فصلنامه مهندسی و مدیریت انرژی
سال دوم شماره 4 (پیاپی 6، زمستان 1391)

This paper presents robust controller design for a wind-driven induction generator system using structured singular value (-synthesis) method. The controller was designed for a static synchronous compensator (STATCOM) and a variable blade pitch angle in a wind energy conversion system (WECS) in order to achieve the required voltage and mechanical power control. The results indicated that this controller offers satisfactory damping characteristics for closed loop systems. Effects of various system disturbances on dynamic performance of the WECS were evaluated in this study by a time domain nonlinear simulation and compared with the output feedback controller. The proposed designed controller showed to be more effective in regulating the load voltage and stabilizing the generator rotating speed for WECS.

In this paper the daily، monthly، seasonally، and yearly optimum slope angles of solar collectors are determined for areas in Iran and new models are developed to calculate the monthly، seasonally، and yearly optimum slope angles for latitudes of to north. To achieve this purpose، the slope and surface azimuth angles of solar collectors for receiving maximum solar radiation were determined in some Iranian cities in different days، months، seasons، and the whole year employing different models. According to the optimum slope angles predicted in this paper and using the optimum slope angles achieved by other researchers at locations out of Iran but in the same range of latitudes، the models are obtained. The outcome of this research is that the optimum slope angle of flat solar collectors has a linear relationship with the latitude of the site.

In this article، the first and second law analysis of diesel engine based cogeneration system was performed. Fuel utilization efficiency، rate of power، and rate of process heat of the plant were determined and various efficiencies based on both energy and exergy methods and the performance assessment parameters are defined for both the system components and the entire cogeneration plant. The objectives of this study are to calculate the exergy destruction within the plant and exergy loss to the environment and apply the performance assessment parameters to the components and entire cogeneration plant. The system under survey in this work was a diesel engine based cogeneration system that produces 250 kW of electricity and increases the temperature of water from 80 ºC to 120 ºC at 2 bar and 1. 75 kg/s. The analysis showed that the least efficient component in the system is diesel engine with the exergetic efficiency of 39. 31%. Also، fuel utilization efficiency of the overall plant was found to be 90. 47% and the exergetic efficiency was 51. 08%.

Making potable water through desalination plants is a very important process in Iran where clean water is highly required. On the other hand، large amount of fossil fuel sources leads to the development of gas turbine power plants all over the country. Furthermore، Persian Gulf in the south and Caspian Sea in the north could be the main sources for supplying potable water in water scarcity areas، especially in the south. Dual-purpose plants are the ones that supply heat for a thermal desalination unit and produce electricity for distribution to the electrical grid. In this paper a gas turbine power plant was combined with a multi stage flash desalination system. Then، energy and exergy analysis for desalination plant، power generation cycle، heat recovery steam generator (HRSG)، and combined power and water cycle were developed. Results showed that by increasing the number of desalination effect، performance ratio، exergetic efficiency، and specific heat transfer area steadily increase. Additionally، the sensitivity analysis showed the relationship between the parameters that are not known precisely or difficult to foresee such as steam pressure of HRSG and compression ratio on the performance parameters of dual purpose plant.

This paper deals with a lumped-parameter dynamic simulation of a single-effect LiBr-H2O absorption chiller. In many studies the thermodynamic properties of LiBr-H2O solution were taken from some approximate relations causing the results to be somewhat inaccurate. These relations were used to solve simultaneous differential equations involving the continuity of species constituting the LiBr-H2O solution، momentum equations، and energy balances. To diminish the effect of these approximate relations on the results، in this study the thermodynamic properties were taken from the EES software. By making a link between EES and MATLAB softwares، the simultaneous differential equations were solved in MATLAB environment and this process was continued until the convergence criterion was satisfied. Moreover، this study considers the effect of quality on the concentration of solution at the exits of generator and absorber. This effect was ignored in the previous works. In other words، the concentrations of solution at the generator and absorber were not assumed to be equal to the corresponding concentration at the exit of those components in this model. Furthermore، a transient analysis of exergy is accomplished. As time passes، both the coefficient of performance and exergetic efficiency decrease، approaching the steady-state values. The results deduced from the dynamic simulation are compared with those in steady-state condition. The comparison shows that the transient simulation predictions are in close agreement with steady-state results.

A non-premixed coaxial oxy-fuel turbulent flame was studied numerically with standard and realizable k-ε turbulence model and a comparison was made between them. The governing equations were solved by finite volume approach and were discretized using the second order upwind scheme. The presumed β-PDF model was applied to model turbulence-combustion interaction. The discrete ordinate radiation heat transfer method was also used. Comparison of numerical and experimental data showed that the realizable model has a better prediction of the axial velocity and NO concentration than that of the standard model.

The natural convection in differentially heated rectangular cavities with a fin attached to the cold wall was investigated numerically. The top and the bottom horizontal walls of the cavities were insulated while their left and the right vertical walls were maintained at a constant temperature Th and Tc، respectively with Th > Tc. The governing equations written in terms of the primitive variables were solved numerically using the finite volume method while the velocity and pressure fields were coupled using the SIMPLER algorithm. Using the developed code effects of pertinent parameters such as length and location of fin، aspect ratio of the enclosure، Rayleigh number، and Prandtl number on heat transfer and fluid flow in the enclosure were investigated. The results showed that for the cavity filled with water، at high Rayleigh numbers، a longer fin placing at the middle of the right wall has more remarkable effect on the heat transfer inside the cavity. Also، it was observed that at low Rayleigh numbers، the effect of fin on heat transfer enhancement in low Prandtl numbers is more than that in high Prandtl numbers. Moreover، it was found that the fin has more effect for narrow enclosures.