Thermopile: What Are They (And How Do They Work?

What Is A Thermopile

What is a Thermopile?

A thermopile is an electronic device that converts thermal energy to electrical energy. It is a serial-interconnected array of thermocouples, consisting of two different materials with large thermo-electric power and opposite polarities. It is less commonly connected in parallel.

How a Thermopile Works?

A single thermoelectric cell’s voltage output is very tiny, so many of these cells are arranged in series or parallel to achieve a larger signal output. This thermocouple stack arrangement is known as the thermopile.

Thermopile works on the thermoelectric effect principle. The direct conversion of temperature differences to electrical voltage and vice versa is the thermoelectric effect.

A voltage is generated in a thermopile when its dissimilar metals (thermocouples) are exposed to temperature differences.

Adding more pairs of thermocouples in series to form a thermopile, it increases the voltage output. Thermopiles can be constructed with a single pair of thermocouples, consisting of two thermocouple junctions or multiple pairs of thermocouples.

A thermopile is a series of thermocouples, each of which consists of two different materials with large thermoelectric power and opposite polarities, interconnected in series.

The thermocouples are placed through the hot and cold areas of the structure and the hot junctions are thermally isolated from the cold junctions.

Structure of a thermopile
Structure of a thermopile

In response to the temperature difference across the material, the voltage output of the thermopile is known as a thermoelectric coefficient or Seebeck coefficient. It is calculated per kelvin (V/K) or mV/K in volts.

For heat transfer, all thermopile sensors use infrared radiation and are also used for non-contact temperature measurements.

As a consequence, it is used in infrared thermometers, industrial pyrometers, lifecare instruments, moving object temperature control, etc.

Thermopile connections:

With a few thermocouples linked in a chain. At temperature T1, the two top thermocouples are a connected junction, whereas the two bottom thermocouple junctions are at temperature T2.

The output voltage from the delta-V thermopile is directly proportional to the temperature difference, or T1-T2, across the thermal resistance layer and the number of pairs of thermocouple junctions.

Connection of a thermopile
Connection of a thermopile

Thermopile vs Thermocouple

The key distinction between the thermocouple and the thermopile is that the thermocouple is a thermoelectric device and the thermopile is a device that transforms electrical energy into thermal energy.

Thermocouple

A thermocouple consists of two different metals connected in series. In order to detect radiation, one junction is blackened to absorb radiation.

The temperature increase of the junction (with respect to another non-irradiated junction) generates a voltage. This effect, discovered by Seebeck, is the basis for all thermocouple temperature sensors.

A transducer consisting of two distinct metals welded together at each end is a thermocouple which generates a voltage that is proportional to the temperature difference between the two junctions, one of the junction is typically kept at a temperature that is known. Due to this, it is also known as the thermoelectric transducer.

A thermocouple is thinner and has pointed end. It is operated at a lower voltage between 8 mV to 30 mV. It is made of a single pair of wires.

It operates by measuring the difference in temperature from their point of connection to the point at which the output voltage of the thermocouple is measured.

Circuit of a thermocouple
Themocouple circuit

The closed-circuit is made of one metal. Due to the temperature difference between junctions and points of transition from one metal to another a current is generated due to the difference of potential between the hot and cold junction.

Thermocouples can be linked in series with a junction located on either side of a thermal resistance layer as thermocouple pairs.

The output of the thermocouple pair will be a voltage that is directly proportional to the temperature difference between the thermal resistance layer and the heat flux through the thermal resistance layer.

In science and industry, thermocouples are extensively used. It is used in applications like calculation of kiln temperature, gas turbine emissions, diesel engines, and other industrial processes.

The mathematical equation for a thermocouple is

    \begin{align*}V_{out} = S*(T_{x}-T_{ref})\end{align*}

Thermopile

A thermocouple is an electronic device that converts electrical energy into thermal energy and typically constructed using a thermocouple series-combination.

Compared to a single thermocouple, thermopile produces greater voltage output. It has a set of thermocouples arranged for measuring small quantities of radiant heat.

In addition, the thermopile is a non-contact-based temperature sensor device which utilizes infrared radiation along with the IR Absorber system for temperature measurement. Whereas thermocouple is a contact-based temperature sensor.

It consists of alternate antimony and bismuth bars, or any two metals with different heat conduction capacities. It is connected with an astatic galvanometer which, when exposed to heat, it is very sensitively affected by the electrical current induced in the bar system.

The thermopile is bigger and thicker compared to a thermocouple. It has voltage output which is higher compare to the thermocouple and it is in the order of tens or hundreds of mV. It is basically a sensor which converts thermal energy into electrical energy.

Circuit of a thermopile
Thermopile circuit

Thermopiles are also used for heat generation from electrical components such as solar wind, radioactive materials, laser radiation or combustion to produce electrical energy.

Thermopile with N thermocouples generates a voltage N times greater than that produced by a single thermocouple, increasing the sensitivity of the transducer.

Thermopiles do not react to absolute temperature but produce a voltage output proportional to the difference in temperature or the gradient of temperature.

This can be seen in the infrared thermometers which are widely used to measure temperature. They are also used widely in heat flux sensors and pyrheliometers and gas burner safety controls.

The mathematical equation for a thermopile is

    \begin{align*}V_{out} = N*S*(T_{x}-T_{ref})\end{align*}

Thermopile sensor

The sensor device that uses non-contact-based temperature sensing with the help of more than one thermocouple is known as a thermopile sensor. It has more voltage output than the conventional thermocouple sensor device.

It uses the concept of infrared radiation absorption from the object being measured. The electrical output of the thermopile sensor is proportional to its temperature. Hence, it is known as a thermoelectric transducer.

Working of Thermopile Infrared Sensor

A thermopile infrared (IR) sensor has many series-connected thermocouples with “hot” junctions attached to a very thin IR absorber. A micro-machined membrane on a silicon chip is normally an absorber.

The absorber temperature increases or decreases due to the difference in temperature between the absorber and the material. The exchange of infrared radiation between the target and the absorber depends on this.

Structure of a thermopile sensor
Thermopile sensor

To calculate the temperature of the object, it should be calculated that the object fully fills the field of view of the sensor.

This would ensure that only from the object of interest and not from its background comes infrared radiation.

The use of filter and lens components will greatly enhance the performance of thermopile sensors. A filter in front of the thermopile, therefore, assures, that only the desired IR
radiation hits the absorber. It protects the thermopile structure from contamination by dirt or gases.

Silicon is widely used as a micro-machine lens. This is because silicon is fully opaque to visible light, but it is transparent to wavelengths longer than 2μm.

Thermopile sensors are therefore designed to measure the temperature of the object (moving or fixing) from a distance by detecting the infrared energy emission of the object.

Applications of Thermopiles

The applications of thermopiles include:

  • Non-contact measurement of temperature in the process field
  • Handheld calculation of non-contact temperature
  • Scanner Thermal Line
  • Industrial building HVAC and monitoring of lighting
  • Human presence and identification for protection
  • Detection of black ice and early warning
  • Monitoring of Blood Glucose
  • Automatic power over HVAC
  • Detection of fires in transit tunnels
  • Flame and Fire Detection for Aircraft
  • Analysis of automatic exhaust gas

How To Test Thermopiles?

Millivolt gas fireplaces have a pilot light that heats up a sensor usually a thermocouple or thermopile.

The thermopile and thermocouple generate electricity when heat is applied to it. This small amount of voltage is sent to the wall switch.

When the switch is turned on, it will send that voltage back to the fireplace and signal to the fireplace to turn on the flame.

After some time, the connections can get rusty, corroded, or broken within the switch, allowing the switch to lose this voltage.

So it is not enough to signal the fireplace to turn on by the time the small amount of voltage is sent back to the fireplace.

After ruling out the faulty wall switch and bad pilot flame issues, we need to check the thermopile.

When heated up by the pilot flame, the thermopile will produce some voltage similar to a thermocouple. By using a digital multimeter, we can measure the voltage that the thermopile is giving off.

We will take the example of a fireplace where you need to check the condition of a thermopile. A multimeter is used to test the thermopile leads.

They are connected to the gas control valve, so the very first thing we should do is to locate the valve.

Generally, the main control valve is typically located underneath the lower grill of the fireplace.

Below the main control valve, the thermopile sensor will be found. It will have a wire that runs from the pilot assembly to the main control valve.

It consists of two wires that usually have a metal sheath that protects the wires. The end of the thermopile wire branches off into two leads usually in red & white colors.

The leads of the thermopile will be attached to the main valve. We can test the voltage using a digital multimeter on DC millivolt setting on the terminals that the thermopile is connected to. The DC setting will either say “DC” on your meter or a symbol.

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About Vidya Muthukrishnan

Vidya Muthukrishnan is currently employed as a Senior Engineer in a product based IT company. She has 5 years of professional teaching experience, previously Assistant Professor in the Department of Instrumentation and Control Engineering at the Sri Krishna College of Technology. She also has 1 year of industry experience with TCS, India. Vidya completed her B.Tech Electronics and Instrumentation from SASTRA University and M.Tech in Biomedical Engineering from VIT University Vellore.