POWER February 2015 - 57

FUNDAMENTALS
ciency loss within the turbine. Nozzle roughness,
erosion, and repair can account for 35%
of efficiency loss, turbine deposits 15%, and
bucket erosion and roughness 10%. Problems
in these areas can result in significant
efficiency losses: Turbine deposits have been
known to cause nearly a 5% efficiency loss
and turbine casing leaks as much as a 3% efficiency
loss.
It's vital to know that the turbine is part of
a much larger steam and water system that
includes condensers, cooling towers, feedwater
heaters, deaerators, pumps, and piping-all
of which have their own efficiency
losses. For example, an increase in condenser
backpressure due to dirty tubes of 0.4 inches
of mercury can reduce the turbine cycle efficiency
by 0.5%. A single split partition
plate in a feedwater heater can reduce turbine
cycle efficiency by 0.4%. Leaking extraction
lines and stuck drain valves can reduce your
feedwater heater efficiency, resulting in net
cycle losses of greater than 0.5%.
Turbine blade improvements are available
for most steam turbines, with improvements
of up to 2% possible with a complete replacement
of the low-pressure turbine. Even
renewable energy can assist with heat rate
improvement, as some generators have explored
the prospect of solar feedwater heating
to boost their turbine cycle efficiency,
with some designs able to achieve a peak efficiency
improvement of more than 5%. Of
course, with all upgrades, you have to examine
the economics (see sidebar).
Electrical Efficiency
For the generator system we are not so concerned
about the conversion efficiency of
rotational energy to electrical energy, as modern-day
generators tend to convert between
the two energy types with 98% or greater efficiency.
However, a significant portion of the
inefficiency seen within this box involves the
station service or auxiliary power consumption
of the power plant itself.
As most large power-consuming systems
at a power plant are needed, little can be
gained by eliminating or turning off major
equipment systems. Even sacrificing ancillary
electrical consumption can have unintended
consequences. One scorchingly hot
June, I was stationed at a power plant in its
engineering office, when a young man from
the corporate office had the clever idea of
turning off the office lights, bumping the air
conditioning to 85F, and unplugging the coffee
makers, water fountains, and soda machines.
The reasoning was that power prices
were more than $1,000/MWh, so he wanted
to be able to sell every last Watt possible.
What the gentleman had not considered were
the potential ramifications of placing a group
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February 2015 | POWER
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57
of plant engineers in a dark, hot office with
no cold drinks or coffee. It was not a pretty
sight.
As more than 80% of the electrical usage
at a power plant is via electrical motors, these
should be the primary focus when improving
your electrical efficiency. Just the main
power plant fans (primary air, forced draft,
and induced draft) can consume as much as
2% to 3% of the plant's gross output. One
option for reducing fan power consumption
is to use variable-frequency AC drives, especially
if the plant tends to operate at lower
loads for extended periods of time. Switching
all of your main plant fans from conventional
to variable-frequency drives could improve
your NPHR by more than 0.5%.
Air and gas leakage can account for up to
25% of fan power consumption, so reducing
leakage in the air heaters and ductwork can
result in a significant fan power savings. Reducing
your boiler excess air will reduce fan
demand as well. Electrostatic precipitator optimization
programs can both increase electrical
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POWER February 2015

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