6 Measuring Irradiance, Temperature, and Tilt
6-6
Entering Irradiance Manually
When this option is selected, the user manually enters an irradiance value obtained by
another method such as a hand-held sensor or an array mounted reference cell. This
option has several disadvantages:
• It is often difficult to accurately orient hand-held irradiance sensors in the plane
of the array.
• A greater and more variable time delay between the I-V curve and irradiance
measurements translates into irradiance error under conditions of rapidly
changing irradiance.
• Hand-held irradiance sensors may have poor accuracy, especially in their cosine
response. This introduces very significant irradiance error when the sun is off
axis of the array and under diffuse light conditions.
Measuring PV Module Backside Temperature with a
Thermocouple
Measurement of backside temperature is one of the traditional methods used by I-V curve
tracers. It has several benefits and limitations.
The Backside Thermocouple method provides these benefits:
• It is a direct method with no dependence on a PV model.
• It enables you to choose the location for temperature measurement; it’s best to
choose an average temperature location and avoid locations that are much hotter
or much closer to the cooler edges of the array.
The Backside Thermocouple method has these limitations:
• The predictive PV model wants to know the cell temperature, but the
thermocouple measures the slightly cooler backside temperature. The PVA
software partially compensates for this by adding up to 3-degrees Celsius to the
measured backside temperature. The amount of compensation varies in
proportion to irradiance, since at high irradiance the difference between cell and
backside temperature is higher.
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