One of the thorniest issues that seemed to be facing large, utility-scale PV systems was sudden changes in PV system output due to the movement of single clouds on an otherwise sunny day. Now it seems that this issue has been resolved favorably, in an economical manner. See: thesolarreview.org

1. The Problem of Fast Cloud Transients and PV Ken Zweibel Director, GW Solar Institute zweibel@gwu.edu

2. Fast Cloud Transients on Sunny Days

3. The Worst Case: System about the Size of a Fast Moving Cloud Small, Isolated System: Almost 100% Fast Variability

4. The Problem Restated The problem is not loss of output – we already know the approximate output of systems includes lost output from less-sunny weather The problem is unpredictability and rapidity of change Unpredictability, because it reduces the ability to compensate for the change Rapidity, because it limits the kind of response or even makes it impossible

5. Different Arrays Cause Very Different Fast Variability Effects Small, Distributed Systems: Under 10% Fast Variability

6. Different Arrays Cause Very Different Fast Variability Effects Small, Distributed Systems: Under 10% Fast Variability Large System: Under 10% Fast Variability

7. Observations Fast cloud transients are a serious problem only for single, isolated systems that are about the size of a cloud on a day of scattered clouds They are a very minor problem for either Widely separated systems whose output is aggregated Like residential systems Very large systems that are much bigger than single clouds Scattered or partly cloudy conditions result in predictably reduced output, but not unfavorable variability Cloud fronts are not a problem because they are less frequent and are predictable

8. Acknoweldgement Clean Power Research (Tom Hoff and Richard Perez) for almost all technical content