www.PowerEngineeringInt.com 27 Power Engineering International June 2017
they do indeed perform this function. But
increasingly, both combined-cycle and
simple-cycle plants are fnding a new role: grid
balancing and load following to compensate
for intermittent renewable generation. In parts
of the US and Europe, grid authorities typically
prioritize renewable energy.
One consequence of this policy is that
natural gas plants are being forced into a
new mode of operation. Many combined-cycle plants were designed with baseload
operation in mind.
Simple-cycle plants, on the other hand,
were originally conceived as peakers – only
turned on when extra power was required on
the grid. Peakers might come online on some
days in the mid-afternoon to satisfy heavy
demand. But for much of the year, many of
these peakers would not see any starts at all.
There are also load following power plants
that make use of gas turbines. They sit between
baseload and peaking: perhaps running for a
good portion of the day and being turned off
or ramped down in the evening.
But there is a new kid in town: renewable
balancing and load following. As renewable
output varies annually, seasonally and day-to-day, it has to be supported by fast-start and
fast-ramping gas turbine plants.
Case study
In California, gas turbine-based power is
required from late afternoon to early evening.
When called, these plants have to produce
their power rapidly.
There are areas with somewhat predictable
wind and solar output peaks and troughs
over the course of the day. In other places,
renewable output can suddenly decrease
without warning. Despite sophisticated
weather models, it can be impossible to
predict these changes with more than a few
minutes’ notice. Gas turbine plants must ramp
up and down to meet these shortfalls. That
ramping causes thermal stress, which leads to
decreased maintenance intervals.
What is emerging, then, is a whole new
mode of operation,with gas turbines operating
in ways their designers never accounted for.
This might entail several starts and stops over
the course of the day, or cycling – whereby
the plant ramps up to full capacity and then
ramps down to partial load many times daily.
In Texas, wind power accounts for nearly
13 per cent of power production. Gas turbines
are often required to start and stop twice per
day as wind power output fuctuates. In other
parts of the world, the growth of renewables is
impacting traditional plant functions.
One NV Energy combined-cycle plant
near Reno, Nevada, for example, saw a major
shift it its operating profle between 2006
and the present day. It went from high hours
and low starts to many starts but far fewer
operating hours. Once heavy cycling and
rapid starts began, the plant saw an impact
in terms of lower availability. Plant personnel
noted the need for more replacement parts,
a greater occurrence of metal creep and a
higher incidence of blade damage. Boiler
tube failures went from two in several years to
three in a month.
OEMs have observed this, too. A GE Gas
Turbine rotor specialist stated at a recent user
group that there is a big difference between
a 5000-start rotor and a 200-start rotor in terms
of fatigue and fracture mechanics. A starts-based machine will tend to suffer more from
low cycle fatigue, he said.
How inlet fogging can help
Unlike other forms of power augmentation
such as chillers and media-type evaporative
coolers, fog systems have stages of fog output.
Each stage corresponds to a few degrees of
evaporative cooling or a fraction of a percent
of wet compression. This makes it possible
to control gas turbine output by varying the
number of fog stages in operation at any
given time.
Fog systems have typically been turned on
only when the gas turbine is above baseload.
They are usually designed to stage up to
maximum cooling – 100 per cent relative
humidity at the gas turbine compression inlet
– whenever they are activated. However, this
control scheme is not ideal given the new
operating environment, particularly for gas
turbines that are running under Automatic
Generation Control (AGC).
If a fog system is set to turn on when
baseload is reached, and if the fog system
then ramps up to full cooling, that can cause
gas turbine output to spike above the power
level that is currently needed. This, in turn, will
cause an AGC signal to ramp the turbine
down. But when the turbine gets below
baseload, the fog turns off and the turbine has
to ramp back up to compensate. With current
fog system control logic, the only solution is to
operate the stages manually or to disable the
fog system.
There is a better way to control fog systems
for gas turbines that are on AGC. The fog
stages can be integrated with the gas turbine
controls so the AGC signal adds or removes
fog stages to get the output needed.
Adding and removing fog stages produces
a nearly instantaneous change in output
power. The fog stage increments can be made
very small, so the grid controller would see
little or no difference between changing gas
turbine fuel fow, or inlet guide vane position,
and changing fog staging.
This control scheme will improve plant
effciency and reduce cycling stresses, as
fewer gas turbines would need to be operated
at any given time. Five GTs operating with
20 per cent power boost would produce the
same power as six GTs operating at baseload
and one gas turbine would not have to be
operated. This would signifcantly reduce wear
and tear on gas turbines operating on a grid
with a large amount of intermittent renewable
power.
To accomplish load following by varying
fog fow, it would be benefcial to allow fogging
at less than baseload. This would ensure fog
stages are available when needed. Where
part load is accomplished by closing IGVs,
the amount of fogging in operation can be
adjusted, in proportion to the reduction in air
mass fow, to avoid inadvertent overspray.
Evaporative and wet compression fogging
are usually thought of as power augmentation
systems, but fog system operation at part load
has advantages. The heat rate is improved
when fogging is used when a gas turbine is
operating below baseload. A considerable
economic beneft can be realized from part
load fogging because the cost of the water is
trivial compared to the cost of the fuel.
Thus gas turbine operators can realize
signifcant economic beneft from evaporative
fogging and wet compression. The power
boost obtained can allow fewer gas turbines
to do the same amount of work with less fuel
burned and reduced maintenance costs.
Fogging at below baseload can save fuel
and improve operating fexibility. The secret is
in how the fog systems are controlled.
Thomas Mee III is chief executive of Mee
Industries, a supplier of inlet fogging and wet
compression systems. www.meefog.com
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