Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
Large photovoltaic (PV) plants, as have been proposed in the Southwestern U.S. and southern
California, have the potential to place extremely fast ramping resources on the power system.
Under certain weather conditions, PV installations can change output by +/- 70% in a time frame
of two to ten minutes, many times per day. Therefore, these plants should consider incorporating
the ability to manage ramp rates and/or curtail power output.
2.4.2.1.Concentrating Solar Thermal Technology
Concentrating solar thermal plants (CSP) use mirrors by focusing direct normal irradiance (DNI)
to generate intense heat used to drive an electric generator. The fact that concentrating solar
plants use DNI limits their geographic application within NERC’s footprint, limiting large-scale
application to the southwestern U.S. and northern Mexico. The most widely deployed form of
concentrating solar thermal generates steam, which ultimately drives a steam turbine-generator.
Concentrating solar thermal plants that use steam turbines typically make use of a “working
fluid” such as water or oil; molten salt may be used for energy storage. Solar thermal plants that
use a working fluid can make use of several optical geometries including: parabolic trough,
power tower, and linear Fresnel. The characteristics described in this section can generally be
applied to these geometric designs.
The mass of working fluid in concentrating solar thermal plants results in these types of plants
having stored energy and thermal inertia. There are several important attributes of thermal
inertia associated with solar thermal plants. First, the electric output can be predicted with a high
degree of certainty on a minute-to-minute basis in the absence of clouds or adverse ground
conditions (e.g. dust storms). Secondly, due to their energy storage capability, the electrical
output ramps of a solar thermal plant can be less severe and more predictable than other forms of
solar power and variable renewable sources. Third, a solar thermal plant will require some
period of time after sunrise to begin electrical production as the working fluid heats up. A solar
thermal plant can produce electrical output after sunset by drawing on the thermal energy stored
in the working fluid. Figures 2.9 and 2.10 demonstrate the variation in output of a 64 MW solar
thermal plant on sunny and partly-cloudy days, respectively.
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