فصلنامه مهندسی و مدیریت انرژی
سال چهارم شماره 3 (پیاپی 13، پاییز 1393)

This paper investigate the properties of the circulating current and load sharing in parallel microgrid inverters. The existing methods cannot minimize efficient circulating current for the case of nonlinear load. The one-cycle control (OCC) technique and virtual complex impedance loop are included in the proposed controller in order to proper load sharing and reduce the circulating current due to impedance mismatch. Simulation results are presented to prove the improvement achieved by the new controller. Compared to the pulse width modulation (PWM)، the proposed method، under different system impedances، reduces more effectively the voltage total harmonic distortion (THD) and offers advantages such as: constant switching frequency، simple circuitry، improved dynamic behavior of the control system during load change and start up.

In this study، heat transfer coefficients are measured for diesel-water two-phase flow in a horizontal pipe for dual continuous، dispersion of oil in water and dispersion of water in oil flow patterns. Flow patterns are observed using high speed photography for diesel-water mixture and two phase heat transfer data are obtained under a uniform heat flux boundary condition. The test section is a 1. 1 mm ID pipe with a 1. 8 m length. The Reynolds number is distributed from about 350 to 3،700 for diesel and from about 1،300 to 13،700 for water. The results show that the heat transfer coefficient of dual continuous pattern is higher than that of dispersion of water in oil pattern.

In this paper، the performance of arefrigeration cycle with double stage ejector is studied. Also the effect of some working fluids، the ejector geometry including the diameter change in the part of constant diameter at the first and second stages and operating parameters on the entrainment ratio of the first and second stages in the ejector and coefficient of performance cycle (COP) are studied. A 2D model of the ejector is considered andthe continuity، momentum، energy، and state equations are numerically solved. The thermodynamic analysis is carried out to study the performance of the double stage ejector. The results show that using R245faand water as the working fluids in the cycle leads to the lowest and highest coefficients of performance (0. 799 and 1. 808 at TG=120oC and TC=12oC) respectively in comparison with other refrigerants. Increasing diameter of the mixing chamber in the second stage causes decrement in the COP at a constant diameter in the mixing chamber of the first stage. Also، increasing the diameter in the mixing chamber of the first stage causesincrease in the COP at a constant diameter in the mixing chamber of the second stage.

Iran has an average of 5. 5 KWh per square meter solar radiation and 300 sunny days per year on 90% of the land. Regarding this amount of solar radiation and the necessity for solar potential zoning for better efficiencies، drawing solar potential maps is essential. In this study، the monthly data of 39 synoptic of Iran meteorological stations over years (1991-2000) has been used as the input data to the MATLAB software and artificial neural network (ANN). In the ANN، a multi-layer feed forward model is used. After applying the input data to the network with desired architecture، in output layer the solar radiation is predicted. The solar radiation anticipated by ANN is highly in accordance with meteorological data so that the final correlation coefficient is 0. 96، depicting the great accuracy of the data derived from the software. By selecting the predicted data of ANN as input to ArcGIS software، the annual solar potential map of Iran is obtained.

Shell and tube heat exchanger، singular baffles have been stimulated by means of the computational fluid dynamic technique and Fluent software. Several elliptical cross sections for the tubes are examined. So each model drawn by using Gambit software. In stimulating، 3D meshes is used for irregular Couper and tetrahedral networks. Also the performance of each of two shell and tube exchanger is compared with، one of the optimized set of elliptical tubes، and the other with circular tubes according to heat transfer coefficient and pressure drop. The results accuracy is compared with Bell’s theory plus using an engineering equation solver software. The main objective at this stage is finding the best geometry for elliptical tubes used in the shell and tube exchangers as a new idea among the heat exchangers for future use. The results indicate the increase in the heat transfer rate to 59. 9 percent، and the rise in pressure to 9. 5 percent which is negligible in comparison with a boost in heat transfer.