1. No category

Impact of Internal Thermal Bypasses in High Temperature
TE-Modules
Sarah Laubea, Dimitri Tatarinova, Marlis Morschelb and Georg Bastianb
a
Trier University of Applied Sciences, Faculty of Engineering, Schneidershof, 54293 Trier, Germany;
Rhine-Waal University of Applied Sciences, Faculty of Technology and Bionics, Nollenburger Weg 115, 46446
Emmerich, Germany
b
Abstract. High temperature thermoelectric generators (TEG) enable elevated conversion efficiencies, but require suitable
materials and smart packaging. However, internal thermal bypasses given by the setup of the modules may reduce the
efficiency when compared to the theoretical prediction. In a classical TE-module two ceramic plates are separated by the
length of the p- and n-doped semiconductor legs. A bypass heat current can flow through the gaps between the
semiconductor legs, carried by convection as well as thermal radiation. The significance of both contributions scales with the
temperature difference. We present experimental results to evaluate the impact of either bypass.
The influence of convection losses is determined by changing the atmospheric pressure while recording the TE-module
conversion efficiency. Thermal radiation losses are extracted from measured emissivity values for a given module geometry.
Based on our results we propose optimized module geometries, encapsulation and altered material surfaces. Depending on
temperatures and geometries, the electrical output power could be increased by a factor of more than 1.25.
Keywords: Thermal bypass; high temperature TE-modules; thermal radiation; convection; conduction
PACS: 85.80Fi; 72.20Pa;
INTRODUCTION
The optimization of thermoelectric devices is concentrated on semiconductor materials, semiconductor structures
and contacting. However, an important aspect especially regarding high temperature TE-modules, for example for the
application in combustion engines or solar systems, is neglected. The design of a standard TE-module poses the risk of
thermal bypasses due to heat radiation, conduction and convection.
To avoid those thermal bypasses in thermoelectric devices, the different mechanisms of heat transfer have to be
considered. A distinction between solid material, such as semiconductor legs, and fluids, like the air between those
legs, is necessary. By means of conduction the thermal energy is transferred from the hot ceramic plate into the
semiconductor legs and should be converted into electrical energy. Between the two ceramic plates the heat is
transferred by thermal conduction, convection and thermal radiation, which entails an approach of the two
temperatures. This process leads to a reduction of the conversion efficiency. To still ensure a satisfactory efficiency, the
mentioned difficulties have to be taken into account when designing the TE-modules. By optimizing the module
design, the convection, conduction and radiation losses between the plates inside the module, and the transmission of
thermal radiation through the semiconductor, could be compensated or prevented.
The precondition of convection is a flowing medium such as the air between the two ceramic plates. To investigate
the influence of decreasing ambient pressure a measurement setup was developed, which enables vacuum
measurements characterizing the thermoelectric parameters of a TEG.
While preventing conduction and convection losses by generating a vacuum, radiation losses are still problematic
and can lead to thermal bypasses inside the TE-module. The radiation of the hot parts, and subsequently the absorption
of the emitted and transmitted thermal energy on cold parts, has to be avoided. By measuring the emissivity value of a
ceramic plate and of the semiconductor material, the impact of radiation is examined. The roughness of the
semiconductor surface as an influencing factor on emissivity is also determined.
In this paper theoretical analysis of the losses is performed and the measurement setups are presented. Then the
experimental results are introduced and discussed. Finally, an outlook for the necessary optimizations is provided.
MEASUREMENT SETUP
In this section the relevant transport mechanisms are summarized. The consideration of the transport mechanisms is
divided into fluids and solid material.
Thermal Transport in Fluids
thermal transport
n
p
electron, phonon, photon
convection, conduction, radiation
FIGURE 1.Transport mechanisms of thermal energy
In the space between the ceramic plates and the semiconductor legs the thermal energy is transferred by means of
convection, conduction and heat radiation (see Fig. 1). The influence of these thermal bypasses is determined by
different setups. At first, convection, conduction and radiation are calculated as follows:
∙
∙
∙∆
(1)
∙
∙∆
(2)
∙
(3)
∙
represents the free surface of the ceramic plate between the semiconductor legs and the distance between the
two plates. The properties of air are given by
as heat transfer coefficient and
as thermal conductivity. stands
represents the Stefan-Boltzmann constant.
for the emissivity of the ceramic plates and
5.67 ∙ 10
To decrease or prevent the losses of thermal bypasses, caused by convection and conduction, the ambient pressure
shall be reduced. The following equation enables the calculation of the vacuum
which is necessary to have an
effect on thermal conductivity
∙
√ ∙
∙
(4)
k represents the Boltzmann constant, σst the collision cross-section and the system dimensions [1]. The convection
losses in the given TE-module dimensions can be neglected when rough vacuum (1 mbar) is achieved [2].
The measurement setup to detect the influence of the pressure consists of a TE-module with a hot plate and a
cooling element. By knowing the temperature and flow of the cooling circuit and the electrical output power of the
TEG, the thermal and electrical properties of the device are set. The construction can be covered by a vacuum-sealed
lid. Varying the ambient pressure, the effect on the efficiency becomes obvious. A detailed presentation of the setup
can be found in [3, 4].
Afterwards the emissivity of a ceramic plate and of a skutterudite sample is measured to determine the radiation
losses at different temperatures. A hot plate is used as a heat source for the ceramic plate. Four thermocouples attached
to the sample measure the exact temperature. In a fixed distance a thermography camera is positioned and measures the
intensity of the radiation. After calibrating the camera on the temperature of the thermocouples, the emissivity value
can be determined. Additionally, the connection between surface roughness and emissivity is investigated using an
atomic force microscope.
Thermal Transport in Solid Material
The transport of thermal energy through semiconductor legs takes place by conduction (see Fig. 1). Inside the
material three types of transport have to be taken into account: conduction by electrons, by phonons and by
radiation (photons). Especially the last mechanism, which has been neglected so far, may gain influence at higher
temperatures. Equation (5) illustrates the impact of those mechanisms on the figure of merit:
²∙ ∙
(5)
To investigate the impact of conduction by photons, the radiation going in and through a semiconductor sample is
measured. The sample is heated and the emitted radiation is measured through an aperture. As a reference the radiation
of a black surface is measured at the same temperatures. The measurement results might represent a trend of the most
influential mechanism.
MEASUREMENTS AND DISCUSSION
Below the results of the calculation and of the measurements are presented. The parameters of the TEG used are:
Bi2Te3, 4x4 cm or 6x6 cm, packing density 50%. To detect convection and conduction losses a temperature range from
293 K to 523 K is measured. The emissivity of the ceramic plate is determined between 323 K and 573 K and then
extrapolated to 973 K. The emissivity and consequential the heat radiation of two semiconductor samples is measured
at temperatures between 323 K and 598 K.
= 2 W/m²K and
= 10 W/m²K the convection losses are
Using equation (1) with A=0.001568 m²,
_
_
calculated. A maximum of 2.5% of the input power of 400 W can be saved by evacuating the interior of the TEG to
prevent convection at temperatures of nearly 1000 K. With regard to the prevention of conduction, the power gain
amounts to 4.25% at the same temperature calculated with equation (2). To reach this value, and therefore essentially
decrease the thermal conductivity, a pressure of 3 ∙ 10 mbar is necessary. Relative errors in this measurement are in
the range of 3-6%.
500
vacuum
no vacuum no cover
no vacuum with cover
vacuum
500
400
Qwm in W
400
300
300
200
200
100
100
0
(a)
no vacuum with cover
0
300
400
500
Temperature in K
600
(b)
300
350
400
450
500
Temperature in K
550
FIGURE 2. Heat flow through the module with different thermal interface materials: (a) differences in air, (b) almost identical
behavior using thermal compound (5 W/mK)
Figure 2 illustrates the measured influence of a low ambient pressure with different thermal interface materials (air (a),
thermal compound (b)). Qwm represents the heat flow, transported by the cooling circuit. Obviously, the difference
between vacuum and no vacuum is just the quality of the thermal contact between the heat source and the TEG.
Measuring the emissivity of the ceramic plate at different temperatures with the thermography camera, results in an
average value of 0.9. The calculated loss of thermal energy by means of radiation amounts 17.5 % of the total input
power at hot-side temperatures of nearly 1000 K.
The radiation of skutterudite surfaces at temperatures of nearly 600 K is about 0.003 W/mm². To clarify the impact
of the surface roughness of the sample, which depends on processing, fig. 3 shows the relation of emissivity and
roughness.
ε edge
ε side
rms roughness in nm
0,8
emissivity
0,6
0,4
0,2
0
(a)
300
400
500
600
Temperature in K
700
edge
1000
side
800
600
400
200
0
0
(b)
10
20
Number of measurements
30
FIGURE 3. Relation between emissivity (a) and surface roughness (b) of skutterudite sample at different positions
All losses mentioned above sum up to a theoretical thermal power bypass at 1000 K of about 25%. Figure 4 present
the possible effect on the efficiency when preventing all these losses.
with losses
efficiency in %
20
without losses
15
10
5
0
0
500
1000
Temperature in K
1500
FIGURE 4.Possible efficiency of TEG at different temperatures
CONCLUSIONS
The calculations and the performed measurements clearly demonstrate the potential of the thermal savings. The
setup for measuring the impact of different ambient pressure exhibits error bars of about 3%. Thus, the theoretically
estimated impact of convection losses could not be verified experimentally.
To avoid the calculated and measured thermal losses the design of a TE-module has to be optimized. A vacuumsealed encapsulation of the modules can provide the possibility of an interior pressure lower then atmospheric.
Simultaneously, the protection against condensate can be maintained. To decrease or prevent thermal bypasses by heat
radiation of the inner parts, coverage with low emissivity layers is beneficial. Thus a minimum of thermal energy may
flow through the components and a maximum of thermal energy is reflected to the radiating parts.
The given examples concerning how to deal with these sources of potential losses show small interventions on the
modules design. Therefore a reconstruction of consisting applications is not necessary at all.
ACKNOWLEDGMENTS
This work was partly supported by the German Ministry of Science and Education (BMBF).
REFERENCES
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of Thermoelectric Generator Systems”, International Conference on Thermoelectrics, Freiburg, 2009
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