Solmetric PVA-1000S, User Manual

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6 Measuring Irradiance, Temperature, and Tilt
6-9
Measuring PV module temperature with an infrared
thermometer
Some competing measurement solutions rely on infrared measurements of module
temperature. This approach has serious limitations, which are discussed here because the
PVA software allows the user to manually enter temperature values from hand-held
sensor devices.
Since the IR thermometer determines temperature by sensing radiant energy emitted by
the object being measured, the accuracy of the temperature measurement depends on how
closely the emissivity control setting of the instrument matches the actual emissivity of
the object. The emissivity of a material is a measure of its relative ability to emit energy
through radiation. It is the ratio of energy radiated by a particular material to energy
radiated by a black body at the same temperature. A true black body would have an ε = 1
while any real object would have ε < 1. In general, the duller and blacker a material is,
the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity.
Highly polished silver has an emissivity of about 0.02.
Some IR thermometers allow continuous adjustment for emissivity. Some models
provide only a high/medium/low setting, which limits your accuracy. Some use a factory
preset emissivity and are not user-adjustable.
PV module backsides do not all have the same emissivity, so you much either adjust the
emissivity control of the instrument to match the backside surface or change the
emissivity of the backside surface to match the instrument. Flat black electrician’s tape is
commonly used to achieve high emissivity. Using this technique, you can set your
instrument’s emissivity control at 1 and have reasonable accuracy.
If you do not use tape, you can calibrate your instrument against another measurement
method, usually a thermocouple taped to the back of the same PV cell (see guidelines
discussed earlier). Adjust the emissivity until the temperature readings are the same.
Keep in mind that this emissivity setting is calibrated only for this particular type of
module backside.
When using infrared techniques, module temperature should not be measured from the
front side of the module. Glass reflects the heat of other objects, especially the sun. Also,
the glass may not be completely transparent to the wavelength of the IR instrument; as a
result, the temperature reading will be some function of both the glass temperature and
the PV cell temperature.