View source for Thermopile

{{short description|Device that converts thermal energy into electrical energy}}
{{distinguish|Thermophile|Thermopylae (disambiguation){{!}}Thermopylae}}
[[File:Differential Temperature Thermopile.png|thumb|365x365px|Diagram of a differential temperature thermopile with two sets of thermocouple pairs connected in series. The two top thermocouple junctions are at temperature ''T<sub>1</sub>'' while the two bottom thermocouple junctions are at temperature ''T<sub>2</sub>''. The output voltage from the thermopile, ''ΔV'', is directly proportional to the temperature differential, ''ΔT'' or ''T<sub>1</sub>'' - ''T<sub>2</sub>'', across the thermal resistance layer and number of thermocouple junction pairs. The thermopile voltage output is also directly proportional to the heat flux, ''q"'', through the thermal resistance layer.]]
[[File:FluxTeq PHFS01 Heat Flux Sensor.jpg|thumb|443x443px|Picture of a heat flux sensor that utilizes a thermopile construction to directly measure heat flux. Model shown is the FluxTeq PHFS-01 heat flux sensor. Voltage output is passively induced from the thermopile proportional to the heat flux through the sensor or similarly the temperature difference across the thin-film substrate and number of thermocouple junction pairs. This voltage output from the sensor's thermopile is initially calibrated in order to relate it to heat flux.]]
A '''thermopile''' is an electronic device that converts [[thermal energy]] into [[electrical energy]].<ref name=":0">{{Citation|chapter=Woodhead Publishing Series in Energy|date=2016|pages=xiii–xviii|publisher=Elsevier|isbn=9780081003015|doi=10.1016/b978-0-08-100301-5.09002-0|title=Advances in Solar Heating and Cooling}}</ref> It is composed of several [[thermocouple]]s connected usually in [[series connection|series]] or, less commonly, in [[parallel connection|parallel]]. Such a device works on the principle of the [[thermoelectric effect]], i.e., generating a voltage when its dissimilar metals (thermocouples) are exposed to a temperature difference.<ref name=":0" />

Thermocouples operate by measuring the temperature differential from their junction point to the point in which the thermocouple output voltage is measured. Once a closed circuit is made up of more than one metal and there is a difference in temperature between junctions and points of transition from one metal to another, a current is produced as if generated by a difference of potential between the hot and cold junction.<ref>{{Cite book|url=https://books.google.com/books?id=5MIXAQAAIAAJ&q=thermopile+thermal+energy&pg=PA116|title=Johnson's Universal Cyclopedia: A New Edition|last=Adams|first=Charles Kendall|date=1895|publisher=D. Appleton, A. J. Johnson|pages=116|language=en}}</ref>

Thermocouples can be connected in series as thermocouple pairs with a junction located on either side of a thermal resistance layer. The output from the thermocouple pair will be a voltage that is directly proportional to the temperature difference across the thermal resistance layer and also to the heat flux through the thermal resistance layer. Adding more thermocouple pairs in series increases the magnitude of the voltage output. Thermopiles can be constructed with a single thermocouple pair, composed of two thermocouple junctions, or multiple thermocouple pairs.

Thermopiles do not respond to absolute [[temperature]], but generate an output [[voltage]] proportional to a local temperature difference or temperature gradient. The amount of voltage and power are very small and they are measured in milli-watts and milli-volts using controlled devices that are specifically designed for such purpose.<ref>{{Cite book|title=Fundamentals of HVAC Control Systems|last1=Montgomery|first1=Ross|last2=McDowall|first2=Robert|date=2008|publisher=Elsevier|isbn=9780080552330|location=Atlanta|pages=161|language=en}}</ref>

Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the [[infrared thermometer]]s widely used by medical professionals to measure body temperature, or in [[Accelerometer|thermal accelerometers]] to measure the temperature profile inside the sealed cavity of the sensor.<ref>{{Cite journal|last1=Mukherjee|first1=Rahul|last2=Basu|first2=Joydeep|last3=Mandal|first3=Pradip|last4=Guha|first4=Prasanta Kumar|title=A review of micromachined thermal accelerometers|journal=Journal of Micromechanics and Microengineering|volume=27|issue=12|pages=123002|doi=10.1088/1361-6439/aa964d|arxiv=1801.07297|year=2017|bibcode=2017JMiMi..27l3002M|s2cid=116232359}}</ref> They are also used widely in [[heat flux sensor]]s and [[pyrheliometer]]s<ref>{{cite web|url=http://www.novalynx.com/glossary-t.html|title=Glossary of Meteorological Terms (T) - NovaLynx Corporation|access-date=17 November 2016}}</ref><ref>{{cite web|url=http://www.omega.com/literature/transactions/volume1/glossary.html|title=Glossary|access-date=17 November 2016}}</ref> and gas burner safety controls.  The output of a thermopile is usually in the range of tens or hundreds of millivolts.<ref>{{cite web|url=http://pisces.sdsu.edu/ONLINE_LESSONS/glossary.htm|title=Glossary|access-date=17 November 2016|url-status=dead|archive-url=https://web.archive.org/web/20160303202943/http://pisces.sdsu.edu/ONLINE_LESSONS/glossary.htm|archive-date=3 March 2016}}</ref> As well as increasing the signal level, the device may be used to provide spatial temperature averaging.<ref>{{cite web|url=http://www.capgo.com/Resources/Sensors/SensorGlossary.html|title=Capgo - Sensor Glossary|access-date=17 November 2016}}</ref>[[File:Thermopile2.png|thumbnail|Thermopile, composed of multiple thermocouples in series. If both the right and left junctions are the same temperature, voltages cancel out to zero. However, if there is a temperature difference between sides the resulting total output voltage is equal to the sum of junction voltage differentials.]]{{Thermoelectric effect|cTopic=Applications}}Thermopiles are also used to generate electrical energy from, for instance, heat from electrical components, solar wind, radioactive materials, [[Thermopile Laser Sensors|laser radiation]] or combustion. The process is also an example of the [[Peltier effect]] (electric current transferring heat energy) as the process transfers heat from the hot to the cold junctions.

There are also the so-called thermopile sensors, which are power meters based on the principle that the optical or laser power is converted to heat and the resulting increase in temperature is measured by a thermopile.<ref>{{Cite book|url=https://books.google.com/books?id=U9g1DwAAQBAJ&q=thermopile+thermal+energy&pg=PT468|title=Thermoelectric Energy Conversion: Basic Concepts and Device Applications|last1=Pineda|first1=Diana Davila|last2=Rezaniakolaei|first2=Alireza|date=2017-08-22|publisher=John Wiley & Sons|isbn=9783527698134|location=Hoboken, NJ|language=en}}</ref>

==See also==
* [[Seebeck effect]], the physical effect responsible for the generation of voltage in a thermopile
* [[Thermoelectric materials]], high-performance materials that can be used to construct a compact thermopile that delivers high power

==References==
{{reflist}}

==External links==
* [http://www.robot-electronics.co.uk/htm/tpa81tech.htm TPA81 Thermopile detector Array Technical Specification]

[[Category:Electrical components]]
[[Category:Thermoelectricity]]