6 Measuring Irradiance, Temperature, and Tilt
6-5
SolSensor irradiance sensor is corrected for angular effects. One of the benefits of this
correction is better performance under diffuse light conditions.
Determining Irradiance from the Measured I-V Curve
When you select the From I-V option, the PVA software calculates irradiance from the
measured I-V curve. This option has several benefits and limitations.
The From I-V option provides several benefits:
• There is no time delay between measurement of the I-V curve and determination
of irradiance. This is helpful when irradiance is changing rapidly (ramping) due
to moving clouds, a condition under which any time delay between I-V and
irradiance measurements translates into irradiance error.
• It is useful when measuring cell technologies that have a poor spectral match to
the SolSensor silicon irradiance sensor.
• Since the predicted I
sc
value is forced to agree with the measured I
sc
value, it is
easy to notice any deviations between the shapes of the measured and predicted
I-V curves.
The From I-V irradiance option also has certain limitations:
• Uniform soiling is interpreted as reduced irradiance and thus does not cause a
deviation between the measured and predicted I-V curves. You can mitigate this
risk by inspecting the array before measuring, and cleaning it if needed.
• Similarly, uniform degradation of module I
sc
is also interpreted as reduced
irradiance, and thus is not detected. The From I-V temperature option provides
several benefits:
• The From I-V irradiance value is used by the predictive PV model to calculate
the expected value of Isc. Using a measured value to calculate the expected
value is circular, forcing the expected value of Isc to agree with the measured
value.
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