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Bottom-up PV module energy yield and integrated reliability model for site-specific design optimization

Periodic Reporting for period 1 - PVMINDS (Bottom-up PV module energy yield and integrated reliability model for site-specific design optimization)

Reporting period: 2018-02-19 to 2020-02-18

Photovoltaic (PV) module reliability is a critical factor for energy yield predictability, and reduced PV cost of electricity. Today, there is limited understanding of PV reliability issues under real-field conditions; and none of the state-of-the-art energy yield models can predict their long-term performance considering lifecycle degradation and failure propagation. Ultimate objective of the planned research is to develop the first bottom-up reliability model for selected PV failure/degradation modes, coupled with advanced simulation of real-field stress factors.
The implementation of PVMINDS provided the research and technical groundwork to address a two-fold innovation goal:
To investigate on the bottom-up reliability testing and modelling for targeted (i.e. dominant and industry-relevant) failure and degradation mechanism(s) of PV modules.
To identify the failure-/site- specific stress factors, as input parameters for coupling imec’s energy yield simulation framework with reliability models.
Over these 18 months of the project, PVMINDS has achieved most of its key research goals as well as the training and public outreach goals that have been set. In view of the early termination of the project, due to family/professional reasons of the MSCA fellow, the status of the work related to the integration of specific reliability models with the energy yield simulation framework has not been completed yet; though the team at imec will continue the research that has been started.
In order to best align the above goals of PVMINDS with emerging R&D challenges and needs in the PV industry, the overall research plan carried out in the project was particularly adapted and focused on the following activities-objectives:
1. As a start, to identify, through theoretical/review work, the occurrence, diagnostic patterns and impact of dominant failure modes and degradation mechanisms in fielded PV systems.
2. To investigate on the physics, stress factors and reliability testing/modelling needs associated to potential-induced degradation (PID) for bifacial PV modules.
3. To define practices for optimal reliability testing and characterization (indoor and outdoor) of dominant yet repairable failures of PV modules; thus, adding a circular economy/sustainability dimension in the aimed research.
4. To define key design and component-level stress parameters that enable: i) integration with imec’s energy yield simulation model, on one hand, and/or ii) repair/re-use for second-life PV, on the other hand.
"The work in WP1 was comprised in three individual tasks (T). T1, a theoretical research study on PV failure modes and their association with certain degradation rates, performance losses and diagnostic patterns; T2, a theoretical research study on field reliability and fault diagnostics for the case of bifacial PV, being the emerging and future dominant PV technology; T3, PV monitoring and yield simulation studies in real field, i.e. for existing PV test sites.
A particular focus was given on understanding and defining the occurrence of cell crakcs, potential induced degradation and bypass diode failures throughout the early years of the PV plants' operational lifetimes, as well as the electrical and thermal/optical characteristics of different failure modes. In addition, thorough insights were gained into the reliability testing needs, the field reliability and the dominant (expected) failure modes specificially in bifacial PV modules, both in glass-glass and in glass-transparent backsheet configration.
The research activity in WP2 comprised of two main Tasks, T1 and T2. The former shed light on the physics, stress factors and reliability testing/modelling needs associated to PID for bifacial PV modules. On the other hand, the work of T2 added a sustainability perspective, focusing on the characterization/inspection, classification and reliability testing of repairable PV failure modes, for the re-use (second life) of failed/decommissioned PV modules, i.e. in a circular economy framework.
The key findings, in brief, included: i) the identification and underlying physics of the different PID mechanisms (i.e. shunting and polarization type, Fig. 1), ii) the thorough understanding of the different characteristic ""signatures"" of these PID mechanisms and their propagation, i.e. in terms of I-V, EL and EQE characterization measurement, as well as iii) the optimization of current PID testing practices for bifacial PV, to allow correct interpretation and distinguishment of such mechanisms.
Finally, in WP3, in view of the early termination of the MSCA fellowship, the envisaged work has not been fully completed, though it remains an ongoing activity within the research team of imec. The extension of imec’s energy yield simulation framework and its integration with reliability model for certain failure/loss mechanisms (PID, soiling) is (and will) based on the gained insights from WP1, WP2 and on a three-fold ongoing activity:
Monitoring and what-if simulations study at Kuwait’s PV test installation.
Energy yield simulations’ benchmarking study for IEA PVPS Task 13 ST2.1"
Through the aforementioned work progress and innovation activities, there were two main findings/outcomes beyond the state of the art:
Identification of the mechanisms’ physics and impact of individual stress and material parameters associated to PID in bifacial PV modules.
Improved reliability testing for PID in bifacial PV modules. On such basis, this work also raised awareness of false interpretations derived while conducting PID stress according to the existing foil-method as described in IEC62804. This work might be interesting when the PID standard IEC 62804 gets updated for bifacial PV modules. Therefore, we included three possible measures which should be considered when monofacial PID stress tests of bifacial PV modules are conducted:
o shorting the cell and the non-stressed glass cover side (shielding);
o using a floating high voltage source;
o and replacing the glass cover by a PID-resistant cover at the non-stressed side.
Bifacial PID of bifacial p-PERC solar cells when using a glass/glass module configuration