Considerations on Pyranometers

Some considerations on the use of photovoltaic pyranometers


In the hope that it will also affect the non-Italian speaking audience, this article is republished in English.

Just returning from a course on use of pyranometer in PV. For the uninitiated, the pyranometer are instruments for measuring solar radiation on the Earth’s surface. There are three types of pyranometer and are made looks like this:

Pyranometers Typology
Pyranometers Typology

Pyranometers Typology

  1. The first types are mono or polycrystalline silicon solar cell or otherwise known polycristalline pyranometer;
  2. The second uses a photosensitive diode;
  3. The third that is the real pyranometer consists of a blackbody on which is affixed to a thermocouple which generates a small current with changes in temperature of the blackbody.

In PV, pyranometer is used to estimate in advance the manufacturability of a solar field in the project, or estimate the yield of a field into production. In the second case, the yield is tied to penalties that the installer must pay if the performance does not guarantee a certain production plan established in the agreement. This just to give some information on the instrument.

And now I explain  the idea that I made ​​during the course. Generally, in PV world have always used the first two types of pyranometer because it is less expensive and less sensitive in the maintenance although they had the defect of being insensitive for some angles of the sun (mainly in the morning and at sunset), or cut certain frequency spectrum of solar radiation. Now, for marketing purposes is becoming the blackbody pyranometer that does not suffer of angle of the sun and picks up all frequencies. This is presented for more accurate, too bad that only now (after having bought one!), I realized that all this precision is not needed but also leads to over estimate the production of solar field project is being under review. This is because the blackbody pyranometer being more sensitive on all spectrum but the solar panel which is made of mono or polycrystalline silicon captures precisely the same frequencies to detect a solar cell. So apart that this increase instrument cost but  more damage is made for the bad estimation of productivity (even a 25%!) that overestimate the business revenues.

Spettral Sensibility of Pyranometers
Spettral Sensibility of Pyranometers

Spettral Sensibility of Pyranometers

 Also, there is to say that blackbody pyranometer  has many problems of efficiency:

  • often in glass caps forms dewdrops that requires a salt filter that must be changed periodically (every 3-4 days in severe cases);
  • the powders decrease the sensitivity;
  • the blackbody absorber is covered with black paint that peels off after a few seasons, making the instrument unusable. Unless you have an operator who often goes to this site.

So the question is: is useful to buy a blackbody pyranometer if pyranometer does not allow you to compare the results with the productivity of the PV panels to be installed or already installed? The answer seems clear: definitely NO! Therefore, if someone is’ thinking of buying one, also reflects on these few personal considerations and do not hesitate to contact me for a confrontation. These conclusions are reached by many of my colleagues in the audience.

In December 2010, we have installed a blackbody pyranometer and in mid february we begin data acquaring, next I will report the results of data analisys

Recalling that “In real world, the only real performance is economic output!”.