Batteries: thermoelectric and photoelectric – Thermoelectric – One junction element surrounded by another junction element
Reexamination Certificate
1999-10-18
2001-10-30
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
One junction element surrounded by another junction element
C136S203000, C136S242000
Reexamination Certificate
active
06310280
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of semiconductor components, particularly semiconductor thermoelectric components.
DESCRIPTION OF THE RELATED ART
The present invention relates to a semiconductor component as a thermoelement or Peltier element that converts temperature differences into electrical differences in potential and vice versa, exploiting the thermoelectric effect (Seebeck-Peltier effect).
Sensors that can be interrogated telemetrically are superior to conventional systems in a large number of applications. In systems having a large number of sensors, or having measurement points that are difficult to access or are very small, the cost of signal transmission can be reduced significantly using telemetric sensors. Passive telemetric sensors require no energy storage device on the actual sensor, but are limited in functionality and range. Active telemetric sensors based on CMOS technology permit the construction of intelligent systems at a low cost per measurement point. Up to now, one disadvantage has been the necessity of a battery as an energy source in each sensor. The battery requires an expense, limits the useful life, and has to be disposed of separately afterwards. In modem low-power circuits, the power required for the operation of the sensor is very low; somewhat higher power is briefly required only for the signal transmission.
Peltier elements enable the direct conversion of thermal energy to electrical energy. The removal of thermal energy is always possible when heat reservoirs are available at various temperatures. For example, a body having an elevated temperature, and temperature gradient thereof to the surroundings, is sufficient to obtain electrical energy.
The flow of power in a thermoelement depends on the material and geometric parameters, as well as on the electrical current at the junction.
The Seebeck coefficient S (measured in V/K) describes the contact voltage per Kelvin of a material in relation to a reference material.
FIG. 2
shows an equivalent switching diagram of a Peltier element. The Peltier coefficient &pgr;
pn
(measured in W/A) describes the heat transport caused by electrical current. If two different materials with different doping (p, n) are joined, the contact voltage per Kelvin is calculated from the difference of the two Seebeek coefficients. The heat transport corresponds to this difference multiplied by the temperature T (measured in K) of the respective junction.
&pgr;
pn
=T(S
p
−S
n
) (Eq. 1.1)
A Peltier element standardly consists of a number m of junctions that are connected in parallel thermally but in series electrically. The warm (H) and cold (I) junctions are not ideally insulated from one another thermally; there exists a heat conductance value K
thHL
that effects a loss, since the relevant quantity of heat for the conversion is lost, where K
thHL
is the heat conductance value. T
amb
and T
source
designate the ambient temperature and the source temperature, respectively. T
H
and T
L
designate the warm or high temperature, and the cold or low temperature, respectively. Additional losses arise because the electrical current in the junctions and in the regions in between produces ohmic power loss. This power loss I
2
el
R
i
can be distributed symmetrically to (H) and (L).
The flow of heat into the two heat current sources results according to the following:
P
H
=
mS
pn
T
H
I
el
+
I
el
2
R
i
2
(Eq. 1.2)
P
L
=
-
mS
pn
T
L
I
el
+
I
el
2
R
i
2
(Eq. 1.3)
The no-load voltage at the terminals (I
el
=0) depends in linear fashion on the temperature difference between (H) and (L).
U
0
=mS
pn
(T
H
−T
I
) (Eq. 1.4)
The electrical internal resistance results from the specific resistances &rgr;
p
, &rgr;
n
of the materials, the geometry and the number m of elements.
R
i
=
m
(
ρ
p
l
p
A
p
+
ρ
n
l
n
A
n
)
(Eq. 1.5)
Here A
p
, A
n
=cross-sectional surface of the individual element in m
2
, l
p
, l
n
=length between (H) and (L) in m.
The heat conductance value K
thNL
follows correspondingly from the specific heat conductance values &lgr;
p
, &lgr;
n
and the geometry.
K
thHL
=
m
(
λ
p
A
p
l
p
+
λ
n
A
n
l
n
)
(Eq. 1.6)
The output voltage of the Peltier element is:
U
out
=mS
pn
(T
H
−T
I
)−R
i
I
el
=U
0
−R
i
I
el
(Eq. 1.7)
The output power of the Peltier element is
P
out
=mS
pn
(T
H
−T
I
)I
el
−R
i
I
2
el
(Eq. 1.8)
The maximum output power of a Peltier element (with ideal heat sinks K
th1
, K
th2
→∞) results as:
P
out
max
=
m
2
S
pn
2
(
T
H
-
T
L
)
2
4
R
i
(Eq. 1.9)
The following expression is used as a “figure of merit”:
Z
=
m
2
S
pn
2
R
i
K
thHL
(Eq. 1.10)
By substitution of (1.5) and (1.6) and optimization of the geometry, the following is obtained:
Z
max
=
S
pn
2
(
λ
p
ρ
p
+
λ
n
ρ
n
)
2
with
A
p
l
n
A
n
l
p
ρ
p
λ
n
ρ
n
λ
p
(Eq. 1.11)
If the heat conductivities and specific resistances of the materials are equal, Z is independent of the geometry. Peltier elements that are of interest technologically have Z>10
−3
K
−1
.
The expected temperature differences at the heat junctions are small. Despite progress in low voltage circuit technology, circuits require a supply voltage of at least 1 to 1.5 V, although 3 to 5 V is optimal; these can then be DC-convertcd as needed, with a high degree of efficiency. Through series connection of a large number of heat junctions, a sufficient voltage can be produced even given low temperature differences, but this increases the internal resistance of the thermogenerator, and finally limits the output voltage. The target design must be composed of elementary cells that are as small as possible, in order to ensure sufficient degrees of freedom for the adaptation of the output voltage.
The price of the thermogenerator is proportional to the chip surface. The achievable power depends on the chip surface, the thermal relations and material parameters. In order to determine a guide value for the desired power, the following observation should be taken into account:
A small button cell (1.5 V) has a capacity of approximately 50 mAh and an energy content of less than 0.1 J. If an operational life of only one year is assumed, an average power drain of only 3 nW is permissible. A lithium photobattery with 3 V has a capacity of 1.3 Ah and an energy content of 4 J. Given an operating life of one year, there results a permissible average power drain of 0.13 &mgr;W. The average power requirement of circuits in which batteries are to be replaced by thermogenerators is thus very low. While batteries can unproblematically emit higher power for a short period of time, for these cases a thermogenerator (depending on the design) must be supplemented by an energy storage unit. Besides the average power, the power required for brief periods is thus also a dimensioning criterion.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor thermoelement arrangement that converts temperature differences into electrical differences in potential and vice-versa.
These objects are achieved in accordance with the invention in a semiconductor thermoelement arrangement including a doped layer of semiconductor material having a first side and a second side and having contiguous regions. The doped layer is p-doped and n-doped region-by-region in alternating fashion for positive and negative electrical conductivities. A first thermally conductive layer arranged on the first side of the doped layer in relation to a direction perpendicular to a planar extension of the doped layer. A second thermally conductive layer is arranged on the second side of the doped layer in relation to a direction perpendicular to a planar extension of the doped
Aigner Robert
Hierold Christofer
Schmidt Frank
Gorgos Kathryn
Parsons Thomas H
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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