Page 79 - A Comprehensive Guide to Solar Energy Systems

74 A COmPrEHENSIVE GUIDE TO SOlAr ENErGy SySTEmS

FIGURE 4.24 The duck curve during a typical spring day [41].

In tandem with demand response, solar can provide a benefit to the utility that is not
currently accounted for in most states. Utilities often assess commercial and industrial fa-
cilities a demand charge based on their peak kilowatt use in a given month. Often the util-
ity applies a “ratchet clause,” charging for monthly demand based on a percentage of the
peak kilowatt in a year, even if demand is far less during the rest of the year. Therefore, con-
sumers can substantially lower their demand charge by implementing demand response
measures in tandem with utility’s peak demand. Along with energy conservation, consum-
ers can in fact provide a service to utilities during peak demand by maximizing PV system
production and export during such times (i.e., installing west-facing arrays to maximize
afternoon and evening production). These services, however, must still be quantified and
properly valued with respect to their benefit to the grid.
In contrast, the integration of distributed generation (DG) requires system upgrades
to enable bidirectional power flow and energy storage. The issue of intermittency is para-
mount. The “duck curve,” developed by the California Independent System Operator (CAI-
SO), demonstrates the challenge of matching demand and supply with increased rE pen-
etration. The curve (Fig. 4.24) shows that nonsolar supply must be high in the morning,
steeply decline during the “solar window,” the three hours before and after 12 Pm noon,
and surge in the afternoon as the sun begins to set [41].
CAISO has taken several steps to better match supply and demand, including time of
use rates to deter high use in the evening, and flexible ramping. Flexible ramping is the
ability of a generator to start and stop on command. This is useful since during high noon
when solar resources are the most productive, conventional “baseload” generators are still