Electric power generated by photovoltaic (PV) modules depends on the intensity of the light that illuminates them as well as on their temperature. Thus, in order to properly design a PV installation, we need to know how much sunlight will shine on it on average and how the PV modules respond to the environmental conditions that are typical for the chosen location. How can this information be collected and put together for planning future PV systems?
The EURAMET ENG55 PhotoClass project has significantly contributed to give robust scientific answers to this question, by supporting the development of an advanced metrics for PV based on the energy rating approach. Consistently with the two main parameters that affect the electric power of PV modules (i.e. sunlight intensity and PV module’s temperature), two groups of interlaboratory comparisons were run in the project between various European laboratories. The first group of measurements targeted the temperature dependence of the short-circuit current and obtained measurement agreement previously unmatched. The second group of measurements, whose results have been published more recently, represented the very first intercomparison dealing with the dependence of the short-circuit current on the light intensity (i.e. irradiance). Besides assessing the methods involved, the second group of measurements gave also significant inputs to the revision of the respective international standard (IEC 60904-10), which is currently being revised under JRC leadership.
Why is it so important to know the dependence of the short-circuit current on the irradiance?
Under the energy rating approach, measurements of PV modules’ electrical power at various temperatures and irradiances are necessary. The way of performing them is fully standardised in IEC 61853-1. But, how do we quantify the irradiance actually reaching the PV module during measurement?
Cavity radiometers and thermal sensors
The measurement of the irradiance can be done with various types of sensors. Some of them exploit the well-known property of black objects to become rapidly warmer when they are directly exposed to sunlight. The heat they accumulate is then compared to or converted to a voltage that can be easily measured. However, this type of sensor “sees” almost all wavelengths that compose the sunlight spectrum, from the UV down to the far infrared radiation. A crystalline silicon PV module, however, has a much narrower “sight”; it just “sees” or responds to sunlight from near UV (UVA and some UVB) up to near infrared. Therefore, in order to measure with the above sensor the amount of sunlight effectively “seen” by a PV module, we have to calculate a number that accounts for all the spectral quantities involved. This number is called spectral mismatch factor and it is multiplied to the short-circuit current of the PV module to set its electrical performance onto the correct scale.
PV reference devices
The measurement of PV modules against a thermal sensor like the one mentioned above is, however, not the most convenient way of testing thousands of them, because of practical and technical limitations. Therefore, the short-circuit current of a calibrated PV device (called PV reference device and preferably with similar response to sunlight as the one to be tested) is usually used to measure the irradiance. This significantly reduces the spectral mismatch factor (although at the cost of some additional uncertainty) and makes the measurement of PV modules’ power handier. If a careful management of the measurement practice is put in place, an uncertainty in the calibration of small-size PV devices can be achieved which is comparable to the one obtained with the highest level of thermal sensors above.
Testing at Standard Test Conditions
The second method is thus routinely applied when PV modules are measured at Standard Test Conditions (STC) of 25 °C for module’s temperature, 1000 W/m2 of sunlight intensity and a sunlight spectrum as tabulated in the standard IEC 60904-3 corresponding to that occurring near midday with a clear day. STC are used by testing and calibration laboratories like ESTI to verify and certify that a PV module has a certain electrical performance. This allows us to compare different PV modules of the same PV technology (e.g. poly-crystalline silicon) as well as different PV technologies (e.g. poly-crystalline silicon against mono-crystalline silicon or cadmium-telluride). STC are also used by PV module manufacturers to sort their products according to their electrical parameters (maximum power, open-circuit voltage and short-circuit current ).
Short-circuit current linearity measurements and why they are important
However, also the PV reference device is in general only calibrated at STC. Usually its short-circuit current is assumed to be proportional to the irradiance, i.e. halving the latter in principle will halve the former. This is an essential characteristic of a PV reference device, because an unknown deviation from this proportionality function will introduce an unknown error in the irradiance reading and therefore in the calibration of the other PV devices. Considering the low level of uncertainty reached in the calibration of PV devices, it is becoming more and more important to actually assess quantitatively and reliably the function that links the short-circuit current of the PV reference device to the irradiance, at least for the range of irradiances covered by the energy rating approach (from 100 W/m2 to 1100 W/m2). The results published recently have contributed to increase confidence in the methods used to determine this function and derive a quantitative parameter, which can be used to correct for deviations in the proportionality between irradiance and short-circuit current.