Thermocouples, efficiency and operation?

Question

Hey. I was wondering if anyone could give me a thorough explanation as to how a thermocouple works. I understand that as you heat the junction point between two points a voltage will be created, but i have no clue as to the actual physics behind it.

If possible could anyone give me the average efficiency of a thermocouple, are they best at high temperatures, or low temperatures ?

Thanks.

Answer 1

Efficiency; 3%-7%, never more than 10%

Operation:  heating of dissimilar metals in conjunction with each other creates an electrical current.  The heat causes electrons to migrate between the metals, thus creating a current; but not a very strong one.

Most thermocouples are used as temperature sensors to determine how hot something is or to act as a safety mechanism.  They are rarely used as power sources.

The single exceptions are in applications like unmanned Russian lighthouses (which has killed at least one man) and in unmanned spacecraft sent out into space beyond Mars.

The new Mars laboratory experiment may have a thermopile on board to give it increased power and a longer lifespan than would solar panels.  NASA is hoarding the last of its plutonium for just such a mission, international treaties control how much weapons grade radioactive material is produced so it is hard to get a hold of.

When you put a large group of thermocouples together you create a thermopile.  Thermopiles have to be operated with a small yet extremely hot power source the thing that best fits this bill is a radioactive substance like plutonium; thus making being near one fatal (a thief trying to steal the core).

Voyager, Galileo and Cassini are all powered by thermopiles.  The reactor is so hot that it has to be stuck out on a pole so as to not interfere with the delicate instruments.

According to Wikipedia:  http://en.wikipedia.org/wiki/Thermocoupl...

"In electronics and in electrical engineering, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; Kieran Thomas' research shows that system errors of less than one degree Celsius (°C) can be difficult to achieve....

Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators."

According to Wikipedia:  http://en.wikipedia.org/wiki/Radioisotop...

"A radioisotope thermoelectric generator (RTG) is an electrical generator which obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples. RTGs can be considered as a type of battery and have been used as power sources in satellites, space probes and unmanned remote facilities. RTGs are usually the most desirable power source for unmanned or unmaintained situations needing a few hundred watts or less of power for durations too long for fuel cells, batteries and generators to provide economically, and in places where solar cells are not viable.

The first RTG launched in space by the United States was in 1961 aboard the SNAP 3 in the Navy Transit 4A spacecraft. One of the first terrestrial uses of RTGs was in 1966 by the US Navy at the uninhabited Fairway Rock Island in Alaska, where it remained in use until its removal in 1995....

A common application of RTGs is as power sources on spacecraft. Systems Nuclear Auxiliary Power Program (SNAP) units were used especially for probes that travel far enough from the Sun that solar panels are no longer viable. As such they are used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini and New Horizons. In addition, RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12 through 17 (SNAP 27s), excluding Apollo 13 because that mission never reached the Moon's surface. RTGs were also used for the Nimbus, Transit and Les satellites. By comparison, only a few space vehicles have been launched using full-fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A....

Most RTGs use 238Pu which decays with a half-life of 87.7 years. RTGs using this material will therefore lose 0.51 / 87.7 or 0.787% of their capacity per year. 23 years after production, such an RTG would produce at 0.523 / 87.7 or 83.4% of its starting capacity. Thus, with a starting capacity of 470 W, after 23 years it would have a capacity of 0.834 * 470 W = 392 W. However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well; at the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315 W for Voyager 1 and to 319 W for Voyager 2. Therefore in early 2001, the thermocouples were working at about 80% of their original capacity....

RTGs use thermoelectric couples or "thermocouples" to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3-7%. Thermoelectric materials in space missions to date have included silicon germanium alloys, lead telluride and tellurides of antimony, germanium and silver (TAGS). Studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations."

The physics behind the operation of thermocouples is the Peltier effect, the theromoelectric effect and the Seebeck effect.

According to Wikipedia:  http://en.wikipedia.org/wiki/Peltier_eff...

"The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. Simply put, a thermoelectric device creates a voltage when there is a different temperature on each side, and when a voltage is applied to it, it creates a temperature difference. This effect can be used to generate electricity, to measure temperature, to cool objects, or to heat them. Because the direction of heating and cooling is determined by the sign of the applied voltage, thermoelectric devices make very convenient temperature controllers.

Traditionally, the term thermoelectric effect or thermoelectricity encompasses three separately identified effects, the Seebeck effect, the Peltier effect, and the Thomson effect. In many textbooks, thermoelectric effect may also be called the Peltier–Seebeck effect. This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and Estonian-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage difference is applied across a resistive material, is somewhat related, though it is not generally termed a thermoelectric effect (and it is usually regarded as being a loss mechanism due to non-ideality in thermoelectric devices). The Peltier–Seebeck and Thomson effects are reversible, whereas Joule heating is not....

The Seebeck effect is the conversion of temperature differences directly into electricity....

The Seebeck effect is commonly used in a device called a thermocouple (because it is made from a coupling or junction of materials, usually metals) to measure a temperature difference directly or to measure an absolute temperature by setting one end to a known temperature. Several thermocouples when connected in series are called a thermopile, which is sometimes constructed in order to increase the output voltage since the voltage induced over each individual couple is small.

This is also the principle at work behind thermal diodes and thermoelectric generators (such as radioisotope thermoelectric generators or RTGs) which are used for creating power from heat differentials."

For the formulas read the article.

The temperature for best operation varies with the heat and the dissimilar metals involved.

The Peltier Effect is where you have a special junction of dissimilar metals and if you apply current to them then you create a heating effect on one side and a cooling effect on the other.  The small USB powered coolers that you can plug into your computer work with this principle.  They don’t make very good air conditioner units; but they do make a fairly good spot cooling product.

Answer 2

Where the thermocouple operates best depends on what temperature it was built for. Efficiency is never very high, and the voltage out is always in milli-volts. If you are looking for a way to make electricity from heat, you can, it is done, but it takes a lot of thermocouples to do it.

The way they work is the electron exchange between dissimilar metals. That is something best found in a book that a well stocked library should have. You could also ask a company that makes them.