Hello. Measuring solar radiation is a vast field of possibilities. Many instruments exist and for many applications. Here we'll focus on broadband solar radiation, that is the total radiation that comes from the Sun. Here we have a standard radiometric station that is used to measure global horizontally irradiance, GHI, direct normal irradiance, DNI, and diffuse horizontal irradiance, DHI. You might recall these concepts from the previous sequences. The global horizontal irradiance is measured with the pyranometer sitting on the horizontal plane that is exposed to all radiation from all parts of the sky. The diffuse and direct irradiance measurements are possible thanks to this sun tracker, which follows the sun. The black bowl shades the disk for this pyranometer, and thus, it only receives radiation for all parts of the sky except from the solar direction. This instrument is the per kilometer that is used for direct, normal irradiance. It points at the sun direction at all times during day time. Although these are the standard meteorological measurements, most of time for photovoltaics application, what is interesting to know is the plane of array, POA irradiance. That is the irradiance received by the solar panels. This concerns only parameters that are normally tilt. This sequence would be focused on per nanometers. The ISO 9060:2018 standard defines a pyranometer as a radiometer designed for measuring the irradiance on a plane receiver surface, which results from the radiant flux incident from the atmosphere above within the wavelength range from approximately 0.3 micrometers to about 3-4 micrometers. The quality of a pyranometer like this one that measures the pure radiance, it's determined by its specifications that are of many kinds. For example, a spectral response. What is the measurement uncertainty related to the fact that the pyranometer does not see the whole spectrum or response time. How much time does it take for a pyranometer measurement when there is a change in irradiance, or the angular response, how does the sensitivity changes depending on the angle of incidence of light, or stability over time, like aging, how does sensitivity digger dates over time? Or temperature dependence by how much measurements can be affected by temperature. Or the zero offset of electronics what does the pyranometer measure when it should actually measure zero watts per square meter. There is also the response to the low irradiance and many other factors. All these factors affect the quality of irradiance measurements, but they are not the only ones. There is also the calibration procedure. That is, how was the pyranometer calibration factor obtained? Density is a factor that converts from the electrical signal, a voltage to the physical quantity, the irradiance in watt per square meter, or the measurement conditions and maintenance, including cleaning are the instruments ventilated, cleaned, regularly checked, or the environment conditions? Are there shadows affecting the measurements? Also, the data logger and certainty which instruments are used to take the readings from a pyranometer? This is accurate, well synchronized with the standard time. In this photovoltaic Platform, we have three types of pyranometers. As for the ISO 9060:2018, here is a class A, the highest quality pyranometer. Here is a Class C. This one is also a class C. What are the differences? Well, they are quite visible. These two are thermopile pyranometers. They absorb solar radiation through these black surface. The body of the sensory is heated, and this creates a temperature difference that is converted in voltage by the thermopile. Thermopiles have a better spectral response than a silicon photodiode, which is the class C parameter in this platform. Thermopiles are sensitive to the great majority of radiation coming from the sun, from 280 nanometers to about 3000 or 4000 nanometers. Silicon photodiodes respond only to radiation coming from about 350-1,050 nanometers. Any change in the solar spectrum beyond these boundaries, like the absorption caused by the water vapor in the atmosphere is not seen by silicon photodiodes. This thermopile is ventilated, which helps in keeping the dome clean and minimizes the effects of dew, rain droplets, and rhyme. The thermopile has glass domes, which makes their angular response much better than silicon cells. That is, that they are able to better measure irradiance at different angles of incidence. Besides, this one has a double dome, which has an extra advantage to reduce the measurement errors related to heat loss from the black surface, like the thermal offset or thermal loss. One thing for which photodiodes are better than thermopiles is the response time. Photodiodes are much faster than thermopiles. Photodiodes react to changes in irradiance in much less than 1 second. Thermopiles react in the order of seconds. Then which is the best pyranometer to choose? Prices go from few hundreds of Euros to few thousands. That depends on the application for high-quality measurement requirements, such as for climate purposes or for resource assessment, class A or B, well-ventilated thermopiles pyranometers are the best. When fast response irradiance measurements are needed, photodiodes might be the best option. When using photodiodes, redundancy, that is having several of them measuring side-by-side is a good way to watch for quality and stability of the measurements over time. There is a general consensus that for identification of the need for power plant maintenance, a much reference cell that is not shown here, but you see in the picture, in the POA is the best option and the best choice. Here you have several reference documents, some of them freely available online. Thanks for your attention.
