199093

10,323

10,694

Table 3-1 Average Heat Rates for Utility Steam-Electric Plants, 1965-1993. Source: U.S. DoE/EIA

plants have been relatively constant over the past two decades.

Individual steam plant thermal efficiencies can range from under 30% to more than 40%. At the higher end, the more efficient plants use supercritical steam conditions and several stages of reheat. Due to cost and technical considerations, these types of plants have not been widely used in the United States, where, currently, the best steam cycles offer a thermal efficiency of about 34%. Based on these typical thermal efficiencies, a conventional steam cycle releases almost two-thirds of the energy in the fuel into the environment. Figure 3-3 illustrates how this occurs.

Table 3-1 Average Heat Rates for Utility Steam-Electric Plants, 1965-1993. Source: U.S. DoE/EIA

plants have been relatively constant over the past two decades.

Individual steam plant thermal efficiencies can range from under 30% to more than 40%. At the higher end, the more efficient plants use supercritical steam conditions and several stages of reheat. Due to cost and technical considerations, these types of plants have not been widely used in the United States, where, currently, the best steam cycles offer a thermal efficiency of about 34%. Based on these typical thermal efficiencies, a conventional steam cycle releases almost two-thirds of the energy in the fuel into the environment. Figure 3-3 illustrates how this occurs.

Fig. 3-3 Conventional Power Cycle Energy Utilization. Source: Cogen Designs, Inc.

The left stacked bar, entitled "Boiler," represents the energy added to a pound of feedwater as it passes into and through the three sections of the steam generator. Since not all of the energy content in the fuel is available to increase the steam energy content, and since the boiler itself has energy losses, in total, 1,458 Btu (1,538 kJ) of fuel energy must be added per pound of steam to increase the energy content of the steam from approximately 200 Btu/lbm (465 kJ/kg) to 1,500 Btu/lbm (3,488 kJ/kg), a 1,300 Btu/lbm (3,023 kJ/kg) increase. The majority of these energy losses, which, in this example, amount to 12% of the fuel energy content, are rejected to the outside environment through the boiler stack.

Expansion of steam through the steam turbine is represented by the diagonal arrow running from the boiler to the stacked bar on the right entitled "condenser." In this example, expansion releases about 452 Btu of useful mechanical work per pound (1,046 kJ per kg) of steam, which is converted by the generator into electric power. Efficiency losses in the generator itself, and other parasitic and auxiliary electric demands of the generating station, slightly reduce the electric energy available for export from the plant.

By far, the largest source of inefficiency in the conventional steam cycle is fundamental to the thermo-dynamic principles underlying the cycle. The latent heat released through the condenser to the environment from the condensing steam after expansion in the steam turbine serves no useful purpose in the power cycle. In this example, roughly 830 Btu/lbm (1,930 kJ/kg) of steam is lost through the condenser. An additional 150 Btu/lbm (349 kJ/kg) of steam is described as "water heating" on the condenser bar below the condenser losses. This energy represents the steam extracted from the steam turbine and recovered for deaeration and feedwater heating.

In summary, this representative steam cycle example indicates a fuel energy input requirement of 1,458 Btu (1,538 kJ) to produce 452 Btu (479 kJ) of useful work, for an overall power cycle efficiency of 31%. On a heat rate basis, this cycle requires about 11,010 Btu/kWh (11,613 kJ/kWh) of electric energy produced.

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