POWER April 2015 - 72

GENERATION TECHNOLOGIES
Are Simple Cycles or Combined
Cycles Better for Renewable
Power Integration?
It's been called " filling the duck pond, " and it's the increasingly common challenge
worldwide of balancing supply and demand when variable renewables
are not feeding power to the grid. Gas-fired generation is often filling
the pond, but the technology mix matters.
Bonnie Marini, PhD
T
he growing portfolio of renewable
power generation around the world has
made the selection of the appropriate
partnering technology a topic of great interest.
With solar and wind power dependent
on the whims of Mother Nature, reliable,
dispatchable, and flexible power generation
needs to be available to ensure a robust and
reliable supply of electricity. Making the right
technology selection requires balancing concerns
about grid stability, the environment,
and economics to ensure demand is met.
Providing reliable electricity means succeeding
in instantaneously meeting demand
with supply. Though electricity storage continues
to show promise, the amount of practical,
economic storage is limited, so the vast
majority of electricity has to be produced as
it is consumed.
Because electricity demand is constantly
changing, the system has always needed to
be able to increase and decrease the amount
of power generated throughout the day. The
weather dependency of renewables has introduced
a new variable to power dispatchers-a
fluctuating supply of energy from renewable
resources. A concern has been raised by some
power producers that these two drivers could
combine in a way that might require many
facilities to start and stop multiple times a
day to fill the gap. Some have concluded that
the solution is to add more simple cycles, but
recent research with advanced dispatch modeling
suggests that a balanced portfolio of
flexible gas-fired combined cycles and simple
cycles-very similar to a mix one might
have chosen before renewables-is a better
option for the future.
Dispatching with Renewables in
the Mix
Understanding what will happen when more
renewables are on a grid is not simple. The
72
reaction of an individual solar or wind plant
is a localized, time- and weather-dependent
event. One attempt to explain what might
happen was presented by the California Independent
System Operator (CAISO) in 2013.
Figure 1 shows a curve called the California
duck. The lines plotted are net load-
the amount of generation that needs to be
provided by nonrenewables sources-over
the course of a 24-hour day with each line
representing a different year. The top line, or
the back of the duck, is 2013 and the bottom,
or belly of the duck, is 2020, when California
is set to produce 33% of its power from
renewable resources. It is worth noting that
California is currently considering a 50% renewable
portfolio, which would result in an
even lower belly.
What can be seen from the duck is that the
solar power generated during the day will result
in a need for nonrenewable resources to
ramp down in the morning and ramp up in
the evening. A glance at this curve can result
in an impression that most of the other generation
on the grid will need to ramp on and
off in order to support the renewables; however,
the y-axis is cut off at 11 GW. Figure 2
shows the same information with the graph
expanded down to zero. The blue area below
the duck, the nonrenewable generation, is the
largest share of generation. If environmental
protection is a goal of the renewable initiatives,
then it is clear that making the right
choice for this blue " duck pond " is even more
important than the renewable duck itself.
The driving concern for filling the duck
pond is the large ramp at the belly of the duck
going down and at the neck of the duck going
up. Figure 3 shows an example of total and
nonrenewable load over the course of a day.
The blue line depicts total demand and the
lower red line is the net load, which in this
1. The California duck. The lines represent net load (the difference between forecasted
load and expected electricity produced by variable renewables) over the course of a 24-hour day
with each line representing a different year. The top line, or the back of the duck, is 2013 and the
bottom, or belly of the duck, is 2020, when California is set to produce 33% of its power from
renewable resources. Source: California Independent System Operator (CAISO)
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Increased
ramp
Significant change
starting in 2015
Potential
over-generation
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Hour
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Net load (GW)

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POWER April 2015

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