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Publication Title | CHP Case Studies – Saving Money and Increasing Security

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CHP Case Studies – Saving Money and Increasing Security

Stephen F. Gillette Capstone Turbine Corporation


Microturbines are excellent power generators for use in combined heat and power (CHP) systems. Their low maintenance and clean exhaust make them a reliable choice for base load CHP applications. Integrating hot water heat recovery into the microturbine package has proven cost effective, and a growing number of commercial installations are saving money using this technology. Not only do microturbines provide this cost- saving performance day in and day out, but their value is further increased when the cost for traditional backup generation is eliminated. The examples sited in this paper are from the author’s company, Capstone Turbine, but are intended to reflect the potential application of similar microturbine products.


There is no formal definition for a “microturbine,” however the common characteristics of today’s products are simple radial compressor and radial turbine designs using a recuperator to preheat the combustion air for improved efficiency. Most designs use a single shaft onto which the radial compressor, radial turbine and generator are all coupled. This assembly is sometimes called the “turbo generator.” Rotating speeds are extremely high, in the range of 45,000 to 100,000 revolutions per minute. The generator output is therefore high frequency alternating current. Figure 1 illustrates a microturbine turbo generator with annular construction recuperator.

Power electronics are used to rectify this alternating current to direct current, then invert to the three-phase 50 or 60 Hertz power frequency. Insulated gate bipolar transistors (IGBT’s) are commonly used, and the power electronics and controls are similar to technology used in uninterruptible power supplies and variable frequency motor drives. Integral computers control the conversion of

Figure 1 Turbo Generator Engine

power, operation of the turbo generator system, as well as provide convenient man-machine interface and remote communications. Because the designs are often so tightly integrated with the onboard computer, operator interface can be made both convenient and extremely powerful. Protective relay functions are also often built into the system, making microturbines extremely easy and safe to connect in parallel with an electric grid. Figure 2 provides a system diagram showing the key elements of such an electronically controlled microturbine system. One manufacturer uses a gearbox to transform the high-speed turbine output to a more traditional synchronous speed generator. In this case, synchronizing and protective relaying control is done using traditional separate control systems external to the microturbine.

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