Siemens Power Grid_11-12 '14:CAREB 05-09.qxd
11/24/2014
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GAS TURBINES
MEETING THE NEEDS OF THE FUTURE POWER GRID
DISPATCHABLE GENERATION THAT IS COMBINED CYCLE-BASED WILL PARTNER WITH
RENEWABLES TO STABLIZE THE GRID
BONNIE MARINI & MATHEW PALMER
W
ith so many sources of renewable generation now on the
grid, and lots more on the way,
it is important to gain an understanding of how future dispatchable
resources will need to behave in order to
accommodate increased renewable generation. A recent study shed some light
in this area. What is clear from the findings is that combined cycle plants are
due to play a greater role than simple
cycle plants in stabilizing the grid in the
presence of supply fluctuations from
renewable sources.
Modern combined cycle plants are
designed to be flexible while providing
lower greenhouse gas emissions and a
lower cost of generation compared to
simple cycle technologies. Simple
cycle plants are easier to maintain in a
start-ready condition and have advantages for non-spinning reserve. But
study projections show that such capabilities are not broadly needed for
renewable integration.
Combined cycle plants use waste
exhaust heat from the gas turbine exhaust to
power a steam cycle and produce more electricity with no additional fuel. This requires
the presence of a heat recovery steam generator (HRSG) and a steam turbine, and
results in a combined cycle having much
higher plant efficiency and lower operating
costs, but higher capital costs than a simple
cycle with the same gas turbine.
Lower-efficiency simple cycle plants,
with lower capital cost, have typically
been used as low-dispatch plants, available to cover high peaks in demand,
while higher-efficiency combined cycle
plants have traditionally been used for
base- or intermediate-load duty. Modern
combined cycles, such as Siemens FlexPlant, can start up quickly and load follow over a wide output range, offering
the flexibility to react to fluctuations in
renewable generation.
Efficiency directly impacts the cost of
generation, and consequently the frequency
and duration of dispatch. For U.S. grid
operations, the first plant dispatched is the
one which can meet the demand at the lowest cost. Combined cycle plants enable a
much lower levelized cost of electricity
(LCOE) than simple cycle plants and will
therefore be dispatched first and have
greater overall use.
www.turbomachinerymag.com
Figure: Levelized Cost of
Electricity per MWh
CAISO integration study
California Independent System Operator
(CAISO) oversees a grid that needs to
accommodate 33% renewables by 2020,
according to the state’s aggressive
Renewable Portfolio Standard (RPS).
This will amount to as much as 5.5 GW
of solar and 4.2 GW of wind, and perhaps another 3 GW of imported solar
and wind resources.
Grid balancing authorities, such as
CAISO, need to maintain a constant and
precise balance between supply and
demand. To deliver significant quantities
of renewable energy and maintain a stable
grid without curtailing demand, the rapid
changes in the energy supplied by renewables must be balanced by controllable
resources. These balancing resources need
to be flexible to match system demand.
A recent CAISO study used computer
simulation to forecast the energy flow and
balancing requirements for the year 2020.
From that simulation, CAISO determined
that at least 5.3 GW of available resources
would need to be sufficiently flexible to
maintain grid reliability.
CAISO simulated the economic dispatch
of a specific set of replacement resources
that would be needed to satisfy requirements. In the model, the heat rate and startup costs of simple cycle plants closely
matched a 100 MW aeroderivative gas turbine and the combined cycle plant characteristics are consistent with 2x1 F-class triple
pressure technology.
These comparisons revealed the superiority of combined cycle over simple cycle
technology. The study projected that with
increased renewable generation, combined
cycle plants would be well utilized. This is
driven by the expectation that they will be
dispatched based on favorable economics
and will be online to supply balancing services, such as rapid ramping.
The modeled combined cycle plant aver-
aged an output of between 700 MW and 800
MW out of its capacity. Its range of available
output allowed it to provide upward load following and ancillary services between 100 and
150 MW each hour on average, and downward load following and ancillary services of
200 to 230 MW each hour. The CAISO study
concluded that combined cycle plants were
more economical than simple cycle plants and
will be used more, thus allowing them to
amortize their higher capital costs.
CAISO dispatch data was combined
with economic information generated by
the California Energy Commission (CEC).
Plant cost rates from a CEC study were
combined with the heat rates and capacity
factors from the CAISO study to determine LCOE. The estimated LCOE for the
modeled simple cycle plants ranged from
$345 to $580 per MWh, while LCOE for
combined cycle plants ranged from $135
to $140 per MWh (Figure).
In addition to enabling a lower cost of
electricity, higher efficiency results in
lower greenhouse gas generation. A 500
MW plant which dispatches 30% of the
time using high efficiency simple cycles
produces in excess of 400 million pounds
more CO2 than generating the same
power from a combined cycle with an
efficiency of 58%. Flex-Plants with
Clean-Ramp technology also offer the
ability to maintain low NOx and CO
while load following.
Modeling results
Modeling projects that the vast majority
of future load balancing services can be
met by combined cycle plants. They are
projected to be dispatched for longer
periods of time and to stay online for virtually the entire day, meeting ramping
demands through load changes, offering
lower cost electricity, and producing
lower emissions than simple cycle
options for the same generation.
Dr. Bonnie Marini is the Director of Siemens
Power and Gas, Energy Solutions Product Line
Marketing team. In this role she explores the market to understand upcoming needs and defines targets for technology development for Siemens
future power plant product line.
Mathew Palmer is a Marketing Specialist in
Siemens Power and Gas, Energy Solutions
Product Line Marketing team specializing in market evaluation and assessment of new technologies
for various markets. For more information, visit
www.energy.siemens.com
November/December 2014 • Turbomachinery International 31