Thermo-Electric Generators

The history of thermoelectric generation begins in 1821 when Thomas Seebeck found that an electrical current would flow in a circuit made from two dissimilar metals, with the junctions at different temperatures. This is called the Seebeck effect. Apart from power generation, it is the basis for the thermocouple, a widely used method of temperature measurement.

In 1821 Seebeck found that in a closed circuit of using two different metals, if one of the soldered junctions between the metals was heated, then a magnetic needle positioned near the circuit would be deviated, indicating a magnetic field. The deviation angle was proportional to the temperature difference between the hot and cold junctions. It is accepted today that Seebeck was not the first to discover the effect, it had been noticed by Alessandro Volta in 1794. Seebaeck was however the first to make a detailed study of the phenomenon.

Seebeck thought that the magnetic field was being generated directly by the heat. The Danish physicist Hans Oersted, who had discovered that electric currents produce magnetic fields, demonstrated that the heat caused an electric current, which in turn generated the magnetic field that displaced the needle. He named this effect thermoelecricity. It was Oersted who brought thermoelectricity to the attention of several French scientists, such as Becquerel father and son, and Fourier, Melloni, Pouillet, and Nobili.

The voltage produced is proportional to the temperature difference between the two junctions, and is usually measured in tens of microVolts per degree K. The proportionality constant is called the Seebeck coefficient, and is usually quoted as a relative coefficent compared with platinum, as it is hard to measure ansolute values.

A series-connected array of thermocouples was known as a "thermopile", by analogy with the Voltaic pile, a chemical battery with the elements stacked on top of each other. The maximum power is obtained from a thermopile when its load resistance is equal to its internal resistance, as with all electrical sources. Since the internal resistance of a chain of thermocouples is very low, this means that it can supply big currents but only low voltages, unless a large number are wired in series. Not all thermopiles were used for power generation; small versions fitted with a distinctive brass horn were used to measure infra-red radiation.

Seebeck used bismuth and copper in his original junctions, which gives a coefficient of 6.5 + 72 = 78.5 uV/K. He went on to rank 28 materials in order of effectiveness. The popular combination of bismuth and antimony gives an output of 47 + 72 = 119 uV per K, nearly twice as much.

In 1929 the Russian Abram Ioffe showed that some semiconductors have a Seebeck coefficient much higher than metals or their alloys. This eventually led to The Russian Lamp in 1959.

Oersted, working with Joseph Fourier, produced the first multiple-junction thermopiles intended to generate power. A description was published in 1823.

Regettably no illustrations of these early thermopiles have been found so far.

George Simon Ohm was probably the most famous thermopile user. In 1825 he was working on the relationship between current and voltage by connecting wires of differing resistance across a voltaic pile- pretty near short-circuiting it. After an initial surge of current rapid polarization of the pile caused the voltage to decrease steadily, greatly complicating the measurements. Ohm took a colleague's advice and replaced the voltaic pile with a thermopile, and the results were much better.

The thermopile was further developed by Leopoldo Nobili (1784-1835)and Macedonio Melloni (1798-1854). They initially used it for the measurement of temperature and infra-red radiation, but the thermopile was also rapidly put to use as a stable supply of electricity for other physics experimentation, such as Ohm's experiments.

a) Nobili prototype built ca 1829, using bismuth and antimony for the couples, giving an output of 47 + 72 = 119 uV per K. If we take red-hot as 900 degC, that gives an output of around 100 mV per couple, demonstrating why it was necessary to use a lot in series to get a useful voltage. The vertical bars linked at the upper ends can be seen. There are twelve large bismuth and antimony elements which were placed upright in a brass ring secured to an adjustable support, and and screened by a wooden disk with a 15mm central aperture. Nobili built a large number of thermopiles, ranging from 6 to 200 elements of bismuth and antimony, using to supply a steady current for his galvanmeter experiments; he called them 'thermomultipliers'.

b) Incomplete Nobili-Melloni thermopile from ca. 1831. Some accounts state that Melloni introduced the idea of using bismuth-copper as a couple, but others say that the effectiveness of antimony-bismuth had already been discovered by Nobili and Oersted. Melloni was intersted in using his thermopile for temperature measurement rather than power generation. His instruments used blocks of up to 38 bars, each 20mm long and 1 or 2 mm wide.

Both are in the Museo Galileo-Institute and Museum of the History of Science, in Florence.

This, I think, is an early thermopile. Unfortunately I have no details on it, and its operation is obscure. It is not clear where the heat is applied; perhaps one brass tank held hot water and the other cold? If so, that would be a much less effective source of heat than a gas flame.

In each tank, one of the L-shaped pipes appears to go into a glass vessel, for reasons unknown.

Example in CNAM, the Conservatoire National des Arts et Metiers, in Paris. Author's photograph.

Claude Pouillet (1790-1868) was a pioneer in the detection of infra-red radiation. He used a "pyroheliometer"- essentially a water calorimeter- to measure the intensity of solar radiation. The apparatus shown above is NOT the pyroheliometer; however it may be some sort of measuring instrument rather than a power source as such.

Pouillet used an iron-platinum couple.

The gas burners are inside the black body of the device; the spigot for the gas supply pipe is at lower left. The brass tanks hold the cooling water for the cold junctions.

An interesting feature is the sliding contact at the top, which allows the output voltage to be altered by connecting a variable number of junctions into the circuit. The output terminals are at top right.

Heinrich Daniel Ruhmkorff, electrical researcher and instrument maker, is best known for the remarkable improvements he made in the induction coil. However, it appears he was also in the thermopile business. Ruhmkorff was born on 15 January 1803, in Hanover, Germany, and died 20 December 1877, in Paris.

Example in CNAM, the Conservatoire National des Arts et Metiers, in Paris. Author's photograph.

The EMF of a single couple was quoted as "One-twentieth of a Daniel cell" which makes it about 55 milliVolts. The negative metal was a 10:6:6 alloy of copper, zinc and nickel, similar to German silver, and the positive metal was a 12:5:1 alloy of antimony, zinc and bismuth. The iron bar a-b was heated and the lower ends cooled by immersion in water. A defect of this design was a rapid increase in internal resistance as the two alloys oxidised at their point of contact.

Markus' thermopile won a prize in 1864/5 from the Vienna Society for the Promotion of Science.

From "Electricity in The Service of Man" published in its 3rd edition in 1896; the thermopile section appears to have been written much earlier, and certainly before 1888. It was originally published in Germany and was written by Dr A R Von Urbanitsky.

This was invented by M. Edmond Becquerel (1820-1891), at a date unknown. The junctions were composed of copper sulphide for one metal, and German silver for the other.
It appears that D is a trough of cooling water for the cold junctions, supplied at B and exiting at C. There appears to be another trough on the other side of the central burner E. Gas for the burner is supplied via pipe A.

Edmond Becquerel was the father of physicist Henri Becquerel, who discovered radioactivity

From "Electricity and Magnetism", 1891.

This pile, developed by Charles Clamond in association with Louis Mure, used a zinc-antimony alloy for one metal and iron as the other. It was gas-fired, and could liberate 0.7 oz of copper per hour by electrolysis while consuming 6 cubic feet of gas in the same period. The output current was quoted in this outlandish fashion because electroplating was the main application of these devices; possibly practical ammeters did not yet exist.
The diagonal connections that join each ring of couples in series can be seen between the two vertical strips.

The gas pipe can be seen coming in from bottom right. The little coffee-pot thing in the line is actually a gas pressure regulator.

From "Electricity in The Service of Man"

The solid sectors A were made of the alloy, while the cooling fins F were made of sheet iron to act as cooling fins for the cold junctions.

From "Electricity in The Service of Man", a much longer book than "Electricity in The Service of Chameleons"

Note gas feed with tap running into the centre of the pile.

This example is in the History Museum of the University of Pavia in Lombardy, Italy.

Showing the multiple annular burners in the centre of the pile. Gas enters through tube T.

According to the French journal La Nature for 1874, one of these piles was in use at the printing works of the Banque de France, presumably for electroplating.

Picture from La Nature 1874.

The EMF of this pile was no less than 109 Volts, with an internal resistance of 15.5 Ohms. The maximum power output was therefore 192 Watts, at 54 Volts and 3.5 Amps.

This pile was fired by coke. The hot junctions were at C, while the cold junctions D were cooled by sheet iron as in the original design above. What purpose was served by the tortuous path T-O-P taken by the hot gases is unclear, because there seem to have been no hot junctions in the inner sections. This beast was 98 inches high and 39 inches in diameter. It was a very serious piece of machinery, quite capable of delivering a lethal voltage.

The second version of the Clamond and Mure pile used the alloy of Marcus (Zn 66.6% and Sb 33.3%) for the negative material, and iron for the positive material. It surpassed all other similar piles and won the Gold Award of French National Industry. In 1876, the "Thermo-Electric Generator Company" (France) began the mass production of Clamond's thermopiles. But soon it turned out that generators had serious problems: the thermocouples melted, oxidized rapidly, and exfoliatied. This affected the pile's efficiency...

Clamond needed four years to develop new thermoelectric materials and improve the construction of the vulnerable elements. His new "Clamond Improved Thermopile" was based on the alloy of bismuth and antimony (negative material) and on iron (positive material). This efficient and powerful generator was 'free of all imperfections of its prototypes' and was the considered the best pile of that time.

On May 1879, the new Clamond's generator was presented to the French Academy of Sciences.

From "Electricity in The Service of Man"

From Volume 4 of "Electrical Installations" by Rankin Kennedy

The hot junctions are the pointed things directed inwards to the central burner. The cold junctions are cooled by radiation and convection from the vertical strips on the outside.

The inventor, Fr. Noe, came from Vienna. The output EMF of this pile was about 2 Volts, with an internal resistance of 0.2 Ohm. This was for a pile with 128 couples.

From "Electricity in The Service of Man".

The hot junction is a copper pin in a brass case, surrounded by "an alloy" which is presumably the other half of the junction.

The connecting wires visible here on each side were of "German silver". German silver (better known nowadays as nickel silver) is the generic name for a range of bright silver-grey metal alloys, composed of copper, nickel and zinc; it contains no real silver.
These wires were essential to join the thermocouples together, but reduced its efficiency as they conducted heat away from the hot junctions to the cold ones. The problem is elegantly solved in modern semiconductor versions by using alternate P and N type materials that do not require these connections.

From "Electricity in The Service of Man".

This high-performance version is surrounded by little cylindrical fins for cooling the cold junctions, permitting a greater output.

This example is in the History Museum of the University of Pavia in Lombardy, Italy.

The EMF of a single couple was quoted as "0.1 of a Daniel cell" which makes it about 110 milliVolts; this seems rather high to me. The current capacity using 30 couples was "capable of making a platinum wire 1.2 inches long red-hot" which is not a very useful sort of spec, since we have no idea how thick the wire was.
The Hauck pile was fired by gas, using something looking very much like a Bunsen burner. The cold junctions were water-cooled by a series of little cylindrical tanks, and there was an obscure little glass device in the middle; possibly to show the rate of gas flow?

These devices appear to have been produced commercially in different sizes, with two or three placed on a common frame. They were used for science education and electroplating. To put a time marker on this, it was 1843 when Moses Poole took out a patent for the use of thermoelectricity instead of batteries for electro-deposition purposes. This was long before practical dynamos and alternators.

In the days when chemical cells needed a lot of attention, something that provided power at the strike of a match must have had its attractions.

From "Electricity in The Service of Man".

Doctor Stone reads an article on thermopiles.

This gives some interesting practical details on the problems of brittle thermocouple materials and the difficulty of avoiding oxidation when iron was used as one half of the couple, as it was in the Clamond pile. There is also the interesting suggestion that petroleum should be vaporised at the cool junctions, reducing their temperature, and the resulting vapour burnt at the hot junctions. However, should there be a bad contact on the cold side, causing an electrical spark, things might end bad.

Attempts to find out more about Dr Stone have so far failed.

This article comes from the English journal Nature, not to be confused with the French journal of the same name.

It was clear from the early days of internal combustion engines that a lot of heat was being thrown away in the exhaust. Some people thought the way to utilise it was to boil water and generate steam, as in the Simon gas-steam engine or the Still diesel-steam engine. The snag is that this adds a lot of weight and complication, with the added risk of a boiler explosion.

A far better idea (an idea that occurred idependently to me, as well as lots of other people) would be to use a thermopile to generate electricity from the exhaust gases. No extra moving parts, sounds a winner. So why don't we do this? The simple but sad answer seems to be that even modern semiconductor thermopiles are so inefficient that it's just not worth the bother for the relatively small amount of electricity you get.

This annular thermopile was intended to be inserted between the engine block and the exhaust manifold, with the exhaust gases passing through the centre. It would certainly have got hot- probably red-hot. However the cold ends of the junctions are only cooled by their exposure to the air; there no fins or other cooling aids, which suggests to me a certain lack of practicality.

This notion was patented in the USA by Hartwell W Webb in 1918; its claimed innovation was the spot-welding together of the two types of metal (iron for one and nickel-copper for the other, in this case) which would presumably have been a lot more reliable than soldering. Though maybe not when red-hot.

US patent 1,242,499

Not a name that exactly trips off the tongue. The voltmeter on the front registers from 0 to 10 Volts; a suitable voltage range for charging accumulators running 6.3 Volt valve heaters. The black knob below the meter obviously controlled something- presumably the gas supply.

It appears this device was designed to charge lead-acid accumulators rather than power the radio directly. This may have been because output voltage fluctuations due to changing gas pressure would have had little effect on accumulators, but would have been very bad for the filaments of valve heaters, a small over-voltage deceasing their life drastically. Note there are four output terminals, in red-black pairs, with a shorting link between them; it appears that the thermo-electric elements were wired in two separate sections, probably to give a choice of output voltage.

The knob under the voltmeter presumably controlled the size of the gas flame and therefore the output voltage.

This example is in the Science Museum in London. It was made by a small Southhampton company called Attaix, hence the strange name.

Note that the the junctions were connected in series-parallel, and the output fell from 4 Volts to 2 Volts on load, which is optimal for maximum transfer of power.

This version has only one output voltage and so only two output terminals, mounted on the upper cover.
No gas control knob is visible.

The asbestos inside is rather a worry; in 1927 it was known to be dangerous to work with but no-one knew how dangerous. (In 1906, the first documented death of an asbestos worker from pulmonary failure was recorded by Dr Montague Murray at London’s Charing Cross Hospital) It is interesting that the WW staff thought it was efficient- it cannot have been in any strict sense.

This description appeared in Wireless World in November 1927.

In this part of the description the writer recommends connecting the Thermattaix to a flue. I think that would have been a very good idea, because here we have gas flames playing on cooled metal, which is the classic way to generate highly poisonous carbon monoxide.

This description appeared in Wireless World in November 1927.

An A-battery was the low-voltage high-current battery that powered the valve filaments. The B battery provided the HT.

This advertisment appeared on the front page of The Press, published in Christchurch, New Zealand, on 28 June 1928.

An advertisment appearing in Amateur Wireless magazine claimed the Thermattaix had been supplied to "...gas companies, the Italian Airforce, architects of note, and Big Game Hunting Expeditions for use in Africa and India." One can see how the Italian Airforce might use them to power radios, but far from clear what use they would be on hunting expeditions a long way from the nearest gas supply. Were they supposed to be wood-fired in some way?

Well, it was certainly the invention of a generation, but not of the generation that advertised this machine, as you will have seen from the thermopiles above.

It is believed it contained thermopiles (ie series arrays of thermocouples) that produced 2 Volts @ 0.5 amps for valve filaments/heaters and 120 Volts @ 10mA for the HT.

Thermocouples do not generate much voltage, but since they are simply junctions between two kinds of wire, connecting many in series is feasible. One of the most useful combinations is Ni/NiCr, ie nickel/nickel-chromium. This has a thermovoltage of about 4 mV/100K and a usable temperature range up to some 1000 K. This is very likely the type of couple used in this generator; it implies that 40 mV is about the most you can get from each thermocouple, so 50 in series would have been needed for the 2 V filament supply and 3000 in series for the 120 V HT. This sounds possible, though probably rather protracted to assemble and maybe heavy on labour costs. It would be interesting to know what the retail price was.

Picture kindly provided by John Howell.

The automatic control feature is intriguing. Given that any radio of the time would have had a Class-A output stage, whose current drain does not depend on volume, there seems no need to compensate for load changes. What might have been more useful (and possibly what the copywriter meant) would have been control to stabilise the 2V filament supply against changes in gas pressure. Excess filament voltage would have seriously reduced the life of the valves.

You may have heard of "steam radio" but this advertisment offers "gas radio".

Picture kindly provided by John Howell.

This is a complete radio set with a internal thermoelectric generator, built by the Milnes Electrical Engineering Company. One source states that the thermoelectric generator was in a lower compartment lined with asbestos, with the electronics and speaker in the top section. This does not match the appearance of this model.

The high tension supply as well as the filament power was provided by the thermopile; it powered the Milnes HT converter, which was probably a vibrator and transformer system; it produced 120V. The burners were ignited by flash device operated by the volume control knob.

The sets sold for £15, when £5 was a good weekly wage. It was claimed out that the running costs were half that of rechargeable accumulators. It was also pointed out that the radio would help to warm the house- which is fine in winter but you might want to listen to the radio in the summer.

At any rate, they did not succeed. A retired salesman told Historic Gas Times that he had had gas radios in his showroom but had only ever sold one of them.

The model seen here is in the National Gas Museum, Leicester, England.

The thermoelectric generator is on the left, and a Milnes gas radio on the right.

With no sign of a connection for an outside flue, I can't help wondering how much carbon monoxide these things produced. A flame playing on a cooler piece of metal (which is exactly how these generators were constructed) is the classic way for carbon monoxide to be produced.

This picture is also believed to be from the National Gas Museum, Leicester, England. Apologies for poor picture quality.

This advert gives a good deal more information, though the text is only just readable.

Henry Milnes emigrated to New Zealand in the 1950s, complaining that government bureaucracy was interfering with his business.

And also heating up some food at the back there.

The design of the legs seems to have changed.

The Lufo Lamp is a hurricane lamp with an AM/FM radio built into the base. Apparently all you can do with it is listen to the radio (an AM/FM radio can be run on very little power) and charging your phone etc is not an option. Whoever wrote the URL seems to think its some sort of heat-pump, whereas it is the opposite; generating electricity by letting heat flow downhill.

Right now the most important RTG in the solar system is the RTG that powers the Curiosity Mars rover. Solar cells do not work well on Mars, as they cannot function in either the Martian night or the Martian winter. Curiosity is therefore kept rolling by an RTG that contains some 5 kg (10 pounds) of plutonium-238. (not the plutonium-235 used in atom bombs) It is designed to power Curiosity for at least 14 years. The elecrical output is 125W at the start of mission; that will slowly fall to about 100W after 14 years. The initial thermal output is 2 kW. so the efficiency is still only 6% for what is presumably one of the most advanced RTGs in existence.

NASA call it the Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG. In pictures of Curiosity the RTG can be seen as a hefty black cylinder sticking out of the back like a tail.

It has its very own own Wikipedia page.

The price seems quite reasonable...