فصلنامه مهندسی و مدیریت انرژی
سال پنجم شماره 4 (پیاپی 18، زمستان 1394)

The growing worldwide demand for less polluting forms of energy has led to a renewed interest in the use of micro combined heat and power (CHP) technologies in the residential sector. The operation of micro CHP system results in simultaneous production of heat and power in a single household based on small energy conversion units. The heat produced may be used for space and water heating and possibly for cooling load if combined with an absorption chiller¡ the electricity is used within the house. In this paper¡ two typical micro CHP alternatives¡ namely¡ gas engine and fuel cell for residential buildings¡ are analyzed. For each facility¡ two different operating modes including minimum-cost operation and minimum-emission operation are taken into consideration by employing a plan and evaluation model for residential micro CHP systems. The analyses of results show that the fuel cell system is recognized as a better option for the examined residential building from both economic and environmental points of view. With the operation considering optimal economic benefits¡ annual energy cost is reduced by about 26%. On the other hand¡ annual CO2 emissions are reduced by about 9%.

Power system restructuring has increased the take part distribution side of power system in providing power. Extensive influence of distribution resources for generation power in distribution side of power system and transferring power electricity between retailer and household consumers are the reason of that. Among different types of distributed resources, micro combined heat and power systems (μ-CHP) have a large acceptability for using by retail household consumers, high efficiency, using the heat output and controllable output. In this paper, costs of annual electric and heat energy providing for a household consumer has been calculated under two conditions: without using μ-CHP (purchasing electric power demand from electricity retailer and providing heat demand by boiler) and using μ-CHP, then the two types compared. The output of μ-CHP considering two types of controllers, modified heat led controller and price-based controller for showing the effect of controlling system in operating μ-CHP has been determined to reduce the costs of electric energy providing for a household consumer.

The ever raising request for energy and resource limitations in the world, particularly Iran, have ever increasing attention to intense research in optimization, and economization of energy consumption. Since, the application of traditional methods for production of sugar especially in Iran is associated with high consumption of energy and water, using of pulsed electric field (PEF) technology as a non-thermal and eco-friendly process is considered. In this paper, the effect of pulsed electric field (7 kV.cm-1, 100 pulses) on mass transfer (conductivity and brix), energy consumption and extraction efficiency were investigated and compared with aids of thermal treatment. The results of the statistical analysis showed a significant decrease in temperature, time and energy consumption by PEF treatment compared to thermal treatment with the same extraction. Furthermore, the rate of mass transfer from thermal treatment because of relatively complete destruction of tissue cells was higher than PEF.

In this article a CCHP system has been proposed for recovering waste heat of PEM fuel cell and this system can be used for residential applications. Main components of the system are PEM fuel cell, absorption chiller, fuel processor and heat storage tank. Electrochemical equations of PEM have been modeled by MATLAB and the chiller, fuel processor and boiler have been modeled by EES. In addition, the energy and exergy performances and emission reduction of the proposed system have been analyzed; also effect of operating temperature on performance and irreversibility of the system have been studied. Performance of the system has been compared when the system works as a power system or a CHP and a CCHP system. Results demonstrate that the refrigeration capacity of absorption chiller is 12.24 kW, output power of fuel cell 38.63 kW, heat generation of PEM fuel cell 39.12 kW and irreversibility losses of PEM-FC is 84.21 kW. Finally when the system works as power system its efficiency is 32.6%, and in CHP and CCHP modes the efficiencies are 66.1% and 75.8 % respectively. Also by increasing operation current density of fuel cell, exergy efficiency of CCHP system decreases but COP of absorption chiller increases.

In this paper, configuration of ‘Vestas_V47’ wind turbines in a wind farm is investigated using computational fluid dynamics (CFD). First the geometry of wind turbine has been modeled, then unstructured mesh around the blades, nacelle and tower have been generated and finally the SST k-ω model has been used for modeling the turbulence. Afterward wind’s velocity contour for a wind farm with two turbines extracted and characteristics of downstream wake has been calculated with different distances between turbines. Velocity profiles have been extracted by assuming distances 5D and 7.5 D between two turbines. As a result, maximum reduction in velocity has been occurred at nacelle height and in 8D and 12D respectively. According to the calculation and comparison maximum efficiency of wind farm when d=5D and v=12.5 m/s is 79.029% and with d=7.5 D and v=15m/s is 78.525%.

Flat reflectors which have lower price compared with parabolic ones are used in Fresnel concentrator. Each Fresnel collector is comprised of a few flat reflectors which are placed parallel to each other near the ground. If effective width of flat reflectors is more than absorber tube diameter, reflected rays will be partly lost, hence low optical efficiency. By installing a secondary reflector above the absorber tube, lost rays are mostly redirected towards the absorber tube. This leads to a more uniform flux and a higher optical efficiency. In this research, effects of three geometries for the secondary reflector are investigated and the results are presented. One of these designs produces very uniform flux on the absorber tube whereas the second design leads to higher optical efficiency. A computer program has been employed to evaluate these effects.