|
Concentrator photovoltaic (CPV) solar energy systems use optics to concentrate direct normal incidence (DNI) sunlight
onto multi-junction photovoltaic (MJPV) cells fabricated from III-V compound semiconductors on germanium
substrates. The MJPV receiver, which integrates cell and bypass diode, is then mated with its concentrating optic to form
a channel, and several such channels form a CPV module, in which the receivers are connected electrically in series. The
two ends of the module receiver string are brought out to a single pair of electrical connections, at which point the lightcurrent-
voltage (L-I-V) response of the entire module can be tested. With commercial CPV modules commonly sealed
against outdoor exposure, there are no other accessible test points, and field installation on trackers further complicates
access to performance data. There are many physical phenomena influencing module performance, and in early
development and commercialization some of these may not yet be completely under control. Unambiguous diagnosis of
such phenomena from one full-module L-I-V curve is problematic. Simple, fast test methods are needed to develop more
detailed information from full-module on-tracker testing, without opening up modules in the field.
We describe a test protocol, using a simple optical shutter array constructed to fit mechanically over the module. When
module L-I-V curves are recorded for each of various combinations of open and closed shutters, the information can be
used to identify one or more anomalous channels, and to further identify the kind of anomaly present, such as optical
misalignment, conductor failure, series or shunt resistance, and so on. Simulated results from anomaly models can be
compared with the measured results to identify the anomalous behaviour. Results herein are compared with direct single-channel
measurements to verify the technique. The L-I-V response curves were obtained in continuous real time, an
approach found to be more helpful than single-shot capture in understanding field response. A triangular wave function
generator is used to drive the DC power supply, and a four-channel digital sampling oscilloscope displays and stores the
real time response. Where modules exhibit unstable or intermittent response under certain conditions, this is immediately
obvious in real-time display.
|
