Last update
A comparison of Photovoltaic & Thermopile Pyranometers Measurements
Thermopile Pyranometer


Photovoltaic cell pyranometer
A PV cell pyranometer has the same receiving surface of a PV module – a flat glass – but his strength lies in possessing the same receiving capacity of a PV module (non-linear response, see red area in Fig. 1). This means that a quality PV cell pyranometer receives the full spectrum that can be picked up by a PV module, and only that. In this way, the sensor acts as a reference of all the solar irradiance power receivable from a Si-crystalline module. The relation between the output power (normalized at 1000) and the value of only that irradiance power allows the correct measurement of financial performances of the PV system. Thus, a drift towards minor efficiencies of the photovoltaic power system might be observed by a monitoring system that has a PV cell pyranometer with the required precision. These premises aside, there is the ‘pragmatic assumption’ between many workers of the sector (O&M, Asset Manager, etc.) that deviations between the measured values of irradiance carried out by thermopile-type and PV cell pyranometers, are established between 3% and 6%. Contrary to this, preliminary data from a study with the Department of Physics of the University of Milan show how deviations vary between 3% and 13%. Of course, this does not include marginal moments like an abrupt variation of irradiance given by the passage of clouds, spiking the deviations up to 80%. This is understandable, considered the difference in the speed of response between the two types of sensors. Even at sunset, deviations above 15% between the two measured values of irradiances have been noted. The three graphs below show the percentile variation of irradiance detected by a thermopile pyranometer and a monocrystalline photovoltaic cell pyranometer.