Thermoelectric Effects 1 Running head: THERMOELECTRIC EFFECTS A Study of the Seebeck and Peltier Thermoelectric Effects Alan Pnakovich American Heritage School, Plantation, Florida In partial fulfillment of the requirements for Third Period Honors Science Mrs. Page December 1, 2009
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A Study of the Seebeck and Peltier Thermoelectric Effects
Science Fair Project that won First Place in the Intel-ISEF Science & Engineering Fair in Physics & Astronomy at the Florida State Junior Level in April 2010.
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Thermoelectric Effects 1
Running head: THERMOELECTRIC EFFECTS
A Study of the Seebeck and Peltier Thermoelectric Effects
Alan Pnakovich
American Heritage School, Plantation, Florida
In partial fulfillment of the requirements for Third Period Honors Science
Mrs. Page
December 1, 2009
Thermoelectric Effects 2
This embossed seal attests that this project is in compliance with all federal and state laws and regulations and that all appropriate reviews and approvals have been obtained including the final clearance by the SSEF/FFFS Scientific Review Committee.
55th State Science & Engineering Fair of Florida OFFICIAL ABSTRACT and CERTIFICATION
A Study of the Seebeck and Peltier Thermoelectric Effects Alan Pnakovich American Heritage School, Plantation, Florida, Broward County, USA
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This experiment explored two main principles of thermoelectricity, the Seebeck and Peltier effects. The Seebeck Effect generates electricity from temperature differences between two junctions of different metals or semiconductors. The reverse, Peltier Effect, creates temperature differences by applying electricity to junctions of different metals or semiconductors. Eight hypotheses were tested varying the types and thicknesses of metal wire, amount of electricity and temperature difference applied, and exploring how modern semiconductor devices could amplify Seebeck and Peltier effects.
The Seebeck Effect was investigated by applying temperature differences to both metal thermocouple wire and semiconductor-based “Peltier modules.” As hypothesized, these semiconductor devices generated several orders of magnitude more voltage than metal wire. Tests with thermocouple wires proved hypotheses regarding proportionality of voltage generated to temperature difference and Seebeck coefficients of the metals, and the lack of effect of wire thickness.
The Peltier Effect was investigated by subjecting Peltier modules to different current levels, which proved hypotheses regarding their cooling capability using measurements from infrared and thermocouple thermometers. A DC power supply and multimeter with data logging capabilities were used in most tests.
Tests with the Peltier modules confirmed their effectiveness in capturing waste heat and converting it into electricity, an exciting innovation that auto companies are exploring to convert exhaust heat into electricity to increase gas mileage. Thermoelectricity is now becoming a direct competitor to solar as an alternate energy source. Peltier modules are also an effective means of cooling computer CPUs, not possible with compressive cooling in such limited space.
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Thermoelectric Effects 3
INTRODUCTION
Statement of Purpose
The purpose of this project is to investigate “thermoelectricity” using the Seebeck and
Peltier Effects to observe the impact of heat and electricity on metals and semiconductors, where
temperature differences can be converted directly into electricity, or electricity into a temperature
difference, with no moving parts or use of other machines such as compressors or generators.
Depending on which effect is being tested, temperature difference and amount of electricity (i.e.
voltage and current) could be either a control (independent variable) or a variable (dependent
variable), in addition to other independent variables such as type of metal wire used, the
thickness of the wire, the type of Peltier device being tested, and how long the test runs.
Background Research
In everyday life, devices like toasters, ovens, hairdryers, and home heaters are commonly
used to convert electricity into heat. Although most people probably do not know this, you can
convert heat directly into electricity with no moving parts or use of other machines such as
generators. Another little known fact is that electricity can also produce cooling without any
moving parts or machinery required. There are many metals and other materials that can do this,
among them semiconductors have the most dramatic effects.
The two facts above are referred to as thermoelectric effects. If a closed circuit is made
of two different metals and one junction is held at a higher temperature than the other, a current
will flow as long as the difference of temperature is kept. This is known as the “Seebeck effect.”
The Peltier effect is the inverse of the Seebeck effect. If a current is sent through the junction of
two different metals, heat is absorbed by the junction when the current flows in one direction,
and is emitted by the junction when the current is reversed (Eshbach, 1952).
Thermoelectric Effects 4
Tomas Seebeck discovered thermomagnetism in 1821. While studying the effects of heat
on certain metals, he took two different metals and connected them in a loop and applied heat to
one end, which he found could repel a compass needle. Seebeck himself did not fully understand
the cause of this event. He assumed that a magnetic field was created and called the effect
thermomagnetism. The electric current generated by the Seebeck effect is actually caused by
heat flowing from the hot metal to the colder, and also by the thermoelectric properties specific
to each metal. The thermoelectric potential of a metal is measured by what is now called the
“Seebeck Coefficient.” Though it made relatively little impact upon the scientific world for
nearly a century, the Seebeck Effect eventually became the basis upon which all future work in
thermoelectricity was built (McGrath & Travers, 2007). Figure 1 below (Salman, 2009)
illustrates how a simple Seebeck Effect circuit could be set up to measure voltage generated by
applying heat from a candle (at the left side) and cooling from ice (near the center) to the
junctions of two different metal wires (Materials 1 and 2), and connecting these to a multimeter
(at the right side). The larger the temperature difference, and the greater the difference in
Seebeck coefficients between the two metals, the larger the voltage generated.
Figure 1: Simple Circuit to Measure Seebeck Effect Voltage Generated
Jean Peltier discovered the thermoelectric effect bearing his name in 1834. Peltier found
that the junctions of different metals were heated or cooled depending on the direction an electric
Thermoelectric Effects 5
current passed through them. The “Peltier effect” is found to be proportional to the first power
of the current, not to its square, as is the permanent generation of heat caused by resistance
throughout the circuit (Duckworth, 1960). The Peltier effect was an accidental discovery made
during an experiment in which Peltier joined copper wire and bismuth wire together and then to a
battery. When he switched the battery on, one of the junctions of the two wires got hot, while
the other junction got cold. If the cold junction was put inside an insulated box, it became a low-
efficiency refrigerator. Modern students are often introduced to Peltier as a physicist, but might
be surprised to find that he didn't study physics until his retirement from the clock-making
business at age thirty. Figure 2 below (Salman, 2009) illustrates how a simple circuit could be
set up to create a temperature difference (Tc and Th) at the junctions (A and B) between two
different metals (Materials X and Y) by connecting this to a battery or DC power supply
(designated by Vin at the bottom). The larger the current supplied to the circuit, and the greater
the difference in Seebeck coefficients between the metals, the larger will be the temperature
difference created.
Figure 2: Simple Circuit to Create a Temperature Difference from the Peltier Effect
For many years after Seebeck's discovery of the thermocouple circuit, it was used as a
sensitive thermometer of incomparable accuracy and range, from a few degrees above absolute
zero to several thousand degrees Fahrenheit. Beginning in the early 20th century, scientists
Thermoelectric Effects 6
experimented to determine if more heat could generate a more powerful current, possibly
powerful enough to run machinery. For years, scientists worked to find the best combination of
alloys to maximize the output of thermocouples. After World War II, physicists experimented
with thermocouples that ran on the heat from decaying radioactive isotopes. These nuclear
thermocouples are used to power deep-space probes, devices that must run unattended for many
years and are too far from the sun to use solar panels (McGrath & Travers, 2006). With modern
techniques, thermoelectric “modules” can now be produced using semiconductors that deliver
efficient solid state heat-pumping for both cooling and heating. A practical thermoelectric
Peltier module generally consists of two or more elements of n- and p-type doped semiconductor
material that are connected electrically in series and thermally in parallel (Ferrotec, 2009). Many
of these units can be used to generate DC power in special circumstances, such as conversion of
waste heat into useful electricity. New and often elegant uses for thermoelectrics continue to be
developed each day (Tellurex, 2006).
Figure 3: Seebeck Effect with Semiconductors Figure 4: Peltier Effect with Semiconductors
Figure 3 above (Whyte, 2009) illustrates how n- and p-type semiconductors, sandwiched
between heat and cooling sources, are used to create the Seebeck effect. The heat would push
Thermoelectric Effects 7
both the negative and positive chare carriers downward, and create an electric current moving in
a clockwise direction through the semiconductors around the circuit. Figure 4 (Whyte, 2009)
shows how a DC current moving clockwise in series through the semiconductors around the
circuit would create the Peltier effect. The charge carriers in the n-type semiconductor would be
drawn downward (along with their heat) by attraction to the positive side of the DC power
supply, while the charge carriers in the p-type semiconductor would be repelled and pushed
downward (along with their heat).
Figure 5: Peltier Effect Cooling with Two Semiconductor Pairs
Figure 5 (Ferrotec, 2009) shows a more realistic view of how a Peltier cooling module is
constructed, with multiple pairs of n- and p-type semiconductors, a “heat sink” near the middle,
and the object being cooled (e.g. a CPU chip) at the top of the diagram. As in Figure 4, electric
current moves in series counterclockwise around the circuit, and all heat is “pumped” downward.
Figure 6 below (Tellurex, 2006) shows a more complete version of a Peltier module with
many pairs of n- and p-type semiconductors, sandwiched between ceramic substrates with the
positive and negative connecting wires shown at the right side of the diagram.
Thermoelectric Effects 8
Figure 6: Realistic Diagram of Peltier Cooling Module with Many Semiconductor Pairs
Figure 7: Actual Peltier Module with 127 Semiconductor Pairs Opened Up
Figure 7 above (Noll, 2008) shows the insides of an actual Peltier Module with 127 n-
and p-type semiconductor pairs, which is a typical number used in industry. The module is
opened up revealing the semiconductor pellets welded to their connecting plates with their
positive and negative connecting wires on the right side, and the top connecting plates on the left
side. Both top and bottom plates are glued to the white colored ceramic substrate that holds
everything together.
Thermoelectric Effects 9
The Peltier effect has also found significant value in recent years. The main use of the
Peltier effect is as a refrigerator; thermocouples can be cooled to a temperature low enough to
liquefy nitrogen and helium (McGrath & Travers, 2006). It was only after mid-20th Century
advancements in semiconductor technology, however, that practical applications for
thermoelectric devices became realistic. New and often elegant uses for thermoelectrics
continue to be developed each day (Tellurex, 2006). A common application of Peltier modules
is in the cooling of computer CPU’s. Others include low-cost, moderate- and high-capacity,
general-purpose modules for cooling sensitive equipment such as instrumentation, laboratory
equipment, consumer appliances, and for commercial and military applications (Melcor, 2009).
Travel coolers/warmers operating Peltier modules are now produced under a variety of brand
names (e.g. Igloo, Black & Decker, and Koolatron) and marketed through well-known sources
such as Amazon and Sharper Image (Warner, 2004).
Hypotheses
1. If two junctions are made between two different conductive metals and are held at different
temperatures, then a voltage will be created in proportion to the temperature difference and
the Seebeck coefficients for the metals.
2. The amount of voltage created in the above, should be independent of the thickness of the
metal/wire used, similar to connecting batteries in parallel instead of series, and should not
decline over time after reaching a “steady state.”
3. The amount of time required to reach steady state voltage should be directly proportional to
the thickness/surface area of the metal wire.
4. If commercially available Peltier modules made from ‘p’ and ‘n’ semiconductors are used to
generate electricity through the Seebeck Effect, and to create heating and cooling from the
Thermoelectric Effects 10
Peltier Effect, then the results, in terms of voltage generated or temperature difference
created, should be many times greater than that for metal wire under the same conditions.
5. The voltage generated from #4 above should be proportional to the number of pairs of
semiconductor pellets in the Peltier module.
6. The temperature difference created from a Peltier cooling module should be proportional to
the current flowing through the circuit, if the voltage is held constant.
7. A Peltier cooling device should maintain a voltage after disconnecting the power source, and
should decline in a measurable, nonlinear way as the device approaches room temperature.
8. If two Peltier units are “stacked,” then the resulting temperature difference should be close to
the total of each unit alone.
Thermoelectric Effects 11
METHOD
Materials
Table 1: Materials Used in This Project
Item Quantity Size Mastech Variable DC Power Supply, Model HY3010E 1 30v/10a Tekpower Digital Multimeter with Data Logger, Model DT9602R 1 na Mastercool High Temp Infrared Thermometer Model 52225A-SP 1 na Digital Thermometer, Extech Model 39240, -40 °C – 200 °C 1 na
CT Fan 134 CFM, DC 12 volts, 120mm sq. x38mm, FN-120-134 1 120 mm CT Thermal Paste Compound, Ceramique Model TG-CMQ-22G 1 22 gm Laptop Computer (to log temp. vs. time with Peltier modules) 1 na DC 12 volt Power Supply for Fan, Spectrum Model 44-6681 1 12 volt Type J Thermocouple: Constantan-Iron, Nanmac # A13-5-12 1 0.51 mm. Type T Thermocouple: Constantan-Copper, Nanmac A13-14-12 1 0.51 mm Type K Thermocouple: Chromel-Alumel, Nanmac A13-20-12 1 0.51 mm Type E Thermocouple: Constantan-Chromel, Nanmac A13-27-12 1 0.25 mm. Type E Thermocouple: Constantan-Chromel, Nanmac A13-28-12 1 0.38 mm. Type E Thermocouple: Constantan-Chromel, Nanmac A13-29-12 2 0.51 mm Type E Thermocouple: Constantan-Chromel, Nanmac A13-30-12 2 0.81 mm. Soldering Gun, Weller Model 8200 N 1 140 watts Solder, Electrical Rosin Core, 60% Tin/40% Lead, 1.27 mm. Dia. 1 21 gms. Soldering Paste Flux, Alpha Fry No. 51012 1 56 gms. Dewalt Electric Drill, Model DW104 1 na Alligator Clip-End Connecting Wires 10 24 gauge Alligator Clip Manipulator Arms (for holding parts for soldering) 1 na Propane Torch, Bernzomatic, Model TX9 1 0.5 kg Dry Ice Block 1 4.5 kg. Isopropyl Alcohol (99+% pure), Iso-Heat (from auto supply) 1 375 ml. Antifreeze (Ethylene Glycol), Mobil Permazone 1 500 ml. Hot Plate or Gas Grill w/Side Burner (to heat antifreeze outside) 1 na Wire Strippers 1 na Safety Equipment (goggles, protective mask, leather gloves) 2 each na * CT above stands for the company “Custom Thermoelectric”
Thermoelectric Effects 12
Procedure
First, the Seebeck Effect will be used to study the electricity generating potential of two
different base metals connected at two junctions with each exposed to different temperatures.
This will test hypotheses 1-3 above. The independent variables will be four temperature
differences created by hot and cold liquids, the types of metal that the thermocouple wires are
made from, and the thickness of the wires. The dependent variable is the voltage generated.
1) Prepare the seven different thermocouples by cutting one of the wires in the middle and
soldering the loose ends to form a second junction between the two metals. Thermocouples
come welded only at one end of the two 12-inch long wires.
2) Connect each of the remaining loose ends of the cut wire to an alligator clip connector and
then connect these to the positive and negative sockets of the multimeter. The thermocouples
have one of the ends colored red, which should be connected to positive.
3) Bend the uncut 12-inch long wire into a “U” shape, so that one end can be dipped into the hot
liquid and the other into the cold liquid.
4) Prepare a pot of boiling water and an insulated container of ice cubes in water. Have these
sitting side by side.
5) Perform two tests with each of the seven metal wire combinations. The first will put one
junction in the boiling water and the other junction in ice water, for close to a 100 °C
difference. The second will put one junction in ice water and leave the other junction
exposed to room temperature, for a temperature difference of around 23 °C.
6) Set the multimeter to measure in millivolts, and for each test, record the voltages reached
once the reading on the multimeter has stabilized. For each, check and record the
temperature of the ice water and the boiling water as well.
Thermoelectric Effects 13
7) For the type ‘E’ thermocouples, of which there are four different thicknesses, connect the
multimeter to a computer using a data logging device, measure the times taken to reach the
final “steady-state” voltage with one junction in the ice bath and the other in the boiling
water, which will test hypothesis #3 above.
8) Select one of the seven samples and hold it in the ice bath and boiling water for an extended
period, recording voltage using a data logger to measure if there is any decline in voltage
over time, which tests part of hypothesis #2.
9) Move outside and prepare an extremely cold ice bath using chunks of dry ice mixed with Iso-
Heat (near 100% isopropyl alcohol) added to almost fill an insulated container. This mixture
will reach about -64 °C after it has stopped “boiling” from the sublimation of the dry ice.
10) Prepare an extremely hot liquid bath by heating a pot of antifreeze on a hot plate or BBQ grill
side burner. Stay on the “up-wind” side of the burner and wear safety masks (to prevent
inhaling any vapor) and safety glasses and gloves, since this liquid will reach about 190 °C.
The adult sponsor should be the only one handling this pot of hot antifreeze.
11) Perform two tests with each of the seven metal wire combinations. The first will put one
junction in the hot antifreeze water and the other junction in dry ice bath, for about a 250 °C
difference. The second will put one junction in the antifreeze and leave the other junction
exposed to outdoor temperature, for a temperature difference of around 160 °C.
12) For each test, record the voltages reached once the reading on the multimeter has stabilized.
Also for each, check and record the temperatures of the hot and cold liquid baths using the
immersible thermocouple temperature wire.
In the next part of the experiment, four different commercial Peltier modules will be used
to test their maximum potential to generate electricity when one side of each device is exposed to
Thermoelectric Effects 14
very hot temperature and the other side to a very cold temperature. A propane torch to heat the
heat sink on one side, and dry ice to chill the other side, will be used to create the maximum
temperature differences. These will test hypotheses four and five above.
13) Use metal polish on the surface of the heat sink to make it as smooth as possible to increase
thermal conductivity. When complete, it should look close to a mirror-like surface.
14) Prepare the heat sink to measure its temperature just below the surface where the Peltier
modules will be placed, by drilling a 5/32 inch hole parallel to the surface into which a digital
thermometer will be inserted.
15) Suspend the heat sink over the edge of a countertop by attaching the heat sink to the side of a
large, heavy, metal, rectangular cooking pan. This can be done easily if the gap between the
heat sink “fins” is a little larger than the thickness of the cooking pan. The last gap in the
heat sink fins can be slipped over the edge of the pan, and hang out like a cantilever beam.
16) With some “thermal paste” covering the tip of the digital thermometer, insert it into the hole
in the heat sink. The tip should be located around the middle of the heat sink surface.
17) Cover the bottom (hot side) of a Peltier module with a very thin layer of thermal paste.
18) Secure the Peltier module to the center top of the reheat sink (directly over the end of the
digital thermometer) by pressing and rotating it on the heat sink until there is a complete
layer of thermal paste between the Peltier module and the heat sink, and the module feels as
if it is almost glued to the heat sink.
19) If available, attach a very thin thermocouple wire to the top of the Peltier module, to attempt
to measure the temperature difference between the two sides of the Peltiers. This may not be
possible since the surface contact should be as smooth and complete as possible.
Thermoelectric Effects 15
20) Have a piece of dry ice nearby between several towels for easy access. The dry ice should
have sides that are as flat as possible, as it will be pressed to the flat surface of the Peltiers.
21) Using the propane torch, heat the heat sink from underneath until it reaches a temperature of
110-115 °C. Do not go above this temperature, as the Peltier devices have a maximum
operating temperature of 125 °C, after which the solder connecting and holding the pellets
will melt and the device will be completely destroyed.
22) Wearing leather gloves, pick up the piece of dry ice as quickly as possible and press it firmly
to the top side of the Peltier device, while an assistant is carefully watching the voltage meter
(or it is connected to a data logger).
23) As soon as the dry ice is applied, the Peltier module will vibrate vigorously, making an
almost squeaking noise as the ice vaporizes, going straight from a solid state to a gas through
sublimation. It is the formation of the gas between the ice and the flat Peltier surface that
causes the noise and vibration. This should be done carefully and only once if possible, as
the harsh vibration will eventually damage the Peltier.
24) Connect the data logging device from the multimeter to the RS232 port of the computer to
record voltage readings. Remove the dry ice once the researcher has noted on the multimeter
that the maximum voltage has been reached. Make sure that the scale of the multimeter is set
to measure volts, since the larger Peltier modules (with 127 “pellet pairs”) should produce as
much as five or more volts. A millivolt setting is required for the thermocouples, since they
will only produce a few millivolts, even with a much larger temperature difference.
25) Repeat the procedure for each of the four Peltier modules.
In the final part of the experiment, the Peltier Effect is used to study the reverse of the
above, with electricity applied to Peltier modules to generate a temperature difference between
Thermoelectric Effects 16
one side of a module and the other. This will test hypotheses 6-8. The independent variables
will be the four different types of Peltier modules, and the amounts of voltage and current
applied to them. The dependent variable will be the temperature difference created.
26) Connect the fan to the bottom of the heat sink, so that the fan blows air up through the vanes
of the heat sink. The connection can be made simply using duct tape around the outside
edges near the lower ends of the heat sink fins, since this will not become hot like it might at
the surface on which the Peltier modules will be attached.
27) The fan will then need to be mounted to some structure that keeps it elevated several inches
above countertop level, with its intake side having access to free air flow (otherwise it would
not be able to suck in fresh cool air to blow on the heat sink). A simple way to do this is by
taking four extra thick books, and stacking two on each side of the bottom of the fan intake.
Only about a quarter inch of the fan base would need to overlap the edge of the books, and
this could be taped to the books using duct tape.
28) As previously done, the 5/32 inch diameter digital thermometer should be inserted, with
some thermal paste smeared on its tip, into the hole drilled near the surface of the heat sink.
29) Each Peltier module will then be tested in the same manner as follows.
30) First, a thin layer of thermal paste will be spread on the hot side (side to which the leads are
connected) of the Peltier, and it should then be pressed firmly onto the center of the heat sink
(directly above the digital thermometer) while rotating back and forth, until the film of paste
has been evenly distributed around all edges, and it almost feels like it has been glued down.
31) One test method will be to use a flat piece of Styrofoam, about one inch thick, to place on top
of the cold side of the Peltier module, on which about ten pounds in weights (e.g. books)
should be placed to create a firm contact between the Peltier and the heat sink.
Thermoelectric Effects 17
32) A thin wire thermocouple connected from the side of the infrared thermometer should be
inserted between the Styrofoam and the top of the Peltier module.
33) The second method will be to allow the top of the Peltier module to remain open to room
temperature air, to test if more cooling is achieved. Using this method, the infrared
thermometer would be used to measure the temperature on the cold side of the Peltier.
34) Under both methods, the hot side temperature of the Peltier would be measured from the
digital thermometer inserted in the top of the heat sink.
35) For each test with the four Peltier modules, the maximum voltage would be fixed on the
variable power supply according to the specifications for each Peltier.
36) Each test with the different Peltiers would be conducted at several different current levels,
during which the hot and cold side temperatures would be recorded every ten seconds.
37) To test hypothesis seven above, voltage would be recorded with a data logger after the power
supply has been cut off from one of the Peltier modules, and this would be allowed to decline
until near zero to test graphically if the decline is nonlinear, as hypothesized.
38) To test hypothesis eight, the smallest Peltier module would be attached using thermal paste to
either the largest (40 mm sq.) or medium (30 mm sq.) size Peltier module having 127 Pairs of
semiconductor pellets. Calculations will need to be performed to determine whether to use
the large or medium Peltier in the stack.
39) The larger Peltier module would be thermal pasted to the heat sink, and the two Peltiers
would need to be connected in electrical series.
40) A voltage and current would be applied and the overall temperature difference created with
To Use the Seebeck Effect to Generate Electricity (voltage)- One side of Paltier Module subjected to dry ice at -78 C- Other side thermal pasted to heat sink heated to 110 C
Voltage declines quicklyonce dry ice is removed
Thermoelectric Effects 27
Table 7: Test Data for 40mm/127 Pair Peltier Modules Subjected to Various Current Levels