Thermoelectric generator

A thermoelectric generator system. The thermoelectric generator has at least one hot side heat exchanger and at least one cold side heat exchanger and at least one thermoelectric module with thermoelectric elements installed in an injection molded eggcrate. The thermoelectric modules are held in close contact with the hot side heat exchanger and the cold side heat sink with a spring force. A preferred embodiment contains eight modules held in compression between the hot and cold heat exchangers with Belville springs. The eggcrate is molded from a high temperature plastic with ridges provided for extra strength and tapered walls to permit ease of installation of the thermoelectric elements and stop tabs assure correct positioning of the elements. Electrical connections at hot and cold surfaces are preferably made by thermal spraying metallize coatings on the surfaces and then surface finishing the module to expose the walls of the eggcrate.

This invention relates to thermoelectric generators. 
BACKGROUND OF THE INVENTION 
Thermoelectric devices are well known. These devices utilize physics 
principals known as the Seebeck effect discovered in 1821 and the Peltier 
effect discovered in 1834. The Seebeck principle tells us that if two 
wires of different materials (such as copper and iron) are joined at their 
ends, forming two junctions, and one junction is held at a higher 
temperature than the other junction, a voltage difference will arise 
between the two junctions. The Peltier effect describes an inverse effect. 
If current is sent through a circuit made of dissimilar materials, heat 
will be absorbed at one junction and given up or evolved at the other 
junction. 
Most thermoelectric devices currently in use today to generate electricity 
or for cooling utilize semiconductor materials (such as bismuth telluride) 
which are good conductors of electricity but poor conductors of heat. 
These semiconductors are typically heavily doped to create an excess of 
electrons (n-type) or a deficiency of electrons (p-type). An n-type 
semiconductor will develop a negative charge on the cold side and a p-type 
semiconductor will develop a positive charge on the cold side. 
Since each element of a semiconductor thermoelectric device will produce 
only a few millivolts it is generally useful to arrange the elements in 
series so as to produce higher voltages for the generation of electricity 
or to permit use of higher voltages for cooling. Several techniques have 
been developed for arranging the semiconductor elements in series in 
thermoelectric devices. In one such prior art method p and n type 
semiconductors are arranged in a checkerboard pattern and electrical 
jumpers are soldered, each to two different semiconductors, at the cold 
side and at the hot side so as to place all of the semiconductor elements 
in series with each other. This method is a low cost method and well 
established but has some limitations. Above 100.degree. C. the solders can 
defuse into the thermoelectric elements destroying them. In a high 
humidity atmosphere moisture may condense in the spaces between the 
elements and thermally short the module. The structure is not mechanically 
strong and easily fractures. 
Another currently used method is the so-called eggcrate design. Here an 
"eggcrate" made of insulator material separates the thermoelectric 
elements and permits electrical jumpers to be pressed against the elements 
to provide a good electrical connection without solder. In prior art 
designs, the eggcrates are fabricated from individual walls which have 
been cut to shape using a precision laser cutter. All of the elements can 
be connected in series by proper construction of the eggcrate. Obviously 
it is possible in both devices to arrange for any desired number of 
elements to be in series. Thus, several elements in series may form a 
series set and this set could be arranged in parallel with other similar 
sets. 
Prior art thermoelectric generators are expensive due primarily to labor 
costs to assemble the eggcrates to install the elements in the crates and 
to form the modules into a generator unit. What is needed is a lower cost 
thermoelectric generator. 
SUMMARY OF THE INVENTION 
The present invention provides a thermoelectric generator. The 
thermoelectric generator has at least one hot side heat exchanger and at 
least one cold side heat exchanger and at least one thermoelectric module 
with thermoelectric elements installed in an injection molded eggcrate. 
The thermoelectric modules are held in close contact with the hot side 
heat exchanger and the cold side heat sink with a spring force. A 
preferred embodiment contains eight modules held in compression between 
the hot and cold heat exchangers with Belville springs. The eggcrate is 
molded from a high temperature plastic with ridges provided for extra 
strength and tapered walls to permit ease of installation of the 
thermoelectric elements and stop tabs assure correct positioning of the 
elements. Electrical connections at hot and cold surfaces are preferably 
made by thermal spraying metallized coatings on the surfaces and then 
surface finishing the module to expose the walls of the eggcrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention may be described by 
reference to the figures. 
Thermoelectric Generator Unit 
A schematic drawing of a thermoelectric generator 28 is shown in FIG. 22 
and a detailed version of generator is shown in FIGS. 20 and 21. The unit 
comprises of one hot side heat exchanger 40, a first cold side heat 
exchanger 42, connecting hose 43 and a second cold side heat exchanger 44, 
eight thermoelectric modules 45 electrically connected in series and four 
spring loaded compression elements 46. The hot glycol enters hot side heat 
exchanger 40 at A and exits at B as shown in FIG. 22. The cool glycol 
enters one of the cold side heat exchangers 42 at C, passes through it and 
connecting hose 43 (shown in FIG. 20) and then passes through the other 
cold side heat exchanger 44 and exits at D, as shown in FIG. 22. 
Heat Exchangers 
The hot side heat exchanger 40 is in this embodiment a welded steel 
structure. The body of the heat exchanger is 141/2 inches long and 3 
inches wide. It is basically constructed of two identical machined finned 
sections 40 A and 40 B. These two sections are welded together and nipples 
40 C and 40 D are welded at opposite ends of the heat exchanger as shown 
in FIG. 20 to form a finned passage for the hot glycol. Each of the cold 
side heat exchangers is also a welded steel structure. They each comprise 
a finned section 42 A and 44 A and a cover plate 42 B and 44 B which are 
welded together and to nipples 42 C and 44 C to form a finned passage for 
the cool glycol. 
Compression Elements 
Compression elements 46 comprise a steel frame 46 A which is basically a 
four sided rectangular frame with outside dimensions of 5.64 inches high, 
33/4 inches wide and 13/4 inches thick. The frame provides a rectangular 
space 3.12 inches wide and 5.02 inches high for the heat exchangers and 
the thermoelectric modules 45 to fit into. Each frame also comprises a 
thrust button 46 A 1 and a nut plate 46 A 2 containing a hexagonal nut 
matching space. Compression elements 46 each provide about 1000 pound 
compression on the heat exchangers and the thermoelectric modules. This is 
accomplished with a Belville spring stack 46 B. each of which are centered 
over two thermoelectric modules 45 each of which in turn are sandwiched 
between two thin (0.01 inch thick) alumina (Al.sub.2 O.sub.3) wafers 46 C 
as shown in FIG. 21. The load is provided by tightening adjustment screw 
46 D through nut 46 E to compress spring stack 46 B. Torque produced by 
screw 46 D on nut 46 E is resisted by nut plate 46 A 2. Upward thrust 
produced by screw 46 D is absorbed on the top side by adjusting nut 46 E 
and then steel frame 46 A to thrust button 46 A 1 located on the opposite 
side of compression element 46. Downward thrust from screw 46 D is 
absorbed on the bottom side by by load washer 46 H and the spring stack 46 
B . Thus, the two thermoelectric modules 45 are held in tight compression 
between the heat exchangers by opposite forces of about 1000 pounds 
provided by thrust button 46 A 1 and spring stack 46 B. 
Well Head Dehydration Installation 
In one preferred embodiment of the present invention thermoelectric 
generating unit 28 is inserted as shown in FIG. 23 in a well head 
dehydration plant. This thermoelectric generator utilizes as its heat 
source the hot (375 degrees F.) dry glycol exiting reboiler 1 through pipe 
9 and utilizes as its cold sink the cool (60 degrees F.) wet glycol 
entering preheat heat exchanger 8 through pipe 14. 
Thermoelectric Modules 
Following is a description to a preferred process for fabricating an 
eggcrate type thermoelectric modules 45 for use in the thermoelectric 
generator discussed above. 
Injection Molded Eggcrate 
The eggcrate for this preferred embodiment is injection molded using the 
mold pattern shown in FIGS. 15A and B and 16A and B. FIGS. 15A and B show 
the bottom of the mold pattern and FIGS. 16A and B show the top of the 
pattern. The top and bottom are shown in their molding position in FIG. 
18. A high temperature thermo plastic, such as the liquid crystal polymer 
resin, Dupont Zenite, is injected through sprue 24 using well known 
plastic molding techniques in an injection molding machine 26 as depicted 
in FIG. 17. The Dupont Zenite plastic is dried at 275 F. and the barrel 
temperatures of the molding machine range from 625 F. at the rear to 640 
F. near the nozzle. Both the bottom mold and the top mold are maintained 
at a temperature of about 200 F. Zenite melts at about 550.degree. F. In 
the usual manner the fluid plastic passes through sprue 24, runner 28, and 
gate 30 into the mold cavity. The vent is shown at 34 in FIG. 18. The 
finished part is ejected by injection pins 32 as shown in FIG. 17. Initial 
production runs made by applicants supplier have produced excellent 
eggcrates at a rate of about 50 eggcrates per hour. This rate can easily 
be increased to 200 eggcrates per hour for one worker and ultimately the 
process can be completely automated. This compares to a one worker 
production rate of about 3 eggcrates per hour with the prior art method of 
assembling thermoelectric module eggcrates from appropriately slotted 
layers of insulating materials. 
The completed injection molded eggcrate is shown in FIG. 1A. This 
embodiment contains boxes (spaces) for 100 thermoelectric elements. The 
dimension of the elements are 5.1 mm.times.5.1 mm.times.3.0 mm. The 
dimension of the spaces at the bottom of the eggcrate are 5.1 mm.times.5.1 
mm. A top view of the eggcrate is shown in FIG. 1. FIGS. 2 through 9 show 
various sections through the eggcrate. FIG. 10 is a side view and FIG. 11 
is a sectional view which shows an expanded view of one of the boxes 
created by the eggcrate. Note that the upper part of the walls of the box 
is tapered 5 degrees as shown at Y in FIG. 11. In this embodiment the 
straight part of the walls of the box forms a 0.2 inch square as shown at 
X in FIG. 11. This dimension is held to a tolerance of plus 0.001 inch to 
provide a tight fit for thermoelectric elements which are 0.200 inch 
square with a tolerance of minus 0.001. Note that a support ridge 62 as 
shown in FIGS. 11 and 12 is provided around the boundary of the eggcrate 
at the midplane between the two surface planes of the eggcrate. This 
support ridge provides extra strength for the eggcrate and is utilized 
during subsequent stages of module fabrication and can be useful in 
mounting the completed module for use. 
FIG. 12 shows a top view of the eggcrate with the locations indicated for 
the P and N elements. The elements are placed in these locations with the 
installer assuring that each element rest firmly against stops 10 as shown 
in FIGS. 1B and 11. Conductor material is then sprayed on the top and 
bottom of the eggcrate as shown in FIGS. 19A and 19B, and then the 
conductor material at the tops and bottoms are ground down until the tops 
of all insulator surfaces are cleared of conductor material. A preferred 
procedure for loading the eggcrate is discussed in detail below. FIGS. 13 
and 14 show examples of sections of the finished product at location 
13--13 and 14--14 as shown in FIG. 12. Note in FIG. 14 how the effect is 
to connect all the thermoelectric elements in series electrically. In this 
particular section the hot surface is on the top and the electron flow is 
from left to right. 
Thermoelectric Elements 
Thermoelectric elements with dimensions of 5.1 mm.times.5.1 mm.times.3.0 mm 
are prepared using any of several well known techniques such as those 
described in Thermoelectric Materials edited by Sittig, published in 1970 
by Noyes Data Corporation, Park Ridge, N.J. Preferred materials are Lead 
Telluride for high temperature applications and Bismuth Telluride for low 
temperature applications. These elements may also be purchased 
commercially from suppliers such as Melcor Corporation with offices in 
Treton, N.J. One half of the elements should be "n" elements and one half 
"p" elements. Alternatively, ingots of "p" and "n" type thermoelectric 
material may be extruded and then sliced into "p" and "n" type 
thermoelectric elements in accordance with well known techniques. 
Loading the Eggcrates 
The "p" elements are positioned in the appropriate boxes of the egg create 
as shown in FIG. 12. The element should be snug against the stop. The "n" 
elements are also positioned in the appropriate boxes of the egg crate as 
shown in FIG. 12. Each element should be snug against the stop. A 2 inch 
long 1/8 inch wide copper mesh wire lead is inserted at positions 61 and 
63 as shown in FIG. 12. At the location of the junction of the leads to 
the module we provide two "p" elements and two "n" elements side by side 
and electrically in parallel for extra support for the leads to reduce the 
likelihood that the leads would break loose. 
Metallizing the Hot and Cold Surfaces 
Using spring loaded clamps, we clamp a number of modules to a rotatable 
mandrel. In FIGS. 19A and B we show 20 modules clamped to such a mandrel. 
We then grit blast the module/element surface with 180-240 grit Al203 to a 
uniform matte finish with the mandrel rotating at 55 rpm. Then we use 
compressed air to blow the module/element surface clean. Next we apply a 
metal thermal spray coating to the exposed surface using a thermal spray 
coating system as shown in FIGS. 19A and B. These spray techniques are 
well known. Further specific details are provided in Metals Handbook, 
Ninth Edition, published by the American Society for Metals. A variety of 
metals can be used to coat the surface. Our preferred coating is a 
two-layer coating comprising a first approximately 0.006 inch thick 
coating of molybdenum and a second approximately 0.06 inch thick coating 
of aluminum. Both coatings are applied using the system shown in FIGS. 19A 
and B with the mandrel rotating at 55 rpm and the spray gun running back 
and forth at speeds of about 0.2 inch per second. After the first surface 
is coated we remount the modules to expose the unsprayed surface and 
repeat the above described process with the second coating. 
Grind the Module Surfaces 
The surfaces must be ground down to expose the eggcrate walls. To do this 
we position a sprayed module in the mounting chuck of a surface finishing 
machine. We reduce the surface of the module to the appropriate height as 
measured from eggcrate tab 62 shown in FIGS. 11 and 12. We then remove the 
module from the chuck, reverse the module and reduce the opposite face of 
the module until the module surface is the appropriate height from the egg 
crate tab. 
Inspection 
We heat the hot surface of the module to 250 C. and cool the cold side of 
the module to 50 C. We then measure the open circuit voltage of the 
module. It should be about 3.2 volts with bismuth telluride elements. We 
then apply an electrical load to the module until the voltage drops to 1.6 
volts and measure the current. We calculate the power produced by the 
module as P=l X V. The power level should be at least 13 watts for the 
bismuth telluride elements. 
Performance of the Unit 
The total output of the eight thermoelectric modules connected in series, 
with a hot side temperature of 375 degrees F. and a cold side temperature 
of about 65 degrees F., will be about 62 watts at about 12 volts. 
Additional power and higher voltages can be obtained by adding additional 
thermoelectric generator units. As indicated only a very small percentage 
of available waste energy of the dehydration plant was utilized in the 
above described arrangement. 
It is known that power output as a function of glycol flow rate for several 
hot glycol temperatures. For flow rates greater than 10 gpm, it is 
feasible to connect more than one unit in series to provide increased 
power. For flow rates above 30 gpm generators should preferably be 
connected in parallel. For flow rates outside these ranges a redesign of 
the unit may be preferably making it longer or wider. 
Engineers installing the generator in the field may elect to incorporate 
bypass lines and valves to allow them to bypass the generator. This would 
allow the generator to be removed from service if not needed or for 
repair. A flow control valve could be provided if desired although in most 
cases it would not be needed. If the generator is to be used to provide 
cathodic protection, the generator would normally be connected to a 
constant current regulator which will automatically vary the system 
impedance to match system requirements. If the generator is to be used to 
provide power for lighting, instruments or communication, the generator 
would normally be connected to a constant voltage shunt regulator. A 
battery is not required to make the system operate. However, if the 
generator is to be used to provide power for a system, such as 
communication, where the short term peak power requirements is higher than 
the normal power output from the generator, then a battery and battery and 
a battery regulator would be included to carry the short term high peak 
loads. All of this extra equipment can be obtained "off the shelf". 
The foregoing description of the present invention has been presented for 
the purpose of illustration and is not intended to limit the invention to 
the precise form disclosed. It is understood that many modifications and 
changes may be effected by those skilled in the art. For example, 
thermoelectric modules other than the one described could be utilized in 
which case details of the heat exchanger and compression elements would 
probably need to be modified appropriately. It is feasible to make modules 
with many more thermoelectric elements in which case a single 
thermoelectric module may be sufficient to provide the required voltage. 
Other well known methods of holding the thermoelectric elements in good 
thermal contact with the heat exchangers could be used. There maybe 
advantages of tapping into hot and cold glycol lines at locations other 
than those described. The heat exchangers were described as welded carbon 
steel units. These heat exchangers could be made with other well known 
techniques utilizing teachings of this specification. When sales are high 
enough to justify it, Applicants plan to manufacture the heat exchangers 
using aluminum castings. This should greatly reduce the cost. As to the 
thermoelectric modules, many other materials besides Zenite can be used 
for injection molded eggcrates. These include Xydar (manufactured by Amaco 
which is substantially equivalent to Zenite), Rytron produced by Philips, 
polyethylene, silicones, teflons, and many others. Zenite was primarily 
selected because of its superior properties (i.e., melting point, thermal 
stability, etc.) at higher temperatures. Also it should be possible to use 
a ceramic material in the form of a "slip". (This is the term used for 
describing a fine ceramic material suspended in a liquid.) After molding, 
the liquid is removed by drying and /or the mold (typically plaster of 
paris) absorbing the liquid. The components are then sintered to give them 
strength. Zenite, in fact, contains a fine glass powder filler to reduce 
material costs and control other material properties. This filler could be 
some other material such as carbon or come chopped fibers made from fiber 
glass, graphite fibers, etc. Other moldable materials which could be used 
are organic precursors that transform from the organic to the inorganic 
state when heated. Materials of this nature would be very desirable for 
higher temperature eggcrates that would be used with high temperature 
thermoelectric materials such as PbTe and SiGe which operate at 
temperatures greater than 350.degree. C. which is typically an upper limit 
on most organic materials. These materials would allow the eggcrate to be 
loaded to higher values at temperatures where organic materials typically 
lose their strength. Phosphate and silicate pastes and cements might also 
be used for the eggcrate material for high temperature applications. These 
materials could be formed into eggcrates using silk-screening techniques 
used in the electronics industry. Accordingly it is intended by the 
appended claims to cover all modifications and changes as fall within the 
true spirit and scope of the invention.