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Neu Isenburg, Germany

Jaeckel B.,UL International GmbH | Cosic M.,UL International GmbH | Arp J.,PV Laboratory Germany GmbH
2014 IEEE 40th Photovoltaic Specialist Conference, PVSC 2014

In recent years several failure modes of PV modules operated under high potentials were observed. Most dominant today is the degradation mechanism called "potential induced degradation (PID) " of crystalline PV modules [1]. This shunting mechanism of c-Si solar cells can cause dramatic power losses in not grounded installations operated at higher system voltages. For thin film PV modules a phenomenon called TCO corrosion [2] was mainly discussed but other mechanisms exist [3]. Each failure mode might require different environmental conditions to be observable in the field and also to be reproducible in the laboratory by an accelerated test setup. Herein the focus is only on one particular failure mode of c-Si modules, usually called PID. Most studies in the past only focused on finding best methods to degrade modules [4]. But what occurs on the field could be different from currently used methods in laboratories to simulate PID and therefore may need a different approach to prove the real stability of PV modules against the PID effect. Therefore we propose a new test sequence, called CV-PID, which not only investigates the degradation rate but also the recovery rate and by that define new stability criteria. Nagel et al [7] presented a similar approach in 2012 but did not introduce a cyclic voltage application that is expected to occur in the field. This proposed test sequence can also be used to determine appropriate countermeasure if an installation already shows PID. Those countermeasures for example could be simple voltage boxes that apply a certain recovery potential during the night or potential shifting systems, always keeping the module string at a safe potential. Depending on degradation/recovery rates and therefor the ratio of recovery to degradation (DR), one or the other countermeasure is more effective and more sustainable in the long term. Testing of the PV Modules is done by applying a conductive foil to the front side of the module as a front side electrode. Voltage cycling is introduced to examine the degradation and regeneration/recovery behavior of the used PV modules. The paper gives a detailed overview of possible degradation mechanisms [13] and looks into the recovery of modules output power. Different degradation scenarios are explained and data is presented. © 2014 IEEE. Source

Jaeckel B.,UL International GmbH | Cosic M.,UL International GmbH | Arp J.,PV Laboratory Germany GmbH
Israel Journal of Chemistry

Recent research demonstrates several failure modes of photovoltaic modules operating under high electric potentials. In crystalline-silicon modules, the predominant failure mode is potential-induced degradation (PID), causing dramatic power losses in systems under high voltage and critical polarity. Environmental conditions highly influence the degradation behavior. The ability to reproduce field observation in the laboratory is challenging and not all stressors can be checked simultaneously. PID and its root cause are not fully understood, but we know several mechanisms are working simultaneously and at varying rates. The main mechanisms are degradation, characterized by ion diffusion and cell shunting, and recovery, driven by temperature, voltage, and potential. Most studies have focused on simulating module degradation using a constant set of parameters. However, field exposure to high voltage is variable, measured by the hour. In 2012, Nagel presented a module stability test with varying environmental conditions, notably temperature. To investigate PID remedies, this research develops a testing procedure that reproduces field observations while understanding that some modules do not degrade in real PV installations under high potentials. Conductive foil is applied to the front side of the module and voltage cycling is introduced to examine potential-induced degradation and potential-induced recovery behavior. The relationship between the two defines a PID stability criterion. Results show general PID sensitivity and suitable remedies for PID affected systems. PID recovery and protection solutions include applying recovery potential at night, and potential shifting, which regulates potential for the module string. This paper explores possible degradation mechanisms, recovery of module output power and PID stability criteria. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

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