Icy Hot Electricity: The Thermoelectric Effect

Abstract

Objective

In this science fair project, you will explore the thermoelectric effect. Investigate how temperature changes can be used to create an electric voltage, and how an applied voltage can be used to cool or heat an object.

Credits

David B. Whyte, PhD, Science Buddies

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Introduction

The thermoelectric effect is the direct conversion of temperature differences to electric voltage, and vice versa. The basic concept behind thermoelectric technology is the Peltier effect—a phenomenon first discovered in the early nineteenth century. The Peltier effect occurs whenever electrical current flows through two dissimilar conductors; depending on the direction of current flow, the junction of the two conductors will either absorb or release heat. The Peltier effect is used to rapidly cool and heat a block of metal in polymerase chain reaction machines, to cool central processing units in computers, and in many other applications.

Figure 1. This figure shows the working parts of a thermoelectric cooler: a fan, two heat sinks (aluminum blocks/fins), a 12-V plug that connects to a power source, a switch, and the Peltier "sandwich" (inset). The Peltier "sandwich" is located between the two heat sinks. When the switch is set to "Hot," the Peltier sandwich warms the top metal block and cools the lower one. When the voltage is reversed, the Peltier sandwich cools the top metal block and warms the lower one. The black tape helps the infrared thermometer take accurate readings of the metal's temperature. In a thermoelectric cooler, the top block is attached to a metal plate in the storage space to cool or warm food.

Most modern Peltier coolers use solid-state components consisting of n-type semiconductors and p-type semiconductors. Figure 2 shows how a Peltier device can be used as a cooler when a voltage is applied (panel A) and as a voltage source when a temperature gradient is applied (panel B). The websites listed in the Bibliography have good detailed explanations of the thermoelectric effect and its use in consumer products.

Figure 2. The thermoelectric effect is reversible. In panel A, a voltage is applied across the Peltier cooler, which results in one surface cooling down and the other surface heating up. In panel B, a heat source applied at the top of the Peltier cooler results in the production of a voltage. (Wikipedia, 2008).

In this science fair project, you will explore how a voltage can be used to cool or heat an object, using a thermoelectric cooler. Also called Peltier coolers, these devices can serve as solid-state heaters or coolers, depending on the direction of the applied voltage. Since heaters can be made using simpler technologies, Peltier devices are most often used as coolers rather than as heaters in consumer products. Their applications are best for small cooling needs. Because of their low efficiency, they cannot replace the standard kitchen refrigerator.

Thermoelectric coolers are available at many retail stores. They are designed for high portability and plug into the 12-V DC power socket found in most automobiles. They have different designs, depending on the manufacturer, but share certain features: 1) a thin plate that contains the electronic components; when a voltage is applied, one side gets hot and the other side gets cold; 2) metal heat sinks that transfer heat or cold away from the Peltier cooler; 3) a fan to carry away heat from the heat sink; and 4) a power cord to connect the unit to the 12-V power source.

You will be able to perform this experiment without removing the Peltier device from the cooler if you use the cooler listed in the Materials and Equipment section. By removing a single plastic plate, you will have access to both surfaces of the Peltier cooler. You will determine how the temperatures of the hot and cold surfaces vary with time, using an infrared thermometer. You will also measure the voltage created by a temperature gradient across the device. To measure the voltage, you will simply attach the leads from a digital multimeter to the positive and negative terminals on the plug.

Terms and Concepts

• Thermoelectric effect
• Voltage
• Peltier effect
• Conductor
• Polymerase chain reaction machine
• Central processing unit
• Solid-state
• n-type semiconductors
• p-type semiconductors
• Heat sink

Questions

• In what year was the Peltier effect first described?
• What are some commercial or scientific devices that use the Peltier effect, other than the ones already listed?
• What factors limit the size of the temperature gradient in a Peltier device?
• In a Peltier device using semiconductors, how does the flow of charge carriers (electrons or holes) compare to the flow of heat energy?

Bibliography

• Tellurex Corporation. (2008). Thermoelectrics and the 12 Most Frequently Asked Questions About Thermoelectric Cooling. Retrieved November 26, 2008, from http://www.tellurex.com/12most.html#top
• Godfrey, S., Melcor Corporation. (2008). Thermoelectric Coolers. Retrieved November 26, 2008, from http://www.electronics-cooling.com/articles/1996/sep/sep96_04.php
• Winder, E.J., et al. "Thermoelectric Devices: Solid-State Refrigerators and Electrical Generators in the Classroom." Journal of Chemical Education, Vol. 73, No. 10, p. 946. Retrieved November 26, 2008, from http://www.sciencebuddies.com/science-fair-projects/project_ideas/Phys_p070.pdf

Materials and Equipment

This science fair project is based on use of a thermoelectric device with a power source. The easiest way to work with such a device is to purchase a thermoelectric cooler that has easy access to both the hot and cold surfaces, such as the Black and Decker cooler listed below. Other coolers are available, but tend to be more expensive.

• Thermoelectric cooler, such as the Roadpro 7 Liter 12V Cooler, available at Amazon.com
• Infrared thermometer, for example the Mastercool Infrared Thermometer with Laser, available at Amazon.com
• Stopwatch
• Black electrical tape
• Digital multimeter, available at Amazon.com
• Power supply, 12-V, 3-amp
• Ice
• Insulated test/jumper leads, 24-inch, RadioShack Catalog # 278-1157
• Helpers (1-2)
• Lab notebook
• Graph paper

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Experimental Procedure

Section 1: Testing the Thermoelectric Cooler

How cold and hot can the device get? In this first section, you will test the thermoelectric cooler and get some practice using the infrared (IR) thermometer. The goal here is to determine the device's range of temperatures, and the rate at which the electrical power supplied by the 12-V battery is converted into a temperature change. In this section, you will measure the temperature of the metal plate in the food storage area. Later, you will remove an access panel so that you can measure the temperature on the other side of the Peltier device.

1. Open the top of the thermoelectric cooler so that you can see the metal plate inside the food storage area.
   1. This plate is attached to a metal block that is in contact with one side of the Peltier device.
2. Start the stopwatch.
3. Measure the starting temperature of the metal plate inside the storage area, using the infrared thermometer.
   1. Note: Infrared thermometers may have trouble getting accurate readings from shiny metal surfaces. Cover part of the metal surface with a piece of black electrical tape and use this area to measure the temperature.
4. Record the starting temperature of the metal plate in the storage area, as well as the time, in your lab notebook.
5. Set the power switch on the thermoelectric cooler to "off."
6. Plug the cooler into the 12-V power supply.
7. Move the Hot/Cold switch to "Cold."
8. Measure the time and temperature every minute until the temperature remains stable for two readings.
9. Turn off the power and let the metal plate warm up to room temperature.
10. Before performing the next steps, measure the temperature of the metal plate.
11. Record the time in your lab notebook.
12. Move the Hot/Cold switch to "Hot."
13. Measure the temperature of the metal plate every minute until the temperature remains stable for two readings.
14. Turn off the power.
15. Graph your results, with Time on the x-axis and Temperature on the y-axis. Note on the graph that the applied voltage was 12 V.
16. Repeat steps 1–15 at least two more times and average your results.

Section 2: Measuring the Temperature Gradient and the Voltage

What is the actual temperature difference between the hot and cold sides of the Peltier device, and how does the size of this temperature difference relate to the magnitude of the voltage created by the Peltier device? In this section, you will compare the temperature of the two sides of the thermoelectric cooler, and graph the temperature difference vs. the voltage.

In the previous section, the applied voltage created a temperature gradient. In this section, you reverse the process and measure the voltage produced by the temperature gradient across the Peltier device. When you unplug the thermoelectric cooler from the power source, the two leads on the plug are attached to the two sides of the Peltier device (the thermoelectric cooler). The difference in temperature between the two sides creates a voltage difference, which you can measure by attaching the leads of the multimeter to the two leads on the plug. One lead on the plug is the circular knob on the tip of the plug. When it is time to measure the voltage, attach the red lead from the multimeter to this knob. The other lead on the plug is connected to the metal strips on the sides of the plug. Attach the black lead from the multimeter to either of these. If you'd like more information about how to measure voltage, visit the Science Buddies page Using a Multimeter.

1. Turn the cooler off.
2. Unplug the cooler.
3. Remove the semi-circular piece of plastic from the bottom of the thermoelectric cooler.
4. Note the aluminum "fins" from the heat sink inside the unit.
5. Cover an accessible part of the internal heat sink with a piece of black electrical tape. Use this region to measure the temperature.
6. Plug the cooler into the 12-V socket.
7. Start the stopwatch.
8. Record the starting temperature of the external metal plate in the storage area, and the metal plate inside the unit, in your lab notebook.
   1. This is a little tricky because the access panel to the heat sink on the inside of the unit is on the bottom. Try placing the cooler on the edge of a table so that the opening that allows access to the internal heat sink is hanging over an edge. You might have one helper read the temperature from the internal heat sink and another helper record data.
9. Record the time in your lab notebook.
10. Turn the "Cold/Hot" switch to "Cold."
11. Measure the temperature of the cold metal plate in the food storage area every minute.
12. Measure the temperature of the hot metal plate inside the access panel area every minute.
13. Keep measuring until the temperatures stabilize.
14. Note the maximum difference in temperatures between the two sides.
    1. This is the temperature difference the unit produces when there is a voltage of 12 V.
15. Turn on the multimeter and set it to read "DC volts." Attach the alligator clips to the multimeter leads.
16. Turn the cooler off.
17. Unplug the cooler.
18. Attach the leads from the multimeter to the plug.
19. Attach the red lead from the multimeter to the knob on the end of the plug.
20. Attach the black lead from the multimeter to one of the leads on the side of the plug.
21. Record the voltage every minute for 10 minutes.
22. Record the temperatures of the hot and cold metal plates every minute for 10 minutes.
23. Repeat steps 7–22 at least two more times.
24. Graph the voltage, the temperature of the cold metal plate, and the temperature of the hot metal plate vs. time.
25. Subtract the temperature of the cold plate from the temperature of the hot plate to get the temperature difference at each point.
26. Repeat the procedure with the "Hot/Cold" switch set to "Hot". The metal plate inside the unit will now get cold as the plate in the food storage area gets hot.
27. Repeat the procedures above for a total of three trials.
28. Graph the temperatures of the two plates vs. time.
29. Graph the temperature difference vs. time
30. Graph the voltage of the unit vs. the difference in temperature between the two sides.
    1. Include the value you obtained when the unit was still plugged in (12 V).

Section 3: Generating Voltage with Ice

Can you generate a voltage by creating a temperature gradient using ice? Think about it: is it really possible to generate useful energy in the form of a voltage difference just by adding ice? In this section, you will measure the voltage at the plug and you will not plug the unit into the 12-V socket.

1. Unplug the cooler.
2. Allow the cooler to come to room temperature if it is not already there.
3. Turn on the multimeter and set it to measure DC voltage.
4. Attach the leads from the multimeter to the positive and negative leads on the plug with alligator clips.
5. Start the stopwatch.
6. Record the starting temperature of the two plates.
7. Record the time.
8. Pour ice water into the storage area so that it covers the metal plate inside.
   1. The temperature of the external heat sink will now be a constant 0°C.
9. Measure the temperature of the internal heat sink every minute for 10 minutes.
10. Measure the voltage across the plug leads every minute for 10 minutes.
11. Graph the temperature of the internal heat sink vs. time.
    1. Note that the fan is not running, unlike in previous sections.
12. Graph the voltage vs. time.
13. Graph the temperature difference vs. voltage.

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Variations

• Try using a larger thermoelectric cooler.
• Repeat section 3 of the Experimental Procedure, but heat the internal metal plate with a hair dryer.

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Ask an Expert

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Electricity & Electronics Project Ideas
• My Favorites
• Using a Multimeter

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