Refrigeration – Using electrical or magnetic effect – Thermoelectric; e.g. – peltier effect
Reexamination Certificate
2001-05-18
2004-01-06
Esquivel, Denise L. (Department: 3744)
Refrigeration
Using electrical or magnetic effect
Thermoelectric; e.g., peltier effect
C062S003700, C136S200000, C136S208000, C136S225000
Reexamination Certificate
active
06672076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improved thermoelectrics for producing heat and/or cold conditions with greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop a thermal gradient across the material in the presence of current flow. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. These are physically and electrically configured in such a manner that they provide cooling or heating. Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in H. J. Goldsmid, Electronic Refrigeration, Pion Ltd., 207 Brondesbury Park, London, NW2 5JN, England (1986). The most common configuration used in thermoelectric devices today is illustrated in FIG.
1
. Generally, P-type and N-type thermoelectric elements
102
are arrayed in a rectangular assembly
100
between two substrates
104
. A current, I, passes through both element types. The elements are connected in series via copper shunts
106
soldered to the ends of the elements
102
. A DC voltage
108
, when applied, creates a temperature gradient across the TE elements. TE's are commonly used to cool liquids, gases and objects.
FIG. 2
for flow and
FIG. 3
for an article illustrate general diagrams of systems using the TE assembly
100
of FIG.
1
.
The basic equations for TE devices in the most common form are as follows:
q
c
=&agr;IT
c
−½
I
2
R−K&Dgr;T
(1)
q
in
=&agr;I&Dgr;T+I
2
R
(2)
q
h
=&agr;IT
h
+½
I
2
R−K&Dgr;T
(3)
where q
c
is the cooling rate (heat content removal rate from the cold side), q
in
is the power input to the system, and q
h
is the heat output of the system, wherein:
&agr;=a Seebeck Coefficient
I=Current Flow
T
c
=Cold side absolute temperature
T
h
=Hot side absolute temperature
R=Electrical resistance
K=Thermal conductance
Herein &agr;, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges.
Under steady state conditions the energy in and out balances:
q
c
+q
in
=q
h
(4)
Further, to analyze performance in the terms used within the refrigeration and heating industries, the following definitions are needed:
β
=
q
c
q
i
n
=
Cooling Coefficient of Performance (COP)
(
5
)
γ
=
q
h
q
i
n
=
Heating COP
(
6
)
From (4);
q
c
q
i
n
+
q
in
q
i
n
=
q
h
q
i
n
(
7
)
β
+
1
=
γ
(
8
)
So &bgr; and &ggr; are closely connected, and &ggr; is always greater than &bgr; by unity.
If these equations are manipulated appropriately, conditions can be found under which either &bgr; or &ggr; are maximum or q
c
or q
h
are maximum.
If &bgr; maximum is designated by &bgr;
m
, and the COP for q
c
maximum by &bgr;
c
, the results are as follows:
β
m
=
T
c
Δ
T
c
(
1
+
ZT
m
-
T
h
T
c
1
+
ZT
m
+
1
)
(
9
)
β
c
=
(
1
2
ZT
c
2
-
Δ
T
ZT
c
T
h
)
(
10
)
where;
Z
=
α
2
RK
=
α
2
ρ
λ
=
Figure of Merit
(
11
)
T
m
=
T
c
+
T
h
2
(
12
)
R
=&rgr;×length/area (13)
K
=&lgr;×area/length (14)
&lgr;×Material Thermal Conductivity; (15)
and
&rgr;=Material Electrical Resistivity (16)
&bgr;
m
and &bgr;
c
depend only on Z T
c
and T
h
. Thus, Z is named the figure of merit and is basic parameter that characterizes the performance of TE systems. The magnitude of Z governs thermoelectric performance in the geometry of
FIG. 1
, and in most all other geometries and usages of thermoelectrics today.
For today's materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are too low to compete with those of most refrigeration systems employing freon-like fluids (such as those used in refrigerators, car HVAC systems, building HVAC systems, home air conditioners and the like).
The limitation becomes apparent when the maximum thermoelectric efficiency from Equation 9 is compared with C
m
, the Carnot cycle efficiency (the theoretical maximum system efficiency for any cooling system);
β
m
C
m
=
T
c
Δ
T
(
1
+
ZT
m
-
T
h
T
c
1
+
ZT
m
+
1
)
T
c
Δ
T
=
(
1
+
ZT
m
-
T
h
T
c
1
+
ZT
m
+
1
)
(
17
)
Note, as a check if Z→∞,&bgr;→C
m
.
Several commercial materials have a ZT
A
approaching 1 over some narrow temperature range, but ZT
A
is limited to unity in present commercial materials. Typical values of Z as a function of temperature are illustrated in FIG.
4
. Some experimental materials exhibit ZT
A
=2 to 4, but these are not in production. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
Several configurations for thermoelectric devices are in current use in applications where benefits from other qualities of TEs outweigh their low efficiency. Examples of uses are in automobile seat cooling systems, portable coolers and refrigerators, liquid cooler/heater systems for scientific applications, the cooling of electronics and fiber optic systems and for cooling of infrared sensing system.
All of these commercial devices have in common that the heat transport within the device is completely constrained by the material properties of the TE elements. In sum, in conventional devices, conditions can be represented by the diagram in FIG.
5
.
FIG. 5
depicts a thermoelectric heat exchanger
500
containing a thermoelectric device
501
sandwiched between a cold side heat exchanger
502
at temperature T
C
and a hot side heat exchanger
503
at temperature T
H
. Fluid,
504
at ambient temperature T
A
passes through the heat exchangers
502
and
503
. The heat exchangers
502
and
503
are in good thermal contact with the cold side
505
and hot side
506
of the TE
501
respectively. When a DC current from a power source (not shown) of the proper polarity is applied to the TE device
501
and fluid
504
is pumped from right to left through the heat exchangers, the fluid
504
is cooled to T
C
and heated to T
H
. The exiting fluids
507
and
508
are assumed to be at T
C
and T
H
respectively as are the heat exchangers
502
and
503
and the TE device's surfaces
505
and
506
. The temperature difference across the TE is &Dgr;T.
SUMMARY OF THE INVENTION
None of the existing TE assemblies modify the thermal power transport within the TE assembly by the application of outside influences. An improved efficiency thermoelectric device is achieved by generally steady state convective heat transport within the device itself. Overall efficiency may be improved by designing systems wherein the TE elements are permeable to the flow of a heat transport fluid, transport thermal energy to a moving substance, or move the TE material itself to transport thermal energy. It should be noted that the term “heat transport” is used throughout this specification. However, heat transport encompasses thermal energy transfer of both removing heat or adding heat, depending on the application of cooling or heating.
One aspect of the present invention involves a thermoelectric system having a plurality of thermoelectric elements forming a thermoelectric array. The array has at least one first side and at least one second side exhibiting a temperature gradient between them during operation. In accordance with the present invention, at least a portion of the thermoelectric array is configured to facilitate convective heat transfer through the array. To accomplish this, the array is configured to permit flow of at least one convective medium th
BSST LLC
Esquivel Denise L.
Jones Melvin
Knobbe Martens Olson & Bear LLP
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