Reference cells are precisely calibrated silicon solar cells that are used to measure radiation and are well packed to protect them from the environment. At prices ranging from €200 to €400, they are considerably less expensive than pyranometers. Short-circuit current in a solar cell can safely be assumed to be proportional to the irradiance G that occurs at the reference cell (see Chapter 3). Many reference cells integrate a precise measurement shunt that converts the short-circuit current into a voltage (e. g. about 30 millivolts with G — 1 kW/m2) and an additional sensor that allows for precise cell
Figure 2.43 Kipp&Zonen CM 11 pyranometer for measurement of global radiation incident on the horizontal plane
temperature measurement (e. g. a PT-100 sensor, thermocouple, or the like). Figure 2.44 shows two such reference cells.
Unlike pyranometers, solar cells only use a portion of the solar spectrum, i. e. photons whose energy exceeds the band gap energy EG (see Section 2.7 and Chapter 3). Although solar cell calibration technicians try to take account of this particularity as much as possible, as the solar spectrum is not always fully homogeneous (including the presence of exactly the same G and the same AM number), solar cell calibration discrepancies can arise. Many vendors or calibration providers calibrate these reference cells using artificial light sources, which are also used for solar module measurements. Although such light sources are similar to natural sunlight, they do not have exactly the same spectrum as natural sunlight. [Ima92] proposes a reference cell calibration method for use in natural sunlight. Hence, precise and replicable reference cell calibration poses a problem for PV systems.
Figure 2.44 Two reference cells with integrated cell temperature measurement sensors; the ESTI cell is shown above, and the Siemens M1R is below. The M1R is the centre measurement cell, around which eight non-contacted cells are arrayed, thus allowing for highly accurate readings of the thermal conditions in a module
Reference cells should be mounted at a point on the solar generator plane where the solar radiation reflects the mean solar radiation for the site as a whole. Wherever possible, the same cell technology should be used for reference cells that is used in the PV system being measured. If the reference cells are the same types as those used in the solar module, and if they possess a flat glass surface like the cells shown in Figure 2.44 (and preferably are made out of the same type of glass as the module glass), the reference cells will be affected by various key factors that affect the reference cells in exactly the same way the solar modules are affected. Hence variations in the solar spectrum, as well as additional reflection from direct beam radiation incident at small angles of incidence, have the exact same impact on reference cells as on solar modules, thus enabling integrated temperature sensors to provide a point of reference for mean module temperatures that are measured in the solar generator. Reference cells mounted in this fashion are a useful tool for monitoring the correct functionality of a PV system (see Chapter 7). Reference cells with glass domes are less suitable for this purpose since they reflect radiation at a low angle of incidence to a lesser extent and differently relative to solar modules.
The ESTI reference cell shown in Figure 2.44 was widely used in the 1990s in particular. These cells are for the most part well calibrated and exhibit only minor discrepancies between the readings for the various cells. Unfortunately, they also have an endemic defect: that is, radiation at a low angle of incidence can penetrate the lateral edge of the cell, whereupon this radiation is bounced onto the measurement cell via total reflection from the glass surface – particularly in the presence of the kind of white backsheet that is used on many of these sensors. This defect can cause the cell readings to be several per cent too high. ESTI reference cells with a black backing sheet (as in Figure 2.44) are less prone to this error and thus should be used in order to obtain accurate measurements.