Distributed Renewable Energy Operating Impacts and Valuation Study

Section 3
3.6.3 Modeling Results
Cloud Cover
APS provided data from the STAR center to illustrate the effect of cloud cover on PV
generation. Figure 3-11 shows the PV generation in kW over a three-day period with
intermittent clouds. Reduction in PV generation is evident when cloud cover reduces irradiance.
Figure 3-11: Effect of Clouds on PV Generation
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Source: APS
While solar DE can provide generation throughout the year, the dependence of PV generation on
clear sunlight and the possibility of clouds reducing or removing the clear sunlight requires the
distribution facilities to be capable of carrying the full load at any time. Even during peak load
periods, clouds can reduce PV generation substantially as indicated in Figure 3-12, which shows
the hourly irradiance for several peak load days. The various lines indicate the day of the year
on which the peak PV production occurred.
Since adjustments in distribution capacity require dependable local solar DE to reduce load
during peak periods, this analysis re-enforces the finding that capacity deferral can only be
accomplished if the loss of PV output does not increase feeder load above the 10 percent
contingency level, or if solar DE storage is economically viable.
3-30 R. W. Beck, Inc. Arizona Public Service