Distributed Generation: A Primer

Diesel Progress North American Edition, Nov, 1999 by Dan E. Kincaid

Gas Research Institute examines DG technologies and benefits.

When Thomas Edison built Pearl Street Power station to provide the first electric service to customers in New York City, he was essentially following a strategy that today would be called distributed generation -- building power generation within the localized area of use. As the young industry grew, many industrial facilities built their own power plants both to serve their own needs and to sell to customers around them, another example of distributed generation. Rapid technology development led to larger and more efficient generation plants built farther and farther from the end user. Large regional power transmission networks delivered this power to the local distribution systems and finally to the end-user. The industry was regulated so that these changes could occur efficiently without wasteful duplication of facilities, and the economic role of distributed generation became much more limited.

Since the 1970s, however, large central nuclear and coal-fired power stations have become increasingly expensive and more difficult to site and build. At the same time, technological development has improved the cost of performance of smaller, modular power generation options -- from 300 MW gas-fired combined-cycle power plants down to individual customer generation of as little as a few kilowatts. The industry is also restructuring to allow customers to competitively select the optimum combination of energy resources to meet their needs.

Energy service providers and consumers can select from a wide range of distributed power generation technologies. Commercial technologies, such as reciprocating engines and small combustion turbines, already are used in a variety of applications from energy power to combined heat and power. Emerging technologies such as fuel cells, microturbines and photovoltaics will provide additional options for distributed power generation.

Reciprocating engines

Reciprocating internal combustion (IC) engines are widespread and a well-known technology. North American production tops 35 million units per year for automobiles, trucks construction and mining equipment, lawn care, marine propulsion, and, of course, all types of power generation from small portable gen-sets to engines the size of a house, powering generators of several megawatts. Spark ignition engines for power generation use natural gas as the preferred fuel -- though they can be set up to run on propane or gasoline. Diesel cycle, compression ignition engines can operate on diesel fuel or heavy oil, or they can be set up in a dual-fuel configuration that burns primarily natural gas with a small amount of diesel pilot fuel and can be switched to 100% diesel. Current generation IC engines offer low first cost, easy start-up, proven reliability when properly maintained, good load following characteristics and heat recovery potential. IC engine systems with heat recovery have become a popular form of DG in Europe. Emissions of IC engines have been reduced significantly in the last several years by exhaust catalysts and through better design and control of the combustion process. IC engines are well suited for standby, peaking and intermediate applications and for combined heat and power (CHP) in commercial and light industrial applications of less than 10 MW.

Combustion Turbines

Combustion turbines (CT), or gas turbines, are an established technology in sizes from several hundred kilowatts to hundreds of megawatts. CTs are used to power aircraft, large marine vessels, gas compressors and utility and industrial power generators. In the 1 to 30 MW size relevant to distributed generation applications, over 500 CTs were shipped worldwide last year, totaling over 3500 MW for electric power generation. Most of these units are sold overseas; the North American market represents an 11% share of these totals. CTs produce high quality heat that can be used to generate steam for additional power generation (combined cycle) or for industrial use or district heating. They can be set up to burn natural gas or a variety of petroleum fuels or can have a dual-fuel configuration. CT emissions can be controlled to very low levels using dry combustion techniques, water or steam injection, or exhaust treatment. Maintenance costs per unit of power output are among the lowest of DG technology options. Low maintenance and high quality waste heat make CTs an excellent choice for industrial or commercial CHP applications larger than 5 MW.

Microturbines

Microturbines, or turbogenerators, are very small combustion turbines with outputs of 30 kW to 200 kW. Individual units can also be packaged together to serve larger loads. Several companies are developing systems with targeted product rollout within the next two years. Turbo-generator technology has evolved from automotive and truck turbochargers, auxiliary power units for airplanes and small jet engine use for pilotless military aircraft. Recent development of these microturbines has been focused on this technology as the prime mover for hybrid electric vehicles and as a stationary power source for the DG market. In most configurations, the turbine shaft spinning at up to 100,000 rpm drives a high-speed generator. This high frequency output is first rectified and then converted to 60 Hz (or 50 Hz). The systems are capable of producing power at around 25 to 30% efficiency by employing a recuperator that transfers heat energy from the exhaust stream back into the incoming air stream. Like larger turbines, these units are capable of operating on a variety of fuels. The systems are air-cooled and some even use air bearings, thereby eliminating both water and oil systems. Low-emission combustion systems are being demonstrated that provide emissions performance comparable to larger CTs. Turbogenerators are appropriately sized for commercial buildings or light industrial markets for cogeneration or power-only applications.