How are pyranometers used in solar PV plants?

What is a pyranometer?

Pyranometer definition: A pyranometer is an instrument engineers can use to measure the level of solar radiation the sun is producing in a specific location. Until recent years, pyranometers were mainly used for climatological research but have now been adopted as a highly useful tool when designing and building solar PV plants.

A pyranometer works by measuring from a hemispherical field of view incident on a flat surface. It collects global solar radiation and converts that into an electrical signal that can be measured. The SI units of irradiance are watts per square meter (W/m²).

Pyranometers are regulated by the ISO 9060:2018 standard. The latest revision of this standard defines three distinct performance classes:

• ISO 9060:2018 Class-C — These are the most economical types of pyranometers which are mostly used for routine measurements where complete accuracy is not required. Class-C pyranometers include low-cost, single dome, spectrally flat thermopile, and lower-cost acrylic photocell diffuser pyranometers.

• ISO 9060:2018 Class-B — Class-B pyranometers offer higher performance and consist of a double dome optic design or an outer dome optic with an inner quartz diffuser. Pyranometers in this category are often used for meteorological monitoring.

• ISO 9060:2018 Class-A — The highest class pyranometers are spectrally flat in response and are higher in cost. Manufacturers wishing to classify their pyranometers as Class-A need to test and document the device’s performance in writing to prove it is Class-A compliant. These devices are used in solar PV plants to monitor performance and efficiency, as well as any other situations that require the highest degree of accuracy.

How does a pyranometer work?

Pyranometers work based on the Seebeck effect, otherwise known as the thermoelectric effect. The Seebeck effect occurs when a temperature difference between two dissimilar electrical conductors (or semiconductors) produces a voltage difference between them. So, if heat is applied to one conductor, the heated electrons will flow towards the cooler one. 

The pyranometer is a compact device made up of a detector element housed under an optic, one or two hemispheric domes, and a diffuser. This can vary depending on the make and model of the pyranometer. Some may use the detector and a diffuser, some may use the detector and domes, and some may use all those parts.

The optic of a pyranometer acts as a filter which will transmit various shortwave solar wavelengths, while rejecting far infrared wavelengths.

The pyranometer uses one of two detector technologies: 

Silicon photodiode (photocell) detector — A photocell detector is a low-cost device housed in an acrylic diffuser. Pyranometers using silicon photodiode detectors are limited in measurement accuracy as they have a limited spectral response range and spectral-sensitive effects under changing spectral conditions. They can measure shortwave solar wavelengths between 400nm and 1100nm nominal.

Blackbody thermopile detector — The thermopile detector is housed under one or two glass domes. The detector element converts shortwave solar radiation into an electrical signal, which is sampled by the data logger or metering device to which the pyranometer is connected. Devices using the blackbody thermopile detector tend to cost more but are considered to be more accurate than devices using a photocell detector. This is because the thermopile detector responds to the true solar spectrum and does not suffer from spectral selectivity bias. They can measure shortwave solar wavelengths between 285 and 3000nm nominal.

What is a pyranometer used for?

Pyranometers used to be a specialized tool used for climate research and agriculture, but in recent years, engineers have adopted them for solar PV plants.

In agriculture, pyranometers help optimize crop growth and yields. Farmers could measure the amount of solar radiation received by crops, which is essential for the plants to perform photosynthesis and thrive. The data also allows farmers to tailor fertilizer use and water consumption to reflect environmental needs and reduce their carbon footprint.

Meteorologists can use pyranometers to better understand climate conditions and the global shortwave energy balance of the Earth’s climate system. They can measure the amount of solar radiation received at the Earth’s surface and surface-reflected solar radiation from the ground.

Pyranometers are extremely useful for configuring solar PV plants. They can measure the amount of solar radiation a PV panel will receive and locate ideal areas for PV panel placement. They are also essential for maintaining the efficiency of solar PV plants, as they can provide data for calculating a plant’s performance ratio.

What is the difference between a pyranometer and a pyrgeometer?

There is often confusion about pyranometers and pyrgeometers. Both devices measure infrared radiation in the atmosphere, use a thermopile detector, and utilize domes.

The main difference between the devices is the application they’ll be used for. Pyranometers are only capable of measuring shortwave radiation. A pyrgeometer is able to isolate longwave radiation from shortwave radiation, making it suitable for longwave irradiance measurements.

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