7 Interpreting Measured I-V Curves
7-11
Irradiance Is Too Low, or the Sun Is Too Close to the Horizon
Most PV modules exhibit changes in the shape of their I-V curves under low light
conditions. This effect tends to set in below 600 W/m^2 and becomes quite significant
below 400 W/m^2. Also, if sunlight is hitting the module surfaces at a glancing angle -
early or late in the day - a much greater share of the light will be reflected by the module
glass and the cells themselves. Finally, the spectrum of sunlight changes in the course of
a day. For best results, measure PV arrays during the central part of the day, preferably
within a two-hour interval either side of solar noon. See this web site to determine solar
noon for your location: http://www.esrl.noaa.gov/gmd/grad/solcalc/.
Manual Irradiance Sensor Is Not Accurate
Hand-held irradiance sensors vary widely in their calibration accuracy, response to off-
angle light, and spectral match to the array being measured. All of these variables affect
accuracy.
3. Low voltage
An example of this type of deviation is shown in below.
Figure 63. Measurement with less steep than predicted slope in the vertical leg of the I-V
curve.
Potential causes are summarized below, and then discussed in more detail.
Potential causes located in the array include:
• PV cell temperature is hotter than the measured temperature
• One or more cells or modules are completely shaded
• One or more bypass diodes is conducting or shorted
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