This part of IEC 60904 describes procedures used to determine the degree of linearity of any photovoltaic device parameter with respect to a test parameter. It is primarily intended for use by calibration laboratories, module manufacturers and system designers.
Photovoltaic (PV) module and system performance evaluations, and performance translations from one set of temperature and irradiance conditions to another frequently rely on the use of linear equations (see IEC 60891 and IEC 61829). This standard lays down the linearity requirements and test methods to ensure that these linear equations will give satisfactory results. Indirectly, these requirements dictate the range of the temperature and irradiance variables over which the equations can be used.
The methods of measurement described in this standard apply to all PV devices and are intended to be carried out on a sample or on a comparable device of identical technology. They should be performed prior to all measurement and correction procedures that require a linear device. The methodology used in this standard is similar to that specified in IEC 60891 in which a linear (straight-line) function is fitted to a set of data points using a least-squares fit calculation routine. The variation of the data from this function is also calculated, and the definition of linearity is expressed as an allowable variation percentage.
A device is considered linear when the following conditions are met over the temperature and irradiance range of interest. Typically, this range of temperature is 25 ℃ to 60 ℃ minimum, and the irradiance range of 700 W·m to 1 000 W·m minimum.
a) For the curve of short-circuit current versus irradiance, the normalized standard deviation of the slope (σs/m) should be less than 0,02.
b) For the curve of open-circuit voltage versus the logarithm of irradiance, the normalized standard deviation of the slope (σs/m) should be less than 0,05.
c) For the curve of open-circuit voltage and short-circuit current versus temperature, the normalized standard deviation of the slope (σs/m) should be less than 0,1.
d) The variation of relative spectral response at a specified voltage is less than 5 % for the wavelength band.
NOTE 1 - Many of the IEC PV standards require spectral mismatch corrections which in turn require spectral response measurements. Therefore, the linearity of spectral response with respect to temperature and irradiance is important. It can also be significant for new technologies such as photochemical cells.
NOTE 2 - Because the relative spectral response of some devices such as amorphous silicon varies considerably with voltage, it is important that the linearity determination be performed at a fixed voltage. The voltage selected is governed by the ultimate usage. Vmax may be selected if the maximum power regime is the area of interest, and for calibration zero bias voltage might be more appropriate.
NOTE 3 - It should be noted that the relative spectral response of some devices varies with temperature and irradiance. Some of these effects may be accounted for as changes in the items a) and c) short-circuit current non-linearity, but some may not be. The item d) requirement accounts for these non-linearities.
General procedures for determining the degree of linearity for these and any other performance parameter are described in clauses 4 through 6.