Pyranometer vs Pyrheliometer: What's the Difference?

Solar radiation is the electromagnetic radiation that comes from the sun. Its measurement is important for understanding the sun's intensity and its effects on Earth.

There are many fields that need solar radiation measurements, such as meteorology and climatology, renewable energy technology, and health science.

Therefore, instruments that measure solar radiation, like pyranometers and pyrheliometers, are essential.

Before we get into the instruments that measure solar radiation, we need to understand what the different types of solar radiation are.

When we say the “types” of solar radiation, we aren't referring to the different wavelength ranges that make up the electromagnetic spectrum. Instead, we are talking about the different ways the sun's radiation interacts with Earth's atmosphere.

The two types of solar radiation are direct radiation and diffuse radiation.

Direct Radiation

Direct radiation is the sun's rays that travel in a straight line from the sun to Earth with no interaction with Earth's atmosphere.

This type of radiation is also called “beam radiation.” When direct radiation hits Earth's surface, it is called “direct beam solar irradiance.”

Diffuse Radiation

Diffuse radiation is the sun's rays that bounce off of particles in the Earth's atmosphere before reaching the surface.

Diffuse radiation is sunlight that has been scattered by the atmosphere. This type of radiation is also called “diffuse sky radiation.” When diffuse radiation hits Earth's surface, it is called “diffuse solar irradiance.”

Global Radiation

Global solar radiation isn't a third type of solar radiation. Rather, it is the sum of direct and diffuse solar irradiance. Global solar radiation is the total amount of sunlight that hits the Earth's surface.

Global radiation is important for many applications, including solar power, solar hot water, and agriculture.

Because global solar radiation includes both direct and diffuse sunlight, it is a more accurate measure of the sun's energy than either direct or diffuse solar irradiance alone.

The ratio of direct to diffuse radiation

During clear, sunny weather, direct radiation accounts for about 85% of total insolation, while diffuse radiation accounts for about 15%.

With each degree of descent of the sun's position in the sky, the amount of diffuse radiation increases until it reaches 40% at 10° above the horizon.

Clouds and pollution can also increase the amount of diffuse radiation in the atmosphere.

Diffuse radiation makes up the vast majority of the sun's rays on a cloudy day. As a general rule, the higher the percentage of diffuse radiation, the lower the total insolation will be.

There is a significant difference between the amount of sunlight reaching Earth at different latitudes and the amount reaching Earth at different times of the day.

As a result of the cloudier, higher-latitude climates, the amount of total radiation emitted in the winter is higher than in the summer. There is less seasonal variation in the ratio of diffuse and direct radiation in the sunniest places.

Comparing London's wet and mild climate to Aden's dry and hot climate is a good example of the difference between the two cities' climates.

Daily irradiation in London's most sunny month (June) is about 5.5 kWh/m2, with diffuse radiation accounting for about half of it. Radiation levels fall below 1 kWh/m2 in December, and the vast majority of that radiation is scattered.

Average daily radiation is approximately 7 kWh/m2, with less than 30% of it diffuse, in May, Aden's sunniest month. About 35% of the irradiation in December is diffuse radiation, which is 5.25 kWh/m2 on average.

Which type of solar radiation is more important for solar power?

Direct radiation is more important for solar power because it is the sun's rays that travel in a straight line from the sun to Earth with no interaction with Earth's atmosphere.

Diffuse radiation is the sun's rays that bounce off of particles in the Earth's atmosphere before reaching the surface. Diffuse radiation is sunlight that has been scattered by the atmosphere.

The steeper your solar panels are inclined, the less of the sky they face and the less diffuse radiation they receive.

If your solar collectors are slanted at a 45° angle, for example, they are looking away from roughly a quarter of the sky and will only capture about three-fourths of the diffuse radiation in the sky.

Nonetheless, because direct radiation is significantly stronger than diffuse radiation, the amount of energy lost by tilted solar panels is often more than offset by the extra radiation collected by tracking the sun.

Solar hot water and photovoltaic systems can use both direct and diffuse radiation. However, concentrated solar power (CSP) systems require direct sunlight and will not work with diffuse radiation.

Now that we understand the different types of solar radiation, let's move on to the instruments that measure them.

There are two types of instruments that measure solar radiation: pyranometers and pyrheliometers. We have already published dedicated posts on pyranometers and pyrheliometers, which you should definitely check out.

But in case you don't have time to read those posts, we will give a brief summary of what pyranometers and pyrheliometers are in this post.

What is a pyranometer?

A pyranometer is a device used to measure the total amount of solar radiation that reaches the Earth's surface.

This includes both direct and diffuse radiation. The measurement is typically expressed in watts per square meter (W/m²) and is used for various purposes such as meteorology, climate research, and solar energy assessments.

Also Read: Everything to know about Pyranometers

Pyranometers are usually constructed as flat, horizontal surfaces with a sensor that measures the radiation. They are generally less expensive than pyrheliometers due to their simple construction and lack of need for a tracking mechanism.

In terms of sensitivity to orientation, pyranometers are less sensitive compared to pyrheliometers since they measure diffuse light, which comes from all directions.

They do not require a tracking mechanism and are usually stationary. Installation is relatively simple, and only minimal calibration is required.

Pyranometers are ideal for measuring solar radiation in a wide area or over an extended period.

They are commonly used in weather stations and are important for understanding the Earth's energy balance. They can also be used in solar energy systems to estimate the amount of energy that can be harvested from a particular location.

What is a pyrheliometer?

A pyrheliometer is an instrument used to measure direct beam solar radiation.

Unlike pyranometers, which measure both direct and diffuse radiation, pyrheliometers are designed to only measure direct radiation that comes directly from the sun. This measurement is also expressed in watts per square meter (W/m²).

Also Read: Everything to know about Pyrheliometers

Pyrheliometers have a narrow field of view, typically around 5° or less, and are highly sensitive to orientation.

They require precise alignment with the sun and must be equipped with a solar tracking system to stay aligned with the sun's position throughout the day.

Pyrheliometers are tube-shaped instruments with a sensor located at one end, and they are more expensive than pyranometers due to the need for a tracking mechanism.

Pyrheliometers are mainly used for solar resource assessment and concentrator solar power systems.

They provide accurate measurements of direct radiation, which is essential for assessing the potential of a solar energy system.

Concentrator solar power systems use mirrors or lenses to focus sunlight onto a small area, and pyrheliometers are used to measure the intensity of the focused beam.

What are the differences between pyranometers and pyrheliometers?

Here are the key differences between pyranometers and pyrheliometers:

Type of Radiation

Pyranometers measure both direct and diffuse solar radiation, while pyrheliometers measure only direct beam solar radiation.

Pyranometers typically use thermopile sensors or silicon photovoltaic cells that measure the temperature difference between two surfaces exposed to solar radiation, generating a voltage output proportional to the solar radiation intensity.

Pyrheliometers, on the other hand, use blackened thermopile sensors that are aligned with the sun to measure the intensity of direct beam solar radiation.

The sensor is typically housed in a vacuum-sealed tube to minimize thermal interference from the surrounding environment.

Field of View

Pyranometers have a hemispherical (180°) field of view, which means they can measure radiation from all directions.

Pyrheliometers, on the other hand, have a narrow field of view, typically 5° or less, and can only measure radiation from the direction of the sun.

Sensitivity to Orientation

Pyranometers are less sensitive to orientation, as they measure diffuse light from all directions.

Pyrheliometers, on the other hand, are highly sensitive to orientation and must be aligned with the sun to measure direct beam radiation accurately.

Pyrheliometers require regular calibration to account for changes in the sensor's sensitivity due to aging and environmental factors.

Tracking Mechanism

Pyranometers are usually stationary and do not require tracking, as they measure diffuse radiation from all directions.

Pyrheliometers, on the other hand, require a solar tracking system to stay aligned with the sun and measure direct beam radiation accurately.

Construction

Pyranometers are typically a flat, horizontal surface with a sensor, while pyrheliometers are tube-shaped instruments with a sensor at one end.

Pyrheliometers are typically more complex and have a vacuum-sealed tube to minimize thermal interference.

Applications

Pyranometers are commonly used in meteorology, climate research, and solar energy assessments, where measuring the total solar radiation is essential.

Pyrheliometers, on the other hand, are mainly used in solar resource assessment and concentrator solar power systems, where measuring the direct beam radiation is critical.

Measurement Units

Both pyranometers and pyrheliometers measure solar radiation in watts per square meter (W/m²).

The measurement units used by both pyranometers and pyrheliometers are standardized by the World Meteorological Organization (WMO).

The WMO defines one standard solar constant of 1361 W/m², which represents the average solar radiation intensity at the top of the Earth's atmosphere.

Pyranometers and pyrheliometers are calibrated to this standard to ensure accurate and comparable measurements across different instruments.

Price

Pyranometers are generally less expensive than pyrheliometers, primarily because they do not require a solar tracking system.

However, the cost of a pyranometer can vary depending on the brand, model, and features.

Installation Complexity

Pyranometers are relatively simple to install, with minimal calibration required. Pyrheliometers, on the other hand, are more complex to install, requiring precise alignment and regular calibration to measure direct beam radiation accurately.

Conclusion

Choosing between pyranometers and pyrheliometers depends on the specific application and measurement requirements.

Pyranometers are suitable for measuring total solar radiation, while pyrheliometers are necessary for measuring direct beam radiation accurately.

While both instruments measure solar radiation in watts per square meter, they differ in construction, the field of view, sensitivity to orientation, tracking mechanisms, and installation complexity.

Understanding these differences can help you choose the right instrument for your needs.