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A pyranometer () is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density (W/m2) from the hemisphere above within a wavelength range 0.3 μm to 3 μm.
A typical pyranometer does not require any power to operate. However, recent technical development includes use of electronics in pyranometers, which do require (low) external power (see heat flux sensor).
To make a measurement of irradiance, it is required by definition that the response to "beam" radiation varies with the cosine of the angle of incidence. This ensures a full response when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0° angle of incidence), zero response when the sun is at the horizon (90° angle of incidence, 90° zenith angle), and 0.5 at a 60° angle of incidence. It follows that a pyranometer should have a so-called "directional response" or "cosine response" that is as close as possible to the ideal cosine characteristic.
The light sensitivity, known as spectral response, depends on the type of pyranometer. The figure here above shows the spectral responses of the three types of pyranometer in relation to the solar radiation spectrum. The solar radiation spectrum represents the spectrum of sunlight that reaches the Earth's surface at sea level, at midday with A.M. (air mass) = 1.5.
The latitude and altitude influence this spectrum. The spectrum is influenced also by aerosol and pollution.
In all thermopile technology, irradiation is proportional to the difference between the temperature of the sun exposed area and the temperature of the shadow area.
In the modern thermopile pyranometers the active (hot) junctions of the thermopile are located beneath the black coating surface and are heated by the radiation absorbed from the black coating. The passive (cold) junctions of the thermopile are fully protected from solar radiation and in thermal contact with the pyranometer housing, which serves as a heat-sink. This prevents any alteration from yellowing or decay when measuring the temperature in the shade, thus impairing the measure of the solar irradiance.
The thermopile generates a small voltage in proportion to the temperature difference between the black coating surface and the instrument housing. This is of the order of 10 μV (microvolts) per W/m2, so on a sunny day the output will be around 10 mV (millivolts). Each pyranometer has a unique sensitivity, unless otherwise equipped with electronics for Calibration.
The solar energy industry, in a 2017 standard, IEC 61724-1:2017, IEC 61724-1:2017 has defined the type and number of pyranometers that should be used depending on the size and category of solar power plant. That norm advises to install thermopile pyranometers horizontally (GHI, Global Horizontal Irradiation), and to install photovoltaic pyranometers in the plane of PV modules (POA, Plane Of Array) to enhance accuracy in Performance Ratio calculation.
To use the data measured by a pyranometer (horizontal or in-plane), quality assessment (QA) of the raw measured data is necessary. This is because the pyranometer measurements typically suffer from environment-induced errors but also handling and neglect errors, such as:
Each of the above issues appears as a specific pattern in the measured time series. Thanks to this, the issues can be identified, the erroneous records flagged, and excluded from the dataset. The methods employed for data QA can be either manual, relying on an expert to identify the patterns, or automated, where an algorithm does the job. As many of the patterns are complex, not easily described, and require a particular context, manual QA is very common. A specialist software with suitable tools is required to perform the QA.
After the QA procedure, the remaining ‘clean’ dataset reflects the solar irradiance at the measurement site to within the uncertainty of measurement of the instrument. The ‘clean’ measured dataset can be optionally enhanced with data from a satellite-based solar irradiance model. This data is available globally for a much longer time period (typically decades into the past) than the data measured by the pyranometer. The satellite model data can be correlated (or site adapted) to the pyranometer-measured data to produce a dataset with a long time period of data accurate for the specific site, with a defined uncertainty. Such data can be used to perform bankable solar resource studies or produce Solar potential maps.
For monitoring of operational PV power plants, pyranometers play an essential role in verifying the solar irradiance available at any given time or over a certain time period. Due to weather variability, redundancy, and the spatial scale of contemporary solar plants (above 100MWp), multiple pyranometers are installed to provide accurate solar irradiation for each section of the PV power plant. IEC 61724-1:2017 international standard for example calls for at least 4 Class A thermopile pyranometers to be installed at 100MWp PV power plant at all times.
Solar measurements that were QA’d could be used to derive Key Performance Indicators (KPI) such as Performance ratio* - metrics used in asset health monitoring or various contractual scenarios relating to energy produced (billing) or asset management (i.e. O&M). In these calculations, the measured sum of in-plane irradiation over a certain period is used as the determinant to which normalized produced PV electricity is compared to. Due to the difficulty of obtaining reliable in-plane measurements, especially in operational power plants, Energy Performance Index is increasingly being used instead of the older Performance ratio metric.
Some secondary standard pyranometers are equipped with integrated dome heating systems designed to reduce measurement errors caused by dew, frost, or snow accumulation on the sensor. These heating mechanisms help maintain the optical clarity of the dome surface in cold or humid environments, ensuring uninterrupted and accurate solar irradiance readings. For example, the MS-80SH model by EKO Instruments incorporates such a heating system in compliance with the ISO 9060:2018 Class A standard, and is used in high-latitude or alpine regions where frost-related interference is common. [3]
Reference PV Cell or Solar Irradiance Sensor may have up to 5 inputs ensuring the connection of Module Temperature Sensor, Ambient Temperature Sensor, Wind speed sensor, Wind Direction Sensor, and Relative Humidity, with only one Modbus RTU output connected directly to the Datalogger. This combination is called “weather station” which is suitable for monitoring the Solar PV Plants. This feature is one of the main differences between the Thermopile Pyranometer and the Reference Cell Solar Irradiance Sensor.
The latest version of ISO 9060, from 2018 uses the following classification: Class A for best performing, followed by Class B and Class C, while the older ISO 9060 standard from 1990 used ambiguous terms as "secondary standard", "first class" and "second class".,
Differences in classes are due to a certain number of properties in the sensors: response time, thermal offsets, temperature dependence, directional error, non-stability, non-linearity, spectral selectivity and tilt response. These are all defined in ISO 9060. For a sensor to be classified in a certain category, it needs to fulfill all the minimum requirements for these properties.
‘Fast response’ and ‘spectrally flat’ are two sub-classifications, included in ISO 9060:2018. They help to further distinguish and categorise sensors. To gain the ‘fast response’ classification, the response time for 95% of readings must be less than 0.5 seconds; while ‘spectrally flat’ can apply to sensors with a spectral selectivity of less than 3% in the 0,35 to 1,5 μm spectral range. While most Class A pyranometers are ‘spectrally flat’, sensors in the ‘fast response’ sub-classification are much rarer. Most Class A pyranometers have a response time of 5 seconds or more.
The calibration is typically done having the World Radiometric Reference World Radiometric Reference (WRR) as an absolute reference. It is maintained by PMOD PMOD in Davos, Switzerland. In addition to the World Radiometric Reference, there are private laboratories such as ISO-Cal North America ISO-Cal North America who have acquired accreditation for these unique calibrations. For the Class A pyranometer, calibration is done following ASTM G167, ASTM G167 ISO 9847 ISO 9847 or ISO 9846. ISO 9846ISO 9846:1993 -Calibration of a Pyranometer Using a Pyrheliometer Class B and class C pyranometers are usually calibrated according to ASTM E824 ASTM E824 and ISO 9847. ISO 9847
In both standards, their respective traceability chain starts with the primary standard known as the group of cavity radiometer by the World Radiometric Reference (WRR).IEC 60904-4:Procedures for establishing calibration traceability- Table1 and Fig.1
Another solution implies greater immunities to noises, like Modbus over RS-485, suitable for ambiances with electromagnetic interferences typical of medium-large scale photovoltaic power stations, or SDI-12 output, where sensors are part of a low power weather station. The equipped electronics often concur to easy integration in the system's SCADA.
Additional information can also be stored in the electronics of the sensor, like calibration history, serial number.
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