Solar radiation can be accurately measured solely on the horizontal plane, though obtaining such measurements on the solar generator plane is no easy matter. However, in view of the fact that accurate radiation readings are essential for determining whether PV systems are working properly, the key sensors that are used for such measurements and the problems associated with these devices will now be briefly discussed.
In the field of meteorology, virtually all irradiance and irradiated energy are measured worldwide using pyranometers, which are composed of a thermopile comprising a large number of series-connected thermocouples. The black receiver at the tip of a thermocouple absorbs a broad band of total irradiated energy in the 300 nm < l < 3 pm wavelength spectrum. A double hemispheric glass dome prevents additional reflection on the glass at small angles of incidence and helps avert condensation on the glass under unfavourable temperature to moisture ratios. A pyranometer’s output voltage is (with a certain amount of dwell) exactly proportional to irradiance, but is relatively weak since the voltage generated by thermocouples is extremely low. With G — 1 kW/m2, the output voltage of a typical commercial pyranometer such as the Kipp&Zonen CM 11 is around 5 mV.
This relatively low output voltage cannot be processed directly, and must therefore be amplified. Design engineering a precise amplifier that is able properly and accurately to amplify signals ranging from around 10 pV to around 10 mV necessitates a certain amount of electrical engineering acumen. Substantial measurement errors can occur with poorly conceived measuring set-ups (e. g. improper shielding, substandard measurement amplifiers and so on).
Inasmuch as pyranometer sensitivity gradually decreases (of the order of tenths of a per cent per year), these devices should be calibrated every two years in settings where extreme accuracy is needed. Reasonably priced pyranometer calibration services are available from specialized organizations such as the World Radiation Centre in Davos. These calibrations can be readily replicated. Good-quality and accurate pyranometers cost anywhere from €1300 to €2000 and their drying agent needs to be replaced annually. Figure 2.43 shows an example of a pyranometer that is used to measure global radiation incident on the horizontal plane.