Thermoelectric generators

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Thermoelectric generatorsAdolf Cortel

IES Pompeu Fabra, Duran i Canameras 3, 08760 Martorell, Barcelona, Spain

E-mail: acortel@xtec.net

AbstractNowadays thermoelectric generators have only a few uses. Some are easy tobuild from simple materials or commercially available Peltier cells, and theycan be used for demonstration purposes. Experimenting with these lowefficiency devices gives opportunities for class discussions on energyconservation.

IntroductionIn 1821, Thomas Johann Seebeck found that theneedle of a compass was deflected when it wasplaced near a loop made of two different metalsand one of the two junctions was heated. Thedeflection was proportional to the temperaturedifference and depended on the metals used [1].

This resulted in the design of thermoelectricgenerators (thermopiles) comprising a large num-ber of junctions. Since a couple of junctionsgives only some tens of millivolts, many unionsconnected in series, alternately heated and cooled(usually with radiating fins), are necessary to pro-duce a few volts [2].

George Simon Ohm discovered his well-known law around 1825 using this type ofgenerator connected to circuits with wires whichhe had manufactured [3]. In 1834, Jean Peltierproved the opposite effect: the current flowingthrough these junctions gives rise to the absorptionor liberation of heat depending on the direction ofthe current.

Later, semiconductors based on more efficientnon-metallic materials, such as bismuth or leadtelluride, were developed. Even with thesematerials, the conversion efficiency is low (about5% was reached in the 1950s), much below thatof the best photovoltaic cells. This limits theuse of thermoelectric generators for supplyingpower, and the main application of Seebeck’sdiscovery is temperature measurement from the

voltage produced when a junction of two metalsis heated (a thermocouple) [4].

In water-heaters there is a thermocoupleacting over the electrovalve that controls the flowof gas (figure 1); in the past these generators wereused as power supplies for radios, using the heatof a kerosene lamp or a stove [5]; nowadays, theyare used in isolated farms of Northern Swedenon stoves, as a supply of a DC–DC convertercircuit which charges a battery [6]. Thermoelectricgenerators, using the heating from radioactivedecay [7], are the power sources for space probes,such as Pioneer 10 and Pioneer 11, which havebeen sent to the outer planets in the solar system,where photovoltaic cells would not be viable.

Nowadays, Peltier cells are commerciallyavailable devices used mainly in portable coolersfor cars. They are made up of a large number ofjunctions in series, so that a temperature differencehigher than 40 ◦C appears between the two facesof the cell when a current flows. A Peltier cellcan work as a thermoelectric generator simply bycooling one of the faces and heating the other.

Several experiments and demonstrations havebeen described using these generators. A simplethermocouple made from wires of different metalsand a voltmeter allows checking that a voltageis generated when a junction is heated [8]. Acompass can be used to detect the current ina thermoelectric generator made with a loop oftwo metals with two junctions [9], and effective

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Thermoelectric generators

Figure 1. Thermocouple acting on an electrovalve,dismantled from an old water-heater.

Figure 2. Thermomagnet built with copper tubing anda screw. Two cupronickel coins have to be held on theflattened part of the tube using bulldog clips.

demonstrations can be carried out with powerfulthermomagnets [10]. The purpose of this article isto describe how to build thermoelectric generatorsor to use commercially available Peltier cells tosupply electric power.

A thermomagnet with copper tubing andcoinsA weak but effective electromagnet running ona simple thermoelectric generator (thermomagnet)can be built using a piece of copper tubing and acouple of coins of nickel or cupronickel1. Eventhough the voltage obtained with only a couple of

1 The 25 cent US (cupronickel, 8.33% Ni) and the 25 centCanada (pure nickel) coins work very well. In the euro zone,the 2 euro coin, which has a cupronickel (25% Ni) outerring, also works well. Many countries have some of theirpresent or old coins made of cupronickel. As the voltage ofa thermoelectric generator is proportional to the temperaturedifference between the junctions, the bigger the coin, the better.

Figure 3. The magnetic poles of the needle of acompass are alternatively attracted when one arm or theother of the thermomagnet is heated.

junctions is low, the low electrical resistance of thecopper tubing allows a relatively strong current.

A piece of copper tubing (about 45 cm long,6 mm in diameter and 1 mm thick, of the type usedby plumbers), is coiled (5 turns) around a thickscrew. The straight ends of the tubing are bentso that they are parallel and about 2 cm apart; thetube is then flattened with a hammer. Two coinsare held on either side of the flattened part of thetube with bulldog clips (figures 2 and 3).

The screw, lined with paper to insulate it fromthe copper tubing, is used as a core inside thecoiled tubing. Since the resistivity of iron is fivetimes that of copper, lining the iron core withpaper is not strictly necessary. The unlined corewould act as a resistance in parallel with the lowresistance of the copper coil. Isolating the coreincreases the efficiency of the thermomagnet by asmall amount.

The coil is held by a clamp and a compass isplaced in front of the screw. As one of the copperarms is heated with a flame, the needle is quicklyattracted towards the screw. When the other arm

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Figure 4. A paperclip is held when one arm of thethermomagnet is strongly heated.

Figure 5. Thermomagnet built with an Alpaca fork andthick copper wire.

is heated the needle on the compass swings 180◦,proving the change of the direction of the current.When one of the junctions is strongly heated thescrew is able to hold a paper clip (figure 4).

As shown in figure 5, similar thermomagnetscan be built using different parts, such as an Alpacafork and thick copper wire2.

Motor working on thermocouples in seriesA small motor can work with 60 Fe–Cu junctions(30 couples) in series, heated in a flame [11]. Anefficient motor, such as a MABUCHI RF 300C-14 2703, turns using only 5 couples (10 junctions)

2 Alpaca (actually Alpacca) is a tradename for German silveror nickel silver. It contains copper, nickel and zinc. There aredifferent formulations; a representative one is 65% Cu, 18% Ni,17% Zn. It was widely used in flatware and cutlery. See alsopages.zoom.co.uk/leveridge/nickel1.html.3 This motor needs only around 170 mV and 17 mA to startturning. It can be bought from several suppliers, but it is veryeasy to remove a similar motor from an old CD player, since itis often used to move the optical set along its guide.

Figure 6. Motor running on the power of athermoelectric generator made with five couples ofCu–constantan junctions.

of Cu–constantan4 in series, heated strongly in aflame. When red-hot, a couple gives a maximumvoltage of about 40 mV.

The hot junctions can be made by twisting theends of 10 cm long wires or, for a more permanentset-up, soldering them together5. The wires of thecool junctions are inserted in terminal blocks andsecured with screws as shown in figure 6.

Powerful thermomagnetsPowerful thermomagnets able to withstand theweight of a person can be built [10, 12]; however,a lathe and a gas welding torch are required6.

One of these thermomagnets is shown infigures 7 and 87: a thick copper rod has been bent

4 The experiments were made using wires of 0.75 mm ofconstantan (60% Cu and 40% Ni) and copper. Sixteen junctions(8 × 2) give a maximum voltage of 350 mV (open circuit) andan intensity of 150 mA (shorted).5 If the junction is not soldered it easily oxidizes at the flametemperature. An appropriate soldered joint (after twisting theends of the wires) can be made with CastolinTM (e.g. 1810XFC) or another high-temperature solder, but a gas weldingtorch with oxygen and butane is required.6 These thermomagnets can be bought from some suppliers,such as PHYWE (thermoelectric magnet, catalogue number06593.01).7 The cylinder of iron is 75 mm in diameter and 20 mm inheight. After making the groove, the face of the cylinderhas been heated with a gas welding torch to partially oxidizethe surface and decrease its electrical conductivity. Thecupronickel coins (Spanish coins from the 1970s and 1980s)have been soldered according to footnote 5. The pieces of ironto be held with this magnet must have a flat smooth face in orderto close the magnetic circuit. This particular thermomagnet isable to support a weight of about 600 g.

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Thermoelectric generators

Figure 7. Thermomagnet made with a single turn of athick copper rod and two soldered cupronickel coins,inserted in an iron core.

Figure 8. When heated, the magnetized core can holdan iron disc from which successive weights can behung.

to fit into a circular groove made in a cylindricalpiece of iron and two cupronickel coins have beensoldered to the arms of the rod. Prior to heating,it can be shown that washers or other flat pieces ofiron are not attracted, but when one of the armsis strongly heated the current along the copperrod is strong enough to make the iron core anelectromagnet able to hold an iron disc with ahook, from which successive weights can be hung.The other arm of the copper rod can be immersedin a cup of water or ice to increase the temperaturedifference between the junctions.

Figure 9. The heat produced after rubbing hands isenough to run a small motor using commerciallyavailable Peltier cells.

Peltier cells as thermogenerators

A commercially available Peltier cell with a largenumber of junctions can work as a thermoelectricgenerator able to give relatively high voltages evenwith a small temperature difference8,9.

Figure 9 shows how the motor turns whenconnected to two Peltier cells in series on a heatsink, using only the heat created from rubbinghands together.

8 The Peltier cells TEC1-12706T125, manufactured byBeijing Huimao Cooling Equipment (www.huimao.com/series1.htm) are particularly appropriate. There are severalsuppliers at prices from $12 to $20 each. Each cell contains127 junctions and, working as a thermogenerator, can give avoltage of around 1.2 V with a temperature difference of about70 ◦C. Other Peltier cells with a large number of junctions canalso work well. The contact of the cells and the heat sink mustbe secured using a heat-conducting grease.9 A commercial device can be bought from PASCO(thermoelectric converter, catalogue number TD-8550A)or Sargent-Welch (thermoelectric demonstrator, cataloguenumber CP32729-00).

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Figure 10. Heating a piece of aluminium on the upperface of two Peltier cells in series, and cooling the otherface with a heat sink in a tray of water, gives sufficientpower to supply a portable radio.

Radio PeltierThe Peltier cells and the heat sink used in theprevious experiment are put in a plastic tray withwater or ice. A rectangular piece of aluminiumplaced on the cells is heated as shown in figure 10.A voltmeter connected to the cells shows thepolarity and the value of the voltage, so thatthey can be connected correctly to the batterycompartment of a portable radio.

Using the Peltier cells given in footnote 8,a voltage of around 2.4 V is obtained when thealuminium reaches about 100 ◦C, and the radiostarts working10 (with water at 20 ◦C in the tray).When the heating is stopped the radio works for awhile until the aluminium cools.Received 13 June 2006, in final form 1 August 2006doi:10.1088/0031-9120/42/1/012

References[1] www.thermoelectrics.com/introduction.htm

chem.ch.huji.ac.il/∼eugeniik/history/seebeck.html

10 The radio is a conventional portable model which works withfour 1.5 V batteries. It is audible from 2.4 V.

[2] physics.kenyon.edu/EarlyApparatus/Thermodynamics/Thermoelectric Battery/Thermoelectric Battery.html

[3] Rogers E M 1960 Physics for the Inquiring Mind(Princeton, NJ: Princeton University Press)p 519

[4] www.omega.com/prodinfo/Thermocouples.htmlwww.omega.com/temperature/Z/pdf/z021-032.

pdf[5] Freier G D and Anderson F J 1981 A

Demonstration Handbook for Physics(American Association of Physics Teachers)p E-77

see also www.dself.dsl.pipex.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm

[6] www.triz-journal.com/archives/1997/01/a/index.htm

[7] en.wikipedia.org/wiki/Radioisotope thermoelectric generator

[8] Graf R F 1973 Safe and Simple ElectricalExperiments (New York: Dover) pp 81–2

[9] Greenslade T B Jr 2006 A quick thermoelectricitydemonstration Phys. Teach. 44 50–1

[10] Freier G D and Anderson F J 1981 ADemonstration Handbook for Physics(American Association of Physics Teachers)p E-76

Sutton R M 1938 Demonstration Experiments inPhysics (New York: McGraw-Hill) pp 324–5

[11] Meiners H F (ed) 1970 Physics DemonstrationExperiments vol II (New York: Ronald Press)p 901

[12] groups.physics.umn.edu/demo/old page/demo gifs/5E50 30.GIF

rigel.phys.ualberta.ca/demos/Mechanics/HTML/thermomagnet.html

www.oberlin.edu/physics/catalog/demonstrations/em/thermomagnet.html

Adolf Cortel received a PhD inchemistry from Universitat Autonoma ofBarcelona and has taught high schoolphysics and chemistry since 1981. Heenjoys developing new demonstrationsand experiments and, for this work, hehas been honoured with an EiroforumTeachers Award, as well as awards from‘Museu de la Ciencia de Barcelona’ and‘Real Sociedad Espanola de Fısica’.

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