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Everything to know about Pyrheliometers

Solar radiation is one of the main sources of energy for the Earth. It doesn’t only give life to plants and make the Earth warm, but it is also used to produce renewable energy.

There are many ways to measure solar radiation and each method has its own benefits and drawbacks. Therefore, it is important to have tools to measure and quantify solar radiation.

The pyrheliometer is one of them and is used to measure direct solar radiation. In other words, pyrheliometers measure the amount of sunlight that is directly hitting a surface. The instrument is designed solely for measuring solar radiation and not the diffuse sky radiation.

In this post, we will explore everything there is to know about pyrheliometers. Let’s get started!

What is a Pyrheliometer?

A Pyrheliometer is an instrument used to measure the intensity of direct sunlight. When the sun’s rays hit the instrument, a light sensor measures the amount of sunlight and then sends this information to a display.

The instrument is made up of a small hole in a screen and a calibration scale. The hole is used to measure the amount of sunlight that passes through it, and the calibration scale is used to convert that into a numerical value.

Pyrheliometers can be used to measure both short-term and long-term solar radiation levels. They are often used to measure the amount of radiation that is incident on a surface or to calculate the total amount of radiation that is received by a location.

Pyrheliometers together with pyranometers are the main instruments used to measure direct and global solar radiation. However, pyrheliometers are used to measure only direct solar radiation, while pyranometers can measure both direct and global solar radiation.

Why should we measure direct solar radiation when we can also measure global solar radiation?

The main reason is that direct solar radiation is the component of solar radiation that affects the Earth’s climate the most. It is the radiation that reaches the Earth’s surface without being scattered or absorbed by the atmosphere.

When measuring global solar radiation, we are including the radiation that has been scattered and absorbed by the atmosphere. This means that the measurement is not only affected by the radiation that reaches the Earth’s surface, but also by the radiation that is emitted by the atmosphere.

On the other hand, when measuring direct solar radiation, we are only measuring the radiation that reaches the Earth’s surface. This means that the measurement is not affected by the radiation that is emitted by the atmosphere.

This is why it is important to measure both direct and global solar radiation. By doing so, we can get a more accurate understanding of the radiation that is affecting the Earth’s climate.

How does a Pyrheliometer work?

A Pyrheliometer works by measuring the amount of electromagnetic radiation that is directly incident on its surface. The instrument consists of a tube with a small hole at one end and a light sensor at the other.

When the sun’s rays hit the instrument, they pass through the hole and hit the light sensor. This sensor then measures the amount of radiation that has passed through the hole and sends this information to a display.

However, there are pyrheliometers that don’t have a light sensor. Instead, they have a black surface at the end of the tube. When the sun’s rays hit this surface, they are absorbed and create heat that is transferred to the thermopile.

This thermopile then creates an electric current. The electric current passes through the multimeter where it is converted into a numerical value.

The readout from the multimeter is in millivolts and is proportional to the amount of radiation that has been received. This value can be then converted into watts per square meter by using a calibration factor.

Pyrheliometers are installed on a tracking mount that keeps the instrument pointed at the sun. The mount is usually controlled by a computer or by an automatic tracking system.

The mount allows movement on 2 axis: azimuth and elevation. The azimuth mount controls the rotation of the instrument about the vertical axis, while the elevation mount controls the rotation of the instrument about the horizontal axis.

This allows the instrument to stay pointed at the sun no matter what the time of day or season.

How does Pyrheliometer track the sun’s position?

Pyrheliometers are installed on a solar tracker to keep it pointed at the sun. The solar tracker continually adjusts the position of the instrument to ensure that it is always facing the sun.

The ideal position of the pyrheliometer is calculated using a solar position algorithm based on GPS coordinates and the time of day. Some trackers may also employ extra sensors (sun sensors) to fine-tune the position once the sun is visible and achieve tracking accuracies of better than 0.1 °.

It is crucial for trackers to remain stable and level for extended periods of time regardless of weather conditions. Thus, many modern pyrheliometer trackers include wind and tilt sensors to compensate for gusts of wind and uneven surfaces.

Calibration of a Pyrheliometer

Accurate measurement of direct beam solar irradiance is important for many industrial and scientific applications. That is why pyrheliometers need to be calibrated regularly.

The calibration of a pyrheliometer is usually done by pointing it at the sun and measuring the amount of radiation with an incident thermopile. A pyrheliometer is calibrated by adjusting the zero offset and gain of the sensor until the measured value is equal to the value of an incident thermopile.

New pyrheliometers need to be calibrated when they are first installed, and then again every 6 months to 1 year. This is necessary to ensure that the instrument is still accurate and provides reliable measurements.

Because the solar irradiance on a given day can vary significantly, a pyrheliometer should be calibrated against a known standard.

Pyrheliometer measurement requirements are governed by International Organization for Standardization (ISO) and World Meteorological Organization (WMO) guidelines. Intercalibration comparisons between pyrheliometers are performed on a regular basis to determine their accuracy.

The International Pyrheliometer Comparisons which take place every five years at the World Radiation Centre in Davos, aim to assure the global transmission of the World Radiometric Reference.

During these comparisons, all participating stakeholders bring their instruments, solar tracking, and data acquisition systems to Davos to undertake simultaneous solar radiation measurements with the World Standard Group.

What are the benefits of using a Pyrheliometer?

There are a number of benefits to using a Pyrheliometer. Some of these benefits include:

  • The ability to measure the direct sunlight intensity
  • The ability to calculate the amount of radiation that is incident on a surface
  • The ability to measure the amount of sunlight that is focused on a small area

What are the limitations of using a Pyrheliometer?

There are also some limitations to using a Pyrheliometer. Some of these limitations include:

  • The instrument can only measure the amount of direct sunlight
  • The instrument is not able to measure the diffuse sunlight
  • The instrument is not able to measure the reflected sunlight
  • The instrument is not able to measure the thermal radiation

How accurate are Pyrheliometers?

The accuracy of a Pyrheliometer can vary depending on the type of Pyrheliometer that is being used. However, most Pyrheliometers have an accuracy of ±3%.

What are the different types of Pyrheliometers?

There are two types of Pyrheliometers: SHP1 and CHP1.

SHP1

When compared to the CHP1 type, the SHP1 type has a better interface because it includes both improved analog o/p and digital RS-485 Modbus. This type of meter has a response time of fewer than 2 seconds, and the temperature correction ranges from -40°C to 70°C.

CHP1

The CHP1 radiometer is the most commonly used type for directly measuring solar radiation.

This meter has one thermopile detector and two temperature sensors. It generates a maximum o/p of 25mV under normal atmospheric conditions. This type of device strictly adheres to the most recent ISO and WMO standards for Pyrheliometer criteria.

The type of Pyrheliometer that should be used depends on the type of application that is being used. Some factors that should be considered when choosing a Pyrheliometer include:

  • The size of the aperture
  • The wavelength range that is being measured
  • The accuracy of the Pyrheliometer
  • The type of radiation that is being measured

Where are Pyrheliometers used?

Pyrheliometers are used in a variety of applications, including:

Solar energy research

Solar energy studies benefit from the high accuracy and fast response of a pyrheliometer in calculating direct normal irradiance (DNI). This measurement is necessary for the validation of satellite-derived irradiance products and for ground-based measurements.

Solar thermal energy systems

Pyrheliometers are used in conjunction with other sensors to measure the thermal radiation flux. This information is then used to design and optimize solar thermal systems.

Photovoltaic systems

Solar PV systems convert sunlight into electrical energy. When installing a solar PV system, it is important to know the amount of direct sunlight that is available at the site. This information can be obtained by using a Pyrheliometer.

Aircraft instrumentation

Pyrheliometers are used in aircraft to measure the amount of sunlight that is focused on a small area. This information is then used to help maintain the aircraft’s navigation and communication systems.

Weather forecasting

Pyrheliometers are also used in weather forecasting to measure the amount of direct sunlight. This information helps meteorologists to better understand how the sun’s radiation affects the weather.

Maritime navigation

Maritime navigation also relies on the use of Pyrheliometers. These devices are used to calculate the sun’s position, which is then used to help navigate a ship.

Soil moisture measurement

Pyrheliometers can also be used to measure the amount of sunlight that is incident on a surface. This information can be used to help determine the evaporation and transpiration rates of soil.

Land surveying

Pyrheliometers are also used in land surveying to measure the amount of sunlight that is incident on a surface. This information can be used to help determine the albedo of a surface.

Aerospace engineering

Aerospace engineering also uses Pyrheliometers to measure the amount of sunlight that is incident on a surface. In this field, the devices are used to measure the heat load on a structure.

Climate research

Pyrheliometers are also used in climate research to measure the amount of sunlight that is incident on a surface. Climate researchers use this information to help understand how the Earth’s climate is changing.

Pyrheliometers are a valuable tool for measuring solar radiation and can be used in a variety of applications. By understanding the limitations of using a Pyrheliometer, you can make sure that you are getting the most accurate readings possible.

Tips for maintaining your Pyrheliometer

Pyrheliometers require minimal maintenance. However, there are a few tips that can be followed to help maintain the instrument:

  • Keep the Pyrheliometer clean and free of dirt and dust
  • Make sure the aperture is clear and free of obstruction
  • Check the telescope for any damage or wear
  • Keep the instrument in a safe place when not in use

By following these tips, you can help maintain your Pyrheliometer and ensure that it is providing accurate readings.

Pyrheliometer Angles

When using a pyrheliometer, the angle at which it is pointed will affect the accuracy of the measurement. Many people have the misconception that the opening half-angle of the pyrheliometer’s telescope is what we define as a ‘field of view’.

However, this is not the case. The field of view is instead defined as the angle subtended by the instrument at the target. In other words, the angle between the line of sight to the target and the perpendicular bisector of the aperture.

The detector’s physical size is also going to occupy some of the aperture’s field of view, which will decrease the angle that is visible to the target.

Pyrheliometers use a geometrical function called the acceptance function to integrate all radiation. The acceptance function is a plot of the solid angle that is accepted by the detector as a function of the angle from the optical axis.

The acceptance function can be used to calculate the angular response of the detector, which is important when trying to correct any angular errors.

When using a pyrheliometer, it is important to understand the different factors that can affect the accuracy of the measurement. By understanding these factors, we can take the necessary steps to ensure that our measurements are as accurate as possible.

What to look for choosing a Pyrheliometer?

Spectral range

When choosing a Pyrheliometer, it is important to consider the spectral range that the instrument covers. The instrument should cover the wavelength range that is being measured.

Aperture size

The aperture size is also important to consider when choosing a Pyrheliometer. The instrument should have a large aperture size to allow for accurate measurements.

Response Time

Any measuring device needs time to respond to a change in the parameter being measured. The response time of a Pyrheliometer should be considered when choosing the instrument.

Operating temperature

The operating temperature of the Pyrheliometer should also be considered when making a purchase. The instrument should have a wide operating temperature range to allow for use in a variety of applications.

Field of view

In order to measure only direct sunlight, the instrument should have a field of view that is restricted to the direct sun. This will minimize the amount of reflected and diffuse sunlight that is measured.

The sun is seen from outside the Earth’s atmosphere as a disk with an angular distance of around 0.27°. The sun appears much larger from ground level. Its visible size is determined by the atmosphere: the larger the sun in the sky, the darker the sky.

According to WMO (World Meteorological Organization) standards, all modern pyrheliometers have the same field of view, which is defined by an opening half-angle of 2.5 °. This means that the direct solar radiation measurement will have some ‘circumsolar’ radiation.

The amount of direct solar radiation varies substantially depending on the height of the sun in the sky (and thus location on the planet, time of day, and season), as well as climatic and environmental elements such as clouds, aerosols, smog, fog, precipitation, and others.

Typical direct sun irradiance values vary from 0 to the theoretical maximum of the solar constant, which is around 1361 W/m2.

Accuracy

When a pyrheliometer is in use, its performance is correlated to a variety of parameters such as temperature, irradiance level, and so on.

Normally, the irradiances are calculated using the provided sensitivity figure. If the conditions deviate greatly from the calibration conditions, there will be uncertainty in the computed irradiances.

The WMO expects a maximum inaccuracy in hourly radiation totals of 3% for a first-class pyrheliometer. Since some response variations balance each other out if the integration period is extended, an inaccuracy of 2% is expected in the daily total.