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It is important to bear in mind that, when photovoltaic panels are connected in series, the panel producing the least power will limit the overall output of the panels in the string. Soiling and shading of one or more panels, for example, will result in production losses for the entire string.
Enel Green Power and the CEA have developed a DC/DC MPPT (Maximum Power Point Tracker) electronic solution with a maximum power of 500W at module level. It allows the number of panels in a string to be increased in order to increase the overall power, while limiting the voltage output to remain in the low voltage range (<1500Vdc). This electronic add-on also reduces the impact of production imbalance between panels connected in series, due to soiling or partial shading. Its characteristics show performance in real conditions, beyond what currently exists on the market with a conversion efficiency close to 98.5%.
After the design, development and validation phases in the laboratory, 36 prototypes of DC/DC MPPT converters were installed on a CEA demonstrator site in Cadarache to carry out tests in real operating conditions. The demonstrator allows to verify the proper working of the converters and to compare the differences in production with a conventional solar installation connected to a string inverter.
The demonstration plant is equipped with a specific monitoring tool that transmits data on voltage, current at the entrance and exit of the inverter, as well as overall production data. Several electrical tests were carried out to validate the main expected functionalities.
For instance, a shading test of one or more panels was conducted by hiding them with an opaque mask. It was possible to verify that, thanks to the use of DC/DC converters, the output voltages of the converters compensate each other well, without loss of production at the level of those remaining intact.
In the same way, partial shading of a module, by hiding several cells, induces a loss of production for the single concerned module and does not impact the production of the other modules in the string.
The demonstrator, which has been in operation since July 2021, has made it possible to observe how well the converters work. They incorporate an MPPT tracking algorithm that is robust and reliable even when irradiance varies greatly over time, with sunny and cloudy days.
Experimental demonstrator in Cadarache - © CEA
The energy produced by the solar power plant equipped with the solution was compared to the energy produced at the same site by a conventional plant equipped with a conventional inverter. The analysis was done under the assumption that the electrical characteristics of the modules would be dispersed. The results show that the developed solution would allow a gain of 1% of energy produced.
The deployment of the solution on a very large power plant on a 1MW scale is planned in Catania, Italy. This will enable a pre-industrial version to be tested and the technical and economic gains to be refined.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.