Solar irradiance variability due to stochastic cloud dynamics can cause unwanted fluctuations in the output voltage of photovoltaic (PV) modules. These dynamics must in particular be understood at very-short and short time scales if grid interconnection and generation/load balance requirements are to be maintained for PV distributed across the grid edge. Using a recently-created database for Ottawa, Canada, a 6-month longitudinal study was conducted with a specific focus on cloud dynamics. A spectral pyranometer was used to derive full-range spectral and broadband global horizontal irradiance under all sky conditions every 250 ms. Exploiting the infrared (IR) measurement channel of this software-augmented multi-filter radiometer allowed the cloud dynamics to be probed across time scales ranging from the subsecond to minutes. Seven distinct sky conditions were self-consistently determined without sky imaging. Probability distributions, established via kernel density estimates (KDE), allowed the statistical dependence of these conditions on the spectral clear-sky index to be found. The stochastic nature of the spectral irradiance variability was probed using spectral clear-sky index increments, over time steps that were found to span three distinct variability regimes.
Matthew Wilkins, Richard Beal, Joan Haysom, Jeffrey Wheeldon, Philippe Mulet, Graeme Jamieson, Nashed Youssef, Dhan Balachandreswaran, Jennifer Fan, Trevor Hall, Stefan Myrskog, Karin Hinzer
An on-sun Concentrating Photovoltaic (CPV) test site with a multiplexed current-voltage measurement capability has
been installed at the University of Ottawa. Herein, we present details of the instrumentation, which is designed to
provide in-situ I-V measurements of individual solar cells within a functioning, series connected, string. A 4-wire
multiplexing system enables current-voltage measurements of individual cells. Where other multi-cell test systems have
left cells at open-circuit or connected to a fixed resistor when not selected by the multiplexer, in this system each test
cell operates within a string of cells controlled by a maximum power point tracker. This allows us to maintain the test
cells at realistic operating conditions for all irradiance conditions. Using the current-voltage data, a range of parameters
of interest can be monitored over time, including the solar cell temperature, photovoltaic efficiency and optical
efficiency. This information is correlated with time-synchronized spectrometer and pyrheliometer data, allowing
automated collection of performance data from several strings of optic/solar cell assemblies. The test site includes two
20m2, dual-axis, ring mounted solar trackers and is being used to test new CPV systems which incorporate novel lightguiding
optics designed and manufactured by Morgan Solar Inc.
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