ble border=0 cellpadding=7 cellspacing=0 width=100% bgcolor=ccccff> « back to results for ""
Below is a cache of http://www.tetech.com/publications/pubs/ICT96RJB.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
THEORETICAL ANALYSIS OF THERMOELECTRIC COOLING PERFORMANCE ENHANCEMENT VIA THERMAL AND ELECTRICAL PULSING THEORETICAL ANALYSIS OF THERMOELECTRIC COOLING PERFORMANCE
ENHANCEMENT VIA THERMAL AND ELECTRICAL PULSING
Authors:
Richard J. Buist and Paul G. Lau
TE Technology, Inc.
1590 Keane Drive, Traverse City, Michigan 49686 USA
Abstract
This paper is an introduction and theoretical investigation of
the fast-transient cooling characteristics of a TE module under
applied high-current electrical pulses. A temperature-
dependent, finite element model was developed to accurately
model the fast-transient performance. Analysis of
experimental data is presented to verify the accuracy and
validity of the model and the conclusions derived therefrom. It
has been shown that cold plate temperatures are achievable
from a typical TE module beyond that obtainable by
conventional, steady-state means.
The cooling enhancement is by virtue of the fact that Peltier
cooling is a surface effect and extremely concentrated at the
cold junction, whereas, Joule heating is a volume effect and is
distributed throughout the volume of the TE pellet. As such,
most of the Joule heat takes a longer time to reach the cold
plate than the Peltier cooling effect. This phenomenon is
theoretically demonstrated by applying a high-current pulse
after the minimum steady-state cold plate temperature has been
established. Calculations have shown that cold plate
temperatures can be reduced by 16 K below that via steady-
state means.
These transient enhancements are admittedly short-lived and
have limited effectiveness. However, the results presented
herein suggest that further exploitation of the fundamental
differences between Peltier and Joule heat are possible. A
concept is re-introduced which consists of thermally and
electrically separating the cold electrode from the TE pellet.
This pulse cooling concept was originally conceived over 30
years ago by Reich[1] at the Borg-Warner Research Center.
Introduction
Dr. Allen Reich[1] did a lot of pioneering work in the field of
thermoelectric cooling analysis in the early 1960's. He may
have been the first to recognize the potential for pulse-TE
cooling by capitalizing on the fundamental differences
between Peltier and Joule heat. Unfortunately, however, he
preceded the age of high-speed personal computers and the
accompanying analytical technologies which now provide the
means for detailing the many complicated intricacies of nature.
He had to rely on key, clever simplifying assumptions and
solutions of difficult and complex differential equations in
order to gain an analytical glimpse into the physics of
thermoelectric semiconductors. Nevertheless, his work
suggested the possibility of significant net cooling by
thermoelectric pulsing in order to segregate the localized
Peltier cooling from bulk Joule heating.
Therefore, the work presented herein is essentially
computerized numerical update of a dormant, but not
necessarily fruitless, idea. On the other hand, this paper is
only a precursor to the real potential for pulse-TE cooling but
clearly and rigorously re-establishes the existence of the
potentials of enhanced TE cooling via pulsing.
Thermal Model
The key to understanding and accurately characterizing the
fast-transient properties of a TE module is the development of
a thermal model which incorporates as much of the real-world
technical details as possible. The starting point was the finite-
element, steady-state thermal model for TE pellets by Buist[2].
This model employed the speed and power of a high-speed
personal computer to take advantage of simplicity but accuracy
and rigor of a finite element thermal model. By making each
finite element small enough, they were accurately quantified
using the familiar constant parameter theory. Of course,
however, the ultimate precision of this model was a
consequence of accurately testing the key kinetic
thermoelectric material parameters. For that model, the
temperature-dependent TE material parameters were measured
using the test system as described by Buist[3].
The transient model is very similar but included the added
dimension of time dependence. Thermal nodes were generated
for each finite element. The interactions between them were
derived from the steady-state model. The details of the
transient model was presented by Lau and Buist[4] together
with a thorough analysis of its features and attributes. A
summary of the key elements of this thermal model is given in
Table 1:
Table 1
Node
Description
42
Cold Ceramic Substrate
41
Cold Copper Tabs
40
Top "Half-Slice" of TE Pellet
1-39
Imaginary "Slices" of TE Pellet from
Bottom to Top
0
Bottom "Half-Slice" of TE Pellet
-1
Hot Copper Tabs
-2
Hot Ceramic Substrate
-3
Infinite Heat Sink
Physically, the configuration of system is shown in Figure 1.
A typical TE module was thermally bonded to a large
aluminum block which served as an infinite heat sink for the
low current testing used to verify and validate the fast transient
characteristics of this system. This same module was not only
used for transient testing but was also tested for its TE material
properties: Seebeck coefficient, electrical resistivity, thermal conductivity and figure of merit, Z. These parameters were
incorporated into the transient model.
Model Verification Testing
Some initial testing at 3% of Imax was performed on the TE
module as shown in Figure 1. This was performed by the same
high-resolution, high-speed, integrating A/D board used in the
test system[3]. However, it was set up to rapidly test and retest
the voltage across the TE module upon the application of an
applied current. These data are shown in Figures 2 and 3
together with the calculations produced but the transient
model. Figure 2 clearly establishes the long-term accuracy of
the model by the excellent closure throughout the entire cool-
down process.
Figure 3 was generated to examine the accuracy of the fast-
transient characteristics of the transient model. For this case,
the module voltage was calculated for the same times as the
test data. As observed, the calculations agreed with the test
data to within 0.5%. This closure is very good, especially
since the actual time for the test points with respect to the true
power-on instant was not exactly known. It could be off by as
much as 0.05 seconds. In any case, the shape of the calculated
curve closely matches that of the test data thereby validating
the transient model for the calculations presented.
Calculations for Imax
The first step in establishing the feasibility for enhancement
via pulsing was to calculate the performance of the TE module
at Imax. Imax is defined as the current which produces
maximum hot-to-cold temperature differential (delta-T),
holding the hot junction to a constant 297K.
The calculated temperature for each node (as defined in Table
1) at selected instants of time are shown in Figures 4-6. Figure
4 depicts the temperature profiles at 10 time steps of 5
milliseconds each.
It is evident that the main body of the TE pellets is heated
(even the hot junctions adjacent to the "infinite" heat sink) but
the Peltier cooling is clearly overcoming Joule heat at the
localized area near the cold junction. It is also clear that the
delta-T between the main body of the TE pellet and the cold
junction interface is considerably larger than that between the
Fig. 1 Test/Model configuration
0
10
20
30
40
50
60
70
80
90
100
0.20
0.22
0.24
0.26
0.28
0.30
0.32
0.34
0.36
Elapsed Time (Seconds)
V
o
l
t
a
g
e
(V
ol
ts
)
TEST DATA
CALCULATIONS
Fig. 2 Power-on to TE module until stabilization.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.214
0.216
0.218
0.220
0.222
0.224
0.226
Elapsed Time (Seconds)
V
o
l
t
a
g
e
(V
o
l
ts
)
TEST DATA
CALCULATIONS
Fig. 3 Initial power-on to TE module
-4 0
4
8 12 16 20 24 28 32 36 40
296.0
296.5
297.0
297.5
298.0
298.5
Node
Te
m
p
e
r
a
t
ur
e
(
K
)
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
Fig. 4 Temperature profiles during first 50 mill-seconds of
cool-down, Current = Imam
Time (Sec)
0.050
0.00 heat sink and the cold ceramic plate. This was intuitively
obvious but the cool-down speed of the cold ceramic plate was
slower than anticipated. However, it did close quite well with
experiment as illustrated in Figure 2.
Figure 5 illustrates the profiles over a longer time period (one
second). It is obse