Pyranometers – Solar Irradiance Sensors

A pyranometer is a solar irradiance sensor that measures solar radiation flux density (W/m²) on a planar surface.

Pyranometer for solar irradiance
Kipp and Zonen Pyranometer

Widely used within the solar energy sector, pyranometers provide high-quality data for feasibility studies and monitoring photovoltaic performance of established solar projects.

Modern pyranometers utilise a thermopile sensor (collection of thermocouples which turn thermal energy in to a small electrical current). However, silicon semiconductor sensors are also utilised in some pyranometers. Thermopile sensors absorb a greater spectrum of solar radiation and have a near-perfect cosine response. As a result, they are considered superior. Kipp and Zonen CMP and SMP pyranometers use thermopile sensors. 

How does a Pyranometer work?

Thermopiles’ black coating absorbs all solar radiation. The absorbed radiation causes a temperature difference between the hot and cold junctions of the thermopile. Which in turn, generates a very small voltage proportional to the solar radiation absorbed.  Therefore, a greater temperature difference produces a larger voltage output. However, output on a sunny day may only reach 10mV. 

A prominent feature of thermopile pyranometers is the glass dome. These allows full 180° of solar radiation to reach the sensor. The dome acts as a radiation shield against convection consequently maintaining the temperature difference between the ‘hot’ and ‘cold’ junctions of the thermopile.

Where to use a pyranometer

Kipp and Zonen pyranometer with ventilation unit
Pyranometer with ventilation unit measuring solar irradiance in the field.

Pyranometer precision is classified by three standards; Class A, B and C. Previously named; secondary standard, first class and second class.

Class A (Secondary Standard) pyranometers have the highest standard and produce the most accurate data. Subsequently, large solar farms and meteorological weather stations utilise them.

Class C pyranometers are slightly less stable and as a result, produce slightly less accurate data. Consequently, they are ideal for none critical observations and building automation, where the accuracy of data is not vital 

Pyranometers capture 180° of solar radiation allowing their orientation on solar farms to be in the same plane of array as the photovoltaic panels. Pyranometers positioned horizontally measure global horizontal irradiance. GHI is a useful metric for the planning of solar farm locations.

Bi-facial panels (photovoltaic panels which have two sides to capture direct radiation and radiation reflected radiation off the ground) utilise pyranometer mounted ‘back to back’, which forms an Albedometer. This defines the ratio of irradiance reflected to the irradiance received by a surface.

Albedometer using two CMP11 pyranometers.

To adapt to the various uses Kipp and Zonen produce two ranges of pyranometers the CMP and SMP.

For remote areas with no power source, CMP pyranometers are ideal, as they use the electrical energy generated in the thermopile to provide a very small analogue output (V or mA) which can be read by data loggers with an accuracy better than 10 microvolts. 

For facilities requiring sensors compatible with a SCADA system then the SMP range is best. With both analogue and RS 485 Modbus outputs the SMP pyranometers can easily be integrated.

If you would like advice on which pyranometer would suit your specific application then please contact us for information.