فصلنامه مهندسی و مدیریت انرژی
سال سوم شماره 1 (پیاپی 7، بهار 1392)

Economic dispatch (ED) is one of the most important issues in power system operation. The main objective of ED is to determine each power plant generation in order to minimize the total operation cost while satisfying demand and generation constraints. In recent years، due to increasing environmental concerns and regarding to the role of pollution produced by fossil-fuelled power plants، environmental economic dispatch (EED) problem has been introduced، that minimizes operation cost and emission of power plants، simultaneously. In this paper، the economic and emission objectives are combined using pollution penalty factor and the problem is modeled as a single objective optimization problem. To solve this problem، an accurate and fast meta-heuristic optimization algorithm called modified shuffled frog leaping is employed and applied. To demonstrate the efficiency of the proposed algorithm، it is tested on 6 and 11 unit test systems and the obtained results are compared with those of other existing methods. The results show the superiority of the proposed approach.

The goal of this article is to present electrochemical and economic models for a tubular solid oxide fuel cell stack. To carry out this research، first، a complete electrochemical model has been employed for a single cell of the fuel cell stack، and then its activation، ohmic and concentration losses have been obtained. The results show that with the increase of the working temperature and pressure of the fuel cell، the overall voltage loss diminishes and the cell’s performance improves. On the other hand، the rise of the cell’s working temperature leads to the increase of electrical efficiency and the reduction of thermal efficiency in the cell; while the increase of the cell’s working pressure doesn’t have much effect on its electrical efficiency. In this research، for the tubular solid oxide fuel cell stack at the working temperature of 1273 K، an optimal fuel utilization coefficient of about 0. 8-0. 85 and an inflow air-to-fuel ratio of 8. 0 have been obtained. The results of economic analyses indicate that the rise in the cell’s working temperature causes the power generation capacity of the cell to increase، which reduces the price of the generated electricity as well as the costs associated with the purchase، installation and startup of the system. At the working temperature of 1273 K، the price of the generated electricity and the cost of a power generation unit with fuel cell generator have been estimated as 14 cents per kW and 1900 dollars per kW، respectively.

The application of heat sources as an alternative to the electric energy has been an important issue in the refrigeration technology. Adsorption chillers are capable of employing heat sources in wide-range temperature and recovering low-grade waste energy. The adsorption cycle is working on the basis of adsorption and desorption processes of a refrigerant in porous media. In this study، a numerical model is developed to investigate the effect of heating water temperature on the performance of silica gel/water adsorption chiller. The simulation consists of solving heat transfer equations for fins، metal tube and thermal fluid along with the heat and mass transfer equations for an adsorbent bed simultaneously. The numerical results show that the cycle time reduces with the increase in heating water temperature. Coefficient of performance increases sharply with the increase in heating water temperature at temperature of 50-70 ºC and reach to 0. 74 for temperature of 100 ºC while the specific cooling power increases almost linearly with the increase in heating water temperature.

In this study، exergy analysis of a flat plat solar collector in a closed circuit has been performed using existing experimental data and the effects of utilization of the collector with upper، lower and side reflectors alone and together with lenses on exergy efficiency have been considered. Then، based on a mathematical model، optimization analysis has been performed. Unlike the past studies، the collector overall heat loss coefficient is not constant and the fluid inlet temperature to the collector is not the same as the ambient temperature; instead the experimental results are used. The optimization analysis has been undertaken by studying the effects of the fluid inlet temperature and flow rate، ambient temperature، optical efficiency، solar radiation flux and collector area. The results show that exergy efficiency increases always with increasing solar radiation flux as well as optical efficiency، but decreases with increased ambient temperature. For every condition there are an optimum flow rate and inlet temperature for which the exergy efficiency becomes optimum. Utilization of the collector with reflectors and reflectors-lenses improves the collector exergy efficiency، but using reflectors alone is better.

The Smagorinsky and One-Equation sub-grid scales (SGS) for modeling of large scale pool fire using Large Eddy Simulation (LES) has been used and compared. The fire heat release rate was 2. 07 MWatt and the fuel was Methane. Based on the mixture fraction approach and infinity fast chemistry assumption، the mean velocity profile and turbulence kinetic energy are calculated and compared against experimental measurements. The predicted time-averaged velocity is in good agreement with the experimental data. The computed velocity time history is better predicted using One-Equation SGS model than Smagorinsky SGS model at about 5 percent. The computational time of One-Equation is about 16 percent higher than Smagorinsky for the test case. The difference between time-averaged vertical and horizontal velocity are about 4 and 5 percent. In the area where the pulsating behaviour of the fire is important، the One-Equation model is recommended. For the one which averaged values are dominated، Smagorinsky SGS recommend reasonable results.

The behavior of the flow between two coaxial conical cylinders with the inner one rotating and the outer one stationary is studied numerically. The angular velocity of the inner cone cylinder was raised step by step from the rest until reaching its final speed. In this work we first present a numerical simulation of the flow characteristics and the heat transfer mechanism of a fluid in the space between the two incomplete coaxial rotating cones. The problem is studied when the heated inner wall which rotates around the common axis with constant angular velocity and outer wall is at rest in the constant temperature، endplates are assumed adiabatic. The unsteadiness and the rotational speeds of such flows make them highly turbulent and thus challenging to predict by numerical simulations. Computational Fluid Dynamics (CFD) provides detailed insight for these types of flows، at a cheaper cost than experimental studies، thus allowing engineers to quickly improve the design of rotating machines. Governing equation and numerical simulation of the problem are carried out by using the commercial CFD code and the effects of the different parameters on the heat transfer and flow are considered. This condition such as local and average radial Nusselt number is computed with axial and rotational Reynolds numbers in low Prandtl number (Pr). The axial Reynolds numbers is 30-1200 and rotational Reynolds numbers are 30-2922.