will cause
two issues in the electrical part of the system. Since, the
output voltage V of the TEG is a function of the temperature
difference (∆T ) over the TEMs, the output voltage will
follow the temperature variations. At the same time as the
temperatures and the voltage of the TEMs change, the internal
resistance (R
in
) of the TEMs will change as well. Due to the
reduction of mobility of the carriers in the semiconductor with
increased mean operating temperature (T
m
), the resistivity of
the semiconductor in a TEM increases. Therefore, a TEM
produces less power for same ∆T at a higher T
m
. In Fig. 4
the behavior of a HZ-20 module from HiZ as a function of T
m
is shown. Here, the ∆T is kept constant at 50
◦
C but it can be
seen that R
in
is increasing with increased T
m
. It should also be
noted that the Seebeck coefficient of thermoelectric elements is
also temperature dependent therefore the open circuit voltage
of the TEM is also varying although ∆T is fixed. The variation
of the internal resistance and the voltage of the TEMs give rise
to the characteristics shown in Fig. 5, where two different ∆T
results in two voltage-, current- and power functions.
Figure 4. The internal resistance, open circuit voltage and power of a HZ-20
module as a function of the mean operating temperature T
m
. Temperature
difference, ∆T , is kept to 50
◦
C.
Since the voltage of the electrical system of the vehicle
controlled by the alternator while the voltage and power
of the TEG vary, clearly a power conditioner between the
TEG and the electrical system is needed. The aim of the
power converter is to control the voltage and search for the
maximum available power at different ∆T s. Generally TEMs
have a low conversion efficiency, and for that reason the power
conditioner and the other part of the system have to operate
with highest possible efficiency over a large range of power.
Figure 5. Variations of the temperature in the exhaust system give rise to
variations in the internal resistance and different V-I-P curves, as a function
of ∆T . This is an example showing the behavior of the output power and
voltage of a TEM at two different ∆T s as functions of current.
A. Connection of Thermoelectric Modules
The design of the HX, based on the available space and
the heat power, dictates the number of TEMs. Moreover, the
type of the HX will determine the temperatures at the hot- and
cold sides, allowing to predict the electrical power each TEM
will generate. However, the analytically calculated power for
each TEM can not be added together to predict the total power
of the TEG. In fact, the TEG power will be affected by the
electrical connections between the TEMs.
6 IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION, 2018
The maximum delivered power from a TEM (or a TEG)
to a load is obtained when R
in
= R
Load
, i.e. the internal
resistance of the module is matched with the load. It is
important to mention that R
in
of a module (or a TEG), which
has to be considered for the maximum power tracking, is
the effective value of the internal resistance. The effective
internal resistance is a function of thermal conductance of the
module(s) itself, as well as the thermal conductance between
the heat- source and sink (κ
hot
and κ
cold
) [31], see fig. 6.
The maximum output power P
RLmax
of a single module
can be determined by,
P
RLmax
=
V
2
4R
in
, (5)
while the maximum output power of n number of TEMs
creating a TEG, connected in series or in parallel is obtained
by
P
RLmax−ser ies
=
1
4
(ΣV
n
)
2
ΣR
n
, (6)
and
P
RLmax−par allel
=
1
4
(Σ
V
n
R
n
)
2
Σ
1
R
n
, (7)
where V
n
is the open circuit voltage and R
n
is the internal
resistance of each specific TEM. Clearly, when the TEMs
operate in similar conditions, the voltages and the internal
resistances of all modules are the same. Therefore, the power
obtained by (6) and (7) will have the same value, i.e. the
electrical connection of the TEMs does not affect the total
output power [32]. However, in practical applications there
are constraints that give rise to different operating conditions
for each TEM. Y. Apertet et al. and A. Vargas-Almeida et
al. investigated the impact of TEMs’ thermal and electrical
connections on the performance of a TEG [33], [34]. In order
to analyze a TEM thoroughly the model in Fig. 6 was used.
Employing the Onsager compact expression [35] where Joule
losses are disregarded, the system can be described by
I
I
Q
=
1
R
in
−1 α
αT α
2
T + R
in
κ
I=0
∆V
∆T
, (8)
where I and I
Q
are the electrical- and thermal current
respectively, and ∆V is the voltage over the electrical load.
∆T is the temperature difference (T
hM
− T
cM
) across the
module, T is the average temperature and κ
I=0
denotes the
thermal conductance at zero electrical current. From (8) the
electrical current and thermal current are obtained as
I =
α∆T
R
in
+ R
L
, (9)
I
Q
= αT I + κ
I=0
∆T. (10)
Figure 6. The electrical and thermal model of a TEM.
Combining (9) and (10) yields the relation between the
thermal current I
Q
, zero current thermal conductance κ
Cond
and the thermal conductance associated with the electrical
current κ
Conv
, i.e.
I
Q
= (
α
2
T
R
in
+ R
L
+ κ
I=0
)∆T, (11)
which can be expressed as
I
Q
= (κ
Conv
+ κ
Cond
)∆T = κ
T EG
∆T, (12)
where
κ
Conv
=
α
2
T
R
in
+ R
L
. (13)
Equations (11) and (12) show that not only the electrical
current but also part of the thermal current can be controlled
by the total resistance in the circuit. This is an excellent tool
when the heat source and the cooling system have a limited
capacity, as in the case of vehicle applications. This gives
a capability of controlling the thermal current by adjusting
the electrical current, and vice versa whenever it is necessary,
for instance to protect the TEMs or cooling circuit from over
heating. Furthermore, since a large electrical current increases
the thermal current through a module, a low-current TEG is
preferred in a system with limited cooling capacity, in order
to keep the ∆T on an acceptable level. This can be achieved
by connecting a number of TEMs in series to increase the
internal resistance of the TEG. In [36] Gao Min showed that
the relation between thermal and electrical current can also
be used to increase the conversion efficiency by 40 %, and
thereby the output power by operating a TEM in an electrical
pulse-mode system.
Another issue regarding the connection of the TEMs is the
mismatch caused by the applied mechanical force on different
TEMs in the system. In real applications, where a large number
of TEMs are used, it is impossible to ensure identical applied
force of all TEMs. A force mismatch that causes a lower
κ
hot
and κ
cold
on one module than the others, will lower
the internal resistance and voltage of that particular module.
If all modules are connected in parallel and the requested
current from the load is low, or in the case of failure when the
