Refrigeration – Using electrical or magnetic effect – Thermoelectric; e.g. – peltier effect
Reexamination Certificate
2001-07-31
2003-07-29
Jones, Melvin (Department: 3744)
Refrigeration
Using electrical or magnetic effect
Thermoelectric; e.g., peltier effect
C062S003700
Reexamination Certificate
active
06598405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improved thermoelectrics for converting a temperature differential to electrical energy with greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop an electrical potential across their terminals in the presence of a temperature differential across the TE. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in Angrist, Stanley W.,
Direct Energy Conversion,
3
rd
edition,
Allyn and Bacon, Inc., Boston, Mass. 2210, USA, (1976). 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 an assembly
100
between two substrates
104
. The thermoelectric elements
102
are connected in series via copper shunts
118
soldered to the ends of the elements
102
. A temperature differential is applied via the thermal source
106
at temperature T
H
and a thermal sink
108
at temperature T
C
across the device. The Peltier effect causes a voltage
110
(V) to be generated at the device terminals
116
that can be used to drive a current
112
(I) through a load
114
(R
0
).
FIG. 2
shows the flow of power within the system of FIG.
1
. For simplicity, only two TE elements
202
are shown. The TE elements
202
are sandwiched between hot and cold substrates
204
and are electrically connected in series by shunts
218
. The source
206
of input heat energy is maintained at temperature T
H
and the cold side source
208
is maintained at T
C
. Power is extracted at the terminals of the shunts
218
and provided to the load where work (W)
214
is done. Heat Q
H
enters at the left with waste heat Q
C
leaving at the right. Internal losses I
2
R are distributed evenly, half each to the hot and cold sides.
The basic equations for TE power generating devices in the most common form are as follows:
q
C
=&agr;IT
C
+½
I
2
R+K&Dgr;T
(1)
q
H
=&agr;IT
H
−½
I
2
R+K&Dgr;T
(2)
W=q
H
−q
C
=&agr;I&Dgr;T−I
2
R=I
2
R
L
(3)
where q
C
is the heat exiting from the cold side, q
H
is the heat entering at the hot side, and W is the power dissipated in the load, wherein:
&agr;=Seebeck Coefficient
I=Current Flow
T
C
=Cold side absolute temperature
T
H
=Hot side absolute temperature
&Dgr;T=T
H
−T
C
, the temperature difference
R=Electrical resistance of the thermoelectric device
K=Thermal conductance
R
L
=Electrical resistance of the external load
Herein &agr;, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges. It is also assumed that heat and current flow are one-dimensional, and that conditions do not vary with time.
Further, to quantify the performance of the generator, the efficiency is given by:
η
=
W
q
H
(
4
)
combining (2) and (3) yields:
η
=
I
2
R
L
α
IT
H
-
1
2
I
2
R
+
K
Δ
T
(
5
)
To achieve maximum performance, the generator internal resistance must be suitably matched to that of the load. Introducing:
m
=
R
L
R
(
6
)
as the ratio of load resistance to internal resistance Equation (5) can be rewritten as:
η
=
m
Δ
T
T
H
(
1
+
m
)
-
1
2
Δ
T
T
H
+
(
1
+
m
)
2
ZT
H
(
7
)
where;
Z
=
α
2
RK
(
8
)
is a material property known as the figure of merit
The optimum value of m is:
m
max
={square root over (1+
ZT
A
)} (9)
wherein:
T
A
=½(
T
H
+T
C
) (10)
the average temperature
Substituting (9) in (7), the maximum efficiency achieved is therefore:
η
max
=
(
m
max
-
1
)
Δ
T
T
H
m
max
+
T
C
T
H
(
10
)
FIG. 3
depicts the efficiency of a thermoelectric generator for different hot side temperatures and different values of the figure of merit, Z. As can be seen from the graph, high values of Z and T
H
are needed to make thermoelectric generators efficient. Commercially available materials have ZT
A
≈1 and some new, experimental materials have ZT
A
≈1.5. Materials commonly used in thermoelectric generators include suitably doped lead telluride (PbTe) for T
H
≈500° C. or silicon germanium (SiGe) for T
H
≈1000° C. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
From
FIG. 3
it can be seen that theoretical efficiencies over 25% are possible. Practical considerations of unavoidable losses, present material limitations, and reliability have limited actual efficiencies to 4% to 8%. For today's materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are generally too low to compete with those of other types of electrical generators. Nevertheless, several configurations for thermoelectric devices are in current use in applications where benefits from other qualities of TEs outweigh their low efficiency. These include applications requiring multi-year reliability without maintenance, heat flux sensing, conversion of waste heat, and power supplies for certain interplanetary spacecraft. In sum, in conventional devices, conditions can be represented by those described above.
SUMMARY OF THE INVENTION
The commercial devices have in common that the heat transport within the device is constrained by the material properties of the TE elements. None of the existing devices modifies the heat transport within the TE assembly.
An improved efficiency thermoelectric power generator is achieved by generally steady state convective heat transport within the device itself. Overall efficiency may be improved by designing systems wherein the TE system (elements or arrays) are configured to permit to the flow of a heat transport fluid, transport thermal energy to a moving substance, or move the TE material itself to transport heat. As an alternative to, or in combination with improved efficiency, generally steady state convection can be employed to reduce q
C
, the heat flux to the waste (cold) side.
One aspect of the present invention involves a thermoelectric power generation system using at least one thermoelectric array. The array may be made up of a plurality of individual elements, or one or more arrays. The array has a hot side and a cold 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 transport through at least one array. To accomplish this, the array is configured to permit flow of at least one convective medium through at least a portion of the array to provide generally steady-state convective heat transport from one side to the other side of at least a portion of the array. In one embodiment, the flow is from the cold side to the hot side of at least a portion the thermoelectric array.
In one embodiment, the convective medium flows through at least some of the thermoelectric elements or along (between and/or around) the thermoelectric elements. In another embodiment, the convective medium flows both along and through the thermoelectric elements. In one preferred embodiment, to permit flow through the thermoelectric elements, the elements or the arrays may be permeable or hollow. A combination of both permeable and hollow elements may also be used in an array. In one embodiment, the elements are porous to provide the permeability. In another embodiment, the elements are tubular or have a honeycomb structure. All such flows may be generall
BSST LLC
Jones Melvin
Knobbe Martens Olson & Bear LLP
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