Chinese Academy of Sciences presents new way to assess HJT solar modules

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To address concerns about heterojunction solar module degradation, a team of scientists led by the Chinese Academy of Sciences, has developed a model to predict the lifetimes of heterojunction modules.

The model considers factors such as temperature, humidity and UV irradiation and was used by the research team to predict the lifetimes of modules made with ultraviolet cut-off POE encapsulant films.

The scientists stressed that ultraviolet-induced degradation in heterojunction panels is “largely caused by a decrease in short-circuit current” and that it can be “effectively suppressed” by using encapsulants made of POE, as it has durability properties.

The group tested its approach on heterojunction modules fabricated with a double glass configuration and 144 pieces of half-cut M2-sized cells. One set of panels was laminated with an ultraviolet passive POE (UVP) encapsulant and the other set was laminated with an ultraviolet cut-off POE (UVC) encapsulant.

The samples were exposed to accelerated UV irradiation tests from 60 kWh to 300 kWh in the ultraviolet chambers at the National Center of Inspection on Solar Photovoltaic Products Quality of China.

The modeling and UV tests enabled the team to draw some early conclusions. “Overall, the UVC modules have a longer lifetime than the UVP modules, which can be extended by an average of about 4.2 years across different regions. This advantage is mainly attributed to the UV cut-off POE, which blocks the UV photons’ damage to the HJT solar cells,” the academics said.

As temperature and humidity also affect the module degradation rate and the group did not want to rely on UV data alone, it conducted subsequent damp heat (DH) tests were conducted only on the UVC encapsulant material. Mini modules were fabricated with one M2-sized HJT cell in a double glass configuration and laminated with UVC films.

Fifteen samples of the one-cell mini-modules were prepared. They were divided into three groups to be exposed to damp heat conditions as follows: 85 C at 85% relative humidity, 55 C at 85% relative humidity, and 25 C at 85% relative humidity.

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All mini-modules were then subjected to the damp heat (DH) conditions for 500 h, and the current-voltage output parameters were measured every 50 h at standard test conditions of the irradiation of 1,000 W/m2 at 25 C.

Next, the team used the model to make degradation predictions about HJT modules based on specific climate parameters for five locations across China corresponding to different climatic zones – Daqing, Hainan, Qinghai, Ningxia, and Shanghai.

The analysis showed that the systems located in Daqing and Ningxia may achieve lifetimes of over 30 years, as these locations have lower UV irradiation of 48 kWh/m2 and lower relative humidity, less than 55%.

“Evidently, the lifetime of HJT modules can be easily calculated when the temperature, humidity and irradiation data from a specific location are input into the Peck model,” concluded the team.

The details of the research appear in “Predicting the lifetime of HJT modules towards the outdoor real-world environment,” which was recently published in Solar Energy Materials and Solar Cells.

 The researchers participating in the study were from China’s State Key Laboratory of Materials for Integrated Circuits, University of Chinese Academy of Sciences, Tongwei Solar, Huaneng Clean Energy Research Institute, and Huaneng Gansu Energy Development.

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