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Solar Radiation

The electromagnetic radiation of the Sun is radiated by its photosphere, which is the region of the Sun we are able to observe. The Sun’s photosphere has an approx temperature of 6000K and emits electromagnetic radiation similar to a ‘black body’ at that temperature. The total power of the radiation emitted by the Sun is approximately 9.5×1025W. This power is radiated by the Sun towards all the surrounding space and only a small part of it reaches the Earth.

Sun’s structure

The average energy emitted by the Sun that per unit of time reaches outside the surface of the Earth’s atmosphere has an average value of 1353 W/m2.

Only a portion of the radiation that affects outside the earth’s atmosphere reaches Earth’s surface. This is due the fact that the atmosphere has many effects on the radiation that passes through it: part of the radiation is absorbed, part is reflected, in varying ways depending on the individual frequencies. The solar radiation that reaches the earth’s surface is the sum of the radiation that comes directly from the Sun and the indirect one that the earth’s atmosphere spreads in every direction.

The combination of these effects determines an alteration of the spectral distribution and of the solar constant which corresponds to the total radiation that reaches Earth’s surface. It is indeed attenuated: the power value is approximately 1000 W/m2 even if locally and temporarily we frequently see irradiance values around 1100 W/m2.


Distribution of the incoming radiation on Earth outside and inside the atmosphere


The productivity of a photovoltaic solar cell depends on many factors, the main ones being the following:

  • the photovoltaic solar cell does not respond uniformly to all frequencies of radiation;
  • the efficiency of a silicon photovoltaic solar cell is maximum in the frequency range of visible light;
  • the productivity of a photovoltaic solar cell and consequently of a photovoltaic system depends on the incidence of radiation hitting its surface.

Another factor that affects the productivity of a photovoltaic solar cell is the temperature because the operation of a photovoltaic cell is based on the existence of a ” band gap ” specially created in the semiconductor. The band gap determines, under the effect of the irradiation of the semiconductor, the formation of electron-hole pairs. An increase in temperature modifies the ‘band gap’ and decreases the production speed of the electron-hole pairs: the consequence is a decrease in the production of electricity.

In summary, an increase in temperature causes a reduction in energy production in the photovoltaic system.

If the energy production of a photovoltaic system is a determinable function of these factors, it is possible to identify unexpected decreases in energy production and understand if they are caused by failures or maintenance deficiencies or if there are margins for optimizing the system in order to implement the actions to optimize the economic result.

To make this possible, at present it is necessary to know at all times how much energy a photovoltaic system can produce and therefore it is very useful to know how much energy is reaching the surface of the photovoltaic modules at any time. It could be even more useful to know how much solar radiation reaches the photovoltaic modules in the part of the spectrum included in the range 300 nm – 1100 nm (silicon cells are sensitive to wavelengths in this range), so that you can know how much energy the system PV should produce at all times.

Sensors that measure solar radiation are capable of measuring solar radiation at the point where they are installed. They are called pyranometers and, for applications in photovoltaic systems, two types of them are essentially used: the irradiance sensor (photovoltaic cell pyranometer) and the thermopile pyranometer. A schematic representation of the different solar radiation measurement technologies can be viewed at the following link:


The irradiance sensor (or photovoltaic cell pyranometer) is an instrument used to measure the flow of solar radiation. It uses the photovoltaic effect to measure the amount of solar radiation that reaches a given surface.


Working principle of a photovoltaic cell

Since an irradiance sensor uses the photovoltaic effect, it provides similar responses to a photovoltaic system: it produces an electrical signal as a function of the incident solar radiation. More specifically, it mainly responds to visible light and its output depends on various factors, including the temperature of the cell. It absorbs waves approximately in the range 300 nm – 1100 nm. To obtain a temperature independent reading, the values measured with a photovoltaic cell solar meter must be corrected to offset the impact of temperature variations. Not all irradiance sensors are equipped with such a compensation system which, if present, must have a high level of accuracy.


Thermopile pyranometers (commonly called simply ‘pyranometers’) are instruments used to measure the flow of solar radiation.

They work by measuring the temperature difference between the part exposed to radiation and the one not exposed. This temperature difference is measured using a thermopile. A thermopile consists of a series of connected thermocouples which are constitute by a junction between two different metals that, producing a ‘temperature-dependent potential’, are commonly used to measure the temperature difference between two points.


Working principle of a thermopile

Therefore, by using a thermopile, the potential gap generated by the temperature difference between the exposed surface and the surface not exposed to radiation makes it possible to obtain a measurement of the global solar radiation without, however, a useful selectivity between the different wavelengths.

The response of this type of thermopile pyranometer typically covers the entire spectrum of the wavelengths of the solar spectrum throughout the range 300 nm – 3000 nm.

Thermoelectric pyrometers that do not use a thermopile have recently been developed. However, exploiting the thermoelectric effect and the temperature difference, they have similar response to thermopile pyranometer, therefore not differentiating from them except for response times of the order of 20 s.


Distribution of irradiance absorbed by a pyranometer (thermopile) and by a photovoltaic cell

It should therefore be noted that the spectrum width detectable by a thermopile pyranometer is wider than that measurable by a irradiance sensor with a photovoltaic cell or it also includes the radiation that photovoltaic systems cannot convert into electricity.

The difference in the spectral response of a thermopile pyranometer compared to a photovoltaic cell irradiance sensor can be seen in the following two illustrations:

Spectral response of a thermopile pyranometer and photovoltaic irradiance sensor

For this reason, using the thermopile pyranometer to test the correct operation and performance of a photovoltaic system may result in incorrect performance measurements under certain environmental conditions. On the contrary, in the case of using a irradiance sensor (which is equipped with photovoltaic cells), the values given in each environmental condition are similar to those of the plant because the spectral portion that determines the operation of a photovoltaic system is the same as that which determines the operation or the measurement signal of this instrument.


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