Pyranometer
A pyranometer (from Greek πῦρ (pyr) 'fire' and ἄνω (ano) 'above, sky') 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.
However, recent technical development includes use of electronics in pyranometers, which do require (low) external power (see heat flux sensor).
Depending on the type of pyranometer used, irradiance measurements with different degrees of spectral sensitivity will be obtained.
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.
The figure here above shows the spectral responses of the three types of pyranometer in relation to the solar radiation spectrum.
[2] 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.
The thermopile generates a small voltage in proportion to the temperature difference between the black coating surface and the instrument housing.
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.
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.
The ‘clean’ measured dataset can be optionally enhanced with data from a satellite-based solar irradiance model.
IEC 61724-1:2017[5] international standard for example calls for at least 4 Class A thermopile pyranometers to be installed at 100MWp PV power plant at all times.
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.
The photodiode converts the aforementioned solar spectrum frequencies into current at high speed, thanks to the photoelectric effect.
Photodiode-based pyranometers are implemented where the quantity of irradiation of the visible solar spectrum, or of certain portions such as UV, IR or PAR (photosynthetically active radiation), needs to be calculated.
When invested by a luminous radiation in the mentioned range, it produces current as a consequence of the photovoltaic effect.
[13] In addition to the World Radiometric Reference, there are private laboratories such as ISO-Cal North America[14] who have acquired accreditation for these unique calibrations.
It is considered a "weak" signal, and as such, rather vulnerable to electromagnetic interferences, especially where the cable runs across decametrical distances or lies in photovoltaic systems.
Thus, these sensors are frequently equipped with signal conditioning electronics, giving an output of 4-20 mA or 0-1 V. 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.
