The emphasis of this article is on the power aspects of Distributed Generation, and only a cursory description of the relevant issue with the technologies is given. Also, the Internet contains a multitude of resources on Distributed Generation. A word of caution- As with all things on the Internet, it is good to maintain a healthy scepticism of any material found there.
1. Reciprocating Engine Generator Set:
The most commonly applied Distributed Generation technology is the reciprocating engine generator set. This technology is generally the least expensive Distributed Generation technology, often by a factor of 2.
Reciprocating gas or diesel engines are mature technologies and are readily available. Utilities currently favor mobile gensets mounted on trailers so that they can be moved to sites where they are needed. A common application is to provide support for the transmission and distribution system in emergencies. The units are placed in substations and interconnected to the grid through transformers that typically step up the voltage from the 480 V produced by the generators. Diesel gensets are quite popular with end users for backup power.
One of the disadvantages of this technology is high NOx and SOx emissions. This severely limits the number of hours the units, particularly diesels, may operate each year to perhaps as few as 150. Thus, the main applications will be for peaking generation and emergency backup.
Natural gas-fired engines produce fewer emissions and can generally be operated several thousand hours each year. Thus, they are popular in combined heat and power cogeneration applications in schools, government, and commercial buildings where they operate at least for the business day.
Reciprocating engine generator set have consistent performance characteristics over a wide range of environmental conditions with efficiencies in the range of 35 to 40 percent. They are less sensitive to ambient conditions than combustion turbines whose power efficiency declines considerably as the outside air temperature rises. However, the waste heat from a combustion turbine is at a much higher temperature than that from a reciprocating engine. Thus, turbines are generally the choice for combined heat and power applications that require process steam.
2. Combustion (Gas) Turbines:
Combustion turbines commonly used in cogeneration applications interconnected to the distribution system generally range in size from 1 to 10 M W. The turbines commonly turn at speeds of 8000 to 12,000 rpm and are geared down to the speed required by the synchronous alternator (typically 1800 or 3600 rpm for 60-Hz systems).
Natural gas is a common fuel, although various liquid fuels may also be used. One new combustion turbine technology the microturbine has been responsible for some of the renewed interest in Distributed Generation.
One of the major advantages of this technology is that installations are clean and compact. This allows deployment near living and working areas, although there may be some issues with the high-pitched turbine noise in some environments.
There are niche applications where microturbines are used strictly for electricity generation. Because microturbines have compact packaging and low emissions, they make convenient and environmentally friendly standby and peaking generators. They are also used in some base load applications; have the ability to accept a wide variety and quality of fuels; and are a convenient means to extract energy from biomass gas, flare gas, or natural gas that is not economical to transport to pipelines.
3. Fuel Cells:
Fuel cell technology also occupies a relatively small footprint, is very quiet, and has virtually no harmful emissions during operation. Fuel cells are efficient electricity generators and may be employed in combined heat and power applications to achieve among the very best possible energy-conversion efficiencies. Those who see the future energy economy based on hydrogen see the fuel cell as the dominant energy-conversion technology.
A fuel cell is basically a battery powered by an electrochemical process based on the conversion of hydrogen. It produces dc voltage, and an inverter is required for interfacing to the ac power system.
The main drawback to fuel cells at present is cost. Fuel cell technologies are on the order of 10 times more expensive than reciprocating gensets. This will limit the implementation of fuel cells for electricity production to niche applications until there is a price breakthrough. Many expect this breakthrough to occur when the fuel cell is adopted by the automotive industry.
4. Wind Turbines:
Wind generation capacity has been increasing rapidly and has become cost competitive with other means of generation in some regions. A common implementation is to group a number of wind turbines ranging in size from 700 to 1200 kW each into a “wind farm” having a total maximum capacity range of 200 to 500 MW.
Large farms are interconnected to the transmission system rather than the distribution system. However, smaller farms of 6 to 8 MW have been proposed for applications such as ski resorts, and they would be connected directly to distribution feeders. The chief power quality issue associated with wind generation is voltage regulation.
Wind generation tends to be located in sparsely populated areas where the electrical system is weak relative to the generation capacity. This results in voltage variations that are difficult to manage. Thus, it is sometimes impossible to serve loads from the same feeder that serves a wind farm.
There are three main classes of generator technologies used for the electrical system interface for wind turbines:
i. Doubly fed wound-rotor induction machines that employ power converters to control the rotor current to provide reactive power control.
ii. Conventional squirrel-cage induction machines or wound-rotor induction machines. These frequently are supplemented by switched capacitors to compensate for reactive power needs.
iii. Non-power frequency generation that requires an inverter interface.
5. Photovoltaic Systems:
The recent power shortages in some states and the passage of net metering legislation has spurred the installation of rooftop photovoltaic solar systems. A typical size for a residential unit would be between 2 and 6 kW. Once installed, the incremental cost of electricity is very low with the source of energy being essentially free while it is available. However, the first cost is very substantial even with buy-down incentives from government programs. Installed costs currently range from $5000 to $20,000/kW.
Despite this high cost, photovoltaic solar technology is favored by many environmentalists and installed capacity can be expected to continue growing.
Photovoltaic solar systems generate dc power while the sun is shining on them and are interfaced to the utility system through inverters. Some systems do not have the capability to operate stand-alone—the inverters operate only in the utility-interactive mode and require the presence of the grid.