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Distributed
Power Generation
 There
are a number of technologies commercially available for generating
electric power or mechanical shaft power on-site or near the site
where the power is used. Following are the three major categories
of technologies for distributed generation:
Combustion
turbines
Combustion
turbines are a class of electricity generation devices that use
natural gas or fuel oil to produce high-temperature, high-pressure
gas to induce shaft rotation by impingement of the gas on a series
of specially designed blades. Some turbines also use a heat exchanger
called a recuperator for utilizing some of the thermal energy in
the turbine exhaust heat for preheating the air/fuel mixture for
the combustor section of the combustion turbine system.
The
efficiency of electric power generation for combustion turbine systems,
operating in a simple-cycle mode (i.e., without external use of
heat in the turbine exhaust), ranges from 21 to 40 percent. Combustion
turbines produce high quality heat that can be used to generate
steam and hot water for other applications, including heating and
cooling (using absorption chillers).
Utilization
of thermal energy in the combustion turbine exhaust significantly
enhances the efficiency of energy utilization. Maintenance costs
per unit of power output for combustion turbines are among the lowest
of all power generating technologies.
Power
output rating of all combustion turbines is based on inlet temperature
of 59oF. Output capacity of these turbines decreases
with increase in ambient air temperature. Therefore, in hot weather
climates or on hot days, cooling of turbine inlet air has been found
to be cost effective for many power plants for boosting power output.
Three
types of combustion turbines are commercially available:
- Industrial
turbines
- Mini
turbines
- Micro
turbines
Some
discussion on each of these turbines is given below:
Industrial
turbines
Industrial
turbines represent one of the well-established technologies for
power generation. These turbines also represent "high" end of power
generating capacity equipment. These can provide 1 MW to more 100
MW of electric power. Most CHP systems need capacities below 20
MW, enough for large office buildings, hospitals, or small campuses
of offices and commercial buildings. Energy efficiency of gas turbines
for power generation ranges from 25 to 40 percent.
Schematic diagram of an industrial combustion turbine and generator
For
information on the development of advanced gas turbines, visit the
DOE Website:
(http://fossil.energy.gov/programs/powersystems/turbines/)
Mini
and micro turbines
Mini
and micro turbines are the newer generation of smaller turbines.
The capacities of mini turbines range from 100 kW to 1000 kW and
micro turbines range in capacities from 25 kW to 100 kW. It is not
uncommon to ignore the differentiation between mini- and micro-
turbines. For the purpose of discussion at this Web site all turbines
smaller in capacity than 1MW will be referred to as microturbines.
 These
turbines can use natural gas, propane, and gases produced from landfills,
sewage treatment facilities, and animal waste processing plants
as a primary fuel. The fuel source versatility of microturbines
allows their application in remote areas.
Microturbines
evolved from automotive and truck turbochargers, auxiliary power
units for airplanes, and small jet engines used on pilotless military
aircraft. Microturbines have far fewer moving parts than conventional
generating equipment of similar capacity. Therefore, these machines
have the potential to significantly reduce maintenance and operating
costs.
By
using recuperators, existing microturbine systems are capable of
energy efficiencies for power generation in the 25-30 percent range.
These turbines have a tremendous potential for on-site power generation
for CHP systems.
For
more information on Microturbines please visit the DOE/DER Technology
Primer on Microturbines and the DOE
Microturbines Program.
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Engines
 A
reciprocating engine, either 4-cycle internal combustion or diesel,
is used for producing mechanical shaft power. The shaft power can
be used to operate a generator to produce electric power. It can
also be used to operate other equipment, including a refrigerant
compressor for process or space cooling. Both of these applications
of engines are very well known and widespread. Engines can use natural
gas, propane or diesel fuel and are available in capacities ranging
from 5 kW to 10 MW.
Reciprocating
engines for power generation are low capital cost, easy startup,
proven reliability, good load-following characteristics, and heat
recovery potential. Reciprocating, or piston-driven, engines are
the fastest selling distributed generation technology in the world
today. Existing engines achieve efficiencies in the range of 25
percent to over 40 percent. The incorporation of exhaust catalysts
and better combustion design and control has significantly reduced
pollutant emissions over the past several years.
Thermal
energy in the engine exhaust gases and from the engine cooling system
can be employed to provide space heating, hot water, or to power
some absorption and desiccant equipment.
Emissions
of engines tend to be somewhat higher than those of microturbines
and fuel cells. In some locations, depending on local air quality
standards, engine emissions may limit its applications for CHP systems.
For
more information please visit the DOE/DER Technology
Primer on Gas-Fired Reciprocating Engines and the
DOE Gas-Fired Reciprocating Engines Program.
Gas
Engine-Driven Chillers
In
a gas engine-driven chiller, the engine produces mechanical shaft
power that is used for operating a refrigeration compressor. This
chiller is very similar to a conventional electric chiller. The
only difference is that an electric motor that drives a refrigeration
compressor in an electric chiller is replaced with a gas engine.
A Real
Video streaming media presentation of natural gas engine driven
chillers is provided here in (telephone
56K) or (broadband
DSL/T1) formats.
Animation of a Natural Gas Engine Driven Chiller
(Courtesy of InterEnergy Software, Inc.)
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Fuel
cells
 Fuel
cells produce electric power by electrochemical reactions between
hydrogen and oxygen without the combustion processes. Unlike turbines
and engine generator sets, fuel cells have no moving parts and thus
no mechanical inefficiencies.
Phosphoric
acid fuel cells (PAFCs) are commercially available. More than two
hundred PAFC units, most in the size range of 200kW, are operating
worldwide. PAFCs are realizing efficiencies of up to 40 percent.
The
only byproducts of PAFC operation are water and heat. However, hydrogen
fuel is produced by subjecting hydrocarbon resources (natural gas
or fuels) to steam under pressure (called reforming or gasification).
This process often requires combustion and chemical reactions that
produce carbon dioxide and other environmental emissions.
Even
though a fuel cell produces direct current (DC), it comes in a complete
package in which the fuel cell is integrated with an inverter to
convert the direct current to an alternating current (AC).
There
are three other types of fuel cells: proton exchange membranes (PEM),
molten carbonate (MCFC), and solid oxide (SOFC). These fuel cells
are at various stages of technology demonstration and are not commercially
available. Each type of fuel cell has its own "preferred" range
of capacities and waste heat temperatures that determine where they
can be used to best advantage in CHP systems.
For
more information please visit the DOE/DER Technology
Primer on Fuel Cells and the
DOE Fuel Cells Program.
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