مدل سازی، بهینه سازی عددی، و کاهش برگشت ناپذیری از فشار دوگانه بازگرمکن سیکل ترکیبی

Optimizing the gas-turbine combined-cycle is an important method for improving its efficiency. In this paper, a dual-pressure reheat combined-cycle was modeled and optimized for 80 cases. Constraints were set on the minimum temperature-difference for pinch points (PPm), superheat approach temperature-difference, steam-turbine inlet temperature and pressure, stack temperature, and dryness fraction at the steam-turbine’s outlet. The dual-pressure reheat combined-cycle was optimized using two different methods; the direct search and the variable metric. A technique to reduce the irreversibility of the steam generator of the combined cycle was introduced. The optimized and the reduced-irreversibility dual-pressure reheat combined-cycles were compared with the regularly-designed dual-pressure reheat combined-cycle, which is the typical design for a commercial combined-cycle. The effects of varying the inlet temperature of the gas turbine (TIT) and PPm on the performance of all cycles were presented and discussed. The results indicated that the optimized combined-cycle is up to 1% higher in efficiency than the reduced-irreversibility combined-cycle, which is 2–2.5% higher in efficiency than the regularly-designed combined-cycle when compared for the same values of TIT and PPm. The advantages of the optimized and reduced-irreversibility combined-cycles were manifested when compared with the most efficient commercially-available combined cycle at the same value of TIT.

The gas-turbine combined-cycle has been used extensively in power generation. Many techniques have been used to improve the efficiency of the combined cycle. Most techniques improve the efficiency of the gas cycle or the steam cycle to enhance the efficiency of the combined cycle. The efficiency of the steam cycle can be increased by applying reheating and by reducing the irreversibility of the steam generator. Reheating increases the efficiency of the steam cycle by increasing the average temperature of heat addition of the cycle, and by increasing the efficiency of the expansion process in the steam turbine [1]. The efficiency of such a process increases by increasing the dryness fraction of steam at the turbine’s outlet. Reducing the irreversibility of the steam generator can be done by increasing the steam temperature at the steam-generator outlet [2] and by reducing the stack temperature and the temperature difference for heat transfer. Such a temperature difference can be decreased by introducing a multiple-pressure steam generator. Inlet-air cooling, optimizing the compression ratio, and increasing the turbine inlet’s temperature are the main methods for improving the efficiency of the gas cycle. Inlet-air cooling reduces compressor work and enhances the performance of both gas and combined cycles by increasing the power and efficiency of both cycles [3], [4], [5], [6] and [7]. Ebeling et al. [8] reported 29% capacity increase of a combined-cycle power plant by cooling the inlet air using a thermal-energy storage system of ice. Optimizing the compression ratio and increasing the turbine-inlet’s temperature increase the average temperature of heat addition, thus, increasing both Carnot and gas cycle efficiencies [2] and [9]. In recent decades, the development of better materials for turbine blades and the introduction of sophisticated techniques of blade-cooling have increased the turbine-inlet’s temperature significantly. The most advanced cooling-technique can be found in the H-system technology of GE. In such a system, closed-loop steam cooling is used to cool the gas turbine and the first-stage buckets and nozzles are designed with single-crystal materials and a thermal barrier coating [10]. Such features of the H-system allow a turbine-inlet’s temperature of 1477 °C to be achieved. McDonald and Wilson [11] reported that the next generation of gas-turbine engines will have a turbine-inlet’s temperature of above 1500 °C. Such a high temperature has increased the gas-turbine outlet’s temperature, thus, permitting the application of reheating and the multiple-pressure steam generation for the steam cycle. Therefore, dual-pressure reheat and the triple-pressure reheat have become the most common combined cycles. The dual-pressure reheat-cycle is simpler in design, has an advantage of about 0.1% in equivalent forced outage rate, and has a higher power output than the triple-pressure reheat-cycle at higher gas-turbine exhaust’s temperature [12]. Improving the efficiency of the combined cycle can also be done by optimization. Most investigators have focused on optimizing the gas cycle or the steam cycle, separately [12] and [13], to increase the efficiency of the combined cycle. Sarabchi and Polley [14] modeled and optimized a single-pressure combined cycle. Bassily [15] modeled and optimized the dual and triple-pressure non-reheat combined cycles. The layout for the triple-pressure cycle was mainly chosen to reduce the NOx emission using steam injection. A literature review has shown that the dual-pressure reheat combined-cycle has not been modeled or optimized. In this paper, the dual-pressure reheat combined cycle is modeled and optimized. A technique to reduce the irreversibility of the steam generator is introduced. The optimization and irreversibility reduction results are compared with the results of the regular design case, which is the usual design case for a commercial combined cycle. The effects of varying the gas-turbine inlet’s temperature and pinch-point temperature-difference are presented and discussed.

The dual-pressure reheat combined-cycle was modeled and optimized at different values of TIT and PPm. A technique to reduce the irreversibility of the steam generator of the combined cycle was introduced. A comparison study among the optimized cycle, the reduced-irreversibility cycle and the regularly-designed cycle leads to the following conclusions: • The global optimum of the dual-pressure reheat combined-cycle was found at the maximum value of TIT and the minimum value of PPm. The active constraints during the optimization process were PP1, PP2, PP3, PP4, δTsuperheat, and Xsto. The most efficient reduced-irreversibility or regularly-designed combined cycle was also found at the maximum value of TIT and minimum value of PPm. • The optimized dual-pressure reheat combined cycle was found to be up to 1% higher in efficiency than the reduced-irreversibility dual-pressure reheat combined-cycle, which was found to be 2–2.5% higher in efficiency than the regularly-designed dual-pressure reheat combined-cycle, when compared at the same values of TIT and PPm. The reduced-irreversibility combined-cycle had the highest values of specific work (per kg of gas) at all values of TIT. • The optimized combined cycle operates at relatively high values of air compression ratio and lower values of pressure and temperature for the steam cycle, which may make the optimized cycle more attractive economically. • Reducing the irreversibility of the steam generator of the combined cycle enhances both its efficiency and specific work. • The regularly-designed combined cycle is expected to be as efficient as the most efficient commercially-available combined cycle at the same combustion temperature. • More research is needed to model and optimize the triple-pressure reheat combined cycle and reduce the irreversibility of its steam generator.