Cogeneration Combined Cycles

Cogeneration can also enhance the thermal efficiency of advanced power cycles, such as the combined cycle. Figure 3-11 is a simplified heat balance diagram for combined-cycle cogeneration, in which steam turbine exhaust serves a process heating load. The result is a significant upgrade in the net efficiency of the power cycle, from 43 to 50% without cogeneration, to 65% or greater. A comparison of Figure 3-11 with Figure 3-5 shows that only 10% of the fuel energy input is converted to power in the steam turbine in the cogeneration combined cycle as opposed to 15% with the standard combined cycle. However, this shortfall in power generation is more than balanced by delivery of 44% of input energy to a process application, as opposed to losing 39% of input energy to the condenser in the standard combined-cycle system.

The performance of this type of cogeneration cycle can be compared with other power cycles through the concept of a heat rate based on fuel-chargeable-to-power (FCP). The combined cycle cogeneration plant would have an FCP heat rate of 6,500 Btu/kWh (6,856 kJ/kWh) or less, compared with 10,000 to 11,000 Btu/kWh (10,550 to 11,600 kJ/kWh) for a conventional steam cycle and 7,000 to 8,000 Btu/kWh (7,380 to 8,440 kJ/kWh) for a combined-cycle power plant.

If the cogeneration combined-cycle is designed from the outset to maximize energy recovery for the power and process steam applications, still further efficiency gains are possible. For example, adding a second evaporator section to the boiler, operating at the process steam application pressure, can reduce stack losses and produce additional process steam without the addition of more fuel. The result is an overall cycle efficiency of nearly 72% and an FCP heat rate of only 6,180 Btu/kWh (6,520 kJ/kWh).

This cycle is twice as efficient as the average conventional steam cycle and nearly three times as efficient as several of the conventional steam cycle plants currently being operated across the country.

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