Solmetric PVA-1000S, User Manual

Page: 123 / 152

6 Measuring Irradiance, Temperature, and Tilt
6-7
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Backside temperature varies substantially across a PV module, string, or array.
This variation is caused by patterns of convective cooling that depend on degree
of ventilation, relative location in the array, and exposure to wind. For this
reason, your temperature measurements can be biased higher or lower than
actual.
Selecting a thermocouple wire gauge
Choose a relatively fine thermocouple wire gauge, ideally #24 or #30. Some users prefer
#24 for extra ruggedness and handle-ability.
There are three reasons for using one of these finer wire gauges.
1. The first reason for this choice is that for accurate temperature measurements,
the tip of the thermocouple must be kept in good physical contact with the
backside surface. An air gap between thermocouple and backside surface
translates into a lower temperature reading. We typically use tape to hold the
thermocouple in place, and stiff thermocouple wire is not compliant enough to
allow the tape to do its job.
2. The second reason is that because heavier gauge thermocouple wires are more
massive, the thermocouple is not able to respond as quickly to temperature
variations caused by wind or changes in irradiance. Under changing conditions,
a measurement delay translates into a measurement error.
3. The third reason for using a relatively fine wire gauge is that the thermocouple
wires themselves drain a small amount of heat away from the tip of the
thermocouple. This heat drain causes a slight temperature drop in the module
backside material, which has very poor thermal conductivity.
Selecting a thermocouple tip
A variety of tip styles are available. The simple beaded tip is a good choice because it is
very rugged and reliable and has relatively low mass, allowing it to quickly track
variations in temperature. Lower-mass tips with integral adhesive strips can also be used,
but experience has shown that these devices are easily damaged.
Selecting a tape for thermocouple attachment
A good tape for this application will:
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strongly adhere to the backsheet
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keep the thermocouple in physical contact with the backsheet
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meet these objectives at temperatures up to 70 C