5. 3 Operation, Performance and Maintenance of PV Systems
Summary / Abstract:
Cost-effective tools for on-site defect detection help to ensure the reliable power production of photovoltaic (PV) plants. On-site examination is essential to minimize PV module damage and electrical yield loss caused by additional transport to the laboratory (off-site examination). Outdoor inspection may be carried out by using highly sensitive imaging techniques, such as luminescence or thermography imaging. Nowadays, electroluminescence (EL) is used to detect defects such as increased series resistance, local cell irregularities, shunts and electrical disconnections in solar cells and modules . While manual and quasi-stationary acquisition of EL images in the field is slow, mobile and aerial EL image recording setups, featuring a much higher throughput, were recently presented , . The drawback of aerial EL measurements is that they require electrical excitation of the module. This either necessitates special inverters, or requires a time consuming electrical connection and disconnection of modules and strings to an external power source . To avoid this issue, we replaced intrusive electrical excitation by non-contact excitation with light, and measured photoluminescence (PL). Our excitation source consists of twelve broadband, white, high power, 100 W, chip on board light emitting diodes (LEDs) with attached low cost short pass filters. The light source is suitable for InGaAs and silicon (Si) detectors and mobile outdoor applications, which makes this setup a tool for on-site PL inspections for large area modules. The PL setup presented in this paper is capable of measuring large area PV modules of up to 72 cells. In Fig. 1, we compare a PL image and an EL image of one of the investigated Si PV modules. One main difference between EL and PL is that PL characterizes the whole solar cell, not only the electrically connected areas (which appear dark in EL). However, with suitable defect detection algorithms it may still be possible to identify inactive areas. Thus, performance-relevant defects, like cracks and cells with low parallel resistance even under challenging conditions (similar to what is encountered outdoors in the field, like inhomogeneous illumination) can be extracted.