Power Engineering

Small, low-cost, highly efficient gas turbines provide the utility industry with a

Small, low-cost, highly efficient gas turbines provide the utility industry with a fourth-generation technology that features numerous benefits and potential applications. These include firm power to isolated communities, commercial centers and industries; peak shaving for utility systems to reduce the incremental cost of additional loads; peak shaving for large commercial and industrial establishments to reduce demand charges, as well as standby, emergency power and uninterruptible power supply (UPS).

Turbine track record

It is important to recognize small gas-turbine generators are not a new technology but are being developed for the electric utility industry based on technology that is supported by more than 25 years of field experience. The generators discussed in this article are being developed by International Power and Light in association with Allison Engine Co. (a division of Rolls Royce plc) and General Electric Co. Allison will develop the turbine generators, and General Electric will design the controls and inverter and will be responsible for unit site engineering, installation and field maintenance.

Small gas-turbine engines were initially developed by Allison in the 1960s for ground transportation. The first major field trial began in 1971 with the installation of Allison GT404 turbine engines in six Greyhound buses. The 310 bhp turbine engines were described as a two-stage type, with a twin-regenerator system that recycled heat from the gas path to pre-heat incoming air, while cooling exhaust to no more than 500 to 600 F. The power unit required no water-cooling system, had half the moving parts of a diesel engine and had a life of more than 500,000 miles between overhauls.

By 1978, these six testbed buses had logged more than 1 million miles, and the turbine engine was viewed by Greyhound management as a technical breakthrough for intercity coach transportation.1

In 1976, Allison began development of a generator powered by the GT 404 turbine to supply power to the radar set and engagement control station of the U.S. Army Patriot Missile System. The major objectives of this program included placement of two 150 kW generator sets that provided 100 percent backup in a single container to be carried on an Army 5-ton truck. Other goals included minimizing fuel consumption by the use of twin, rotating ceramic-disk regenerators and developing a reliable, multifuel capability without adjustment. In 1978, Allison began the design, development and construction of five military specification turbine-powered generator sets for field testing. The completed generator sets were tested at the Aberdeen, Belvoir, Elgin and White Sands facilities with these results:

z Fuel consumption was reduced from 48 to 16 gallons per hour as compared with previous generators.

z A 0.1 percent frequency stability at rated load was obtained.

z Free-shaft starting to minus 50 F was accomplished without heaters.

z Multifuel capability was demonstrated on diesel, JP and gasoline.

z All reliability requirements were met.

z Sound level standards of less than 90 dBA were met.

In December 1981, Allison delivered an initial order of 200 generator sets to the U.S. Army. More than 2,000 such generator sets have been delivered to-date for the Patriot system, which was employed during the Gulf War. These generators have logged more than 1 million hours of operation without major problems.2

Micro-Turbine features

The turbine has a single rotating shaft with the generator, air compressor and turbine mounted on air bearings, so no lubrication is required. The power plant is air-cooled, with air brought in through an inlet to cool the generator. The air is then compressed before it is ducted through the regenerator into the combustion chamber. The regenerator is a ceramic disk which rotates slowly in front of the exhaust and the inlet to the combustion chamber. The disk is heated by hot exhaust gas, which increases the compressed inlet air temperature, further improving fuel efficiency. Shaft speed is approximately 80,000 rpm, with the generator providing high-frequency ac. The installed power electronics convert ac to dc, with a dc inverter providing either 480 V, three-phase, 60 Hz or 230/400 V, three-phase, 50 Hz.

The small size and weight of the gas-turbine generators shown in Table 1 enables a utility to install such units at almost any location. Any units needing maintenance or repair can be replaced at the generation site and brought into a central shop; even the 250 kW-size unit can be transported in a pickup truck. For comparison purposes, the dimensions and weights of typical 50 and 250 kW diesel units are included in Table 1. For these types of plants, Table 2 provides estimated purchase and installed costs per kW. Additionally, operations are simple since the plants are fully dispatchable from a central operating center via any two-way communication link, or they can be monitored and controlled locally. Low maintenance and overhaul expenses--less than $0.005 per kWh, which includes a major overhaul every 30,000 hours or roughly every three to four years--are other Micro-Turbine power plant features.

Equipment applications

Two firm-power case studies featuring this technology have been developed. Case 1 is based on a 250 kW load with six 50 kW generators, and Case 2 is based on a 750 kW load with four 250 kW generators. In each case, annual load factors are 52 and 100 percent. Estimated annual costs and the costs per kWh are summarized in Table 3. Firm power at less than 5 cents per kWh from multiple assemblies fueled by natural gas is obviously competitive with most power from central station generators delivered over traditional transmission and distribution facilities. The cost of power for the previous cases rises to a little more than $0.09 per kWh, with diesel fuel at $0.85 per gallon--which is still competitive in many areas.

The efficiency of small gas turbines supplying only firm power approaches 30 percent. This efficiency can be increased to 75 percent as a cogeneration project by using exhaust heat for heating water, absorption refrigeration or cooling, space heating and industrial processing. As a cogeneration application, the project can be economically feasible, even with more expensive fuels such as diesel.

Small gas-turbine generators enable utilities to shave peaks economically and at the same time provide capacity for emergencies. This can increase overall system efficiency, which will reduce investments in traditional generation, bulk transmission and distribution facilities. Shaving also enables a utility to serve incremental load growth in areas where there is a substation and/or distribution feeder capacity shortage. The example in Table 4 provides an estimate of annual costs to install a 250 kW turbine generator and provide fuel and maintenance to run the unit daily for three hours.

This cost can be compared with demand-side management (DSM). The annual cost experiences of a major northeast utility to install and operate a DSM system to control water heaters, air conditioning and space heating are summarized in Table 5 and compared with the peak shaving cost of a gas turbine.3

Small turbines can provide peak shaving for 30 to 50 percent of the cost of DSM and eliminate problems associated with controlling customer appliances. Furthermore, small generators near load centers can also provide emergency power. Utilities need to study the real impact on system operations of shaving peaks with distributed generation. However, the potential savings are certainly sufficient to justify indepth study.

Customer considerations

Every utility has demand charges for their major commercial and industrial customers. Small turbine generators can be applied by or for these customers to reduce demand charges. Table 6 provides the costs to a customer with 250 kW of peak load at two different demand charges. Small gas-turbine power plants are ideal choices for UPS and standby emergency power because of their low initial cost, minimum maintenance requirements and high level of reliability. The power plants can be installed as individual generators or can be arranged in multiple assemblies to provide the level of power required by the loads.

When used for standby service, a small turbine plant could be connected to distribution circuits to serve emergency loads, such as hospital operating rooms, critical-care facilities, emergency lights, communications, refrigerators, freezers, elevators, security systems and cash registers. The electronic control of the power plant constantly monitors the service supplied by the main power source.

If service is interrupted, the control causes the secondary circuits to be disconnected from the main power source and connected to the power plant. The power plant is started via the system battery, and the power plant provides power to the secondary circuits until central service power is restored. z


Walter G. Scott, P.E., received his bachelor of science and master of science degrees in electrical engineering from the University of Arizona. He is a senior member of the Institute of Electrical and Electronics Engineers (IEEE); a member of the National Society of Professional Engineers; and a registered professional engineer in Arizona, Michigan, Missouri and Ohio. This paper was originally presented at the 1997 IEEE Rural Electric Conference, April 20-22, in Minneapolis, Minn.


1 R. Rose, "Heavy-Duty Gas Turbine Upgrading and Commercialization: Gas Turbine Transit Bus Demonstration Program," Symposium on Automotive Propulsion Systems, Dearborn, Mich., October 1980.

2 S.C. Laux, Allison Gas Turbine Division & R.N. Ware, U.S. Army, "Application of a Vehicular Designed, Heavy-Duty Gas Turbine Engine to a Military Generator Set," Paper 85-GT-125, Association of Mechanical Engineers Gas Turbine Expo, Houston, Texas, March 18-21,1985.

3 H. Lee Willis and Rackliffe, G.B., "Introduction to Integrated Resource T&D Planning," published by ABB Power T&D Co., 1984.

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Micro-size gas turbines create market opportunities

By Walter G. Scott, International Power and Light

Power plants in the 25 to 250 kW-size range will enable utilities, IPPs and ESCOs to provide economic power for a variety of applications

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Small regenerative-cycle turbine engines were initially developed by Allison Engine Co. in the 1960s for ground transportation applications. Today, the company is working with International Power and General Electric to develop 50 and 250 kW gas turbines for power generation. Pictured are Don Frazier (left), Allison deputy project manager, and Duyane Parsons (right), shop foreman, with a 40 kW-class automotive derivative of a gas turbine engine.

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