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Text | Combined Heat and Power Technology Fact Sheet Series | 002
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Gas turbines are constant pressure open cycle heat engines that are characterized by the Brayton thermo- dynamic cycle. Primary gas turbine hardware subsystems include a com- pressor, a combustion chamber, and an expansion turbine. Figure 1 shows an industrial gas turbine configured for CHP. The CHP arrangement includes a gas turbine that drives an electric generator with exhaust heat used to produce steam in a heat recovery steam generator (HRSG).
Figure 2 highlights the key compo-
nents of a simple cycle gas turbine.
The compressor heats and compresses
the inlet air which is then further
heated by the addition of fuel in the
combustion chamber. The hot air and
combustion gas mixture drive an expansion turbine, produc-
ing enough energy to provide shaft power to the generator or mechanical process and to drive the compressor. The power produced by an expansion turbine and consumed by a compres- sor is proportional to the absolute temperature of the gas passing through the system. Consequently, it is advantageous to oper- ate the expansion turbine at the highest practical temperature consistent with economic materials and internal blade cooling technology and to operate the compressor with an inlet air flow temperature as low as possible. Higher temperature and pressure ratios result in higher efficiency and specific power, or power-to- weight ratio. Thus, the general trend in gas turbine advancement has been towards a combination of higher temperatures and pressures. While such advancements increase the manufacturing cost of the machine, the higher value, in terms of greater power output and higher efficiency, provides net economic benefits.
The efficiency of the Brayton cycle is a function of several factors, including pressure ratio, ambient air temperature, turbine inlet air temperature, compressor energy use, turbine blade cool- ing requirements, and specific engineering design requirements (e.g., recuperation, intercooling, inlet air cooling, reheat, steam injection, simple cycle, or combined cycle). Higher temperatures and pressure ratios result in higher efficiency, and the general trend in gas turbine advancement, therefore, has been towards a combination of higher temperatures and pressures. As indicated in Table 2, overall CHP efficiencies for gas turbines are typically in the range of 65% to 70%, although higher efficiencies can be achieved depending on site specific conditions and engineer-
ing design configurations. The power to heat ratio generally increases with gas turbine size (ranges from 0.58 to 1.03 for
the representative systems shown in Table 2). A changing ratio of power to heat impacts project economics and may affect the decisions that customers make in terms of CHP acceptance, sizing, and the desirability of selling power. It is generally recommended to size a CHP system based on a site’s thermal load demand; therefore, such power to heat ratios are important characteristics to consider. When less than full power is required from a gas turbine, the output is reduced by lowering the turbine inlet temperature. In addition to reducing power, this change in operating conditions also reduces efficiency. Typically, emissions increase as well at part load conditions, especially at half load and below.
Capital and O&M Costs
A gas turbine CHP plant has many interrelated subsystems. The basic package includes a gas turbine, gearbox, electric generator, inlet air and exhaust ducting, inlet air filtration, starting system, and an exhaust silencer. The basic package does not include extra
Figure 1. Gas turbine configuration with heat recovery.
Graphic credit Energy Solutions Center.
Figure 2. Components in a simple cycle gas turbine.
Graphic credit ICF International.
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