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Active Balancing Power Converters for Photovoltaic Energy Systems

Final Report Summary - ACTIVRENERGY (Active Balancing Power Converters for Photovoltaic Energy Systems)

- Summary description of the project objectives.

The project is within the framework of power electronics applications for photovoltaic (PV) energy systems, which play a key role in the generation and storage of carbon-free energy. The main objective of the project is the development of new, smart PV modules, enhanced thanks to the intelligent use of power electronics devices. These power electronics devices replace bypass diodes and allow differential power processing between submodules (or substrings) in order to mitigate mismatch effects due to partial shading, soiling, temperature gradients, manufacturing tolerances and ageing. As a result, the proposed smart PV modules will be virtually immune to mismatch and will be able to operate during more years. This will increase the energy yield of PV systems, while decreasing cost and easing the operation of downstream power electronics.

- Description of the work performed and main results.

With the goal of demonstrating the advantages and the feasibility of integrating power converters in PV systems, the project has tackled two aspects. First, the creation of a simulation tool that enables the generation of long-term predictions of the operation and the energy yield of photovoltaic systems featuring different architectures of power electronics converters. Secondly, the realization of high-power density converters that can operate and mitigate mismatch effects with no supervisory controls or additional sensors.

The first result of the project is a cell-level simulation tool for photovoltaic systems including power electronics converters. The tool can be used to predict the behavior and performance of a photovoltaic system during its entire lifetime. It can solve the different scenarios very efficiently so that the statistical models of long term ageing effects such as degradation rates and their associated coefficients of variation can be taken into account following Montecarlo methods. The tool is comprehensive and includes the conventional architectures with central or string inverters, more recent approaches such as those with module-level microinverters and DC optimizers and also the recently introduced architectures with submodule-level converters and differential (or partial) power processing. The power rating (or limit) of the converters can be taken into account with the simulation tool, such that the relation between cost and efficiency of each approach has been successfully identified. It has been verified that PV modules featuring submodule-level power converters achieve 35 years lifetime, instead of the usual 25 years. The results of this part of the work have been published in several high impact journals such as IEEE Transactions on Power Electronics, Progress in Photovoltaics and Solar Energy.

The second result of the project is a very high frequency (6.78 MHz) resonant power converter suitable as submodule-level power converter for mismatch mitigation. The converter can operate in open loop thanks to a self-oscillating control method that enables operation near the resonant frequency. Also this part of the project has been reported in several high impact journals.

In order to enhance the outreach and visibility of the project, the simulation tool has been published as free software in the personal web page of the principal investigator:

- Potential impact and use of the results.

This project has allowed to identify the advantages of power electronics converters integrated in PV modules. Not only power electronics converters can increase the energy yield of PV systems by mitigating mismatch, but they can also extend the lifetime of the modules and prevent some modes failures. All these factors enhance the performance, cost and reliability of PV modules, effectively decreasing the cost-per-watt-hour of these systems. If submodule-level power converters were introduced in European PV systems, service life and warranty of PV systems could be extended up to 35 years. Thus, the competitiveness of the region on PV renewable energy would increase and it would put the European PV industry on the top world level. Secondly, the payback price of PV installations would be improved, such that PV companies would install more systems and jobs would be consequently created. Finally, the energetic dependence of the EU area with respect to other countries would be reduced, saving significant economic resources of the EU countries.