Editing Pyranometer (section)

=== Thermopile pyranometers ===
A '''thermopile pyranometer''' (also called [[Thermoelectric materials|thermo-electric]] pyranometer) is a sensor based on [[thermopile]]s designed to measure the broad band of the solar radiation flux density from a 180° field of view angle. A thermopile pyranometer thus usually measures from 300 to 2800&nbsp;nm with a largely flat spectral sensitivity (see the spectral response graph) The first generation of thermopile pyranometers had the active part of the sensor equally divided in black and white sectors. Irradiation was calculated from the differential measure between the temperature of the black sectors, exposed to the sun, and the temperature of the white sectors, sectors not exposed to the sun or better said in the shades.

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.

==== Design ====
[[File:SR20 pyranometer linedrawing.pdf|thumb|229x229px|Linedrawing of a pyranometer, showing essential parts: (1) cable, (3) pyranometer and (5) glass domes, (4) black detector surface, (6) sun screen, (7) desiccant indicator, (9) levelling feet, (10) bubble level, (11) connector]]
In order to attain the proper directional and spectral characteristics, a thermopile pyranometer is constructed with the following main components:
* A [[thermopile]] sensor with a black coating. It absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nanometer range, and has a near-perfect cosine response.
* A glass dome. It limits the spectral response from 300 to 2,800 nanometers (cutting off the part above 2,800&nbsp;nm), while preserving the 180° field of view. It also shields the thermopile sensor from convection. Many, but not all, first-class and secondary standard pyranometers (see ISO 9060 classification of thermopile pyranometers) include a second glass dome as an additional "radiation shield", resulting in a better thermal equilibrium between the sensor and inner dome, compared to some single dome models by the same manufacturer. The effect of having a second dome, in these cases, is a strong reduction of instrument offsets. Class A, single dome models, with low zero-offset (+/- 1 W/m<sup>2</sup>) are available.

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.<ref>{{cite web |url=http://www.kippzonen.com/News/572/The-Working-Principle-of-a-Thermopile-Pyranometer# |title = The Working Principle of a Thermopile Pyranometer - Kipp & Zonen}}</ref> 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|signal calibration]].

==== Usage ====
[[File:Thermopile pyranometer as part of MeteoStation.jpg|thumb|left|Thermopile pyranometer as part of a meteorological station]]
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Thermopile pyranometers are frequently used in [[meteorology]], [[climatology]], [[climate change (general concept)|climate change]] research, [[building engineering physics]], [[photovoltaic system]]s, and monitoring of [[photovoltaic power stations]].

The solar energy industry, in a 2017 standard, IEC 61724-1:2017,<ref>[https://webstore.iec.ch/publication/33622 IEC 61724-1:2017]</ref> 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.<ref>{{Cite web |date=25 Mar 2022 |title=Growing Pain #3: On-site measurements in large-scale solar |url=https://solargis.com/resources/blog/best-practices/growing-pain-3-on-site-measurements-in-large-scale-solar }}</ref> This is because the pyranometer measurements typically suffer from environment-induced errors but also handling and neglect errors, such as:

* Pollution of the glass dome (e.g. deposition of atmospheric dust, bird droppings, snowfall), which reduces the measured irradiance
* Issues with positioning, resulting in measurements in a different plane (i.e. not horizontal or in-plane with PV modules) than expected 
* Data logger errors resulting in e.g. static values, oscillations, or data capped to a certain value
* Reflections and shading from the surrounding objects resulting in inaccurate measurements (i.e. not corresponding to solar irradiance)
* Calibration issues of the instrument, leading to measurement errors, offset, or drift over time
* Dew, snow, or frost on the glass dome on lower-end pyranometers not equipped with heating units

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 irradiance#Solar potential maps|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<ref>{{Cite web |title=IEC 61724-1:2017 {{!}} IEC |url=https://webstore.iec.ch/en/publication/33622 |access-date=2024-09-04 |website=webstore.iec.ch}}</ref> 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.