Overall efficiency combined cycles

The combined cycle is also appHcable to dedicated power production. When the steam from the waste heat boiler is fed to a condensing turbine, overall conversion efficiencies of fuel to electricity in excess of 50% can be achieved. A few pubHc utihty power plants use this cycle, but in general utihties have been slow to convert to gas turbines. Most electricity is generated by the cycle shown in Figures 5d and 6d. 

A comparison of the effect of the various cycles on the overall thermal efficiency is shown in Fig. 29-40. The most effective cycle is the Brayton-Ranidne (combined) 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 the combined cycle is between 800- 1200 per kW while that of a simple cycle is about 300- 600 per kW. Repowering of existing steam plants by adding gas turbines can improve tne over plant efficiency of an existing steam turbine plant by as much as 3 to 4 percentage points. 

This cycle produces an increase of 30% in work output, but the overall efficiency is slightly decreased as seen in Figure 2-15. An intercooling regenerative cycle can increase the power output and the thermal efficiency. This combination provides an increase in efficiency of about 12% and an increase in power output of about 30%, as indicated in Figure 2-16. Maximum efficiency, however, occurs at lower pressure ratios, as compared with the simple or reheat cycles. 

Studies carried out in the United States, Russia, and Japan indicate that a combined cycle MHD-steam plant should be able to achieve an overall power station efficiency of at least 60 percent which is about 20 percent more than offered by a conventional steam plant. This should be possible at capital costs comparable with existing steam plants. 

In open cycle, the diesel has the higher efficiency, making it more attractive for light fuel oils. However, in combined cycle, the gas turbine often has the highest overall efficiency, but there is the penalty associated with the additional cost for boiler and turbine. Nevertheless, unless the prime fuel is of low cost, the use of combined-cycle gas turbine plant will prove to be the more economic. 

The overall efficiency of the combined cycle is the total output work (lVi + IV2) divided by the heat input, Qi. Referring to Fig. 5.3, we have... 

Well-to-wheel thermodynamic efficiencies for electric transportation with or without H2 as intermediate. (From Kreith, F. and West, R., ]. I. nerg. Resource Technol., 126, 249, 2004.) (a) Well-to-Fuel, Fuel-to-Grid and Well-to-Grid efficiencies for electricity production from natural gas. All routes allow effective C02 removal but routes including H2 conversion seem most effective in this respect, ng natural gas P production SD storage/distribution C compression GT/ST combined cycle FC fuel cell R reforming (b) Well-to-Wheel efficiencies for the best and worst well-to-grid efficiencies of a. The H2/Fuel option reduces the overall Well-to-Wheel efficiencies to half their values. El Electrolysis B battery eM electric motor. 

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