An overview on solar radiation measurement sensors
The Sun’s surface, called the photosphere, is at a temperature of about 6000K and its behavior approximates that of a “black body” . Considering this, it is possible to calculate the total power emitted by the Sun : it is approximately 9.5×10^25W. The total power emitted by the Sun is not composed of a single wavelength but of many.
Moreover, only part of the Sun’s radiation reaches the Earth.
The average energy radiated by the Sun per unit of time on a normal surface located outside the Earth’s atmosphere is called the solar constant and its average value is about Ics = 1353 W/m^2 .
The presence of the atmosphere has however different effects on the radiation that reaches the Earth’s surface.
The radiation undergoes a power reduction due to absorption, scattering and reflection in the atmosphere. The spectral content of the solar radiation also changes due to the increased absorption or scattering suffered by the long wavelengths. Moreover, there are components of diffuse and indirect radiation.
The result of all this is that the total radiation that reaches the Earth’s surface has less strenght: power approximately around 1000W/m^2 and a different spectral distribution.
The productivity of a solar cell
It depends on several factors: first of all a solar cell does not respond consistently to all frequencies of the incident radiation. The efficiency of a silicon cell is maximum in the frequency range of visible light.
Secondly , the producibility of a solar cell and consequently of a photovoltaic system depends on the incident radiation on its surface.
There is another effect that influences the performance of a PV system: the temperature. Like all the other semiconductor tools, solar cells are sensitive to temperature. An increase in temperature reduces the “band gap” of a semiconductor affecting most of the parameters of semiconductors.
High values of temperature causes a reduction in the energy production of a PV system.
Since the energy production (and the costs) of a photovoltaic system is a predictable function of these factors, a unexpected decline in energy production must be interpreted as failure or breakdown for which corrective actions must be taken.
So, in order to know at any given moment how much energy a photovoltaic system should produce, you have to know how much energy is reaching the surface of the photovoltaic modules in that moment. It would be more useful to know how much solar radiation (silicon cells are sensitive to 300nm-1100nm wavelenghts) reaches the photovoltaic modules, so that you know how much energy should produce a PV system at any time of the day.
The solar radiation measurement sensors are able to detect how much solar radiation reaches the place where they are installed.
There are essentially 2 types of devices:
A solarimeter, also called “silicon cell pyranometer”, is an instrument used for measuring the flow of solar radiation. It uses the photovoltaic effect to measure the amount of solar radiation reaching a given surface.
A solarimeter using the photovoltaic effect has the same response of a photovoltaic system: it produces an electrical signal as a function of the incident light. It responds mostly to visible light and its output depend on the temperature of the cell.
More specifically, a solarimeter with silicon cell is able to capture light waves from approximately 330nm to 1100nm.
In order to obtain a measure not influenced by the temperature, the values measured by a solarimeter which uses the photovoltaic effect must be corrected according to the temperature of the photovoltaic cell. This measurement can be made thanks to a thermocouple, while the fix factor must have high precision levels.
Pyranometers are instruments used to measure the global radiation on a surface (direct and diffuse radiation). Their functioning is generally based on the measurement of the difference between the temperature of a clear bright surface and a dark one. A dark surface can absorb most of the solar radiation while a clear surface tends to reflect it, absorbing less heat.
This difference in temperature is measured using a thermopile, and the difference of potential generated in the thermopile* (due to the temperature gradient between the two surfaces) allows to measure the value of the global incident solar radiation.
The response of this type of pyranometer can cover the entire range of wavelengths of the solar spectrum: approximately from 300 nm to 2800nm.
Note that the spectral range detectable by a pyranometer is wider than the one measurable with a solarimeter with silicon cell. For this reason, using a pyranometer to test the proper operation and performance of a PV system might lead you to think that the plant is not working properly under certain environmental conditions.
Instead, with a silicon-cell solarimeter, the given values are synchronized to the plant since the spectral portion necessary for the operation of a photovoltaic system is the same as that measured with this device.
Besides, the response of a pyranometer is much slower (on the order of tens seconds) than the response of a silicon-cell solarimeter (from fractions of a second to a second).
*A thermopile consist generally in thermocouples connected in series. A thermocouple is a junction between two different metals used to measure the temperature difference between two points. A thermocouple produces a potential that depends on the temperature gradient.