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Ammonit News 2018-02

Accurate and cost-efficient solar resource assessment with RSI in Brazil described by our customer Fotovoltec


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Dear Reader,

The days are getting longer and the Ammonit team is looking forward to the first evening in one of Berlin's beer gardens. Thinking about this some questions pop up: How long does the sun shine and how strong is the sun irradiation at this time in the year? Set up one of our solar measurement stations and you will know as our Brazilian customer Fotovoltec. The company successfully implemented the Rotating Shadowband Irradiometer for solar resource assessment projects.

Fotovoltec installs, operates and maintains a number of solar measurement stations in Brazil. The company is specialized in performing resource assessment campaigns while ensuring highest quality of the measurement data in order to prepare accurate feasibility studies for solar energy power plants. Ammonit is the main supplier for Fotovoltec regarding sensors, data loggers as well as power supply and communication systems.

Fotovoltec focuses more and more on the Rotating Shadowband Irradiometer (RSI) for sun irradiation measurement. By implementing this sensor in the solar resource campaigns for PV and CSP projects using tracking systems, you can conveniently determine all three components of the sun irradiation.

We have published the full story on our website, please click here.


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Most PV power plants in Brazil are designed and built with single-axis tracking systems. The energy production of those PV power plants located close to the equator is almost proportional to the direct irradiation. Hence, the direct irradiation and the uncertainty of the P50 and P90 energy yield data are in the focus for irradiation and energy yield studies.

If the direct irradiation is measured with an RSI, the uncertainty of the long-term corrected DNI data can typically be reduced below 5%. Thus the P50 and P90 yield data can be calculated more accurately. So the RSI can be considered as a profitable investment for planned large PV projects, which generate several Million Euros of reliable revenues per year. more


No matter global and direct irradiation can also be measured by implementing a pyranometer and a pyrheliometer on a sun tracker. In comparison to an RSI installation the measurement would be more precise, but also more cost-intensive and would take more time for servicing.

According to certain studies the RSI has proven its superior performance with respect to soiling compared to common thermopile sensors. The results of a comparative measurement presented only 0.07% loss per day for the irradiation measurement of the RSI compared to 0.79% loss per day for the pyrheliometer. see RSI data sheet

A measurement station with sun tracker requires qualified service staff and continuous support in order to get reliable measurement data. Hence, the RSI does not require qualified staff and only weekly cleaning. more

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In order to give reliable measurement data at different irradiation angles and spectres, the RSI uses certain correction algorithms. The validation has been performed at the BSRN station in Petrolina, Brazil. The comparison of a 3-month measurement campaign presented only 0.2% deviation in the direct irradiation between the RSI and the BSRN reference station.

The validation indicates that RSI-measured DNI is comparable to the data of sun tracker measurements. Considering the service efforts for the sun tracker, the RSI is the preferred solution for very remote sites. more


The RSI has been designed to measure direct horizontal irradiation for project development purposes. The measurement is performed by two silicon photodiode radiation detectors, located in the center of a rotating shadowband.

The shadowband rotates typically once per minute for approximately one second around the radiation sensors. During rotation the shadowband blocks the direct sun irradiation onto the sensors for a brief moment. This causes a drop of the photodiode signals for global horizontal irradiation. The measured value is the diffuse horizontal irradiation.

Having the measured values for global and diffuse irradiation, direct irradiation can accurately be calculated for the location. see data sheet

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Do you want to know more about RSI measurements in Brazil or further technical details, please visit our website:

The US-based institute NREL published the "Best Practice Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications" (Feb. 2015/63112). This handbook can be considered as a kind of a guideline for solar energy projects. It describes required sensors and its quality, e.g., the RSI. For further details and the PDF of the handbook, click here.


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