Irradiance measurements to calculate the Performance Ratio (PR) for solar parks sometimes cause controversy: A few years ago, I visited an O&M conference, where cheating with irradiance measurements was openly discussed on the podium. There were about 200 attendees in the room; And after a presentation on irradiance measurement at a solar conference, our R&D manager was asked whether Kipp & Zonen could make instruments that were less (!) accurate.
This shows that irradiance measurements can cause headaches; but they are absolutely necessary for calculating the Performance Ratio. Besides the output generated relative to the solar irradiance received on the panels, PR is the figure looked at by most players involved in the PV power plant business.
Irradiance measurement sources
Irradiance data can be collected on- site and via satellites. There are significant differences between the two sources, depending upon the integration period.
The satellite data is for Global Horizontal Irradiance (GHI) and the graph illustrates why it can only be used over relatively long time periods for performance ratio calculations. Data from a horizontal high quality Secondary Standard pyranometer can be used in real time. The measurement uncertainty of the daily total irradiance is typically around 2%, but can be down to 1% for the best models.
Performance Ratio is best calculated with irradiance measured in the Plane Of Array (POA). This monitors the actual amount of light received by the solar panels; direct and diffuse, and the effects of shading and reflections from the surroundings. This type of measurement can only be achieved with Secondary Standard pyranometers, placed next to the PV arrays and installed at the same tilt and orientation as the panels.
One source is not enough
The present best practice is a combination of both satellite and local horizontal pyranometer data. Many advisors and O&M parties see this as an ideal combination because these two independent sources can be verified against each other. Satellite data can suffer from aerosols and clouds, and pyranometer data is subject to proper instrument installation and maintenance.
In addition to these two independent sources for GHI, we advise using at least two POA pyranometers on a plant, ideally three or more:
- With one pyranometer there is no redundancy, in case of a failure or being off-site for recalibration, and no cross-check in case of installation or operation errors
- With two pyranometers you have back-up, but in the case of strongly differing measurements which one is correct?
- 3 or more pyranometers allow you to calculate a mean irradiance value, and see if a particular instrument has unusual readings and requires investigation
In daily practice we see poor irradiance measurements due to pyranometer misalignment, often caused by wrong installation or a small collision with a grass mowing or panel cleaning tractor, and lack of maintenance (cleaning the dome and changing desiccant).
So, choose the best solution for calculating the Performance Ratio of your solar energy plant: multiple pyranometers and satellite data!