Application of global optimization strategies to refinery hydrogen network

In this paper, a new method is presented in optimization of hydrogen network. The mixed integer non-linear programming (MINLP) and non-linear programming (NLP) problems have been solved with two methods, simultaneously. The linearization technique for non-linear programming models which proposed by McCormick (1976) and also a new method proposed by Faria and Bagajewicz (2011) have been used to solve these problems. Application of this new method is presented in global optimization of MINLP/NLP, and hydrogen network problem.     

大型发电机转子轴系横振评判标准分析

为了保证大容量汽轮发电机组转子轴系在电厂的长期安全运行,其横振特性在设计及运行阶段均须满足相应的评判标准.总结了国内外标准及行业标准中对于发电机组转子轴系横振特性的评判要求,运用Riccati轴系传递矩阵法分别计算了600 MW、1 000 MW等级大型发电机组轴系临界转速,并结合权威测试机构对机组的现场升降速试验数据进行了调研,将多组计算和试验得到的转子轴系临界转速与相关标准进行了对比.分析结果表明,现行横振评判标准具有较好的适用性,并建议可将频率、响应、敏感度等指标结合,以评判发电机组横振特性的优劣,进一步提高标准的适用性,从而提高大型汽轮发电机组轴系设计的可实现性.     

Simulation based size optimization of a PV/wind hybrid energy conversion system with battery storage under various load and auxiliary energy conditions

In this paper, size of a PV/wind integrated hybrid energy system with battery storage is optimized under various loads and unit cost of auxiliary energy sources. The optimization is completed by a simulation based optimization procedure, OptQuest, which integrates various heuristic methods. In the study, the main performance measure is the hybrid energy system cost. And the design parameters are PV size, wind turbine rotor swept area and the battery capacity. The case study is realized for Izmir Institute of Technology Campus Area, Urla, Turkey. The simulation model of the system is realized in ARENA 12.0, a commercial simulation software, and is optimized using the OptQuest tool in this software. Consequently, the optimum sizes of PV, wind turbine and battery capacity are obtained under various auxiliary energy unit costs and two different loads. The optimum results are confirmed using Loss of Load Probability (LLP) and autonomy analysis. And the investment costs are investigated how they are shared among those four energy sources at the optimum points.     

Comparison results of two optimization techniques for a combined wind and solar power plant

Two optimization techniques have been tested on an hour-by-hour simulation of a combined wind and solar power plant. The system also includes a battery storage system as well as a group of diesel generators. The two optimization techniques are: simplex from the package of MINUITS written at CERN and a modified steepest descent algorithm. Both techniques are suited to hour-by-hour simulation for the above system since the function being minimized is monotonically decreasing towards a minimum. The comparison results showed that the steepest descent algorithm converges slightly faster than the simplex one. Moreover, the application of the techniques for two different sites with different load profiles let us conclude that the results are stable.     

Geometry optimization and performance analysis of a new tapered slope cathode flow field for PEMFC

In proton exchange membrane fuel cell (PEMFC), the cathode flow field structure affects the performance of PEMFC. In a previous study, we proposed a new tapered slope flow field (TSFF). In this study, Ansys Fluent software was used to simulate a PEMFC with a tapered slope cathode flow field structure. The results show that the performance of the TSFF is superior, the drainage efficiency is higher, and the oxygen mass fraction distribution is more uniform. Furthermore, comparing double-sided TSFF with different lengths, the PEMFC performance first increases and then decreases as the length of the tapered slope increases. In particular, the oxygen mass fraction and current density distributions are more uniform in the double-sided TSFF with L = 1.2 mm and the PEMFC performance is the best, and compared with the serpentine flow field, the maximum power density of PEMFC is increased by 5.89%. A detailed analysis of the geometric structure of the flow field can help us understand the reasons why the TSFF structure improves the performance of PEMFC and comprehensively evaluate the flow field performance. The TSFF enhances the flow rate of reactant diffusion to the CL and enhances the mass transfer downstream of the flow field. In particular, when L = 1.2 mm, the relative magnitude of the reactant flow resistance loss in the double-sided TSFF was 1.86% smaller than that of the serpentine flow field.     

Simulation and optimization of a novel solar-powered adsorption refrigeration module

A mathematical model has been developed to simulate and optimize the performance of a solar-powered adsorption refrigeration module with the solid adsorption pair of domestic type of charcoal and methanol. The module is composed of a modified glass tube having a circular generator (sorption bed) at one end and a combined evaporator and condenser at the other end. The charcoal is mixed with small pieces of blackened steel to enhance the heat transfer in the sorption bed. Simple arrangement of plane reflectors is used to heat the generator. The angles of inclination of the reflectors are chosen according to optical and thermal analysis to receive maximum solar energy at noontime. The model accounts for transient heat and mass transfer inside the bed of the module. After an experimental validation based on the results of previous tests of this module, the effects of design and climatic conditions on the module performance all year round are discussed and optimized.It is found by virtue of using the proposed reflector arrangement, solar energy input to the system increases especially in cold climate. This increase ranges from 10% for hot climate to 30% for cold climate.The effects of using steel additives inside the sorption bed and using glass shell over the bed are investigated. It is found that the percentage increase in desorped methanol ranges from 3% in the hottest month to about 19% in the coldest month as a result of using a mass of steel pieces equal about 33% of the mass of the charcoal, (M_st/M_ch = 0.33). These improvements increase to 7% and 43% in the hottest and coldest months respectively when glass shell is used over the bed. Generally, the improvements are more pronounced in cold months than hot ones.The ratio M_st/M_ch inside the sorption bed is optimized and found to be 0.75. Comparison between the model results for the steel additives ratio M_st/M_ch = 0.33 and results obtained with the optimum ratio shows that, the yearly average ice production increases from 0.23 to 0.25 kg/day, the yearly average bed efficiency increases from 55.2% to 58.5%, and the yearly average net COP increases from 0.146 to 0.1558.The effect of climatic conditions on the module performance all year round is also investigated. It is found that, about 28% of the cooling energy is lost due to climate effect in hot months and this ratio reaches 17.5% in cold months.     

Simulation-based design and optimization of refrigeration cassettes

A model-driven design and optimization methodology for sizing the components of refrigeration cassettes for light commercial applications (i.e., cooling capacities ranging from 0.5 to 1.5 kW) is presented. Mathematical models were devised for each of the system components and their numerical results were compared with experimental data taken with different cassettes. It was found that the model predictions for the working pressures, power consumption, cooling capacity and coefficient of performance (COP) showed maximum deviations of ±10%. A genetic optimization algorithm was used to design the condenser and evaporator and also to select the compressor model based on an objective function which considers both the COP and cost. The optimization led to two improved cassette designs, which were assembled and tested. One of the optimized cassettes showed a COP/cost ratio approximately 50% higher than that of the baseline system.     

Size optimization of a PV/wind hybrid energy conversion system with battery storage using simulated annealing

In this paper, we perform Simulated Annealing (SA) algorithm for optimizing size of a PV/wind integrated hybrid energy system with battery storage. The proposed methodology is a heuristic approach which uses a stochastic gradient search for the global optimization. In the study, the objective function is the minimization of the hybrid energy system total cost. And the decision variables are PV size, wind turbine rotor swept area and the battery capacity. The optimum result obtained by SA algorithm is compared with our former study’s result. Consequently, it is come up with that the SA algorithm gives better result than the Response Surface Methodology (RSM). The case study is realized for a campus area in Turkey.     

A fuzzy global efficiency optimization of a photovoltaic water pumping system

This paper presents an on-line fuzzy optimization of the global efficiency of a photovoltaic water pumping system driven by a separately excited DC motor (DCM), a permanent magnet synchronous motor (PMSM), or an induction motor (IM), coupled to a centrifugal pump.The fuzzy optimization procedure stated above, which aims to the maximization of the global efficiency, will lead consequently to maximize the drive speed and the water discharge rate of the coupled centrifugal pump. The proposed solution is based on a judicious fuzzy adjustment of a chopper ratio which adapts on-line the load impedance to the photovoltaic generator (PVG). Simulation results show the effectiveness of the drive system for both transient and steady state operations. Hence it is suitable to use this fuzzy logic procedure as a standard optimization algorithm for such photovoltaic water pumping drives.     

Economic evaluation and optimization of solar systems for space and domestic water heating

Solar energy will be utilized only if it proves to be economical. In this paper, we evaluate the economic feasibility of the use of solar energy for space and domestic water heating systems for a house in Benghazi, Libya. A comprehensive evaluation considerering 324 cases representing the proper ranges of economic and load factors and based on the annual equivalent cost approach is given. The optimum collector area for each case is determined. The results obtained and relationships developed permit generalizations that can be applicable in other locations.     

Size optimization of a PV/wind hybrid energy conversion system with battery storage using response surface methodology

This paper aims to show the use of the response surface methodology (RSM) in size optimization of an autonomous PV/wind integrated hybrid energy system with battery storage. RSM is a collection of statistical and mathematical methods which relies on optimization of response surface with design parameters. In this study, the response surface, output performance measure, is the hybrid system cost, and the design parameters are the PV size, wind turbine rotor swept area and the battery capacity. The case study is realized in ARENA 10.0, a commercial simulation software, for satisfaction of electricity consumption of the global system for mobile communications (GSM) base station at Izmir Institute of Technology Campus Area, Urla, Turkey. As a result, the optimum PV area, wind turbine rotor swept area, and battery capacity are obtained to be 3.95 m², 29.4 m², 31.92 kWh, respectively. These results led to $37,033.9 hybrid energy system cost, including auxiliary energy cost. The optimum result obtained by RSM is confirmed using loss of load probability (LLP) and autonomy analysis.     

A new constraint handling method for wind farm layout optimization with lands owned by different owners

For wind farm optimizations with lands belonging to different owners, the traditional penalty method is highly dependent on the type of wind farm land division. The application of the traditional method can be cumbersome if the divisions are complex. To overcome this disadvantage, a new method is proposed in this paper for the first time. Unlike the penalty method which requires the addition of penalizing term when evaluating the fitness function, it is achieved through repairing the infeasible solutions before fitness evaluation. To assess the effectiveness of the proposed method on the optimization of wind farm, the optimizing results of different methods are compared for three different types of wind farm division. Different wind scenarios are also incorporated during optimization which includes (i) constant wind speed and wind direction; (ii) various wind speed and wind direction; and (iii) the more realistic Weibull distribution. Results show that the performance of the new method varies for different land plots in the tested cases. Nevertheless, it is found that optimum or at least close to optimum results can be obtained with sequential land plot study using the new method for all cases. It is concluded that satisfactory results can be achieved using the proposed method. In addition, it has the advantage of flexibility in managing the wind farm design, which not only frees users to define the penalty parameter but without limitations on the wind farm division.     

Comparative study of various correlations in estimating hourly diffuse fraction of global solar radiation

Proper design and performance predictions of solar energy systems require accurate information on the availability of solar radiation. The diffuse-to-global solar radiation correlation, originally developed by Liu and Jordan, has been extensively used as the technique providing accurate results, although it is latitude dependent. Thus, in the present study, empirical correlations of this type were developed to establish a relationship between the hourly diffuse fraction (k_d) and the hourly clearness index (k_t) using hourly global and diffuse irradiation measurements on a horizontal surface performed at Athalassa, Cyprus. The proposed correlations were compared against 10 models available in the literature in terms of the widely used statistical indicators, rmse, mbe and t test. From this analysis, it can be concluded that the proposed yearly correlation predicts diffuse values accurately, whereas all candidate models examined appear to be location-independent for diffuse irradiation predictions.     

Exergoeconomic analysis and optimization of a new hybrid fuel cell vehicle

This paper introduces thermodynamic and economic analyses on a newly developed energy system for powering hybrid vehicles based on both energy and exergy concepts. The proposed hybrid propulsion system incorporates a liquefied ammonia tank, ammonia dissociation and separation unit (DSU), an internal combustion engine (ICE), and a fuel cell (FC) system. The exhaust gases released from the ICE are exploited to supply the necessary thermal energy to decompose ammonia thermally into hydrogen and nitrogen on board. The ICE is fuelled with a blend of ammonia and hydrogen generated from the DSU. The additional hydrogen released from the DSU will also be provided to the fuel cell system to run the FC and generate electric power, which will be supplied to the electric motor to provide the required traction to the vehicle. An optimization study is also performed to identify optimum design variables. The parametric studies are included in this investigation to evaluate the influence of varying the different operational parameters on the system energy and exergy efficiencies and both total cost rate and exergoeconomic factor values of the system.     

A novel meta-heuristic optimization methodology for solving various types of economic dispatch problem

The increasing costs of fuel and operation of thermal power generating units warrant development of optimization methodologies for economic dispatch (ED) problems. Optimization methodologies that are based on meta-heuristic procedures could assist power generation policy analysts to achieve the goal of minimizing the generation costs. In this context, the objective of this study is to present a novel approach based on harmony search (HS) algorithm for solving ED problems, aiming to provide a practical alternative for conventional methods. To demonstrate the efficiency and applicability of the proposed method and for the purposes of comparison, various types of ED problems are examined. The results of this study show that the new proposed approach is able to find more economical loads than those determined by other methods.     

Global failure criteria for positive/electrolyte/negative structure of planar solid oxide fuel cell

Due to mismatch of the coefficients of thermal expansion of various layers in the positive/electrolyte/negative (PEN) structures of solid oxide fuel cells (SOFC), thermal stresses and warpage on the PEN are unavoidable due to the temperature changes from the stress-free sintering temperature to room temperature during the PEN manufacturing process. In the meantime, additional mechanical stresses will also be created by mechanical flattening during the stack assembly process. In order to ensure the structural integrity of the cell and stack of SOFC, it is necessary to develop failure criteria for SOFC PEN structures based on the initial flaws occurred during cell sintering and stack assembly. In this paper, the global relationship between the critical energy release rate and critical curvature and maximum displacement of the warped cells caused by the temperature changes as well as mechanical flattening process is established so that possible failure of SOFC PEN structures may be predicted deterministically by the measurement of the curvature and displacement of the warped cells.     

Process simulation and optimization for enhanced biophotolytic hydrogen production from green algae using the sulfur deprivation method

This article explores the modeling, simulation and optimization of a biophotolytic cyclic process for enhanced hydrogen production from microalgae, employing the sulfur deprivation method. To achieve sulfur deprivation, each process cycle contained two temporally separated steps of sulfur-controlled algae growth and sulfur-deprived anaerobic hydrogen production.Reaction kinetics were modeled via an empirical logistic model. Reaction times, sulfate concentrations, and medium pH levels of each cycle were controlled to optimize the rate and yield of hydrogen production. Consequently, 65% and 23% improved values were obtained, respectively, with a smaller total process time (?11%), higher ratio of algae growth-to-hydrogen production time (29% vs. 21%), buffered pH (7.8), controlled sulfate injection and intermediary algae concentrations. Two- and 15-times higher hydrogen yields were obtained for 2- and 12-times lower initial algae concentrations. The proposed method is a significant tool for the design and optimization of a process for enhanced hydrogen production from microalgae.     

Conceptual design and optimization of a novel hydrogen liquefaction process based on helium expansion cycle integrating with mixed refrigerant pre-cooling

Due to the safety of operation and the development of high-efficiency helium refrigerators, the development of helium refrigeration cycles in hydrogen liquefaction is continuously promoted. To reduce the energy consumption and exergy loss of this energy-intensive process, a novel hydrogen liquefaction process integrating with mixed refrigerant (MR) pre-cooling is simulated by Aspen HYSYS and optimized by genetic algorithm (GA) to improve performance under the premise of safe production and multi-faceted analyzed based on the helium expansion refrigeration cycle. A new MR with reasonable composition and ratio is used in the pre-cooling cycle to improve the matching of heat transfer curves. Energy, exergy and economic analyses are applied to evaluate the liquefaction process, and horizontal comparison is also used to evaluate the rationality and superiority of the process design. The output of 5 t/d of liquid hydrogen (21.7 K, 1.5 bar) can be achieved and the overall specific energy consumption (SEC), exergy efficiency (EXE) and coefficient of performance (COP) are 9.703 kWh/kg _LH2, 39.1%, and 0.1333, respectively. Compared with similar processes, the proposed process shows better performance and potential development prospects.     

Techno-economic analysis of adiabatic four-stage CO2 methanation process for optimization and evaluation of power-to-gas technology

Owing to increasing demands for clean energy, caused by global warming, renewable energy sources have attracted significant attention. However, these sources can affect the reliability of electrical grids owing to their intermittency. Power-to-gas technology is expected to help address this issue. In this study, the CO₂ methanation process, which yields synthetic natural gas (SNG) via the synthesis of CO₂ and H₂ through proton exchange membrane (PEM) water electrolysis using surplus electricity generated from renewable energy, was evaluated and optimized based on techno-economic analyses. Requirements for the introduction of SNG produced through CO₂ methanation in domestic natural gas markets are presented by considering various scenarios. Results indicate that, even if the electricity costs, including system marginal price and renewable energy costs, are minimal, the costs for PEM water electrolysis and CO₂ methanation must be reduced by ~$550/kW and 25%, respectively, relative to current levels for the viable introduction of SNG in domestic markets.     

Optimization and evaluation criteria of water-gas transport performance in wave flow channel for proton exchange membrane fuel cell

Flow channel optimization is an important method to improve the water-gas transport capacity and enhance the performance of proton exchange membrane fuel cells. In this study, the size ratio of the opposite sinusoidal wave flow channel (OSWFC) is optimized, and three-dimensional numerical models are developed to evaluate the comprehensive performance. The results show that OSWFCs obtain performance enhancement compared to the parallel flow channel. 1:1 wave channel has a maximum improvement of 19.53% in mass transfer capacity by the most frequent flow disturbance, while 1:3 has a maximum increasement of 37.5% in water removal performance by the virtue of gentle wave structure. Large pressure loss decreases the mass transfer efficiency, although it with the maximum mass transfer capacity, the mass transfer evaluation criterion of 1:1 wave channel is less than that of 1:3. Moreover, a significant correlation between water-gas transport capacity and electrochemical performance is demonstrated by Pearson correlation coefficient.