Reciprocating engines, developed more than 100 years ago, were the first among DG technologies. Both Otto (spark ignition) and Diesel cycle (compression ignition) engines have gained widespread acceptance in almost every sector of the economy. They are used on many scales, with applications ranging from fractional horsepower units that power small tools to enormous 60 MW baseload electric power plants. Smaller engines are primarily designed for transportation and can usually be converted to power generation with little modification. Larger engines are most frequently designed for power generation, mechanical drive, or marine propulsion.

Reciprocating engines can be fueled by diesel or natural gas, with varying emission outputs. Almost all engines used for power generation are four-stroke and operate in four cycles (intake, compression, combustion, and exhaust). The process begins with fuel and air being mixed. In turbocharged applications, the air is compressed before mixing with fuel. The fuel/air mixture is introduced into the combustion cylinder and ignited with a spark. For diesel units, the air and fuel are introduced separately with fuel being injected after the air is compressed. Reciprocating engines are currently available from many manufacturers in all size ranges. They are typically used for either continuous power or backup emergency power. Cogeneration configurations are available with heat recovery from the gaseous exhaust.

Microturbines are an emerging class of small-scale distributed power generation in the 30-400 kW size range. The basic technology used in microturbines is derived from aircraft auxiliary power systems, diesel engine turbochargers, and automotive designs. A number of companies are currently field-testing demonstration units, and several commercial units are available for purchase.

Microturbines consist of a compressor, combustor, turbine, and generator. The compressors and turbines are typically radial-flow designs, and resemble automotive engine turbochargers. Most designs are single-shaft and use a high-speed permanent magnet generator producing variable voltage, variable frequency alternating current (AC) power. Most microturbine units are designed for continuous-duty operation and are recuperated to obtain higher electric efficiencies.

Combustion turbines range in size from simple cycle units starting at about 1 MW to several hundred MW when configured as a combined cycle power plant. Units from 1-15 MW are generally referred to as industrial turbines (or sometimes as miniturbines), which differentiates them both from larger utility grade turbines and smaller microturbines. Units smaller than 1 MW exist, but few have been installed in the U.S.  Industrial turbines are currently available from numerous manufacturers. Historically, they were developed as aero derivatives, spawned from engines used for jet propulsion. Some, however, are designed specifically for stationary power generation or compression applications in the oil and gas industries. Multiple stages are typical and along with axial blading differentiate these turbines from the smaller microturbines described above.

Combustion turbines have relatively low installation costs, low emissions, and infrequent maintenance requirements. However, their low electric efficiency has limited turbines to primarily peaking unit and combined heat and power (CHP) applications. Cogeneration DG installations are particularly advantageous when a continuous supply of steam or hot water is desired.

Although the first fuel cell was developed in 1839 by Sir William Grove, it was not put to practical use until the 1960’s when NASA installed this technology to generate electricity on Gemini and Apollo spacecraft. There are many types of fuel cells currently under development in the 5-1000+ kW size range, including phosphoric acid, proton exchange membrane, molten carbonate, solid oxide, alkaline, and direct methanol. One company, International Fuel Cells/ONSI, currently manufacturers a 200 kW phosphoric acid fuel cell for use in commercial and industrial applications. A number of companies are close to commercializing proton exchange membrane fuel cells, with marketplace introductions expected soon.

Although the numerous types of fuel cells differ in their electrolytic material, they all use the same basic principle. A fuel cell consists of two electrodes separated by an electrolyte. Hydrogen fuel is fed into the anode of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode. With the aid of a catalyst, the hydrogen atom splits into a proton (H+) and an electron. The proton passes through the electrolyte to the cathode and the electrons travel in an external circuit. As the electrons flow through an external circuit connected as a load they create a DC current. At the cathode, protons combine with hydrogen and oxygen, producing water and heat. Fuel cells have very low levels of NO_x and CO emissions because the power conversion is an electrochemical process. The part of a fuel cell that contains the electrodes and electrolytic material is called the "stack," and is a major contributor to the total cost of the total system. Stack replacement is very costly but becomes necessary when efficiency degrades as stack operating hours accumulate.

Fuel cells require hydrogen for operation. However, it is generally impractical to use hydrogen directly as a fuel source; instead, it must be extracted from hydrogen-rich sources such as gasoline, propane, or natural gas. Cost effective, efficient fuel reformers that can convert various fuels to hydrogen are necessary to allow fuel cells increased flexibility and commercial feasibility.

In 1839, French physicist Edmund Becquerel discovered that certain materials produced small electric currents when exposed to light. His early experiments were about 1 to 2 percent efficient in converting light to electricity and precipitated research into these photovoltaic effects. In the 1940’s material science evolved and the Czochralski process of creating very pure crystalline silicon was developed. This process was used in 1954 by Bell Labs to develop a silicon photovoltaic cell that increased the light to electricity conversion efficiency to 4 percent.

Photovoltaic systems are commonly known as solar panels. Photovoltaic (PV) solar panels are made up of discrete cells connected together that convert light radiation into electricity. The PV cells produce direct-current (DC) electricity, which must then be inverted for use in an AC system. Current units have efficiencies of 24% in the lab and 10% in actual use, below the 30% maximum theoretical efficiency that can be attained by a PV cell.

Insolation is a term used to describe available solar energy that can be converted to electricity. The factors that affect insolation are the intensity of the light and the operating temperature of the PV cells. Light intensity is dependent on the local latitude and climate and generally increases as the site gets closer to the equator.

Photovoltaic systems produce no emissions, are reliable, and require minimal maintenance to operate. They are currently available from a number of manufacturers for both residential and commercial applications, and manufacturers continue to reduce installed costs and increase efficiency. Applications for remote power are quite common.

Windmills have been used for many years to harness wind energy for mechanical work like pumping water. Before the Rural Electrification Act in the 1920’s provided funds to extend electric power to outlying areas, farms were using windmills to produce electricity with electric generators. In the U.S. alone, eight million mechanical windmills have been installed.

Wind energy became a significant topic in the 1970’s during the energy crisis in the U.S. and the resulting search for potential renewable energy sources. Wind turbines, basically windmills dedicated to producing electricity, were considered the most economically viable choice within the renewable energy portfolio. Today, attention has remained focused on this technology as an environmentally sound and convenient alternative. Wind turbines produce electricity without requiring additional investments in infrastructure such as new transmission lines, and are thus commonly employed for remote power applications. They are currently available from many manufacturers and improvements in installed cost and efficiency continue.

Wind turbines are packaged systems that include the rotor, generator, turbine blades, and drive or coupling device. As wind blows through the blades, the air exerts aerodynamic forces that cause the blades to turn the rotor. As the rotor turns, its speed is altered to match the operating speed of the generator. Most systems have a gearbox and generator in a single unit behind the turbine blades. As with PV systems, the output of the generator is processed by an inverter that changes the electricity from DC to AC so that the electricity can be used.