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Dynamic System Optimal Design Model: Designing and Evaluation of Turboexpander System Operation

Ebrahimi Saryazdi, Mohammad | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54519 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Saboohi, Yadoolah
  7. Abstract:
  8. According to Iran’s energy policies, the natural gas transmission pipeline has expanded significantly over the last decade. Moreover, improving energy efficiency is known as a key strategy in energy policies and programs. Therefore, the need for expansion and development of the rational use of energy has received more and more attention. The use of technologies aimed at energy recovery will be an effective step in this regard. Pressure regulators are most commonly used to reduce this pressure in Iran as one of the significant sources of exergy destruction in Iranian gas transmission system. The turbo-expander system is a kind of technology applied in gas pressure reduction stations to convert the exergy of high-pressure natural gas to high-quality exergy flow. On the other hand, heaters such as gas-fired heaters and bath-type heaters are generally applied in the conventional configuration of pressure reduction stations. Not only does the heater burn a remarkable portion of the passing natural gas, but it also has low energy efficiency. Therefore, it is necessary to focus on replacing the conventional preheating systems with novel ones. Given the points mentioned above, the present study aims to investigate and propose various configurations to reduce mechanical and heat loss. Waste heat recovery system and solar preheating system with heat storage tank are proposed to prevent heat losses in fired heater. Various configurations of expansion turbine (with variable inlet nozzle guide vane) such as systems with single turbine, two parallel turbines, two series turbines, and four series-parallel turbines (combined configuration) are also presented to avoid mechanical exergy destruction in pressure relief valve. The first step in this research is to model and simulate the detailed design of each component. Our developed mathematical models for radial turbine, fired heater, and shell and tube, solar collector, heat storage tank, and pressure relief valve were applied to simulation and optimization of different configurations of gas reduction station. These models are developed by an original code. The results of mentioned models have been validated by comparing with the results of previous studies. In this regard, the optimal design of the mentioned configurations is obtained using economic and exergetic objective functions.Due to the fluctuations of the gas inlet flow rate, pressure, and temperature and also the problems of expansion turbine systems installed in Iran, the main goal of this project is to provide a model to optimize the design and the performance of dynamic systems based on advanced exergo-economic and exergo-enviromental analysis. In this section, by presenting a nested algorithm, optimal design and optimization of system performance were done simultaneously. The optimal design of a combined expansion turbine configuration with a fired heater in variable boundary conditions was considered in this stage of the research. Development of design principles based on advanced exergy analysis can be a solution for the optimal design of dynamic systems that sometimes operate at the off-design point. Therefore, the main purpose of this research was to design and optimize the performance of the system simultaneously by considering the relationship between components, the effect of their reciprocal interaction, and the impact of changes in the boundary condition on optimal performance of the system. In this regard, the optimal design of the system was achieved by developing the functions of both levelized average avoidable exogenous cost and environmental impact based on advanced exergy analyses. The optimal value of control parameters was obtained applying the iterative optimization method. Then, the mentioned objective functions were computed. The control parameters are the inlet nozzle angle of turbines and high-pressure turbine inlet temperature. The effect of simultaneous design and performance optimization and interaction of components on optimal design were investigated using three optimal design methods including conventional, proposed optimal design, and simultaneous design and performance optimization, and then their results were compared.Results of optimal design of waste heat recovery preheating system showed that the optimal designs suggested by MCDM methods in this configuration provide a substantial reduction in exergy destruction up to 78%, compared with the conventional preheating system (using fired heater). The results of optimal design and performance evaluation of solar heating system equipped with heat storage tank showed that this configuration can meet the heat requirement of system in hot months of the year, without using fired heaters. The best solar system, based on the results of this section, is a system that meets 70% of the heat demand. The results of evaluating different configurations of expansion turbines showed that by connecting turbines in series (in combined and series configurations), turbines and consequently systems have better performance. Moreover, the exergy efficiency of these two configurations are higher than base and parallel ones. However, the levelized cost of electricity in these configurations is higher than base and parallel ones. The importance of setting inlet guide vane angle of turbines to maintain stability is clearly visible so that at 60% and 50% of the mass flow rate ratio, the efficiency increases with this technology to about 40% and 60%, respectively. The results of comparing the optimal design algorithms were indicated that from the perspective of exergy efficiency and levelized cost, our proposed optimal design leads to increased exergy efficiency and levelized cost compared to the conventional optimal design (considering economic and exergy analysis). However, considering the evaluation parameters of the system that take into account off-design conditions (our proposed objective functions), the results of the simultaneous design and performance optimization algorithm were significantly different from the others (without considering off-design performance), showing the importance of considering performance optimization in the design process of the system. Furthermore, the impacts of design parameters on system parameters and advanced exergy analysis parameters were demonstrated through a contour plot
  9. Keywords:
  10. Optimal Design ; Expansion Turbine ; Solar Heating ; Performance Optimization ; Gas Pressure Reduction Station ; Advanced Exergo Economic Analysis ; Advanced Exergoenvironmental Analysis

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