POWER September 2010 - 50

COMBUSTION TURBINES
Flexible Turbine Operation Is Vital
for a Robust Grid
Renewable electricity generation has many environmental advantages, but
adding large amounts of far-flung renewable resources to a grid requires
increased operating flexibility from dispatchable generators when the
wind doesn't blow or the sun doesn't shine. One promising option: A
combined-cycle plant based on Alstom's GT24/GT26 combustion turbine
can be " parked " at approximately 20% plant load while producing emissions
comparable to those during baseload operation-with little loss in
thermal efficiency. When demand returns, the combined cycle can return
to baseload within minutes.
By Dr. Robert Peltier, PE
R
enewable generation capacity, mainly
solar and wind, has grown rapidly over
the past couple of years in the U.S. and
several other countries, outpacing all other
forms of generation except natural gas. Utilities
are developing these alternative generation
resources for reasons that range from altruism
(it's good for their customers and the environment)
to compliance (a renewable portfolio
standard that must be met). Regardless of the
reason, adding nondispatchable renewables
to an electricity transmission and distribution
grid designed for instantaneous demand response
adds an additional layer of complexity
for power generators and grid operators.
Large generators, such as coal and nuclear
plants, were traditionally designed for baseload
power supply. As renewable resources
have entered service, coal-fired plants in
some regions of the U.S. have transitioned
into intermediate load plants, and many combined-cycle
plants, also originally designed
as baseload, are now daily peaking plants.
In some locations, combined-cycle plants
are " two-cycled " daily: The plant is brought
online for the morning peak, shut down or
reduced to minimum load, and then started
again to meet an evening peak.
To be sure, the fate of gas-fired combinedcycle
plants is intimately tied to the price of
natural gas. As a result, many are operated
only during peak hours of the summer months.
The point is that the dispatch order of plants is
in flux, as every utility or grid operator uses its
unique economic rules and predictive methods
to determine how much spinning reserve
and dispatchable backup power is required to
handle potential grid demand excursions.
The options for providing spinning reserve
or backup power for renewable generation are
few, and they are all expensive. Sometimes,
50
purchased power can fill the deficit, assuming
that sufficient reserve capacity is available.
Some electricity suppliers have built
small, reciprocating engine or simple-cycle
gas turbine plants for the sole purpose of grid
stabilization and renewable backup power.
Many regions rely on gas-fired simple-cycle
or combined-cycle plants as renewable generation
backup and live with the practical limits
of that decision: The turndown of industrial
gas turbines is limited, part-load efficiency is
poor, start-up from cold iron is slow, emissions
at part-load operation usually exceed
permit limits, and adding more start-up hours
shortens equipment life.
Usually, the decision about what type of
backup power to provide entails making do
with current assets rather than installing new
ones to optimize evolving spinning reserve
and backup power demands on the grid.
An Elegant Solution
If you ask a grid operator or dispatcher responsible
for meshing renewable and conventional
generation to describe the characteristics of the
ideal dispatchable plant, the likely response
will be " a gas-fired plant that has quick startup
capability to respond to system emergencies,
offers exceptional turndown with rapid
load response, stays close to design point efficiency,
and meets emissions limits at all loads. "
Multiple small gas-fired reciprocating engine
plants come close but are not economic when
considering a power block of several hundred
megawatts. Existing combustion turbines can
turn down to about 50% load, but their thermal
efficiency drops and emissions skyrocket.
Unlikely as it seems, there is a combined-cycle
design that has the ideal specifications.
Alstom has developed a Low Load Operation
Capability (LLOC) for its KA24 and
1. The reheat combustion turbine. The sequential combustion system is a key technology
of the GT24/GT26. Compressed air is heated in the first combustion chamber (EnVironmental
or EV combustor) by adding about 50% of the total fuel (at baseload). The pressure is
halved after the combustion gases expand through the single-stage high-pressure turbine. The
remaining fuel is added into the second combustion chamber (Sequential EnVironmental or SEV
combustor), where the combustion gas is heated a second time to the maximum turbine inlet
temperature. The gas then expands through a four-stage low-pressure turbine. Source: Alstom
Retractable SEV fuel lance
24 SEV burners
Annular SEV combustor
Annular EV combustor
Retractable EV burners
with EV fuel lances
(GT24=20. GT26=24)
LP turbine
Compressor
HP turbine
www.powermag.com
POWER | September 2010

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POWER September 2010

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