Maximizing a manufacturer’s combined heat and power plant

A Midwestern manufacturing facility opted for a hybrid combined-cycle steam turbine generator solution.


Learning objectives

  • Demonstrate the obstacles a manufacturing facility overcame as a combined heat and power (CHP) central plant mission changed.
  • Examine the efficiency benefits of a hybrid combined-cycle steam turbine generator.

Figure 6: This is an example of a project using COMcheck's space-by-space method showing compliance with the energy code. The space-by-space method may be more tedious to input data, as compared with the whole building method, but it may result in a proje Following a scale-down in manufacturing production, a Midwestern manufacturer's combined heat and power (CHP) central plant mission changed. Identifying opportunities to optimize existing generating assets, the owner is increasing system efficiency with a hybrid combined-cycle steam turbine generator solution.

Totaling 2.5 million sq ft of conditioned space, the owner's manufacturing and technical center complex has been served by a central plant that generates steam for heating and chilled water for cooling the complex. The central plant facility originally included:

  • A CHP system consisting of three nominal 15-MW combustion turbine (CT) prime movers and associated heat-recovery equipment
  • A nominal 5-MW backpressure steam turbine generator (STG)
  • A 3-MW backpressure STG.


The electrical power generated by the CHP system was indirectly used by the manufacturer's campuses and other adjacent facilities. Steam was used at a pressure of 250 psig by industrial production facilities and space-heating needs, and at 15 psig by absorption chillers.

The real-time costs of electricity and gas are very dynamic and depend on many factors. Assuming a static cost of electricity of 6 cents/kWh and a fixed cost of natural gas of $6/decatherm (purchased utility heat rate of 10,000 Btu/kWh), the original operations of the central plant reduced fuel costs by approximately $3 million annually. CHP is the most cost-effective approach to generate electricity and has the least impact on regional air emissions. The CHP process is approximately 70% efficient as compared with utility-generated power at 35%.

As the owner reduced manufacturing production output at the complex, facility steam and chilled-water use declined and fewer central-plant-generated utilities were required. Because central-plant-generated chilled water was no longer needed by the complex, the absorption chillers were deactivated.

Although the dispatch of the CHP portion of the plant has changed significantly from the original intent, the central plant has been operated well and remains in good condition. Following the manufacturing scaledown, the central plant correctly operated in a temporary hybrid CHP mode of traditional CHP operation with the steam generated being used for heating purposes.

When there hasn't been adequate heating demand and the electric costs were favorable as compared with natural gas costs (spark spread), the recovered steam has been condensed to cost-effectively generate additional electrical power with the existing steam turbine generators. The present operation of the plant within the limits of the existing equipment has been excellent with the proper staging and loading of the various generating components.

The ability to operate in this temporary hybrid mode has been reduced with the elimination of the absorption chiller usage and the 5-MW backpressure STG. These limitations led the owner to examine the replacement of the existing steam turbine generators.

Correcting capacity/load imbalance

Figure 2: This Midwestern manufacturer added a new nominal 12-MW condensing steam turbine generator with extraction capabilities and an exhaust pressure of 2.0 psia to provide lost capacity and more efficiently condense steam than with an existing steam tThe Affiliated Engineers Inc. team examined multiple options to replace the existing steam turbine generators. The study evaluated the current system's operation efficiency and capacity, calculated future steam loads based on an independent campus planning effort, and compared multiple options to replace the existing steam turbine generators. Theoretical CHP dispatch models were developed and compared against present operating strategies.

The electric output of the plant is used in a commercial pricing node (CPN). The use of a CPN allows for cost-effective purchase of electricity for the complex as well as the owner's adjacent installations. The purchase of electricity is based on the optimum mix of the following components:

  • Self-generation
  • Block purchases
  • Day-ahead pricing
  • Real-time pricing.


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