Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
(non-California ISO) interconnection applications for large solar increased from four
interconnection requests representing 920 MW in November 2007 to 33 requests representing
8,013 MW in December 2008. 34
There are several methods of converting electromagnetic radiation received directly from the sun
into useful electricity. Generally speaking, all of the methods described in this section are
classified as “solar” energy. However, it is important to recognize that considerable differences
exist in the technical characteristics from one form of solar technology to another. One
important characteristic shared by all types of solar power is their diurnal and seasonal pattern
(i.e. peak output usually occurs in the middle of the day and in the summer). This is an
important characteristic as it is well correlated with the peak demand of many power systems.
Another characteristic of solar energy is that its output may be complementary to the output of
wind generation and may be produced during the peak load hours when wind energy production
may not be available. The example in Figure 2.8 illustrates this phenomenon and compares the
average demand with the aggregate wind and solar plant output in California. 35 Variability
around these average demand values, especially for individual wind and solar resources, can
fluctuate significantly on a daily basis. However, as illustrated in Figure 2.8, the solar and wind
plant profiles when considered in aggregate can be a good match to the load profile and hence
improve the resulting composite capacity value for variable generation.
Figure 2.8: California average wind and solar output, along with net demand, July 2003.
34 Source: SWAT Renewable Transmission Task Force Presentation, January 2009
35 http://www.uwig.org/CEC-500-2007-081-APB.pdf on page 40
Accommodating High Levels of Variable Generation 24