Thermal galvanic cells

The efficiency of thermal galvanic cells is enhanced by establishing a temperature gradient along the electrodes, in addition to the temperature gradient between the electrodes, and/or by optimizing electrode geometry. Optimization of electrode geometry may comprise segmenting the electrodes while retaining the desired total electrode area or controlling the depth of immersion of the electrodes into the electrolyte. Further performance improvement may be obtained through the addition of a silica containing material and/or a thermal barrier to the electrolyte.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to cell-type devices characterized by a pair of 
electrodes separated by an electrolyte and particularly to thermal 
galvanic cells. More specifically, this invention is directed to the 
conversion of thermal energy into electrical energy. Accordingly, the 
general objects of the present invention are to provide novel and improved 
methods and apparatus of such character. 
(2) Description of the Prior Art 
While not limited thereto in its utility, the present invention is 
particularly applicable to the conversion of thermal energy into 
electrical energy and especially to thermal galvanic cells for effecting 
such energy conversion. The present state of the art with regard to 
thermal galvanic cells is believed to be exemplified by the disclosure of 
U.S. Pat. No. 4,211,828. The disclosure of the said U.S. Pat. No. 
4,211,828 is hereby incorporated herein by reference. 
The cells of the referenced patent represent a significant step forward in 
the art of thermoelectric energy systems. These improved cells are 
characterized by a pair of electrodes which are separated by an 
electrolyte. The generation of current by the cell is induced through the 
establishment of a temperature gradient across the cell and specifically 
by causing the two electrodes to assume different temperatures. 
There is, of course, an ever present desire to enhance the efficiency of 
all devices and this is particularly true in the case of energy conversion 
and especially those devices or systems wherein electrical energy is 
directed produced from thermal energy. In the case of the cells of the 
referenced patent, enhanced performance may be measured in terms of 
electrode efficiency. Electrode efficiency, as this term is employed 
herein, is the electrode/electrolyte interfacial resistivity. Cell 
efficiency is defined as the electrical output power divided by the 
thermal power input. Cell performance may be measured in terms of the cell 
resistance per unit area of electrode (ohm cm.sup.2). The latter quantity, 
while not resistivity in the classical sense; i.e., not ohm cm; will be 
referred to as such herein and will mean the externally measured 
resistance of the cell multiplied by the area of one of the electrodes. 
The open circuit voltage, V, of a thermal galvanic cell of the type being 
discussed is a function of the temperature difference between a pair of 
oppositely polarized electrodes. The open circuit voltage may be obtained 
by multiplying temperature difference by the "Seebeck" coefficient. The 
"Seebeck" coefficient is also known as the thermal galvanic cell constant 
of thermoelectric power, in common usage, is defined as the change in open 
circuit voltage per degree Celsius expressed as mv/.degree.C. The output 
power which may be derived from a thermal galvanic cell may be expressed 
as: 
EQU P=I.sup.2 R (1) 
where: 
R is the load resistance; and 
I may be expressed as: 
##EQU1## 
where: V is the open circuit voltage; and 
Ri is the internal cell resistance as per Thevenin's theorem. For maximum 
power output, as can be proven by maximizing the derivative of P with 
respect to R, Ri must be equal to R. Combining equations (1) and (2) 
above: 
EQU P=V.sup.2 /4Ri (3) 
The open circuit voltage for a thermal galvanic cell is: 
EQU V=(S) (dT.sub.1) (4) 
where: 
S is the Seebeck coefficient; and 
dT.sub.1 is the temperature difference between the two electrodes. 
Therefore, equation (3) above reduces to: 
EQU P=(SdT).sup.2 /4Ri (5) 
Accordingly, for any given cell with a constant Seebeck coefficient, the 
power output for cells of equal geometries and temperatures is a function 
of internal resistance only. If the power per unit area is desired, for 
purposes of comparison, the above definition of resistivity may be 
employed since, under equal temperature differentials, the voltages would 
be the same. In summation, the efficiency of the thermal galvanic cell of 
the referenced patent could be enhanced if the resistance per unit area of 
electrode of the cell, hereinafter the "resistivity", could be minimized. 
SUMMARY OF THE INVENTION 
The present invention relates to novel and improved electrical devices of 
the type which include at least a pair of electrodes spaced by an 
electrolyte. This invention also relates to methods of enhancing the 
efficiency of thermal galvanic cells including the cells described in the 
referenced patent application. 
In accordance with the present invention the efficiency of a thermal 
galvanic cell is enhanced by establishing a second temperature gradient 
across one or both electrodes. This dT.sub.2 is in addition to the 
temperature gradient, dT.sub.1, utilized in prior cells of similar 
character. Accordingly, in one embodiment of the present invention, the 
thermal energy input to the "hot" electrode of a thermal galvanic cell 
constitutes a "point" source of heat coupled to the electrode at a central 
region thereof while a similar "point" source, i.e., a heat sink, was 
established with the "cold" electrode. This arrangement resulted in a 
temperature gradient across the electrodes and extending radially 
outwardly from the points of connection to the heat source and sink and a 
corresponding unexpected decrease in cell "resistivity" when compared to 
prior art cells constructed in the manner disclosed in the referenced 
patent. In accordance with a second embodiment, the electrodes extended 
into the electrolyte toward one another and temperature gradient dT.sub.2 
was established across the electrode from the external heat source and 
electrolyte sink; this dT.sub.2 temperature gradient being in addition to 
the temperature differential dT.sub.1 between the two electrodes. A cell 
constructed in accordance with this second embodiment is also 
characterized by reduced "resistivity" when compared to cells of the type 
disclosed in the referenced patent. In accordance with a further 
embodiment of the invention, parallel electrodes are respectively heated 
and cooled along first edges while the second, oppositely disposed edges 
are immersed in the electrolyte. This arrangement also produces a dT.sub.2 
temperature gradient in the direction of immersion, as well as an average 
dT.sub.1, and results in a significant decrease in "resistivity". 
Also in accordance with the present invention, the efficiency of a thermal 
galvanic cell may be enhanced by optimizing electrode geometry. The 
optimization of electrode geometry may best be accomplished through 
reducing the depth of immersion of the electrodes into the electrolyte; 
i.e., by employing elongated electrodes that extend a short distance into 
an electrolyte. An improvement in performance may also be accomplished, 
for a given electrode area, by segmenting that area, all of the segments 
being electrically commoned. In both cases, the cell exhibits a decrease 
in "resistivity" when compared to the prior art as exemplified by the cell 
of the referenced patent. 
The improvements described above, when combined, provide a dramatic and 
totally unexpected increase in cell efficiency for various electrolytes 
and electrode materials. This increase in efficiency may be further 
enhanced by adding, to the electrolyte, between 1% and 15% by weight of a 
silica containing material and/or by inserting a thermal barrier between 
the electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the drawings, a thermal galvanic cell of the type 
disclosed and claimed in referenced U.S. Pat. No. 4,211,828 is shown in 
cross-section in FIG. 1. The cell of FIG. 1 was fabricated, for purposes 
of comparison with the cells of the present invention, so as to have 
disc-shaped electrode areas exposed to the electrolyte. The electrodes, 
which are indicated at 10 and 12, were clamped against an annular 
polytetrafluoroethylene spacer 14 and the space between the facing 
surfaces of electrodes 10 and 12 was filled with an electrolyte 16. 
Commensurate with previous practice, electrode 10 was uniformly heated to 
a temperature T.sub.H.sbsb.1 by bringing a heated metal plate 18 into 
contact with the outwardly facing surface thereof. Electrode 12 was 
similarly caused to assume a second temperature, T.sub.C.sbsb.1 by means 
of placing the outwardly facing side thereof in contact with a second 
metal plate 20. The temperature difference dT.sub.1 between electrodes 10 
and 12, with appropriate selection of electrolyte 16, results in the 
establishment of current flow through the cell and a voltage V may be 
measured across the cell. The cell open circuit voltage is given by 
equation (4) above. 
Referring to FIG. 2, a cell in accordance with a first embodiment of the 
present invention is shown in cross-section. The cell of FIG. 2 differs 
from that of FIG. 1 only in that the metal plates 18 and 20, which 
respectively insure the establishment of a uniform temperature across 
electrodes 10 and 12, have been removed. In the FIG. 2 embodiment, a 
conduit 22 is attached to the center of the outwardly facing side of 
electrode 10 as shown while a second conduit 24 is coupled to the center 
of electrode 12 as shown. A "hot" fluid is circulated through conduit 22 
whereby this conduit appears to electrode 10 as a point source of heat. A 
"cold" fluid is circulated through conduit 24 whereby conduit 24 appears 
to electrode 12 to be a point heat sink. Employing the arrangement of FIG. 
2, there is an average temperature differential dT.sub.1 between 
electrodes 10 and 12 and, additionally, there is a transverse temperature 
differential preceding radially outwardly from the points of contact of 
conduits 22 and 24 respectively with electrodes 10 and 12. With the 
temperature of the fluid flowing through conduit 22 selected so as to 
provide an average electrode temperature of 75.degree. C., a radial 
temperature differential, dT.sub.2, of 4.degree. per centimeter was 
measured on a copper electrode 10 of 0.5 mm thickness. As will be 
explained below, the establishment of this transverse temperature 
differential, dT.sub.2, produced an unexpected and significant increase in 
cell efficiency. 
Referring jointly to FIGS. 3A and 3B, an electrode in accordance with a 
further embodiment of the present invention is shown on an enlarged scale. 
That portion of the electrode of FIG. 3 which is contacted by electrolyte 
16 has been segmented. In the FIG. 3 embodiment, the electrode is formed 
into a plurality of strips as indicated at 10' and 10". These electrode 
strips are separated by any suitable non-conductive material as indicated 
at 26. All of the electrode strips are electrically and thermally 
connected to a plate 28 and the plate 28 may be heated in the same manner 
as depicted in FIG. 1. Employing the electrode configuration of FIG. 3 in 
the cell of FIG. 1, an unexpected and significant improvement in 
performance is achieved. This improvement in performance, which will be 
discussed in greater detail below in the description of FIG. 7, is 
evidenced by an increase in current density and it is possible to take 
full advantage of this phenomena by retaining the same total electrode 
area and dividing this area into segments which are electrically commoned. 
With reference to FIG. 4, yet another embodiment of the present invention 
is shown schematically. As will be obvious from the discussion below, the 
embodiment of FIG. 4 takes advantage of the discoveries embodied in the 
embodiments of FIGS. 2 and 3. In FIG. 4, rather than employing the 
disc-shaped electrodes of FIGS. 1 and 2, the electrodes 10 and 12 are in 
the form of rectangular plates. In the FIG. 4 embodiment, electrode 10 is 
heated by means of passing a "hot" fluid through a conduit 22' attached to 
the electrode along one of the long edges thereof which extends above the 
surface of electrolyte 16. The temperature of electrode 12 is similarly 
controlled by means of passing a "cold" fluid through conduit 24'. This 
technique results in establishment of an average temperature differential 
dT.sub.1 between electrodes 10 and 12 and a transverse temperature 
gradient dT.sub.2 along each electrode in the direction of immersion into 
electrolyte 16. With the area of the electrodes 10 and 12 of the FIG. 4 
embodiment which are immersed in electrolyte 16 being identical to the 
electrode area that is in contact with the electrolyte in the embodiments 
of FIGS. 1 and 2 and with the same electrode separation, S, an unexpected 
and very significant increase in efficiency is achieved and this increase 
in efficiency is maximized by reducing the depth of immersion, d, of the 
electrodes of the FIG. 4 embodiment into the electrolyte 16. 
FIGS. 5 and 6 represent further embodiments of thermal galvanic cells in 
accordance with the present invention. FIG. 6 is a version of the FIG. 2 
cell while FIG. 5 embodies the features of the cells of FIGS. 3 and 4. 
Before discussing the embodiments of FIGS. 5 and 6, however, it is 
believed desirable to refer to the test results depicted graphically in 
FIGS. 7-10 inclusive. These four FIGURES are plots of cell "resistivity", 
in ohms per cm.sup.2, versus current density, in mA/cm.sup.2 on a log-log 
scale for the cells of FIGS. 1-4. Considering first FIG. 7, for all six 
different cells tested both electrodes were comprised of copper of the 
same thickness and the electrolyte was CuSO.sub.4 adjusted to a pH of 1.4 
with tartaric acid. The "hot" electrode 10 was either uniformly heated to 
75.degree. C. or had an average temperature of 75.degree. C. while the 
average temperature of the "cold" electrode 12 was 24.degree. C. In all 
cases the separation S between electrodes 10 and 12 was 1.9 cm and each 
cell was tested with load resistances of 2, 5 and 16 ohms. The "CASE" 
numbers indicated on FIG. 7 are commensurate with the FIGURE number of the 
embodiment being tested; i.e, CASE 1 is a cell of the type depicted in 
FIG. 1, etc. In CASES 1 , 2 and 4, the area of each of electrodes 10 and 
12 was ten (10) cm.sup.2. 
Continuing with a discussion of FIG. 7, the curve labeled CASE 1 was 
plotted from data derived from measurements taken on the cell of FIG. 1. 
The data was obtained by varying the load resistance across the cell and 
then calculating the cell parameters from the known open circuit voltage 
and the measured voltages at the specified load resistance. The higher 
values of "resistivity" are at the higher load resistances and, of course, 
the lower current density. 
The curve indicated as CASE 2 was obtained with a cell constructed as shown 
in FIG. 2; i.e., the cell geometry was identical to that of FIG. 1; but a 
radial temperature gradient established at each electrode. In the test 
being described, the measured dT.sub.2 was approximately 4.degree. C./cm 
and the average temperature difference between the electrodes, dT.sub.1, 
was maintained the same as in CASE 1. With all of the geometries and 
parameters being maintained the same, it can be seen from FIG. 7 that the 
FIG. 2 embodiment, and particularly the establishment of a transverse 
temperature gradient, resulted in a dramatic decrease in cell resistivity 
and an increase in current density. 
Proceeding to the curve labeled CASE 3, the electrodes of FIG. 3 were 
substituted for the copper discs of FIG. 1 and, with all other parameters 
remaining the same, it may be seen that the segmented electrode also 
produces a decrease in cell "resistivity" and an increase in current 
density when compared to the prior art cell of FIG. 1. 
Continuing with a discussion of FIG. 7, the curve labeled CASE 4 was 
obtained from a cell having the configuration depicted in FIG. 4; i.e., 
the cell of FIG. 4 has rectangular electrodes of the same area as the 
cells of FIGS. 1 and 2 and the same average dT.sub.1 was established 
across the cell. Because of the manner in which heat was delivered to 
electrode 10 and removed from electrode 12, this resulted in a temperature 
gradient dT.sub.2 in the direction of immersion of approximately 
15.degree. C./cm. As may be seen from FIG. 7, the cell configuration of 
FIG. 4 produces a very significant improvement in cell efficiency when 
compared to the prior art embodiment of FIG. 1 and also provides an 
improvement when compared to the embodiments of FIGS. 2 and 3. This 
improvement is believed to be attributable to the transverse temperature 
differential dT.sub.2 and also partly to the change in electrode shape. 
The curve for CASE 4 was obtained with a cell having electrodes which were 
10 cm in length and which extended into the electrolyte to a depth of 1 
cm. 
Further improvement, in terms of both lowered "resistivity" and increased 
current density, was achieved by taking the cell of FIG. 4 and decreasing 
the depth of immersion of the electrodes while maintaining all other 
parameters constant. Thus, the data from which the curve labeled CASE 4' 
was plotted was measured on a cell identical to that of FIG. 4 with the 
exception that the depth of immersion of the electrodes was reduced to 0.3 
cm. This constitutes proof of the unexpected phenomena, which will be 
further discussed below in the descriptions of FIGS. 11-15, that cell 
"resistivity" per unit of electrode area is decreased and current density 
per unit area increased through reduction of electrode immersion depth in 
a thermal galvanic cell. A further improvement in cells of the type 
described above, from which the curve for CASE 4' was plotted, results 
from the addition of a limited quantity; i.e, between 1% and 15% by 
weight; of a silica containing material to the electrolyte. Thus, the data 
from which the curve labeled CASE 4" was plotted was obtained from 
measurements on the same cell as tested with CASE 4' with 5% by weight of 
the material known in the art as Cab-o-Sil having been added to the 
electrolyte. The material known as Cab-o-Sil consists of sub-micron sized 
SiO.sub.2 particles. While the manner in which Cab-o-Sil functions is not 
entirely understood, it causes the electrolyte to gel and may reduce 
convection currents within the electrolyte while promoting the creation of 
hot spots at the surface of the "hot" electrode 10. The effects of the 
silica containing material will be further described below. 
FIG. 8 depicts a series of curves representing data obtained in the manner 
described above with cells constructed in accordance with FIGS. 1, 2 and 
4. However, in obtaining the data for FIG. 8, a different electrolyte was 
employed and the temperature of the "cold" electrode was maintained 
slightly above that of the previously reported tests (FIG. 7). 
Specifically, the pH of the cells from which the FIG. 8 data was derived 
was achieved by employing H.sub.2 SO.sub.4 rather than tartaric acid. It 
may be seen from FIG. 8 that creation of either a radial dT.sub.2 
temperature gradient (CASE 2) or a transverse dT.sub.2 temperature 
gradient coupled with a change in electrode geometry (CASE 4) results in 
the cell "resistivity" being decreased when compared to the prior art 
(CASE 1). FIG. 8, and this is also true of FIGS. 9 and 10 to be described 
below, thus further shows that electrode efficiency in a thermal galvanic 
cell can be increased by establishing a temperature gradient across the 
surface of an electrode or the electrodes. In each of FIGS. 7-10, during 
the testing of the prior art cells identified as CASE 1, heat was applied 
uniformly over the surface of the "hot" electrodes and thus there were no 
measurable transverse temperature gradients. However, when heat is added 
and removed at a point in the center of each electrode to develop a radial 
heat flow over the electrode, thereby causing a transverse temperature 
gradient as is the situation for the FIG. 2 embodiment (CASE 2), cell 
"resistivity" decreased dramatically and there was an increase in current 
density. In the testing of the FIG. 2 embodiment with various 
electrolytes, the electrode temperatures were monitored by means of 
thermistors mounted on the electrode surfaces. It was presumed that the 
average electrode temperature, for purposes of comparison with tests on a 
cell of the FIG. 1 configuration, occurred at a point on the radius 
corresponding to one-half of the electrode area. The radial temperature 
gradient dT.sub.2 measured for all of the tests on a cell of the FIG. 2 
configuration was approximately 4.degree. C./cm. In the testing performed 
on the FIG. 4 embodiment, CASE 4, the measured dT.sub.2 was approximately 
15.degree. C./cm and, as noted above, in each case the electrode tested 
was 10 cm in length with an immersion depth into the electrolyte of 1 cm. 
Every test on the FIG. 4 embodiment showed a significant decrease in 
"resistivity" when compared to the prior art as exemplified by a cell of 
the FIG. 1 configuration. As indicated by the curves for CASE 4', when the 
electrode immersion depth of the cell of FIG. 4 is decreased, there is a 
further reduction in "resistivity" and there is also a significant 
increase in current density. 
FIG. 9 shows the results of tests similar to that of FIGS. 7 and 8 on cells 
having the configuration of FIGS. 1, 2 and 4. In FIG. 9, however, a third 
electrolyte was utilized while all other parameters remained the same as 
those employed in the test results represented by FIGS. 7 and 8. 
Referring to FIG. 10, the results of tests performed on cells having the 
configurations of FIGS. 1, 2 and 4 with a fourth electrolyte are 
represented. FIG. 10 depicts the totally unexpected phenomena that the 
addition of 1% SiO.sub.2 (Cab-o-Sil) to a cell of the FIG. 4 type, wherein 
the electrode immersion depth has been reduced to 0.3 cm, produces a very 
significant decrease in "resistivity" with a modest increase in current 
density (CASE 4") while the addition of 5% Cab-o-Sil to the same cell 
(CASE 4'") causes an increase in "resistivity" and an increase in current 
density. With a dT.sub.1 of 45.degree. C. an electrode separation of 1.9 
cm, the maximum power output obtained for each CASE is as follows: 
CASE 1=7.mu. watts/cm.sup.2 
CASE 2=16.mu. watts/cm.sup.2 
CASE 4'=29.mu. watts/cm.sup.2 
CASE 4"=52.mu. watts/cm.sup.2 
CASE 4'"=212.mu. watts/cm.sup.2 
With up to 1% SiO.sub.2 added to the electrolyte, and a dT.sub.2, there is 
a decrease in "resistivity" which may result from the silicon dioxide 
functioning as a thermal barrier. As may be seen by comparing CASE 4" with 
CASE 4'", with a dT.sub.2 the addition of more than 1% SiO.sub.2 causes a 
slight increase in "resistivity" but also causes a dramatic increase in 
Seebeck coefficient. 
FIG. 16 is a log-log plot of "resistivity" versus current density for the 
prior art cell of FIG. 1 and the electrolyte of FIG. 7 with a dT.sub.1 of 
55.degree. C., all other parameters remaining the same and being as noted 
above, as the percent by weight of sub-micron SiO.sub.2 added to the 
electrolyte is increased. FIG. 16 shows that cell "resistivity" decreases 
and current density increases as the amount of SiO.sub.2 increases without 
a dT.sub.2. Tests have shown, however, that there is an upper limit of 
approximately 15% of the silica containing material. Exceeding this limit 
results in mechanical contact problems between the electrolyte and the 
electrodes. 
It is also to be noted that, with cells having the configuration depicted 
in FIG. 4, with a copper fluoborate electrolyte, there is a steady 
logarithmic increase in the Seebeck coefficient with time when up to 15% 
Cab-o-Sil is added to the electrolyte. Since the power output of a thermal 
galvanic cell is proportional to the cell voltage squared, while only 
directly proportional to cell conductance, a doubling of cell voltage will 
result in quadrupling of output power. The increase in Seebeck coefficient 
with time for a cell constructed in accordance with the FIG. 4 embodiment 
is shown on FIG. 17. The curves indicated on FIG. 10 as CASES 4" and 4'" 
were plotted with the cells having reached equilibrium. 
In order to demonstrate the improvement in thermal galvanic cell efficiency 
with decreasing electrode immersion depth, an experimental "bridge" cell, 
depicted schematically in FIG. 11, was constructed. In the arrangement of 
FIG. 11, the "hot" electrode 10 was heated by means of an electrical 
heater 28 along a first edge while the oppositely disposed edge was 
immersed in the electrolyte 16 in a first container 30. Container 30 was 
positioned on a hot plate 32 whereby the electrolyte temperature could be 
maintained at approximately 90.degree. C. The "cold" electrode 12 was 
immersed in electrolyte 16 in a second container 34 and was maintained at 
ambient temperature. Both of electrodes 10 and 12 were connected to 
laboratory jacks so that the immersion depth, d, thereof could be varied. 
Communication between the electrolyte in containers 30 and 34 was 
established by means of a "bridge" conduit 36 whereby ion or electron flow 
between electrodes 10 and 12 was possible. In order to insure that there 
would be no temperature gradients along the surface of the "hot" 
electrode, the temperature of the electrode was measured immediately above 
the electrolyte surface and the electrolyte temperature was also measured. 
The current flow to heater 28 was adjusted to equalize the temperatures of 
the electrolyte and the expose end of the "hot" electrode 10. FIGS. 12-15 
are plots, for various electrode and electrolyte materials, of cell 
"resistivity" vrs current density for varying electrode immersion depths. 
FIGS. 12-15 show that in each test the "resistivity" decreased and current 
density increased as the electrode immersion depth was decreased. 
Referring again to FIGS. 7-10, the test results indicated as CASE 4' 
confirm that the combined effects of reducing electrode area through 
reduction in immersion depth and establishing a transverse temperature 
differential, dT.sub.2, result in much improved electrode efficiency and 
thus improved power output. The reduction in the immersion dimension, d, 
decreases cell resistivity per unit of electrode area. The establishment 
of the transverse temperature gradient also results in the area 
perpendicular to the transverse temperature gradient becoming additive. 
Without the dT.sub.2, addition of electrode length for a given immersion 
depth produces no significant increase in power output. Thus, by combining 
the decreased "resistivity" per unit area incident to reduction in 
immersion depth with establishment of a transverse temperature gradient, a 
thermal galvanic cell may become larger with a concomitant increase in 
output power. A practical operating cell which takes advantage of both of 
the aforementioned features is depicted in FIG. 5 and will be described 
below. Cells in accordance with the present invention have their 
electrolytes adjusted to be below pH 5 and preferably below pH 2. 
To summarize the foregoing, the open circuit voltage of a thermal galvanic 
cell is, for cells of the same chemistry, equal to a constant multiplied 
by the temperature difference between the electrodes and the power output 
is inversely proportional to the internal resistance of the cell. If the 
internal resistance of a cell can be halved, by way of example, then the 
power output can be doubled. The practical design for an operating cell, 
accordingly, dictates obtaining the largest possible temperature gradient 
dT.sub.1 between the electrodes while reducing the internal resistance as 
much as is economically feasible. The above test results demonstrate that 
internal resistance may be minimized by constructing the cells with a low 
dimension in the heat flow direction; i.e., the immersion depth, d; while 
simultaneously establishing a high transverse temperature gradient, 
dT.sub.2. 
Referring to FIG. 5, a cell may be constructed with a plurality of parallel 
"hot" and "cold" electrodes, respectively indicated at 10'" and 12'", 
supported by respective plates 40 and 42. Plates 40 and 42 are comprised 
of a material which may be both an electrical and a thermal insulator. The 
plates 40 and 42 separate a housing 44 into three separate compartments. A 
hot fluid is delivered, via conduit 46, into a first of these compartments 
48 while a relatively cool fluid is delivered into a second compartment 50 
via conduit 52. The fluids delivered to compartments 48 and 50 
respectively deliver heat to electrodes 10'" and remove heat from 
electrodes 12'". The space 54 between the electrodes will, of course, be 
filled with the electrolyte. The depth of immersion of the electrodes; 
i.e., the distance the electrodes protrude into space 54 from their 
respective supports 40 and 42, will be kept low and typically will be less 
than 0.3 cm. The separation between adjacent electrodes will typically be 
less than 1.5 cm. It will be understood that the electrodes could be of a 
comb-like construction whereby all of electrodes 10'" will be commoned by 
a bridging member which could be plate 40 and there would be a similar 
bridging member for electrodes 12'". FIG. 5b shows that the length of the 
electrodes 10'" and 12'" is quite long when compared to the immersion 
depth. 
It should also be noted that the electrolyte compartment of the cell of 
FIG. 5 can be divided in half by means of a barrier which serves to 
isolate the electrodes thermally to thereby increase the temperature 
differential dT.sub.1. Such a thermal barrier may, for example, comprise 
an ion permeable membrane, such as Nafion or polyoxphenolic, which lowers 
the heat transfer as well as allowing the maintenance of two different 
concentrations of electrolyte within the cell for increased performance. 
Referring to FIG. 6, the performance of a cell of the type depicted in FIG. 
1 is enhanced by obtaining a series of thermal gradients dT.sub.2 on the 
electrode by affixing a plurality of conduits 22' and 24' to the outwardly 
facing sides of electrodes 10 and 12. Conduits 22' and 24' respectively 
carry the heating and cooling fluids. In the FIG. 6 embodiment the cell 
can most economically be designed to have a rectangular shape with the 
tubes 22' and 24' being oriented parallelly with respect to one another. 
As indicated by the broken line 60 in FIG. 6, a membrane may be included 
within the cell to function as a barrier to heat transfer between 
electrodes 10 and 12. 
While preferred embodiments have been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, the present invention 
has been described by way of illustration and not limitation.