Instruments to measure solar radiation in the field of renewable energy
DELTA OHM SOLAR RADIATION METER
In recent years, we have seen a marked increase in the demand for the construction of plants for the production of energy from renewable sources.
Many efforts have been undertaken for the production of energy from sunlight, solar photovoltaic and solar thermal Energy.
An accurate measurement of solar radiation is essential for the proper choice of the PV module orientation and placement. In highland areas, but this is true generally speaking, where there may be problems of shading, the plant position must be handled with care to ensure maximum efficiency of the system used.
Only a measurement campaign using appropriate instruments guarantees uniqueness of the results and therefore the effectiveness of the investment made.
At the same time other parameters of interest to solar radiation for photovoltaic systems are: temperature of the panels due to the effective performance of the system; air speed for a correct dimensioning of the support structures;
rainfall to schedule the cleaning of the panels and then keep them at full efficiency.
DeltaOhm is able to provide instruments to measure all the parameters listed above.
To measure solar radiation, in addition to temperature sensors, anemometers and rain gauges, DeltaOhm offers a complete set of pyranometers:
DeltaOhm WMO Classification ISO Classification
LP PYRA 10 High quality Secondary Standard
LP PYRA 02 Medium quality I Class
LP PYRA 03 Moderate quality II Class
To monitor the efficiency of the plants (small and medium-size) over time, it is recommended to use a Class I pyranometer. This instrument, combined for example with the indicator HD2302.0, is also recommended for panel installers who want to run a quick check to verify the correct installation operation of the plant. To monitor the efficiency of the plant (medium to large size) over time, it is recommended to use a Class I pyranometer. This instrument can also be used for measurements campaign meant to optimize the orientation and placement of photovoltaic Installations. To study the feasibility of large photovoltaic plants it is recommended to use a secondary standard pyranometer to determine orientation and placement. Installation of a pyranometer.
The pyranometer is mounted on a plane parallel to the module to be assessed. For small systems, where the panels have the same orientation and are distributed over a small area (tens of square meters), it is sufficient to use a single pyranometer.
When the photovoltaic field is large, then it is necessary to consider the use of several pyranometers, especially if the various parts of the field have different orientation (or inclinations). In this case you should use a pyranometer for each orientation.
The use of higher quality pyranometer is subject to a maintenance that has to be performed on the instruments themselves. All pyranometers have a (special) glass outer dome meant to protect the sensor from the atmospheric agents. The more the dome is clean, the more the measurements obtained will be correct (so as the performance of the panels lowers when they are dirty). The regular cleansing and maintenance of the pyranometer is the best way to obtain accurate measurements regardless of the class of
the pyranometer used. If for the thermo-dynamic solar the only available instrument on the market is the pyranometer, for solar photovoltaic there are reference solar cells and solar meters on the market (instruments that use silicon sensors).
In these cases the use of a pyranometer has a much greater accuracy than measurement systems performed with silicon sensor. Let’s see the main reasons why a pyranometer is more appropriate than a cell reference / solar meter (using silicon sensors):
1. First of all the measurement performed by a pyranometer is independent of the temperature of the instrument and this is not applicable for a silicon reference cell. So, to get a proper measurement with the reference cell, it is necessary to apply a correction by talking into account the temperature.
2. About the 97% of solar energy is within the spectral range from 300nm-3000nm, therefore, to correctly measure the efficiency of a system, you need an instrument that has a flat response in this spectral range, in fact in the calculations of efficiency are necessary to know the total energy that arrives on the panel regardless of its wavelength.
The spectrum of light that reaches the panel depends on the sky conditions at the time of the measurement and changes depending on the presence of pollutants, haze or partly cloudy sky.
If the measurement is performed with a reference cell that has a limited spectral range (typically 400nm-1050nm), you can make very large errors in the measurement of solar radiation (> 10% - 30%).
3. Another source of error of the sampling cells is their response as the angle which is not Lambertian (cosine law). In this case, the measured value is always lower than the actual one because of the inability of the cell to measure light that does not strike perpendicular to its surface.
In conclusion, a reference cell / solar meter or in any case silicon sensor is able to give a good measure of solar radiation only when used under the same conditions (spectrum of light measured, the same tilt of sun rays etc. etc) used to its calibration, which are difficult to repeat and usually are not stated by the
manufacturer.
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