Solmetric PVA-1000S, User Manual

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6 Measuring Irradiance, Temperature, and Tilt
6-4
Orienting and mounting the irradiance sensor
To provide a valid reference irradiance value, the sensor must be mounted in the plane of
the array. This assures that the PV modules and the irradiance sensors present the same
proportion of their area to the sun at all times of day (a cosine effect) and that reflective
albedo effects are as similar as possible.
See Setting Up SolSensor for details on mounting SolSensor on the module frame.
NOTE Mount SolSensor along the upper horizontal edge of the module whenever possible. This
allows you to achieve better irradiance accuracy earlier and later in the day, extending the
useful work day. It also generally improves wireless transmission distance.
When mounting SolSensor overhead, it is recommended that it be secured to the racking
structure using the provided tool lanyard, to prevent injury or sensor damage in the event
that SolSensor is dropped.
SolSensor can also be oriented correctly by simply placing it on the surface of a PV
module. If you take this approach, be aware that SolSensor is shading cells, which will
cause a step in the I-V curve of that string.
Another mounting approach is to place the irradiance sensor on a tripod and orient the
sensor to the correct azimuth and tilt. See
Mounting SolSensor on a tripod for details.
Albedo effects
PV arrays – and irradiance sensors – can pick up significant amounts of irradiance
reflected off of surrounding surfaces. Examples of albedo effects include reflections from
adjacent roof surfaces, building walls, and other PV arrays. The strength of the albedo
effect is not as much a function of the perceived color of the surface as one might think.
Even the surface of a blacktop parking lot reflects substantially.
If your array under test is located in a built-up environment with lots of reflective
surfaces, there is not much you can do about it, other than selecting an irradiance sensor
location that represents the typical irradiance conditions. Of course, the I-V curve
measurements themselves will register the albedo effects. This is another reason for
conducting your important performance measurements in the central four hours of the
day, when albedo effects are likely to be minimized in relation to the direct irradiance.
Diffuse light
As sky conditions become hazier, a greater fraction of the sunlight is scattered. This
scattered or ‘diffuse’ portion of the irradiance is incident on the array from all directions
and angles. Depending on the irradiance sensor construction, highly diffuse irradiance
may seriously degrade the accuracy of irradiance measurements. For example, some
hand-held irradiance sensors have poor cosine response and their accuracy is specified
only for direct normal irradiance, that is, clear days and pointed directly at the sun. Using
a similar-technology reference cell will reduce this error, but not eliminate it. The