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Measuring diffuse irradiance with a shade disk

Diffuse irradiance refers to all the solar radiation coming from the sky except for solar radiation coming directly from the sun and the circumsolar irradiance within approximately three degrees of the sun.
Measurement of diffuse radiation is difficult because one needs to shade the pyranometer from the direct normal and the circumsolar irradiance. Fortunately automatic trackers exist that perform such functions. (See a photo of a Kipp and Zonen 2AP tracker in Eugene).
At first glance the accuracy of the reading appears to be limited only by the absolute accuracy of the pyranometer. If a pyranometer has an absolute accuracy of four percent, one would think that the diffuse values should be accurate to within 4%.
Unfortunately, other factors come into play. On clear days, the diffuse readings are typically under 100 watts per square meter. Therefore, any offset error can have a significant effect. The PSP sensor radiates to the sky and this give the sensor a 10 to 20 watts per square meter offset that is a large source of error for the diffuse measurement if it is not corrected.

Explanation of the offset caused by the radiation of the sensor to the sky.

The PSP is a thermopile based pyranometer that has a black disk at the center attached to one side of the thermopile. The other side is connected to the heavy base of the pyranometer. The temperature difference between the black disk and the body of the pyranometer is the source of the voltage difference that generates the output from the sensor. The black disk is also a good radiator of infrared radiation and it radiates to the sky. This is the source of the negative five to ten watts per square meter values measure for horizontal PSPs that are observed during the night.
In our data we average the nighttime values and subtract them from the daytime values. Unfortunately, the radiation to the sky is greater in the day when the temperature of the sensor is greatest. It is very difficult to measure the effect of this radiation to the sky, although there is work being done on this problem elsewhere. To help evaluate the problem locally, we mounted a disk shaded Schenk pyranometer next to a disk shaded PSP.
The Schenk is a star type pyranometer that has alternating black and white wedges. The output of the instrument is based on measurements of the temperature difference between the black and white wedges. The black and white wedges both should radiate equally to the sky. This is born out by nighttime measurements that show no offset and hence an equal amount of radiation to the sky.
The figure below shows the comparison of the PSP and the Schenk data. During the morning and evening hours when the offset should approximate the nighttime offset, the values of the diffuse reading from the two instruments are about the same. During the middle of the day, the difference between the two instruments is about eight to ten watts per square meter. It is difficult to say for sure the exact cause of this difference, but the difference is similar to the re-radiation offset found elsewhere.
Graph of

      Schenk Star vs. Eppley PSP
The examination of this radiation factor was a result of test and modeling efforts associated with the ARM program. In 2000, Ibrahim Reda of the National Renewable Energy Laboratory and colleagues are publishing a paper on their finding. Understanding this small effect of radiating to the sky may not seem important, but in the long term it will likely lead to better instrument design and more accurate solar resource estimates.

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Last revised: March 22, 2022.

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