Tadanori Tanahashi

Acceleration of potential-induced degradation by salt-mist preconditioning in crystalline silicon photovoltaic modules

We examined the sequential effects of salt-mist stress followed by high-system-voltage stress on the power loss of crystalline silicon photovoltaic (PV) modules to determine whether a crucial failure as potential-induced degradation (PID) is accelerated by material-property changes caused by the long-term effects of a less harmful stress such as salt-mist spraying. Degradation profiles confirmed in this study show that PID is accelerated by certain types of salt-mist preconditioning. For the acceleration of PID, the contribution of sodium ions liberated from the front glass of the PV module seems to be excluded. Therefore, we consider that the sodium ions penetrating into the PV modules from the ambient environment may also cause degradation according to the proposed mechanisms of PID, as the sodium ions existing in the front glass cause PID. Furthermore, this type of degradation may indicate the wear-out phenomenon after a long-term exposure in the field (especially near the coast).

Electrical detection of gap formation underneath finger electrodes on c-Si PV cells exposed to acetic acid vapor under hygrothermal conditions

For the current collection failure induced by the acidic corrosion in finger electrodes on photovoltaic (PV) cells, the formation and evolution of gaps underneath the electrodes were able to be found by the monitoring of AC impedance signals and I-V characteristics of PV cells. The similar degradation behavior was also observed in PV modules under damp heat test. Then, the degradation mechanisms of crystalline silicon PV module under hygrothermal conditions are discussed, to contribute the lifetime prediction of PV modules installed in fields.

Early Failure Detection of Interconnection with Rapid Thermal Cycling in Photovoltaic Modules

To accelerate the degradation with thermal fatigue in crystalline silicon photovoltaic modules, the modules were exposed to dry thermal stress with rapid thermal cycling, and module impedance was monitored in situ during this testing. The spikelike increase in module impedance at a temperature-alteration point was observed in the early stage of this rapid thermal cycling. The pattern of increase in module impedance proceeded step-by-step, from the early stage, via the double-spikelike pattern at two temperature-alteration points (the middle stage), and finally to the successive increases in module impedance in the high-temperature period (the late stage). The nondestructive analyses suggest that the interconnector failures without the defects of photovoltaic cells occurred. From these results, it is suggested that the pattern of increase in module impedance is related to the interconnection degradation of modules, and that the rapid thermal cycling with in situ monitoring of module impedance would be a useful procedure for the earlier detection of interconnection failures in photovoltaic modules.

Causes of Degradation Identified by the Extended Thermal Cycling Test on Commercially Available Crystalline Silicon Photovoltaic Modules

To assess the contribution of the thermomechanical stress on the long-term reliability of photovoltaic (PV) modules, we applied extended thermal cycling tests up to 600 cycles to 13 models of commercially available PV modules. The extensive testing, using 5–10 PV modules for each model, revealed that the levels of power loss, induced by thermal fatigue during this extended testing, differed in each model of the PV module. Degradations by solder bond failure and bypass diode failure, which were observed in a few models of PV modules, were most likely to result from incorrect soldering that may be completely avoidable through appropriate implementation of a quality management system. Therefore, we suggest that, in most of the latest PV modules, their durability to thermal fatigue was established, as evidenced by the extended thermal cycling test with 600 cycles. However, solder bond failure due to inappropriate manufacturing process quality, which is not identifiable by the conventional thermal cycling test (200 cycles), would be detected by this extended test in a few models. Based on these suggestions, we discuss the evaluation procedures of these failures caused by the quality-control and design issues of PV modules.

Direct evidence for pn junction without degradation in crystalline Si photovoltaic modules under hygrothermal stresses

Origin of degradation in output power for crystalline Si photovoltaic modules by hygrothermal stresses was studied. The direct evidence that pn junction is hardly damaged and the origin is damage in Ag finger electrodes was obtained through the experiments composed of removal of degraded finger electrodes and reconstruction of electrodes. It was suggested that electrode is final defense for reliability of photovoltaic modules under hygrothermal stresses. The most important issue for highly reliable photovoltaic modules may be the development of electrode materials with toughness against acetic acid.

Issues and Solutions Concerned in the Coefficient of Thermal Expansion on Photovoltaic Modules

太陽光発電システムは,2014 年末までに全世界で約 177 GW(合計設備容量:規定条件下での発電能力の合計)導 入されており,世界全体の電力需要の 1%超を供給している と見積られている 。また,2015 年は 57 GWの新規導入が 速報されており ,太陽光発電による電力供給は,地球温暖 化対策などを目的に,先進国・新興国を問わず今後も拡大す るものと見込まれている(2019 年末時点で最大:540 GW程 度の導入量への到達が予想されている[現導入量の約 3倍])。 このようにエネルギーインフラの一翼を担う可能性を持ち始 めた太陽光発電においては,当然のこととして長期の安定し た稼働が求められる。このため,屋外設置という厳しい環境 条件においても,十分な長期信頼性を確保するための様々な 研究開発が行われている。本稿では,現在導入されている太 陽電池モジュール(太陽電池パネルと同意)のうち 90%以上 を占める結晶シリコン系太陽電池モジュール(以下,c-Si PV モジュールと略記)について ,屋外設置環境における温度 変化と熱膨張率を主題に概説する。