Observed Impacts of Transient Clouds on Utility-Scale PV Fields

Variability is perceived to be a major issue for large-scale photovoltaic (PV) systems. The sudden change in PV system output due to partly cloudy weather conditions is the primary concern. Because PV modules respond instantaneously to changes in solar irradiance, it has been speculated that PV fields (regardless of size) could have large and frequent ramp events that may create challenges for electric grid operators. The recent completion of Florida Power & Light Company's 25 MW DeSoto Next Generation Solar Energy Center (henceforth referred to as DeSoto) near Arcadia, Florida, offers us a unique opportunity to observe the effects of transient cloud fields on utility-scale PV systems. Figure 1 depicts the aggregate electricity production output from one day in October 2009 from DeSoto. Clear skies in the morning gave way to a partly cloudy afternoon on this day. {Plotted at the bottom of Fig.1 are the ten second rates of change of the energy production of the full site (25 MW) and a smaller 1 MW subsection region of DeSoto.} We see a significant reduction in the amplitude of power fluctuations when we scale up from 1 MW to 25 MW in PV capacity; frequent small cumulus clouds have less full site production impact as compared to their effect on a smaller subsection of DeSoto. Our paper will focus on results and statistics highlighting the effects transient clouds of all scales have on the DeSoto PV field. We will demonstrate the dampening effects utility-scale solar PV plants have on production ramp events caused by the passage of transient clouds. In addition we will present results of a study that correlates pyranometer measurements to power output from a collocated plant subsection. As pyranometer measurements are more ubiquitous than plant output, results from this study will aid us incharacterizing plant output ramps using pyranometer measurements at future sites.