Pyranometer

A pyranometer is a special tool used to measure how much sunlight reaches the Earth. The name comes from two Greek words: πῦρ (pyr), meaning 'fire,' and ἄνω (ano), meaning 'above, sky.' It helps scientists know how strong the sunlight is on a flat surface.

Pyranometers work by catching sunlight from all directions above and measuring its energy. They can tell us the amount of solar energy in watts per square meter (W/m²).

Most traditional pyranometers do not need any electricity to work. But newer versions sometimes use a little bit of power to help them give even more accurate readings. These tools are important for studying the weather, climate change, and how we can use solar power to keep our planet healthy.

Explanation

Sunlight that reaches Earth has different wavelengths, roughly from 300 nm to 2800 nm. Pyranometers can measure this sunlight, depending on the type used.

When measuring sunlight, the device works best when the sun is straight above and less well as the sun moves away. This is called a "cosine response," meaning it works best when sunlight hits the sensor straight on.

Types

Following the ISO 9060, there are three types of pyranometer. They use two different technologies: thermopile technology and silicon semiconductor technology.

The light sensitivity, called 'spectral response', depends on the type of pyranometer. The picture shows the spectral responses of the three types in relation to the solar radiation spectrum. The solar radiation spectrum shows the sunlight that reaches the Earth's surface at sea level, at midday with A.M. (air mass) = 1.5.
The latitude and altitude affect this spectrum. The spectrum is also affected by aerosol and pollution.

Thermopile pyranometers

A thermopile pyranometer (also called thermo-electric pyranometer) is a sensor based on thermopiles. It measures the broad band of the solar radiation flux density from a 180° field of view angle. It usually measures from 300 to 2800 nm with a largely flat spectral sensitivity. The first generation had the sensor divided into black and white sectors. Irradiation was calculated from the temperature difference between the black sectors, exposed to the sun, and the white sectors, not exposed to the sun.

In all thermopile technology, irradiation depends on the temperature difference between the sun-exposed area and the shadow area.

Design

To work properly, a thermopile pyranometer has these main parts:

• A thermopile sensor with a black coating. It absorbs all solar radiation, covers the 300 to 50,000 nanometer range, and has a near-perfect cosine response.
• A glass dome. It limits the spectral response from 300 to 2,800 nanometers, while keeping the 180° field of view. It also shields the sensor from convection. Some pyranometers have a second glass dome for better thermal balance.

In modern thermopile pyranometers, the active parts of the thermopile are under the black coating and are heated by the radiation absorbed. The passive parts are protected from solar radiation and in contact with the housing, which acts as a heat sink. This stops any problems from yellowing or decay when measuring temperature in the shade.

The thermopile creates a small voltage based on the temperature difference between the black coating and the housing. This is about 10 μV (microvolts) per W/m², so on a sunny day the output is around 10 mV (millivolts). Each pyranometer has a unique sensitivity, unless it has electronics for signal calibration.

Usage

Thermopile pyranometers are used in meteorology, climatology, climate change research, building engineering physics, photovoltaic systems, and monitoring of photovoltaic power stations.

The solar energy industry has standards for the type and number of pyranometers to use based on the size of solar power plants. These standards advise installing thermopile pyranometers horizontally and photovoltaic pyranometers in the plane of PV modules to improve accuracy.

To use pyranometer data, quality assessment (QA) of the raw data is needed. This is because measurements can have errors from the environment or handling, such as:

• Pollution of the glass dome
• Issues with positioning
• Data logger errors
• Reflections and shading
• Calibration issues
• Dew, snow, or frost on the dome

These issues show specific patterns in the data. They can be found, flagged, and removed. QA can be done manually by an expert or automatically by an algorithm. Manual QA is common because some patterns are complex. Specialist software is needed for QA.

After QA, the clean data shows the solar irradiance at the site within the instrument's uncertainty. This data can be improved with satellite-based solar irradiance models. These models provide data for many years past and can be matched to the pyranometer data to create a long and accurate dataset. This is useful for solar resource studies or making Solar potential maps.

For monitoring solar power plants, pyranometers help verify the solar irradiance available. Because of weather changes and the size of solar plants, multiple pyranometers are installed to give accurate irradiation for each section. Standards call for at least 4 Class A thermopile pyranometers at a 100MWp PV power plant.

Solar measurements can be used to find Key Performance Indicators (KPI) for asset health monitoring or contracts about energy production. Some pyranometers have heating systems to reduce errors from dew, frost, or snow. These are used in cold or humid areas.

Photovoltaic pyranometer – silicon photodiode

Also known as a photoelectric pyranometer, a photodiode-based pyranometer detects part of the solar spectrum between 400 nm and 1100 nm. The photodiode changes these frequencies into current quickly, thanks to the photoelectric effect. The change is affected by temperature.

Design

A photodiode-based pyranometer has a housing dome, a photodiode, and a diffuser or optical filters. The photodiode is small and acts as a sensor. The current it makes is proportional to irradiance; a circuit, such as a transimpedance amplifier, changes this to a voltage. The output is usually millivolts, like thermopile pyranometers.

Usage

Photodiode-based pyranometers are used where the amount of visible solar spectrum or parts like UV, IR, or PAR (photosynthetically active radiation) needs to be measured. They are the core of luxmeters in photography, cinema, and lighting. They are sometimes installed near photovoltaic systems.

Photovoltaic pyranometer – photovoltaic cell

Made in the 2000s, the photovoltaic pyranometer is an update of the photodiode pyranometer. It was made to have a single reference photovoltaic cell when measuring the power of cells and modules. Each cell and module is tested by manufacturers using flash tests, and thermopile pyranometers are too slow and have a different spectral response. This caused mismatches in power measurement.

The active part of the sensor is a photovoltaic cell working in near short-circuit condition. It makes current when hit by light in the 350 to 1150 nm range, due to the photovoltaic effect. Its sensitivity matches that of Silicon photovoltaic cells.

Design

A photovoltaic pyranometer has these parts:

• A metallic container with a fixing staff
• A small photovoltaic cell
• Signal conditioning electronics

Silicon sensors like photodiodes and photovoltaic cells change their output with temperature. Newer models have electronics that adjust for temperature, removing its effect on the irradiance values. Some models have a board for signal amplification and conditioning of the signal.

Usage

Photovoltaic pyranometers are used in solar simulators and with photovoltaic systems to calculate module power and system performance. Because their spectral response is similar to photovoltaic modules, they can also help find problems in photovoltaic systems.

Reference PV Cell or Solar Irradiance Sensor can connect to sensors for module temperature, ambient temperature, wind speed, wind direction, and humidity, with one Modbus RTU output to a Datalogger. This is a main difference from Thermopile Pyranometers.

Standardization and calibration

Both thermopile-type and photovoltaic pyranometers are made following special rules.

Thermopile pyranometers follow the ISO 9060 standard, which is also used by the World Meteorological Organization. This standard splits pyranometers into three groups. The newest version of ISO 9060 from 2018 uses Class A for the best ones, followed by Class B and Class C.

Photovoltaic pyranometers are standardized under IEC 60904-4 for top-quality samples and IEC 60904-2 for other samples and instruments sold to people.

Signal conditioning

Pyranometers create very small signals, often only tens of millivolts. These signals can be disturbed by electromagnetic interference, especially over long cables or near solar power systems. To fix this, many pyranometers have special electronics that change the signal to a stronger, more stable form, like 4-20 mA or 0-1 V.

Other methods, such as using Modbus over RS-485 or SDI-12, help protect the signal in noisy places like large solar farms or small weather stations. These electronics can also store extra details, like the sensor’s calibration history and serial number.