Pyranometers are radiation sensors able to measure the solar radiation flux density in the range of wavelength between 0.3 μm to 3 μm. Currently, pyranometers can be:
- Thermopile based, for scientific and meteorological use,
- Silicon based,with the traditional photodiode or the newer PV cell, for light’s measures and for benchmarking the perfomance of PV.
Thermopile based technology
The thermopile pyranometer is made of a big quantity of junctions and placed in series.
The hot side is exposed to solar radiation, while the cold side is in the shade: thus, by measuring the voltage between the 2 joints, you can take note of the radiation that is irradiating the sensor. The voltage gives a measure of solar irradiation.
Thanks to that, the thermopile pyranometer is capable of absorbing the widest solar radiation spectrum with a spectral sensibility between 300 and 3000 nm, and a flat response along nearly all this spectrum.
Another important element to ensure the correct sensibility of the instrument is the glass dome.
Following ISO9060 thermopile pyranometer are classified as follow, with costs and precision decreasing from up to down:
- Secondary Standard pyranometers
- First class pyranometers
- Second class pyranometers
Its natural signal is quite weak, thus is rather vulnerable to electric and electromagnetic disturbances. So our pyranometers are equipped with an internal amplifier.
This is the standard solar measuring instrument used for monitoring weather conditions and for highly accurate researches. Can be used for evaluating the efficiency of a photovoltaic system, but for the purpose of the latter, photovoltaic pyranometers based on silicon cell are more suitable.
Silicon cell based technology
The silicon cell based technology is an evolution of silicon diodes and sensors. They both give a similar curved spectral response. Their working principle is different. When photons coming from the sun hit the silicon cell (diode), electrons of cell can ‘jump’ on the upper band and then start the ‘photovoltaic phenomena’
Such value is usually influenced by temperature on a scale of 0,1% * °C circa, unless the electronics contained in the photovoltaic pyranometer provides a compensation in temperature.
A well calibrated pyranometer with temperature compensation allows to perform accurate measurements of PV systems performances, and support diagnostics on malfunctioning PV systems.
Analogue or digital output?
Pyranometers with analog output are ‘read’ by all datalogger with analog (V or mA) input channels.
In contrast you have to consider that medium-large PV systems generates many E.M.C. noises; thus pyranometers with analog output signal are more subjected to disturbances by external factors like E.M.C. disturbances. Indeed, voltage output are more subjected to E.M.C. noises, while current loop output (e.g. 4..20mA) are much less susceptible.
Digital output (e.g. RS485 Modbus) is the right choice for medium-large PV systems. This choice has double advantage of a good rejection to E.M.C. noises, and also the irradiation digital value cannot be altered along the wires and it cannot be affected by conversions errors: the value is transferred without any possibility to altering the measured value.
|Use||Requested Accuracy||Datalogger Input Type||E.M.C. Disturbances level||Model|
|Meteorology||< ± 3%||Voltage, Current||Low, Medium||PYR1-420|
|Meteorology||< = ± 8%||Voltage, Current||Low, Medium||PYR2-420|
|6-20kW PV system||< ± 10%||Voltage, Current||Low||LM1-10V LM1-420|
|30-200kW PV system||< ± 5%||Voltage, Current||Low, Medium||LM1-10V LM1-420|
|200-500kW PV system||< ± 5%||Current; RS485||Medium, High||LM1-420 Sunmeter|
|500-2 MW PV system||< = ± 3%||Current; RS485||Medium, High||Sunmeter An/Dig|
|500-2 MW PV system||< ± 3%||RS485||High||Sunmeter PRO|
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