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Case Study: Solar Measurement with RSI Technology in Brazil by Fotovoltec

Accurate solar resource assessment data for PV projects with tracking systems

In order to design profitable solar power plants the local sun irradiation and further meteorological parameters, e.g., temperature and wind speed, have to be evaluated in a site assessment analysis. Our long-term customer Fotovoltec has been specialised in measuring high quality solar irradiation data for accurate site assessment studies. Fotovoltec installs, operates and maintains a number of solar measurement stations in Brazil. The company is experienced 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 logger as well as power supply and communication systems.

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), which can be considered as a guideline for resource assessment campaigns. The comprehensive handbook describes required sensors and its quality, e.g., the Rotating Shadowband Irradiometer (RSI).

Fotovoltec focuses more and more on the Rotating Shadowband Irradiometer 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.

Source: Fotovoltec

Solar Resource Assessment for PV power plants in Brazil

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.

Source: Fotovoltec

P50 and P90 describe the statistical exceedance probability of the calculated energy yield with a probability of 50% and 90%. So far global irradiation data is used in certain simulation models. Further models are used to separate the global horizontal irradiation into direct normal and diffuse horizontal irradiation. Those models are fraught with uncertainties of approximately 5%. Thus the overall uncertainty of the direct normal irradiation increases.

By measuring the direct normal irradiation with RSI, you can reduce the uncertainty of the long-term corrected DNI data typically below 5%. Hence, P50 and P90 yield data can be calculated more precise. 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.

RSI Technology

The RSI is a meteorological measurement instrument designed to measure direct normal irradiation for project development purposes. The measurement is performed by two horizontally levelled silicon photodiode radiation detectors, positioned in the centre 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. As a subsequent the direct normal irradiation can be calculated from global and diffuse horizontal irradiation and the known solar incidence angle.

Source: CSP Services

For sure, global and direct irradiation can also be measured by installing a pyranometer and a pyrheliometer onto 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.

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.

Solar measurements with RSI in Brazil

In order to give reliable measurement data at different irradiation angles and spectres, the RSI uses certain correction algorithms. The corrections have been validated for a number of locations. The validation has been performed at the BSRN station in Petrolina, Brazil. The location is in the Northeast of the country and is one of the strong regions for solar power plant development. The local climate can be seen as a reference for the whole region. The validation has been performed in co-operation with the research institutes INPE and EMBRAPA. 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 and more project developers recognize the advantages and ask for measurements with the RSI. Meanwhile Fotovoltec has successfully performed solar resource assessment campaigns with RSI in six Brazilian states: Bahia, Pernambuco, Piaui, Ceará, Rio Grande do Norte and Paraná.

Source: Fotovoltec

RSI with cost and service benefits

Fotovoltec is very satisfied with the implemented RSI technology, which has proven its robustness in the field and also its accuracy. Using an RSI in the measurement campaign can significantly reduce the uncertainty for the energy yield forecast for tracking PV power plants. Thus the P90 energy yield can be substantially increased. A further plus for the RSI are its cost and maintenance advantages.

Quality control of your measurement data

Ammonit offers the web-based platform AmmonitOR to monitor measurement stations. AmmonitOR displays system as well as measurement data in various charts. If there are any implausible deviations, you can react immediately before any data is lost, e.g., low voltage or a sensor defect. Additionally, you can check data integrity and plausibility in AmmonitOR, export data in further file formats or generate PDF reports.

In close co-operation with international research partners Fotovoltec developed computer-aided routines to automatically monitor data consistency and plausibility as well as the sensor levelling. Additionally, satellite data is used to compare the measured data of the same period. Thus any occurring problem can early be detected and quickly be solved.

Solar Measurements

Since accurate solar resource assessment for the selected location is a very complex topic only specialised companies like Fotovoltec should be contracted with a site assessment campaign. The Fotovoltec experts are experience with the correct set up of a measurement station including sensors, installation, servicing and monitoring. Hence, a good preparation can later be seen in the feasibility study and finally in the energy yield of the running solar power plant.

If you have any question about your solar project or any measurement instrument, please get in contact with us on Email住址會使用灌水程式保護機制。你需要啟動Javascript才能觀看它 or via +49 30 6003188-0. You can also contact our colleagues of Fotovoltec in Brazil on Email住址會使用灌水程式保護機制。你需要啟動Javascript才能觀看它 or +55 (43) 3037-3523.




Ammonit Measurement GmbH
Wrangelstrasse 100
10997 Berlin

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