Thermoelectric generator for motor vehicle

A thermoelectric generator for producing electric power for a motor vehicle from the heat of the exhaust gasses produced by the engine of the motor vehicle. The exhaust gasses pass through a finned heat transfer support structure which has seat positions on its outside surface for the positioning of thermoelectric modules. A good contact cylinder provides a framework from which a spring force can be applied to the thermoelectric modules to hold them in good contact on their seats on the surface of the heat transfer support structure.

BACKGROUND OF THE INVENTION 
Engine driven "motor" vehicles typically am equipped with a battery to 
supply energy for starting the engine and to supply power for auxiliary 
electric equipment such as headlights, horn and various instruments. The 
power for the battery is normally provided by an alternator or a generator 
which is pulley driven by the crank shaft of the engine. The efficiency of 
these systems is generally less than about 35 percent. 
The gasses exhausted from the engines of motor vehicles is at a relatively 
high temperature. For Diesel engine trucks the temperatures are in the 
range of about 850 to 1000 degrees Fahrenheit, much higher than the 
ambient temperature or the temperature of the engine's cooling water. 
These gasses normally are exhausted to the atmosphere so that the energy 
represented by these high temperature exhaust gasses is completely wasted. 
Thermoelectric modules are currently available which can produce electric 
power when a substantial temperature difference is available. 
Specifically, thermoelectric modules are commercially available having 
dimensions of 2.1 inches.times.2.1 inches.times.0.2 inch thick which will 
produce 13 watts when the temperature difference between its hot side and 
cold side is about 360 degrees Fahrenheit. 
SUMMARY OF THE INVENTION 
The present invention provides a thermoelectric generator for producing 
electric power for a motor vehicle from the heat of the exhaust gasses 
produced by the engine of the motor vehicle. The exhaust gasses pass 
through a finned heat transfer support structure which has seat positions 
on its outside surface for the positioning of thermoelectric modules. A 
good contact cylinder provides a framework from which a spring force can 
be applied to the thermoelectric modules to hold them in good contact on 
their seats on the surface of the heat transfer support structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
General Layout 
A preferred embodiment of the present embedment can be described by 
reference to FIGS. 1 through 4. FIG. 1 is a drawing of a 1,000 watt 
thermoelectric generator 1 for providing electric power for the diesel 
engine driven 14 liter Trailways Truck from the heat of the engine's 
exhaust gasses. FIG. 2 shows the location of the thermoelectric generator 
on the truck. In this embodiment the thermoelectric generator 1 is coupled 
directly to the existing turbocharger 3 using a standard V-Band clamp as 
shown in FIG. 7. Generator 1 is upstream of muffler 5 and is connected to 
the muffler also with a standard V-Band clamp. 
Heat Transfer Support Structure 
As shown in FIG. 1 the generator utilizes a main heat transfer support 
structure 2 made of Cast Ni Resist iron to support 72 1.55 Volt, 13 Watt 
thermoelectric modules 4. As shown in FIG. 2 support structure 2 is 
located in the engines exhaust system at the point where the exhaust 
leaves the turbocharger power turbine outlet 12 of the truck's engine. 
The inside of support structure 2 is generally cylindrical and contains 40 
rows of radially oriented fins 14 which provide an improved heat transfer 
surface. The fins are shown in cross section in FIG. 8. The outside of the 
support is octagonal in shape providing eight flats, each with a width 
wide enough to provide good seats for thermoelectric modules 4 which have 
dimensions of about 2.1 inches.times.2.1 inches with a thickness of about 
0.2 inch. A thin piece of aluminum oxide (not shown) with a thickness of 
0.01 inch is placed between the surface of the support structure 2 and the 
modules 4 for electrical insulation. Both sides of the insulator are 
covered with a layer of heat transfer grease such as Wakefield 120 thermal 
compound at a thickness of about 0.001 inches. Nine thermoelectric modules 
are located in line about 1/2 inch apart on each of the eight flats. 
Thermoelectric Modules 
Thermoelectric modules 4, as stated above, have dimensions of about 2.1 
inches.times.2.1 inches with a thickness of about 0.2 inches. One of the 
2.1.times.2.1 surface is the cold side and the other 2.1.times.2.1 surface 
is the hot surface. Thermoelectric modules of this type are commercially 
available from suppliers such as Teledyne Energy Systems, Inc. Each module 
will produce about 13 watts at a potential of 1.55 volts when subjected to 
a temperature differential in the range of about 400 degrees Fahrenheit. 
Heat Sink 
A watercooled aluminum heat sink 16 is bonded to the cold side of the 
modules as shown in FIG. 1 with a high thermal conductivity silicon 
adhesive such as castable liquid from Castol, Inc. or adhesive-coated 
sheet form from Burquist, Inc. The heat sinks are slightly wider than a 
module and long enough to cover all nine modules in one flat. The inside 
of each heat sink contains heat sink fins 18 providing finned passages for 
good heat transfer from the coolant to the heat sink. The side of the heat 
sink away from the modules has a circular boss located at the center of 
each module location. Each of these bosses provide a location for a stack 
of four Belleville spring washers 22, a circular washer 23, and a spring 
follower 24 as shown in FIG. 5 or FIG. 6. 
Good Contact Cylinders 
Three circular aluminum cylinders 28 are placed external to the heat sinks 
and spring stacks. Each cylinder is long enough to cover three modules. 
The cylinders contain a group of 24 holes arranged in a three-by-eight 
array. The location of each hole in the aluminum cylinder corresponds with 
the location of the center of each module. A piece of channel-shaped steel 
32 machined on one side to fit the curvature of the cylinder is placed 
radially inside each set of three holes in the cylinder. A square nut 26 
with a screw 34 threaded into it is located at each cylinder hole 
location. One end of the screw is rounded and the other end contains a hex 
socket. The hex socket end is placed through the hole inside the channel 
and into the corresponding hole in the cylinder so the screw is oriented 
in the radial direction. The rounded end of the screw rests on the spring 
follower so that when the screw is screwed inward through the nut 26, a 
compressive force occurs between the spring follower and the inside of the 
cylinder 28. The force in the individual screws is adjusted to provide 
about 1000 lb. of force (at room temperature) to assure good contact in 
order to minimize the contact temperature drop between the module surfaces 
and both the hot support structure and heat sink surfaces. I estimate the 
force at operating temperature to be about 1150 lb. 
Inlet Diffuser and Outlet End 
As shown in FIG. 1 an inlet diffuser section 36 is welded to one end of the 
support structure 2. This diffuser connects to turbo-supercharger turbine 
outline flange A length of straight tubing 38 is welded to the outlet end 
of the support structure, and terminates in a flange 40 which is attached 
to the of the inlet end of muffler pipe 40 of the standard exhaust system 
piping of the truck as shown on FIG. 2. 
Exhaust Flow Distributor 
A hollow substantially cylindrical body 42 is located within the center of 
the support structure 2 and extends from somewhat inside the inlet 
diffuser section 36 to the outlet tube 38. This body 42 assures that the 
truck engine exhaust flow is distributed along the inner surface of heat 
transfer support structure 2 Both the inlet end 46 and the outlet end 48 
of the body 42 are tapered so they aid in recovering the pressure that 
would otherwise be lost both in the exhaust gas expansion from the outlet 
of the turbocharger turbine and from the outlet of the thermoelectric 
generator support structure to the exhaust piping. I recommend a 3 inch 
diameter at the inlet and a 3.4 inch diameter at the outlet. The hollow 
substantially cylindrical body 42 is held in place by inlet and outlet 
spider supports 50 and 52. Inlet spider support 50 is welded into the 
inlet diffuser section and outlet spider support 52 is located in the end 
of the outlet tubing and is held in place by a screw as shown at 54. The 
cylinder is assembled in place by passing it through the support structure 
2 from the outlet end and screwing it into the threads located in the 
inlet spider support 50. Outlet spider support 52 support is the slipped 
into place so that an axially located slotted dowel 56 in the outlet end 
of cylindrical body 42 passes through the hole in the center of the outlet 
spider support 52. The slot in the dowel in aligned with a radial hole in 
the hub of the outlet spider support 52, and a threaded radial hole in the 
outer rim of the outlet spider is aligned with a radial hole in the 
generator outlet flange. A cotter pin 58 is then placed through both the 
radial holes in the hub of the outlet spider support 52 and the slot in 
the dowel on the outlet end of the cylindrical body 42. A screw 60 is then 
placed radially through the radial hole in the outlet flange and is 
screwed into the threads in the rim of outlet spider support 52. This 
arrangement prevents the hollow cylindrical body 42 from rotating to keep 
it from becoming detached from the screw thread in the inlet spider while 
allowing the hollow cylinder to expand and contract freely relative to the 
support structure as will occur when the temperature of the system 
changes. The slot in the dowel 56 is made long enough to permit about 0.3 
inch expansion of cylindrical body 42. 
Electrical Circuit Arrangement 
Thermoelectric modules 4 are connected electrically in a series-parallel 
arrangement as shown in FIG. 3 to provide the desired output voltage. Each 
module produces a voltage potential of about 1.55 Volts. For a 14 V 
system, the nine modules in each of the eight rows are wired electrically 
in series, providing a nominal 14 V DC. Adjacent rows of modules are 
arranged with alternating positive and negative polarities. The rows are 
wired in parallel as shown in FIG. 3. The resulting parallel redundancy 
increases system reliability. The system reliability is further increased 
by the cross connections between the center points of the pairs of rows of 
modules as shown at the bottom of FIG. 3. This cross connection 
arrangement results in the loss of only one-eight of the generator power 
should an open failure occur in any one thermoelectric module within the 
generator. (For a 28 Volt system, all rows of the modules are connected to 
the positive and negative busses to provide a nominal 28 V DC at high 
current.) The two electric leads 64 and 66 which result from the above 
wiring scheme are connected to insulated electric terminals 68 located at 
the outlet end of the support structure 2. A diode circuit is placed in 
the electric circuit to prevent reverse flow of current from the battery 
to the generator when the generator is not producing sufficient power. A 
voltage regulator circuit prevents overcharging the trucks battery. 
Cooling Circuit 
In this embodiment cooling fluid from the truck's regular cooling system 
cools the heat sinks. The hydraulic arrangement is shown in FIG. 4. The 
sinks are connected as four parallel systems of two heat sinks each. Each 
pair of heat sinks is in turn connected in parallel with another pair to 
provide two inlet and two outlet coolant connections. This arrangement 
maintains the coolant velocities required for good heat transfer within 
each of the sinks without excessive temperature rise or excessive fluid 
pressure drop. 
Conclusion 
A cylindrical sheet steel cover 30 is bolted to flanges on support 2 as 
shown in FIG. 1. A prototype unit fabricated by my fellow workers and me 
have demonstrated the performance of a pre-prototype of the unit described 
above. The pre-prototype unit produced an output of about 400 watts. 
Testing of this unit has indicated the importance of maintaining good 
mixing of the turbulent flow of exhaust gas through the generator. As 
shown in FIG. 1, the fins 14 are preferable not continuous but broken at 
intervals to force eddies in the flow to assure additional mixing of the 
gas to minimize stratification of cooler gasses at the outer surface of 
the heat transfer support structure. I have calculated that the output of 
the unit described above with a better fin arrangement than the 
pre-prototype unit will produce about 1000 watts. 
While the above description contains many specificities, the reader should 
not construe these as limitations on the scope of the invention, but 
merely as exemplifications of preferred embodiments thereof. Those skilled 
in the art will envision many other possible variations are within its 
scope. For example, the principals of this invention can be applied to 
motor vehicles other than Diesel engines. In addition, the generator could 
be placed immediately ahead of the vertically oriented muffler, moving the 
muffler forward by the length of the generator. This location may be 
preferable when the generator is a retrofit. In this case the exhaust pipe 
ahead of the generator should be insulated. Persons skilled in the art 
will recognize that the water cooled heat sink could be replaced with an 
air cooled heat sink. The heat transfer support structure can also be 
fabricated from various materials having good heat transfer qualities such 
as a carbon-carbon compound or a creep resistant aluminum such as a metal 
matrix of aluminum and silicon carbide either in fibrous or particulate 
form. The structure can be fabricated from flat plates rather than cast. 
Body 42 could be cylindrical rather than substantially cylindrical in 
order to same in fabrication costs. Accordingly the reader is requested to 
determine the scope of the invention by the appended claims and their 
legal equivalents, and not by the examples which have been given.