Multiple different photovoltaic module analysis techniques are available and necessary for the inspection of photovoltaic (PV) modules, the detection of occurring degradation and the analysis of cell properties. The analysis of PV modules during production and operation is an important part in ensuring reliability and thus energy efficiency of the PV technology. Therefore, it is crucial for solar module quality assurance. During their lifetime, PV modules experience severe changes in weather and working conditions, leading to large temperature variations (day - night, summer - winter, irradiance) and mechanical stress (wind, snow, hail). This can lead to an enhanced degradation compared to the usual wearing-out of materials over time, resulting in degradation modes (DMs), which can have an (negative) effect on lifetime and power production. To predict the impact of DMs on a PV module or even a PV system, DM detection and evolution studies are needed. Several different analyses techniques are available, as each visualizes and analyzes different DMs and properties, therefore allows specific statements.
Analysis techniques Some DMs, like snailtracks or glass breakage, are visible by the naked eye. Others, like cell cracks and current mismatches in cells, can be visualized with luminescence techniques, while hot spots can be detected with infrared thermography. This article gives an overview about common analysis techniques used for operation and maintenance (O&M) of PV modules in the field.
Visual inspection As it is the cheapest and fastest method, visual inspection is always first choice. It can be done during every inspection of the PV plant, but also more detailed, following a certain procedure. As visual inspection is subjective, evaluation forms are developed to ensure comparability. Possible defects, which can be identified by visual inspection, are glass breakage, electro-chemical corrosion, burn marks (in front or back sheet), delamination of front glass or back sheet, browning (evoked by atmospheric oxygen or heating), snail tracks, soiling and others.
IV curve measurement A current-voltage curve (IV curve) of a PV module gives information about the relation between current and voltage of the module and therefore about its quality and the solar cell efficiency. It might be distinguished between IV curve measurements in a laboratory under standard test conditions (STC) and a measurement outside in the field. Measurements under standard test conditions (STC: 1000 W/m2, 25 °C, air mass (AM) 1.5 radiation) show the specifications of a PV module and its quality and allows comparison with other modules measured under the same conditions. To assure STC, laboratory conditions and certain equipment are necessary. A solar simulator and a test bench are required: the module (or cell) is mounted in the test bench and then irradiated for a fraction of a second (so called "flashed"). During the flash, the voltage of the module is swept along a defined range and the resulting current is measured, resulting in the IV curve. Usually accuracies of about 3% can be expected for laboratory IV measurements.
For IV curve collection in the field one have to notice that irradiance and temperature are not controllable. Thus, to compare the resulting IV curve to ones taken in different conditions, it needs to be adapted to STC by correction factors for measured irradiance and cell temperature. A calibrated solar device can be used to measure the present sunlight irradiance and a temperature sensor (like a Pt100) to measure the cell temperature of the module under investigation. To measure the IV curve of a single module, portable devices, so called IV curve tracers, are available. Modern inverter or maximum power point tracker (MPPT) are able to measure the IV curve of the connected string (series circuit of multiple PV modules). In a PV module under operation, the extreme conditions are open-circuit and short-circuit. At open-circuit, the voltage is maximum (open-circuit voltage VOC) and the current zero. Whereas at short-circuit the current is maximum (short-circuit current, ISC) and the voltage zero. The power is given by the product of current and voltage and has a maximum at maximum power point (MPP). One parameter to define the quality of a PV module is the fill factor (FF), which is the ratio between maximum power (PMPP) of the module and the virtual power (PT, product of VOC and ISC). All these values can be extracted from a measured IV curve. Furthermore, the IV curve allows to determine the shunt resistance (RSH) and series resistance (RS) of a PV module. The series resistance is the combined resistance of all materials and their transitions, created current has to overcome, to reach the load. An increased RS results in less slope of the IV curve close to VOC. The shunt resistance instead describes the separation strength of the p–n junction in the solar cell. A decrease in shunt resistance results in an increased slope of the IV curve close to ISC. Electrical mismatches between the cells of a module result in a step wise behaviour of the IV curve. The same characteristic can occur from partially shading, which itself creates a mismatch.
Infrared thermography
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