Recently, a new in situ measurement technique has been developed using miniature digital relative humidity (RH) sensors. The measurement technique proved to be a valuable tool not only for in situ monitoring of water concentration in the photovoltaic (PV) modules exposed to accelerated test conditions in climatic chambers but also for long-term outdoor monitoring under field conditions. In this paper, we present a completely new in situ setup, which is based on a wireless radio-frequency identification (RFID) technology. In our design, up to seven digital humidity sensors can be soldered on a 130 μm thick polyimide foil that also integrates RFID antenna and all required readout electronics. Their ultra-small size and wireless design allows placing them anywhere in the PV module either in front or at the back side of the solar cells. All measurement data can be read by a dedicated RFID reader which also provides the power needed during the measurement and communication cycle. The systems were applied in mini-modules with one full-size crystalline silicon solar cell and exposed to the elevated temperature and humidity conditions (40 °C / 85 %RH and 50 °C / 70 %RH). The acquired moisture ingress data agree well with the two-dimensional simulations using the realistic dimensions and diffusion coefficient of ethylene-vinyl acetate.

Moisture plays a critical role in the degradation process of photovoltaic (PV) modules in field conditions. A commonly used approach is to evaluate the properties of PV materials by conducting tests in climatic chambers, and then apply the Fickian diffusion model in simulations to replicate field conditions, however, without experimental verification. This study describes an experimental setup for in situ measurement of moisture in fielded minimodules, using miniature temperature and humidity sensors encapsulated within the modules. Five identical setups are deployed in different climatic regions around the world, enabling a quantitative evaluation of moisture diffusion in the field and a comparison of different climates. The relative humidity measured beside the cell follow weekly weather trends due to breathable backsheet, whereas the sensors in front of the cell react much slower to outside changes and follow seasonal trends. We show that a single 2-D simulation, which is a standard practice in published studies, is insufficient to accurately depict moisture diffusion in front of the cell. Therefore, a two-stage 2-D simulation model, combining Fickian diffusion in vertical and horizontal cross section with carefully set boundary conditions, was introduced. A comparison with measurement results showed the simulation approach to be a good compromise between the simulation accuracy and speed. Finally, the results underscore the significance of understanding the local microclimate surrounding the modules, including the interface between air and backsheet, which is necessary for precise moisture diffusion simulations.

Backsheet cracking is among the most commonly observed degradation modes of photovoltaic modules in the field. Cracks can reduce the ability of backsheets to fulfill their functions, such as protection of the modules from the environment or electrical insulation. This work presents an evaluation of the degradation and cracking propensity of two backsheets during accelerated IEC TS 62788-7-2 (International Electrotechnical Commission) aging under the A3 condition with a fragmentation test using a coextruded polyamide backsheet (AAA), and a laminated multilayer backsheet with a polyethylene terephthalate core and outer layer and ethylene vinyl acetate inner layer (PPE). Results show the surface embrittlement of the AAA outer layer during exposure. A longer time of exposure causes cracks to form at lower strains during stretching and creates deeper cracks. Accordingly, Young's modulus of the outer layer increases, as measured by cross-sectional nanoindentation. PPE exhibits cracking after exposure as well. While no similar increase of modulus or crack depth can be observed, the outer layer of PPE exhibits more obvious signs of erosion during exposure, including progressive morphological changes and thickness losses. A finite element model is developed to simulate surface crack formation, based on initially zero-thickness decohesion elements. Decohesion criteria define a critical stress, at which these elements grow, and cracks can begin to form. These criteria are obtained via parameter optimization by comparison between simulation and experiment. The model is used to interpret crack formation in both backsheets.

The Development of Thermal Coefficients of Photo-voltaic Devices

Light emitting diodes (LEDs) have experienced rapid technological development in the past decade, making them a winning alternative to conventional light sources in many applications. LED arrays allow precise control of the desired irradiance profile in a target area by adjusting the position and output power of individual LEDs. However, despite increased efficiency, many LEDs still transform a large proportion of the input electrical power into heat, requiring an efficient cooling system. This paper presents a modular LED array light source mounted on a water-cooled aluminum plate. Novel electronic LED driver modules, connected via a serial communication bus in a daisy-chain topology, were developed with the ability to set the operating current of individual LEDs. A modular layout of cooling and mounting system and LED driver modules, as well as a specialized design for the LED soldering footprint, was able to house a variety of common commercial LEDs, enabling easy adjustment of the lighting system to the required application and size of the irradiated area. In a prototype of one plate containing 10 LEDs, individual LED radiance was optimized for a better irradiance homogeneity in the target area. Array characterization showed a low standard deviation of the irradiance of 1.8% and a good fit between measured and calculated irradiance. A test of the array at elevated temperatures showed moderate LED radiance degradation and a wavelength shift of the measured spectra after extended use.