POWER April 2015 - 76

GENERATION TECHNOLOGIES
10. Texas two cycles. As in California's solar scenario, combined cycles are an important
technology for responding to variable wind generation in Texas. Source: Ventyx Inc.
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
1
Hour
24
Combined cycle
Wind
Solar
Simple cycle
Combined cycle
Hydro
Gas-fired steam turbine
Coal-fired steam turbine
Nuclear
15,000
10,000
5,000
1
1,000
800
600
400
200
1
ly to use the combined cycles to cover large
demand changes.
Figures 8 and 9 show a projected summer
day in Huntington Beach. Again, the dispatch
of the simple and combined cycles show that
the larger share of demand change is supported
by combined cycles. In this case, the
overall demand is high, and the simple cycles
are dispatched to meet the peak in demand.
This dispatch order on a high-renewable grid
is similar to the dispatch order on a conventional
grid. Combined cycles dispatch first
because they offer a lower cost of generation
and are followed by simple cycles to meet
peaks in demand. There is no indication of a
need for more simple cycles to support load
changes.
Similar data was extracted for a node in
Texas, which has the largest supply of wind
power in the U.S. Figure 10 illustrates that
the same phenomenon can be observed there
as well. The vast majority of load changes
are supported by combined cycles first.
Simple cycles are used primarily for peak
demand and are not critical for supporting
the large ramps in load that were seen in the
past, or the even larger ramps in load that
are expected in the future. Combined cycles
are able to change load quickly and ultimately
dispatch first due to the lower cost
of generation.
While conventional combined cycles offer
advantages over simple cycles for renewable
integration, modern Flex-Plant combined
cycles offer significantly more capability.
Instead of only leveraging the benefits in
ramping capability, these newer, more flexible
plants can start as fast as a simple cycle,
making multiple restarts viable.
Combined Cycles Win, Win, and
Win
It seems rare when a choice is better in functionality,
cost, and environmental footprint,
but for high-dispatch plants, combined cycles
win in all three areas. Flexible combined cycle
power plants support renewables by being
more efficient, cleaner for the environment,
and flexible to meet the change in demand.
76
Hour
24
Higher efficiency results in a lower cost
of generation. Electricity from a combined
cycle can be on the order of one-third the
cost for electricity from a simple cycle.
Higher efficiency, offered in combined cycles,
also means less greenhouse gas generation.
Simple cycle efficiencies are in the
range of 35% to 40%, while combined cycle
efficiencies are in the range of 55% to 61%.
For every MW generated, a combined cycles
burns about 35% less fuel than a simple
cycle and, consequently, produces 35% less
carbon dioxide.
Fewer starts means less CO generation.
Gas turbines typically produce more CO in a
start than they do in 10 hours of operation, so
leaving the plant running at low load is actually
better for the environment than turning it
off for a few hours.
A running engine also supports grid frequency.
Most renewable generation does not
provide rotating inertia, which is needed to
maintain the grid at 60 Hz; however, gas turbines,
help enforce speed stability. Grids without
enough units producing rotating inertia
need to add equipment, like synchronous condensers,
to help stabilize the grid frequency.
Starting and Stopping
Considerations
With all of these advantages, one may wonder
why simple cycles are getting any attention
at all. The reason often brought up is the
economics of starting.
Historically, heavy duty gas turbines were
designed to run for very long periods of time
without stopping and to require service after
a certain number of starts. Aeroderivative
gas turbines, based on aircraft engines,
are designed to start and stop frequently and
don't require service after a set number of
starts. If an engine is started frequently, the
service-related costs can change the economics
of the plant, resulting in an advantage
for an aeroderivative engine. This is why at
first glance, plants expecting to start and stop
multiple times may look at an aeroderivative
simple cycle as a good solution. However,
even in the case of plants expecting multiple
www.powermag.com
Hour
Simple cycle
24
starts on a given day, this costly simple cycle
approach is not optimum.
The worst day doesn't happen every day.
A plant may actually start three times in a
given day, but dispatch modeling shows that
even at very high cycling plants, this happens
infrequently. When looking at service costs,
the number of starts a year is the critical factor.
Even high-starting plants don't start an
average of more than once a day.
Different plants will dispatch differently.
If a simple cycle peaker is compared with a
combined cycle, the duty cycles will not be
the same. The lower levelized cost of electricity
typically drives simple cycles to dispatch
<10% of the time. A combined cycle
will have an overall higher dispatch, have
much longer runs, and will therefore stop
and start less often than the simple cycle.
The same duty cycle shouldn't be used for
both plants.
For those simple cycle plants that are
needed, the plants that fill that 10% dispatch
window, economic evaluation shows
that typically, frame units in simple cycle
are a better economic choice over an
aeroderivative. The benefit of the higher
efficiency of a simple cycle aeroderivative
rarely outweighs the lower capital cost of
a frame unit for low-dispatch applications.
With today's proven dilution selective catalytic
reduction options, frame units can
now meet low NOx
and CO requirements
and often use much less water than some
aeroderivative options.
The Cure Needs to Be Better Than
the Disease
The primary driver for supporting wind
and solar power generation is protecting
the environment. Solar and wind plants
don't create the pollutants that fossilfired
generation creates. Yet, while rules
and legislation are popping up around the
globe supporting or demanding renewable
generation, there is little or no discussion
about the partnering technology that may
be needed to support renewable resources
to firm the power supply.
As this article shows, the " duck pond "
of nonrenewable generation is much bigger
than the " renewable duck " itself. Given
the information and experience available
today, it may be the right time to revisit the
requirements for fossil generation. With
the right approach, we can ensure that the
clean renewable portfolio is supported by
an environmentally conscious mix of generation
that can ensure reliable power for
the future. ■
- Bonnie Marini, PhD (bonnie.marini@
siemens.com) is director, 60 Hz Solutions
Product Line for Siemens Power and Gas.
POWER | April 2015
Generation (MW)
Generation (MW)
Generation (MW)
http://www.powermag.com
POWER April 2015

Table of Contents for the Digital Edition of POWER April 2015
