POWER March 2015 - 36

AUXILIARY SYSTEM EFFICIENCY & RELIABILITY
Save Power with Natural Cooling
for Building Ventilation
Tougher environmental regulations are pushing for more energy efficient coal
plants. Every kilowatt counts, and the boiler building ventilation system
can free up many of them.
Brandon Bell
W
ith the final Clean Power Plan rule
covering existing power plants
scheduled for release this summer,
and the amount of flexibility that has been
afforded to the states to meet emissions targets,
states have a variety of options that can
be explored to meet this regulation. Plant
upgrades, improving energy efficiency, fuel
switching, and promoting renewable energy
are just a few. With these options in mind,
generators that expect coal-fired units to
remain operational in the long term need to
start evaluating all plant systems for potential
auxiliary power savings.
In all thermal power plants a portion of
the electricity produced is needed to operate
the plant's auxiliary systems. These consist
of fans, pumps, compressors, and even plant
lighting. With more efficient plant auxiliary
systems, more electrical energy is available
for sale, and the plant operates at a higher efficiency,
with reduced carbon pollution.
One system to consider when evaluating
potential energy savings is the boiler building
ventilation system. In coal-fired power
plants, a large amount of heat is released during
the combustion process. The intent of this
process is to transfer thermal energy from the
combustion process to a working fluid (water
and steam) to be used for electric power generation.
In order to contain as much thermal
energy in the boiler as possible, thick insulation
is installed on the boiler casing to retain
thermal energy in the working fluid.
Unfortunately, insulation is unable to contain
all the thermal energy, and some heat
is transferred to the ambient surroundings
inside the boiler building. In addition to the
heat from combustion, many other forms
of heat generation exist within these buildings.
All the fans, pumps, and compressors
required to operate the plant are driven either
by electric motors or steam turbine drives.
Electric motors convert electrical energy to
mechanical energy. This conversion of energy
is not ideal, and the inefficiencies result in
heat rejection to the environment. Steam tur36
bine
drives have the same issue as the boiler,
and insulation is unable to contain all their
thermal energy.
Cool-Down Options
If they are not controlled, heat losses from all
sources in the boiler building would increase
internal ambient temperatures to a point where
workers would not be able to enter the building
for safety reasons. To counter the large
amount of heat generated from combustion
and equipment, boiler buildings are equipped
with very large ventilation systems to continuously
draw in cooler air from outside and remove
hot air from within the building.
At the majority of coal-fired boiler buildings,
a forced ventilation system is used to
remove hot air from the structure and draw
cooler air in. Large fans are installed on the
roof, with intake louvers at the base of the
structure to accomplish the needed ventilation.
Because of the large amounts of air being
moved, some boiler building ventilation
systems may require in excess of 450 kW of
operating power for a single boiler.
In addition to using auxiliary power for
operation, forced ventilation systems require
regular maintenance to remain operational.
Routine maintenance tasks include belt replacements,
motor rewinds, bearing replacements,
and fan realignments. Some existing
systems may also contain known hazardous
materials such as asbestos insulation or lead
paint. Over time, the asbestos insulation will
deteriorate and fall off, and lead paint begins
to chip or peel from surfaces. These substances
are hazardous to workers and require
special, costly removal processes.
However, alternatives to forced ventilation
systems exist that both reduce auxiliary loading
and the need for continuous maintenance
activities. Natural ventilation systems, sometimes
referred to as gravity ventilation systems,
are typically used as replacements for
forced ventilation systems. Their designs are
simple in nature, have very few moving parts,
and require little to no maintenance.
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POWER | March 2015
Leveraging the Stack Effect
In a natural ventilation system, large openings
in a structure's roof are used in lieu of
the smaller openings that are common to most
forced ventilation systems. These larger openings
promote movement of hotter, buoyant air
out of the structure, resulting in the stack effect.
Multiple sources of heat rejection inside
the boiler building will drive the ambient air
temperature up until it is higher than the ambient
temperature outside the building.
The difference in temperature creates a
difference in air density and air pressure
(Figure 1). Because the warmer air inside the
boiler building has a lower density than the
cooler air outside, a difference in air pressure
is created, with the higher pressure located
outside of the boiler building.
Due to this developed pressure differential,
cooler, outdoor ambient air will naturally try
to infiltrate the lower portion of the structure
while trying to equalize internal/external air
pressures. In the case of a forced ventilation
1. Stacks are stacks. The same forces
that govern pressure differentials in combustion
system stacks will apply to boiler building
ventilation. Courtesy: National Renewable Energy
Laboratory (NREL)
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POWER March 2015

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