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Fig. 3-7 Performance Comparison of Constant and Variable Speed Reciprocating Engine Operation. Source: Caterpillar Engine Division generation plants because there is no host demand for the available thermal energy — the largest exceptions being large-scale city-wide district heating systems.

Figure 3-8 is a basic heat balance diagram for a cogen-eration-cycle system featuring a gas turbine and heat recovery unit. The recovery of the thermal energy to displace these energy sources provides an added financial value to an on-site prime mover application in the form of an energy credit. Compared with a conventional or combined-cycle central electric generation plant, an on-site cogeneration cycle not only achieves higher overall and net energy efficiencies, but also provides other environmental benefits associated with air quality and non-renewable resource conservation.

Fig. 3-8 Basic Heat Balance for Cogeneration System.

of steam. The process can use all of the latent heat energy in the steam, and the application has a thermal efficiency of about 83%.

Fig. 3-9 Energy Utilization by Power Cycle and Process Loads. Source: Cogen Designs, Inc.

Fig. 3-8 Basic Heat Balance for Cogeneration System.

Cogeneration Cycle Efficiency Example

Figures 3-9 and 3-10 illustrate the potential efficiency improvement provided by cogeneration in meeting local power and thermal loads. In Figure 3-9, independent power and thermal energy conversions are shown. In the steam power cycle on the left side of the figure, roughly 57% of fuel energy input is lost in condenser heat rejection. The right side of the figure illustrates heat generation for a process steam load, such as paper drying, steam distillation, or absorption chilling. In this application, a fired boiler generates steam at a pressure of about 150 psig (11 bar), which is delivered to the process and returned to the boiler as liquid condensate at about 200°F (93°C). Condensate, containing about 167 Btu/lbm (388 kJ/kg), is heated, evaporated, and superheated by the boiler, adding roughly 1,080 Btu of energy to each pound (2,511 kJ to each kg) of condensate. Stack losses result in a fuel energy requirement of about 1,300 Btu/lbm (3,020 kJ/kg)

Fig. 3-9 Energy Utilization by Power Cycle and Process Loads. Source: Cogen Designs, Inc.

Figure 3-10 illustrates energy use by a cogeneration system in which steam is fed sequentially through a steam turbine, followed by the process application. The result is that the heat rejected by the condenser in the conventional cycle is directed instead to the process heating load. This approach also eliminates the process boiler and its stack loses.

While the power production capabilities of the system are reduced as a result of raising the steam turbine exhaust pressure from vacuum to 150 psi (10 bar), the gains due to latent heat utilization and elimination of one set of stack losses result in an overall thermal efficiency of greater than 85%. In this example, for each pound of process steam delivered, the steam turbine generator would also produce 0.05 kWh of electrical energy.

The separate conventional steam cycle power plants and process boilers shown in Figure 3-9 require about

Fig. 3-10 Cogeneration System Energy Utilization. Source: Cogen Designs, Inc.

1,850 Btu (1,950 kJ) of fuel energy, while the cogenera-tion facility shown in Figure 3-10 requires only 1,420 Btu (1,498 kJ) of fuel energy, giving a 23% energy savings overall. This is the benefit of the "sequential use" concept of cogeneration. Similar benefits also accrue to gas turbine and reciprocating engine cogeneration cycles.

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