Thermopile Pyranometers – How Do They Work?

Kipp & Zonen had been involved in the manufacture of a fast and sensitive thermopile used for the measurement of light and thermal radiation since 1913. The design, which was made up of several thermocouples, was by Dr. Moll of Utrecht University.

Dr. Moll’s thermopile prototype drew the attention of several scientists in a meteorological conference at Utrecht in 1923. Following that, Professor L. Gorczynski of the Polish Meteorological Institute constructed a pyranometer and a pyrheliometer after modifying Moll-Gorczynski thermopiles.

The thermopile characteristics ensure small, light-weight and low-cost instruments that produce a continuous voltage output signal. Such instruments avoid the need for control systems or any external electrical power. Following the success of these prototypes, Kipp & Zonen became the pyranometer manufacturers in 1924.

This article explains, in detail, the working principle of thermopile pyranometers.

Working of Pyranometer

A pyranometer operates by the thermoelectric detection principle, which is when a horizontal blackened surface fully absorbs the incoming energy over a wide range of wavelengths. As a result, the temperature of the surface is increased, which is measured using thermocouples connected in series-parallel or series to form a thermopile. Figure 1 shows the schematic of thermopile principle.

Schematic of thermopile

Figure 1. Schematic of thermopile

The absorbed radiation stored in the black coating is used for heating the active, or hot, junctions positioned beneath the blackened receiver surface. The passive, or cold, junctions are, however, in thermal contact with the blackened surface, which acts as a heat-sink. The latest higher performance pyranometers employ a Peltier element - a thermoelectric element that uses dissimilar semiconductors instead of the dissimilar metals of a thermocouple/ thermopile. Figure 2 shows the schematic of pyranometer with dissimilar semiconductors.

 Schematic of pyranometer with dissimilar semiconductors

Figure 2. Schematic of pyranometer with dissimilar semiconductors

The black detector coating needs to be protected against external factors, such as wind, dirt, and precipitation, which can affect the measurement. Almost all types of pyranometers employ an optical quality glass to cover their hemispherical single or double domes. The transmission may vary from 300 nm to less than 3000 nm based on the glass. Double domes tend to insulate the sensor surface from environmental parameters like rapid temperature fluctuations and wind, providing excellent stability under varying operating conditions.

The sensor response when the sun is in close proximity to the horizon, bending the incoming beam of radiation, is improved with respect to the refractive index of the material and shape of the dome. The CMP 22 model from Kipp & Zonen employs Quartz domes that provide a wide spectral response. The high refractive index of the Quartz domes further improves the directional response. The Quartz domes provide improved thermal conductivity compared to glass, offering a range of other performance benefits. Figure 3 shows the schematic of pyranometer with dome.

 Schematic of pyranometer with dome

Figure 3. Schematic of pyranometer with dome

Passive thermopile / Peltier pyranometers, including CMP series, do not need a power supply. A small voltage is produced by the detector in the order of 10 µV per W/m2 proportional to the difference in temperature between the instrument housing and the black absorbing surface. Therefore, the output on a sunny day will be around 10 mV. The signal, in microvolts, is converted into global irradiance, in W/m2, using the unique sensitivity of each pyranometer. Figure 4 shows the schematic of Peltier pyranometers.

Schematic of Peltier pyranometers

Figure 4. Schematic of Peltier pyranometers

Conclusion

The SMP series of Smart pyranometers from Kipp & Zonen have the same detectors as those in the equivalent CMP models, with an additional built-in digital signal processing and performance enhancement. Hence, they do not require external power for operation.

It is suggested that the instrument is recalibrated every two years in order to maintain high performance of the instrument. Further, the process is simplified by the introduction of a high quality water-proof connector for the signal cable.

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This information has been sourced, reviewed and adapted from materials provided by Kipp & Zonen.

For more information on this source, please visit Kipp & Zonen.

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