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Article updated on 11/03/2020 by Brett Smith and Ben Pilkington
A pyranometer is a type of actinometer that gives information about solar irradiance in the desired location by measuring solar radiation flux density. Pyranometers measure solar radiation flux density in watts per square meter (W/m2) within a wavelength range of 300 nm to 3000 nm from a fixed plane at a specific orientation with a hemispherical field of view. The pyranometer uses flat spectral sensitivity to cover this spectrum.
On most occasions, the plane of interest for a pyranometer is horizontal, so that the field of view matches the sky. In this orientation, the assessed quantity is known as global horizontal irradiance (GHI). The direction is sometimes set at an angle, for instance, in solar power applications where the pyranometer is aligned with the orientation of solar panels. In this application, the assessed quantity is called the global tilted irradiance (GTI).
At any given moment, global irradiance is highly dependent on the position of the sun, weather conditions and atmospheric conditions, such as the presence of smog or levels of airborne particulate matter.
In photovoltaic applications, pyranometers are used to assess the efficiency of a solar power system. These devices can be used to compare the actual output of a solar power system to the expected output. If a significant deficit is detected, it may indicate that service is needed. Pyranometers can also be used to determine the suitability of a location for a potential solar power plant.
On rare occasions, the plane of interest is horizontal, but the pyranometer is at a significant height and facing downward. This is to measure the diffuse reflection of light from the Earth’s surface. These measurements can be used to assess net irradiance and albedo, or the amount of light being reflected back into space.
The instrument is standardized by the ISO 9060 standard and has been adopted by the World Meteorological Organization. Pyranometers are calibrated with the World Radiometric Reference, which is maintained by the World Radiation Center in Davos, Switzerland.
Design of Pyranometers
Pyranometers are classified as ‘A’, ‘B’ or ‘C’ based on specifications such as response time, directional response, temperature response, tilt response and calibration method, as dictated by the ISO 9060 standard.
There is also a subclass of pyranometers called ‘spectrally flat’. Widely used, spectrally flat pyranometers can provide highly accurate measurements of solar radiation on cloudy days or when irradiation includes reflected light. “Normal” pyranometers can only measure solar radiation accurately under clear skies. Due to their nature, spectrally flat pyranometers are well-suited to measuring GHI, albedo and net radiation.
The main components of pyranometers are a thermopile, a glass dome, and an occultating disc.
The thermopile is a sensor consisting of thermocouples connected in series and coated in black coloring, which absorbs solar radiation. It exhibits a near-perfect cosine response and a flat spectrum that covers 300 nm to 50,000 nm. It is capable of producing a potential that is relative to the temperature gradient.
The glass dome restricts the spectral response from 300 nm to 3000 nm, from the near-infrared, though the visible light, and into the UV-B parts of the spectrum, for a field of view of 180 degrees.
Moreover, it shields the thermopile from wind, rain, and convection. Ideally, transmission through the dome is 100%. In reality, however, the transmission is around 92%. Occasionally, a second dome is used to boost performance. The occultating disc is used for measuring the diffuse radiation and for blocking (occultating) beam radiation from the surface.
Based on the Seebeck- or thermoelectric effect, a pyranometer is operated based on the measurement of a temperature difference between a clear surface and a dark surface.
The black coating on the thermopile sensor absorbs solar radiation, while the clear surface reflects it. Hence, less heat is absorbed on the clear surface.
The potential difference created in the thermopile owing to the temperature gradient between the two surfaces reveals information about the amount of solar radiation.
The voltage produced by the thermopile can also be measured using a potentiometer. While most pyranometers generate an analog signal, some come with a smart digital interface.
Advantages of Pyranometers
The key benefits of pyranometers are:
- Very small temperature coefficient
- Calibrated to ISO standards
- More accurate measurements of performance index and performance ratio
- Longer response than a photovoltaic cell
- Integrated measurement of the total available short-wave solar energy under all conditions
Applications of Pyranometers
Pyranometers are used as stand-alone devices, or they can be used as part of a meteorological station. These are the major applications of pyranometers:
- Predicting insulation requirements for building structures
- Ideally locating commercial greenhouses
- Designing commercial solar power systems
- Meteorological and climatological studies
- Measuring of solar intensity data
References and Further Reading
Ringoir, R, Kipp and Zonen. (2011) When to use a pyranometer vs a reference cell. [Online] Renewable Energy World. Available at: https://www.renewableenergyworld.com/2011/02/14/when-to-use-a-pyranometer-vs-a-reference-cell/#gref (Accessed on 6 March 2020).
Humboldt State University. Apogee Pyranometer. [Online] Available at: https://engineering.humboldt.edu/resources/equipment-handbook/pyranometer (Accessed on 6 March 2020).
Hukseflux Thermal Sensors. What is a pyranometer? [Online] Available at: https://www.hukseflux.com/applications/solar-energy-pv-system-performance-monitoring/what-is-a-pyranometer. (Accessed on 6 March 2020).
Poling, R. (2015) What is a solar pyranometer? [Online] Solar Power World. Available at: https://www.solarpowerworldonline.com/2015/03/what-is-a-solar-pyranometer/ (Accessed on 6 March 2020).