POWER July 2011 - 32

FUELS
and combined-cycle plants. In combination
with larger quantities of renewable generation,
the requirements for gas turbines will
certainly change:
■ The tendency will be to move toward
smaller and dispersed plants.
■ The type and frequency of cycling operation
will be significantly different than is
seen today.
■ More-frequent cycling will affect component
life and plant efficiency and will result
in much more stringent environmental
emissions at part-load operation.
■ A significant reduction in peak load should
occur due to better grid management and
intermittent renewables, while some baseload
resources will see an increase in the
percentage of their utilization share.
The Gas-Plus-Renewables Option
As wind and PV capacity increases, these
renewable sources are used to meet intermediate
and peaking loads. However, during
periods when PV or wind generation output
is low, additional backup power is needed to
ensure that the grid demand is met. Figure 3
illustrates the need for spinning reserves or
storage. In the absence of large electrical,
thermal, or pumped storage options, providing
backup power and maintaining spinning
reserve will be a major role of fast-starting
and rapid-loading gas turbines.
Gas turbines are particularly well-suited
to operate in conjunction with wind and PV
sources (see sidebar). Their well-known faststart
and fast-ramping capabilities are better
able to meet rapid changes in grid requirements
than coal, steam, or nuclear plants.
Consequently, until alternative solutions are
widely available, there is a real need for manufacturers
to adapt gas turbines specifically to
compensate for renewable power variability.
Grid codes and customers are continuously
demanding more operational flexibility, faster
starts, and accelerated loading response
times.
As a rule of thumb, for each installed
400 MW of wind power, 100 MW of gasfired
backup power is required. Hence, the
requirement that gas-fired generation support
renewable generation is driving modern
power plant design to strongly focus on operational
flexibility.
When compared with a continuous baseload
regime, gas turbine operation over
wide power ranges not only increases fuel
consumption but also impacts NOx
and CO
emissions. The inability to achieve premix
combustion operation at low power levels
makes this particular requirement difficult to
meet.
The good news, according to a Brattle
Group study, is that a wind-plus-gas turbine
plant could achieve at least 75% reduction of
the maximum possible CO2
emissions. It is
obvious that emissions-free power from wind
generation is compensating for some of the
conventional fossil-generated power.
The bad news is that the economics of gas
turbine operation under these conditions are
different than for a conventional standalone
gas plant. All parties involved in determining
the price of electricity must account for
Combining Gas and Photovoltaics
Sempra Generation's El Dorado Energy plant has merged two generation
technologies that are flourishing together in the desert
south of Las Vegas. Sempra Generation, the unregulated power
plant-operating subsidiary of Sempra Energy, sells the output
from the 480-MW gas-fired combined-cycle plant into the Southwest
power markets.
In 2009, it added ten 1-MW thin-film photovoltaic (PV) power
modules arranged on 80 acres adjacent to the plant. The combined
PV power is then connected with the main electrical bus of the
combined-cycle plant to simplify interconnection with the grid
(Figure 4).
The staff of the gas-fired plant have been cross-trained in PV
plant operation and maintenance to reduce operating costs and to
improve overall plant reliability. A single technician is responsible
for managing the PV side of the plant. First Solar, designer and
supplier of the PV modules, provides remote monitoring and maintenance
as part of the ongoing support contract.
For more information on this unique hybrid plant, see " El Dorado
Energy's Solar Facility, Boulder City, Nevada, " December 2009,
in the POWER archives at www.powermag.com.
32
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POWER | July 2011
increased costs incurred by the gas turbine
power generators. Forcing these facilities to
operate at less than full capacity reduces their
revenue stream. Cycling operation also affects
maintenance schedules and gas turbine
availability.
In response to these diverse requirements,
OEMs have developed both heavy-duty and
aero-derivative gas turbines with greater capability
to support a wide range of operational
flexibility enhancements, enabling customers
to effectively use equipment for peak and
cycling applications. The relative ease and
speed of installing gas-fired generation also
gives it an advantage when it comes to meeting
emergent and urgent power demand.
Some features of OEMs' solutions to the
new demands on gas technologies are examined
below.
Simple-Cycle Developments
All major manufacturers have realized the
importance of fast start-up and rapid loading
for gas turbines in simple-cycle operation,
particularly for installations designed for
cycling operation. Current gas turbines can
ramp at the rate of 3% per minute (though
their efficient operating range is narrow),
which is a much higher rate than that of
pulverized coal plants. The coal-fired steam
cycle exhibits substantial power losses each
time a steam turbine is shut down or restarted.
Major ramp-downs in under 15 minutes
may require wasting (venting) the steam, and
major ramp-ups in less than 15 minutes may
be impossible.
Here is a short list of published features
4. Side by side. A photovoltaic installation shares a grid connection
and real estate with the combined-cycle El Dorado Energy
plant. Source: POWER
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POWER July 2011

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