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6 Measuring Irradiance, Temperature, and Tilt
6-1
6 Measuring Irradiance, Temperature, and
Tilt
Introduction
Thorough evaluation of array performance by any measurement method (I-V curve
tracing or conventional methods) always involves comparing measured I-V data to some
form of reference. That reference may be anything from a simple STC maximum power
value to a detailed PV performance model. In any case, you will need to know the
irradiance in the plane of the array and the array temperature in order to properly evaluate
array performance against your reference.
Measuring irradiance and array temperature sounds easy, but in practice, a number of
factors must be considered in order to assure good measurement results. This chapter
provides the background you’ll need to make informed choices for your specific
application.
Measuring irradiance presents a number of requirements and challenges:
•
Irradiance must be measured in the plane of the array (POA)
•
Irradiance may not be uniform across the surface of the array due to shading and
albedo effects, as well as local cloud effects
•
Irradiance can vary rapidly
•
Irradiance sensors may have different spectral responses than the PV modules
themselves
•
The solar spectrum is shifted significantly early and late in the day
•
The shape of the PV module I-V curve changes at low irradiance levels
Each of these topics is discussed in this chapter.
The temperature parameter of interest to the PV model is the average temperature of the
PV cells in the string or module under test. In practice, determining the average cell
temperature poses a number of challenges:
•
The PV cell is embedded in other materials, so measuring the cell temperature
by direct contact is not possible.