United States
                         Environmental Protection
                         Auxiliary and Supplemental Power
                         Fact Sheet:  Microturbines
This fact sheet describes the use of microturbines as
Auxiliary  and  Supplemental  Power  Sources
(ASPSs) for wastewater treatment plants (WWTPs).
Microturbines  are a new, innovative technology
based on jet engines (more specifically the turbo
charger equipment found in jet engines) that use
rotational energy to generate power.

Most microturbines have four main components:
compressor,  combustion  chamber, turbine blades,
and drive shaft. The compressors operate by taking
in  the  surrounding  air at  one  end  of the
microturbine and  condensing it by increasing the
air's pressure and density. This air is fed into the
combustion chamber where it is mixed with fuel,
and  then  burned.  This  combustion  releases
enormous  amounts  of  heat  energy  and high-
pressure exhaust gases.  The exhaust gases are
discharged through exhaust vents into a  series of
turbine  fan blades that are attached to a central
shaft.  As the gases are discharged,  they spin the
turbine fans, which in turn spin the drive shaft at
high speeds (100,000 revolutions per minute).  The
rotational  energy produced by the shaft spins
copper coils, which excite the electrons in the wire,
producing electricity. The quantity of electricity
depends on  how  fast the shaft can spin  in the
magnetic field, the strength of the magnetic field,
and the quantity and arrangement of the copper
coils.   To  produce electricity  at a relatively low
cost, the shaft must rotate at high speeds.

Microturbines  can run  on bio-gas, natural  gas,
propane, diesel, kerosene, methane, and other fuel
sources, making them suitable for backup power in
a variety of applications. Since each individual
microturbine produces anywhere from 15 to 300
kilowatts (kW) of energy, they are often grouped to
produce the required energy for a given application.
Most microturbines are about the size
of a refrigerator and have very low nitrogen oxide

               Microturbine Schematic
                                   turbine Exhaust

There  are  numerous  advantages  that  make
microturbines  appealing.    From  an  economic
standpoint, the microturbine generators are cheaper
to  build  and  run  in  comparison  to  larger
conventional gas or diesel powered generators. The
technology is well  understood  and  has  been
implemented in many applications throughout the
U.S. They are also relatively inexpensive, easy to
manufacture, and have few moving parts.  These
power plants can also use various types of fuels.
Another advantage of microturbines is durability
and reliability; they function for about 40,000 hours
and require little maintenance. These systems can
also be ready to operate  only  ten minutes after
being turned on.   Microturbines create a large
amount of energy relative to their size. Because of
their size, microturbines can be placed on  site,
easing security and maintenance.  Microturbines
have the ability to work alone or in groups. If one
microturbine  fails  while  in use,  this does not
necessarily  mean  that  the  entire system  of
microturbines will fail.

From an  environmental  standpoint, these new
machines pollute less and take up less space. The
increased efficiency means that they use less fuel,
which  means  fewer  emissions  into  the  air.
Increased efficiency and less fuel also result in a
lower reliance  on finding the natural resources
necessary to power the turbines.

One disadvantage of microturbines is a limit on the
number  of  times  they  can  be  turned  on.
Microturbines also run at very high speeds and
high temperatures,  causing  noise  pollution for
nearby residents and potential risks for operators
and maintenance  staff.  It may  also take  several
microturbines set in a  series to provide  enough
energy to power a small WWTP.
addition to saving in peak energy demand charges,
The San Elijo Joint Power Authority also received
a $76,000 rebate check from the San Diego Gas and
Electric's Self-Generation Incentive Program.
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                                                              Large Scale Microturbine Application
Capstone Mictroturbine and Ingersoll Rand are two
of the larger microturbine manufacturers. Each
offer different models of microturbines that depend
on the power out put that is needed. Costs for these
units  can  range  from  $30,000  to  $250,000,
installed, depending on the unit.  Interviews with
several municipalities suggested annual savings of
$25,000 to $216,000 through use of microturbines
over conventional gas or diesel powered.
In 2000, microturbines were installed at San Elijo
Joint  Powers Water Reclamation Facility.   San
Elijo is a small (3 MOD) WWTP. Instead of just
burning the excess gas, San Elijo now uses its bio
gas from their digesters to  fuel the microturbines.
Three microturbines were installed, producing 80
kW of energy. The system produces approximately
15% of the plant's demand. The exhaust from the
microturbines was captured and used to heat water
at the reclamation facility.  This process is known
as combined heat and power (CUP). One generator
system uses one fuel  source to yield two usable
energy outputs with very high fuel efficiency. The
plant  experienced a decline  in  electricity  costs
estimated at  $4,000  per  month and  expects a
payback  on their investment in 3-4  years.   The
microturbine exhaust is also lower in methane and
NOx than emissions from flaring digester gas and
substantially  less that those  from  conventional
reciprocating engine  driven generator sets.  In
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The  mention  of trade  names  or commercial
products does  not  constitute  endorsement  or
recommendation for use by the U. S. Environmental
Protection Agency.

               Office of Water
                 March 2005