Brayton-Rankine cycle

The Combined (Brayton-Rankine) Cycle The 1990s has seen the rebirth of the combined cycle, the combination of gas turbine technologies with the steam turbine. This has been a major shift for the utility industry, which was heavily steam-tnrbine-oriented with the use of the gas turbine for peaking power. In this combined cycle, the hot gases from the turbine exhaust are used in a heat recoveiy steam generator or in some cases in a snpplementaiy fired boiler to produce superheated steam. 

The most effective cycle is the Brayton-Rankine cycle. This cycle has tremendous potential in power plants and in the process industries where steam turbines are in use in many areas. The initial cost of this system is high however, in most cases where steam turbines are being used this initial cost can be greatly reduced. 

Figure 3-19 shows the thermal efficiency of the gas turbine and the Brayton-Rankin cycle (gas turbine exhaust being used in the boiler) based on the LHV of the gas. This figure shows that below 50% of the rated load, the combination cycle is not effective. At full load, it is obvious the benefits one can reap from a combination cycle. Figure 3-20 shows the fuel consumption as a function of the load, and Figure 3-21 shows the amount of steam generated by the recovery boiler. 

Combined Brayton-Rankine Cycle The combined Brayton-Rankine cycle. Figure 9-14, again shows the gas turbine compressor for the air flow to the cell. This flow passes through a heat exchanger in direct contact with the cell it removes the heat produced in cell operation and maintains cell operation at constant temperature. The air and fuel streams then pass into the cathode and anode compartments of the fuel cell. The separate streams leaving the cell enter the combustor and then the gas turbine. The turbine exhaust flows to the heat recovery steam generator and then to the stack. The steam produced drives the steam turbine. It is then condensed and pumped back to the steam generator. 

Figure 9-14. Combined Brayton-Rankine Cycle Fuel Cell Power Generation System... 

The results of the performance calculations for the fuel cell, Rankine cycle heat recovery system, summarized in Table 9-24, indicate that the efficiency of the overall system is increased from 57% for the fuel cell alone to 72% for the overall system. This Rankine cycle heat-fuel recovery arrangement is less complex but less efficient than the combined Brayton-Rankine cycle approach, and more complex and less efficient than the regenerative Brayton approach. It does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger. And in applications where cogeneration and the supply of heat is desired, it provides a source of steam. 

Fig. 5.12. In general, modifications of both the Brayton and Rankine cycles could also be included. Since the net work output is equal to the sum of the two outputs and the heat input is that of the topping cycle alone, a substantial increase in cycle efficiency is possible. Another arrangement of the Brayton/ Rankine cycle, which is a combination of a two-stage reheat Brayton cycle and a two-stage reheat Rankine cycle, is shown in Fig. 5.13. 

A Brayton/Rankine cycle (Fig. 5.12) uses water as the working fluid with 1 kg/sec mass flow rate through the Rankine cycle, and air as the working... 

What is a combined Brayton/Rankine cycle What is its purpose ... 

What is the heat source for the Rankine cycle in the combined Brayton/Rankine cycle ... 

Why is the combined Brayton/Rankine cycle more efficient than either of the cycles operating alone ... 

Figure 5.28b Combined Brayton-Rankine cycle input.

Figure 5.28c Combined Brayton-Rankine cycle output.

Figure 5.28d Combined Brayton-Rankine cycle sensitivity diagram.

FIG. 29-39 Performance map showing the effect of pressure ratio and turbine inlet temperature on a Brayton-Rankine cycle. 

Figure 8-34 Combined Brayton-Rankine Cycle Thermodynamics...

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