Distributed generation (DG) refers to small (a few watts up to 1 MW) power plants at or near the loads, operating in a stand-alone mode or connected to a grid at the distribution or subtransmission level, and geographically scattered throughout the service area. Typically they harness unconventional energy resources such as insolation, wind, biomass, tides and waves, and geothermal.

Small plants powered by site-specific conventional energy resources such as low-head and small hydro and natural gas are also included in this general group. Interest in DG has been growing steadily since the dramatic oil embargo of 1973.

In addition to the obvious advantages realized by the development of renewable energy sources, DG is ideally suited to power small remote loads, located far from the grid. An entire family of small power sources has been developed and employed for space, underwater, and biomedical applications.

Another niche for these systems is in energizing remote rural areas of developing countries. It is estimated that there are more than one million remote villages in the world with no grid connection and minimally sustained by locally available energy sources. Integrated renewable energy systems (IRES), a special subset of DG, are ideally suited for these situations.

General Features
DG will have one or more of the following features:
• Small size
• Intermittent input resource
• Stand-alone or interface at the distribution or subtransmission level
• Extremely site-specific inputs
• Located near the loads
• Remoteness from conventional grid supply
• Availability of energy storage and reconversion for later use

Potential and Future
Globally, the potential for DG is vast. Even extremely site-specific resources such as tides, geothermal, and small hydro are available in significant quantities. Assessments of the future for various DG technologies vary, depending on the enthusiasm of the estimator.

However, in almost all cases, the limitations are economic rather than technical. Concerns over the unrestricted use of depletable energy resources and the ensuing environmental problems such as the greenhouse effect and global warming are providing the impetus necessary for the
continued development of technologies for DG.

Among the powerful motivations for the entry of DG are:
• Less capital investment and less capital at risk in the case of smaller installations
• Easier to site smaller plants under the ever-increasing restrictions
• Likely to result in improved reliability and availability
• Location near load centers decreases delivery costs and lowers transmission and distribution losses
• In terms of the cost of power delivered,

DG is becoming competitive with large central-station plants, especially with the advent of open access and competition in the electric utility industry

DG Technologies
Many technologies have been proposed and employed for DG. Power ratings of DG systems vary from milliwatts to megawatts, depending on the application. A listing of the technologies is given below.

•Photovoltaics (PV)
•Wind-electric conversion systems
• Mini and micro hydro
• Geothermal plants
• Tidal and wave energy conversion
•Fuel cells
•Solar-thermal-electric conversion
• Biomass utilization
• Small cogeneration plants powered by natural gas and supplying electrical and thermal energies

The technology involved in the last item above is mature and very similar to that of conventional thermal power plants and therefore will not be considered in this section.

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