When I was doing research on satellite derived irradiance data I spoke to more than just my colleagues at Kipp & Zonen and providers of satellite data; I also contacted other stakeholders in the solar energy industry. And that is when I came across different myths that were going around in the industry. Here’s my list of the top myths on the subject of satellite and pyranometer measurements that I would like to dispel.
1. The design of my solar plant was checked by an expert, so I know what to expect as the annual energy output of my plant
This is, of course, naïve! Irradiation is not stable over the years, it can easily fluctuate some percent from year to year. Next to this, figures show a rise in the 10 year average irradiation value in the Netherlands, and elsewhere; we are getting more sunlight over the years, so your plants will generate more output as well.
2. My irradiance data is produced by a meteorological station, maintained by the national meteorological institute, at 50km from my plant; that will do.
You can interpolate between at least three well maintained, nearby meteorological stations. However, local circumstances such as near and far shading, micro-climates, woods, hills and mountains all have a large influence on the actual local solar radiation received. So, measure irradiation by pyranometers on site to correlate the estimated data from satellites.
3. I placed a pyranometer on my solar plant years ago, never had a look at it, and it still works fine.
A measurement instrument without maintenance can deliver seriously misleading results. Pyranometers need regular cleaning (sometimes also a change of desiccant) and periodical calibration to measure well.
4. Compared to satellite data, my solar plant in Brazil is performing well
Accuracy and uncertainty of satellite data isn’t constant over the globe. In Europe and North America satellite data is more precise since it is continuously checked against well maintained meteorological ground stations and there is a historical database. You need to ask your satellite data provider how accurate their estimates are for your specific location and conditions, and over what timescales.
5. I use satellite data, so I don’t need a pyranometer.
In the long term (6 months and longer) satellite data and local Secondary Standard pyranometer data for Global Horizontal Irradiance (GHI) have a comparable uncertainty (although a well maintained pyranometer is still better). For complex short-term solar plant performance analysis, only local (well maintained) pyranometers can provide irradiance data with sufficiently low uncertainty to analyse reliably. Local pyranometers are always recommended and can be used to ‘train’ the satellite model.
For measuring the actual irradiation in the plane of array (POA) pyranometers are significantly better. They take local conditions into account, like row-to-row shading at low sun positions and reflections from the ground, which are difficult to model from the satellite GHI data.
So, do you want to find out too late that something was wrong or act as soon as the performance ratio shows a drop?
6. I have pyranometers on my plant, so I don’t need satellite data.
Horizontally placed pyranometers on a plant are used to verify satellite irradiation data, and vice versa. You can find out if there may be a measurement quality issue, for example a mowing lorry accidentally distorting the pyranometer support, by comparing both data sources.
The energy of the sun (irradiation) is harvested by a solar power plant, so accurate irradiance measurement is crucial for determining the performance of your investment. To find out more on this key subject, download our whitepaper; and use both satellite data and pyranometers!