Frequently Asked Questions
Question:
For PV application, what will be the added value of using a shaded pyranometer in comparison to direct and global horizontal measurements?
Answer:
As you need to make direct solar radiation measurements it is required to use a solar tracker. Therefore the use of a shaded pyranometer for the determination of the global (hemispheric) radiative flux is advantageous because of the lower measurement error using the combined measurement (compared to the global measurement without shading). This is due to the following:
The most accurate measurement of solar radiation is obtained from the pyrheliometer, especially because it does not have a cosine-response error, neither does it have a significant thermal-offset error. Usually the measurement error of the pyrheliometer is less than 1% (let's assume to be 1%).
The measurement using a pyranometer is affected by more errors compared to a pyrheliometer (as mentioned: thermal errors due to dome cooling and cosine-response errors). It is rather difficult to estimate the total measurement error of a pyranometer, but lets say it is not more than 3% (lets assume here: 3%).
Now it it is possible to measure the global radiation in two ways: 1) using one pyranometer for the measurement of the global radiative flux or 2) using the sum of one pyrheliometer and a pyranometer, which just measures the diffuse radiative flux. As the measurement error of the global radiative flux is mainly affected by the pyranometer measurement uncertainty, it is possible to minimize the total measurement error by using the pyranometer just for the measurement of the diffuse radiation, which is usually a factor 5 smaller than the global radiation. Therefore I would recommend to use the pyranometer for the measurement of the DIFFUSE radiative flux (as you already have to use the tracker system for the measurement of the direct solar radiative flux).