Solar radiation can be measured according to the Seebeck effect. This thermopile effect is part of the physical phenomenon called thermoelectric effect, in which a temperature difference between two dissimilar electrical conductors produces a voltage difference between the two substances. The potential difference between the two conductors kept at different temperatures is defined as the Seebeck effect.
A thermopile pyranometer is made of a big quantity of junctions as those written above, 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.
Another important element to ensure the correct sensibility of the instrument is the glass dome, which preserves the pyranometer from convection currents and wind. If the glass dome is missing, any air current would abnormally cool down the hot side, thus altering the measurement of irradiation.
Thanks to this principle and these shrewdness, 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.
With some shrewdness, a pyranometer can be used to measure the diffused solar radiation.
According to ISO9060 which defines standard of pyranometers, thermopile pyranometer are classified as follow, with costs and precision decreasing from up to down:
- Secondary Standard pyranometers
- First class pyranometers
- Second class pyranometers
Unfortunately its signal is quite weak, thus is rather vulnerable to electric and electromagnetic disturbances. Then, in areas where there are these disturbances, its signal has to be amplified within the first few decimeters or inside the body of pyranometer.
This is the standard solar measuring instrument used for monitoring weather conditions and for highly accurate researches. Sometimes, it is also used for evaluating the efficiency of a photovoltaic system, but for the purpose of the latter, photovoltaic pyranometers based on silicon cell, like the one of photovoltaic modules, are more suitable.
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