Development of procedures and equipment for indoor characterization of concentrator photovoltaic systems

Resumen

The high concentrator photovoltaic (HCPV) modules, as their name suggests, concentrate the sun light by means of affordable optics onto high efficiency and costly multi-junction (MJ) solar cells (hence reducing the needed semiconductor area). To drive down costs and to increase the commercial opportunities of this technology, the approach commonly taken is to improve the system efficiency. Currently, the record efficiency for CPV modules is in the order of 35%-36% which almost double the numbers given for flat panels. To reach these numbers in production line or even to exceed them, proper characterization procedures specific for the CPV technology are needed. However, there is a distinct lack of methods and testing instrumentation as neither international standards nor commercialized equipment are available. This thesis reviews new methods and tools proposed for the accurate characterization of CPV modules and some of their constituent elements. Specifically, the study of the differentiating components of the concentration technology as the optical system and the MJ solar cell unlike common figures for PV are of particular relevance. Much of the cutting-edge research involved in this thesis is related to the development of a method for the optical-angular properties evaluation of a CPV module. The method consists in measuring the light emitted by the module while it is forward biased in dark conditions. Main metrics resulting from its implementation are the angular transmittance function, and in a novel way, the misalignments between optical system-cell units comprising the module (i.e., differences between the units pointing vectors). The method validation and the guiding principles that must be followed to obtain correct results are thoroughly reviewed at the beginning of this document. Parallel to this, a major task carried out is the design and fabrication of different equipment prototypes to implement the novel measurement. Among all inspected solutions, the system so-called Module Optical Analyzer (MOA) is particularly worthy of mention; it stands out for its optimum integration in production line to perform an exhaustive module characterization taking just seconds. During the course of this thesis, different MOA versions have been produced and tested, even one unit being installed in the manufacturing facility of the CPV company Daido Steel in Nagoya (Japan). With respect to the optical system characterization (in particular, Fresnel lenses), two noteworthy lines of research were pursued. The first line of inquiry is aimed at the study of degradation effects on lenses and the second one is focused on the lens evaluation in manufacturing. With regard to degradation effects in Fresnel lenses, a novel instrument was developed during a short research stay at the National Renewable Energy Laboratory (NREL) in the Photovoltaic (PV) Reliability Research and Development (R&D) group. This instrument measures with a CCD sensor the optical transmittance of the lens while is scanned by a laser The capabilities of the developed tool and other measurement techniques (hemispherical transmittance with integrating sphere and optical efficiency measurements at a solar simulator for CPV) have been reviewed and discussed while measuring different lenses samples that were subjected to different aging conditions. The definition of characterization procedures not only for laboratory but also for industry scenarios is important to ensure the proper development and strengthening of the concentrator technology. An easy to implement quality control is proposed based on measuring the first reflection at the lens entrance surface, and thus check up to identify possible wrapping or deformation defects on lenses. These lens shape anomalies can be caused by either internal or external stress related to manufacturing processes and module enclosure respectively. The electrical losses that these kinds of defects have in the module performance have been quantified by evaluating not only unassembled units but also mounted lenses in a module. It was observed that the warp effect can be even more damaging than the misalignments between components in the module. The electrical performance of MJ cells under non-uniform light profiles is a subject of research addressed at the end of this document. Several CPV module configurations have been studied by means of different developed methods and instrumentation to quantify and distinguish the damaging effect of both spatial and spectral non-uniformities on MJ cells.