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
• Engines
• Fuel Cells

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 59°F. 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

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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 DOE's Industrial Gas Turbines Program.

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 read the EPA Technology Primer on Microturbines and visit 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 EPA Technology Primer on Gas-Fired Reciprocating Engines and the DOE Gas-Fired Reciprocating Engines Program.

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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.

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 read the EPA Technology Primer on Fuel Cells.

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