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Everything to know about Pyranometers

Solar radiation is the radiant (electromagnetic) energy emitted by the sun. It is the primary energy source for Earth and all life. Because without it, the Earth would be a frozen, dark planet.

There are three ranges of the solar spectrum in terms of wavelength. They are ultraviolet radiation, visible radiation, and infrared radiation.

Infrared radiation accounts for 49.4% of the light that reaches the Earth’s surface, whereas visible light accounts for 42.3%, and ultraviolet radiation accounts for slightly more than 8% of total solar radiation.

Each of these spectrums has unique properties that are useful for different applications.

A variety of factors influence the amount and intensity of solar radiation that a location receives. These factors include but are not limited to latitude, longitude, altitude, weather conditions, time of day, and season.

Also, not all of the solar radiation that is emitted by the sun reaches Earth. Although a large portion does, some are reflected or scattered by the atmosphere, which acts as a natural shield for our planet.

Solar energy can be absorbed directly from the sun, which is direct radiation, or it may be obtained indirectly by way of light that has been scattered as it passes through the atmosphere.

When solar radiation is absorbed by objects on Earth, it creates heat. The amount of heat that is produced depends on the wavelength of the radiation that is absorbed, as well as the object’s color and surface texture.

As mentioned, the sun’s radiation is important to us because it is the primary energy source of energy for Earth. That is why human beings have been trying to capture and use solar energy in different ways for centuries.

Nowadays, solar energy is used to power homes, businesses, and vehicles. It is also used to produce electricity, which can be utilized in a variety of other ways.

However, in order to take full advantage of solar energy, we need to understand how it works and how to best use it.

Therefore, it is important to measure solar radiation. With this information, we can better understand what areas on Earth receive the most sunlight and how best to utilize that light to our benefit.

That’s where pyranometers come in. Pyranometers are devices that help us measure solar radiation. They work by detecting the amount of energy that is emitted from the sun in a given area.

In this post, we’ll take a closer look at what pyranometers are, how they work, and some of their key features.

How is Solar Radiation Quantified?

Solar radiation is quantified in wavelengths or frequencies. Because light moves in waves, the wavelength is a distance from one peak of a light wave to the next. Wavelength is quantified in nanometers which are one-billionth of a meter.

Another property of light is its frequency. Frequency is the number of waves that pass by a certain point in one second. It is measured in Hertz (Hz). High frequencies have shorter wavelengths than low frequencies because they contain more waves.

Similarly, the longer the wavelength, the lower the frequency since it takes more time for the waves to pass by a certain point.

The wavelength’s energy increases with the decrease in wavelength. This is because shorter waves have more energy, and they are packed more tightly together than longer waves.

For example, UV radiation has a high frequency and a short wavelength, while infrared radiation has a low frequency and a long wavelength. That is why UV radiation has more energy than infrared radiation.

Multiple studies have shown that UV radiation is the most harmful to our health. It can cause sunburns, skin cancer, and eye damage.

What is a Pyranometer?

Pyranometer

A pyranometer is an instrument used to measure the amount of solar radiation that falls on a given surface from a hemispherical field of view. The output is measured in watts per square meter (W/m²) or kilowatts per square meter (kW/m²).

Previously, pyranometers were primarily utilized for climatological study and weather monitoring. However, the growing global interest in solar power has prompted a resurgence in pyranometer use.

Originally, pyranometers were calibrated using a sun simulator, but researchers have found that they can be calibrated using a clear blue sky as well. Calibration is important because it allows the pyranometer to give accurate readings.

How Does a Pyranometer Work?

A pyranometer consists of a light sensor, typically a silicon photodiode mounted on a blackened sphere.

The sensor is aimed at the sky and measures the amount of sunlight that is reflected off of all surfaces in the hemisphere. This measurement is then converted to watts per square meter.

Pyranometers can be used to measure direct, diffuse, and global horizontal irradiance. Direct irradiance is the amount of sunlight that falls on a given surface without being diffused.

Diffuse irradiance is the amount of sunlight that is scattered by the atmosphere. Global horizontal irradiance is the average amount of sunlight that falls on a given surface from all directions.

The global horizontal irradiance values typically vary from 0 to 1,400 W/m². It is expected to be greater in cases like reflections off buildings or snow-covered surfaces. 

Pyranometers can also be used to measure the irradiance of a target at a set angle. This is known as the pyrheliometric measurement.

In this mode, the pyranometer is calibrated to measure the amount of sunlight that is incident on a target at a specific angle. This is used to calculate the solar concentration or direct normal irradiance.

The global irradiance varies substantially depending on the position of the sun in the sky (and thus the time of day, season, and latitude). The diffuse irradiance is also affected by the weather conditions like clouds and fog.

The pyranometer measures the irradiance averaged over a period of time, typically 15 minutes. This allows for the calculation of an irradiance curve, which can be used to determine the solar resource at a given location.

Pyranometers do not need any external power and can be powered by the sun itself. However, newer pyranometers are equipped with an internal power source to help keep them operational in low-light or cloudy conditions.

Also, newer pyranometers can communicate wirelessly with a computer or data logger to transmit measurements. Thus, a pyranometer can be placed in a remote location and still send data back to a central monitoring station.

The wavelength range of solar energy that reaches the earth’s surface is around 300 nm to 2800 nm. Irradiance measurements with varying degrees of spectrum sensitivity will be produced depending on the type of pyranometer employed.

The reaction to “beam” radiation must vary with the cosine of the angle of incidence in order to be measured as irradiance.

This ensures a full response when solar radiation strikes the sensor perpendicularly (normal to the surface, the sun at zenith, 0° angle of incidence), a zero response when the sun is at the horizon (90° angle of incidence, 90° zenith angle), and a 0.5 response when the sun is at the horizon (90° angle of incidence, 90° zenith angle).

As a result, a pyranometer should have a “directional response” or “cosine response” that is as close to the ideal cosine characteristic as possible.

What are the types of Pyranometers?

According to ISO 9060, there are two pyranometer technologies. Thermopile and silicon semiconductor technologies.

The light sensitivity, also known as ‘spectral response’ varies with the type of pyranometer.

The solar radiation spectrum is the spectrum of sunlight that reaches the Earth’s surface at sea level at midday when A.M. (air mass) = 1.5. This spectrum is influenced by latitude and altitude. Aerosols and pollution also have an impact on the spectrum.

Thermopile pyranometers

A thermopile pyranometer consists of an array of thermocouples. The thermocouples are connected in series and create a voltage that is proportional to the irradiance.

The response of a thermopile pyranometer is not wavelength-dependent and the instrument does not need to be calibrated.

Thermopile pyranometers adhere to the ISO 9060 standard, which has been adopted by the World Meteorological Organization as well (WMO). This standard identifies three groups.

The most recent ISO 9060 standard, from 2018, employs the following classification: Class A for best performance, followed by Class B and Class C, whereas the older ISO 9060 standard, from 1990, utilized confusing words such as “secondary standard,” “first-class,” and “second-class.”

There are a lot of qualities in the sensors that contribute to the different classes: reaction time, thermal offsets, temperature dependence, directional inaccuracy, non-stability, non-linearity, spectral selectivity, and tilt response.

All of these are outlined in the ISO 9060 standard. A sensor must meet all of the minimal requirements for each of these attributes in order to be assigned to that category.

ISO 9060:2018 includes the sub-classifications ‘fast response’ and ‘spectrally flat’. They aid in sensor classification and differentiation.

Sensors with a spectral selectivity of less than 3% in the 0,35 to 1,5 m spectral range are classified as ‘spectrally flat’. In contrast to most Class A pyranometers, sensors classified as ‘fast response’ are uncommon. Most Class A pyranometers take 5 seconds or longer to respond.

Calibration is normally done using the WRR as an absolute reference. PMOD in Davos, Switzerland, manages it. Aside from the World Radiometric Reference, private laboratories like ISO-Cal North America have been accredited for these unique calibrations.

Calibration of a Class A pyranometer is per ASTM G167, ISO 9847, or ISO 9846. Class B and C pyranometers are commonly calibrated to ASTM E824 and ISO 9847.

A thermopile pyranometer is built with the following main components to achieve the desired directional and spectral characteristics:

  • Optical cover to reduce stray light and protect the sensor
  • Aperture to allow a defined beam of radiation to strike the sensor
  • Filter to select a specific wavelength band
  • Housing to protect the sensor and filters
  • Electronics to convert the voltage output of the thermocouples into an irradiance value

Modern thermopile pyranometers have active (hot) connections that are heated by radiation absorbed by the black layer.

The thermopile’s passive (cold) connections are in thermal contact with the pyranometer housing, which acts as a heat sink. This prevents any yellowing or decaying of the thermometer in the shadow, affecting the measurement of solar irradiation.

The thermopile produces a tiny voltage proportional to the temperature differential between the black coating and the instrument housing.

For example, a sunny day will produce roughly 10 mV (microvolts) per W/m2 (millivolts). Unless integrated with electronics for signal calibration, each pyranometer has a distinct sensitivity range.

Thermopile pyranometers are often used for solar radiation measurements in PV power plant monitoring.

Meteorological institutes, universities, and research centers also use these types of pyranometers. Because of the lack of spectral dependence, these pyranometers are often used for long-term measurements and comparisons with other instruments.

Thermopile pyranometers are installed horizontally with the aperture pointing downwards to minimize the effect of direct radiation on the sensor. The angle of incidence should be as close to perpendicular as possible (normal incidence) to get an accurate measurement.

Silicon semiconductor pyranometers (Photovoltaic pyranometers)

A silicon semiconductor pyranometer, also known as PV pyranometer or photoelectric pyranometer, is a type of pyranometer that uses an active silicon photodiode to measure irradiance.

Silicon semiconductor pyranometers are calibrated for a specific wavelength band and cannot be used to measure irradiance over other bands.

A photodiode-based pyranometer can detect the solar spectrum between 400 nm and 1100 nm.

The photodiode uses the photoelectric effect to quickly transform the solar spectrum frequencies into the current. The temperature influences the conversion, with a rise in current caused by a rise in temperature (approximately 0.1% °C).

This pyranometer is made up of a dome enclosure, a photodiode, and a diffuser or optical filter.

The photodiode is a sensor with a tiny surface area. It provides a voltage directly proportional to the photocurrent provided by the photodiode. The output is generally millivolts, similar to thermopile pyranometers.

Photodiode-based pyranometers are used when the amount of visible solar spectrum irradiation, or specific regions of the spectrum such as UV, IR, or PAR (photosynthetically active radiation), needs to be estimated.

This is accomplished by employing diodes with precise spectral responses. The heart of lux meters used in photography, cinema, and lighting techniques is photodiode-based pyranometers. They are sometimes positioned near photovoltaic system modules.

The photovoltaic pyranometer is an advancement of the photodiode pyranometer that was built around the 2000s to coincide with the spread of photovoltaic systems.

It addressed the requirement for a single reference photovoltaic cell when calculating the power of cells and solar modules.

Specifically, each cell and module is flash tested by their individual manufacturers, and thermopile pyranometers lack the required response speed and spectral response of a cell.

This would result in a clear disparity in power measurement, which would need to be quantified. This pyranometer is frequently referred to as a “reference cell” in technical documentation.

The active component of the sensor is a photovoltaic cell that operates in a near-short-circuit state. As a result, the produced current is directly proportional to the solar energy hitting the cell in the 350 nm to 1150 nm region.

When exposed to light radiation in the specified range, it generates current as a result of the photovoltaic effect. Its sensitivity is not uniform, but it is comparable to that of a silicon photovoltaic cell.

A photovoltaic pyranometer is made up of the following components:

  • A metal container with fixing staff
  • A small PV cell
  • Signal conditioning electronics

Silicon sensors, such as the photodiode and photovoltaic cells, change their output in response to temperature. In more current models, the electronics correct the signal with temperature, reducing the influence of temperature from sun irradiance estimates.

The casing of certain variants has a board for signal amplification and conditioning.

Photovoltaic pyranometers are employed in solar simulators and alongside photovoltaic systems to calculate photovoltaic module effective power and system performance.

Because the spectrum response of a photovoltaic pyranometer is comparable to that of a photovoltaic module, it can also be utilized for preliminary detection of problems in solar systems.

Reference PV Cell or Solar Irradiance Sensor can have external inputs to ensure the connection with other devices including data logger, weather station, or electronic balance.

This data is very important for solar PV plants as it helps in performance analysis, yield assessment, and troubleshooting.

What are pyranometers used for?

The Sun is the planet’s primary energy source. Pyranometers are used to measure the amount of energy that falls on a given surface. This measurement can be used for a variety of purposes, such as:

Solar energy studies

Pyranometers are used to measure the amount of solar energy that is available at a given location. This information can be used to help determine the feasibility of installing a solar energy system at a particular site.

pyranometer for measuring irradiance in solar farm

Pyranometers are used to monitor photovoltaic (PV) power plants in the solar energy industry.

The efficiency of a PV power plant can be assessed by comparing its actual output to a pyranometer-based expected output.

Low efficiency may signal PV plant repair. Pyranometers can also be employed to assess the applicability of possible PV power plant sites. Pyranometers are employed in this example to calculate the expected output of a PV installation.

Climate research 

Solar radiation is one of the main forces behind Earth’s weather patterns, making it a significant aspect of weather and climate research.

Pyranometers are commonly employed in such research to measure the GHI and determine the irradiance incident on the earth’s surface.

The GHI measured immediately outside the earth’s atmosphere is reasonably predictable, while irradiance at the earth’s surface is heavily influenced by factors such as cloud cover, aerosol concentration, fog, and smog.

Aircraft design

Pyranometers are used in the design of aircraft to ensure that they are able to withstand the high levels of solar radiation that they are exposed to during the flight.

Agriculture

Pyranometers are used in agriculture to help optimize crop growth by understanding the amount of sunlight that is available at different times of the day.

Land management

Pyranometers are used in land management to help make decisions about land usage based on the availability of sunlight. For example, they can be used to help decide whether to plant crops or install solar panels in a particular location.

Forestry

Pyranometers are used in forestry to help understand how sunlight affects the growth of trees. This information can be used to help make decisions about forest management.

Astronomy

Pyranometers are used in astronomy to study the sun and other objects in space.

Building energy management

Pyranometers are used in building energy management to help optimize energy usage by understanding the amount of sunlight that is available in different parts of the building.

Weather monitoring

Meteorological station at a Solar Energy Plant

Pyranometers are used in weather monitoring to help understand the patterns of sunlight that occur during different types of weather. This information can be used to help make decisions about when to plan outdoor activities.