فهرست مطالب

Energy Equipment and Systems - Volume:6 Issue:3, 2018
  • Volume:6 Issue:3, 2018
  • تاریخ انتشار: 1397/06/15
  • تعداد عناوین: 8
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  • Rahim Moltames *, Mehrdad Boroushaki Pages 235-246
    In this paper, a simple algorithm based on a two-diode circuit model of the solar cell is proposed for calculating different parameters of PV panels. The input parameters required for this algorithm are available from datasheets of the standard PV modules. The values of series and parallel resistances, as well as the recombination factor of Diode 2, are estimated through an iterative solution process. This method is based on maximum power point matching (MPPM) in which the maximum power of PV panel is calculated by the model reaching a minimum error from maximum power proposed in the datasheet. Unlike the other methods, this method is very straightforward and does not require any additional information apart from that of the datasheet. The objective of this paper is to calculate the recombination factor of both diodes in a two-diode PV model, which then leads to further accuracy of the PV model. This novelty in the calculations further improves the accuracy of the model. The simulation is performed in MATLAB, and the effect of altering the temperature of PV cell and level of radiation on the current, voltage, and output power of the PV panel is investigated. The accuracy of the simulation is validated by data extracted from the datasheets of two different PV modules (polycrystalline and monocrystalline).
    Keywords: Solar Energy, Photovoltaic, PV Module, Two-Diode Circuit Model
  • Seyed Morteza Mousavi, Seyed Mohammadali Rahnama * Pages 247-259
    Numerical simulations of a three-dimensional model of impingement and film cooling on a flat plate are presented and validated with the available experimental data. Four different turbulence models were utilized for simulation, in which SST had the highest precision, resulting in less than 4% maximum error in temperature estimation. A simplified geometry with periodic boundary conditions is designed, based on the main geometry, and is used for the optimization procedure. Six geometrical parameters related to impingement and film holes are selected as design variables. To further reduce the time required for optimization, a feedforward neural network is implemented for the function estimation, and 584 CFD observations were performed for randomly generated design points. The data from CFD simulations were fed to network for training and test operations, and the results with good consistency were extracted from the network. The objective of the optimization is to minimize the coolant mass flow rate, subject to maximum temperature and maximum temperature gradient in solid domain being equal to or lower than their values in base design. A genetic algorithm (GA) with 100 population and 50 iterations, coupled with an artificial neural network (ANN), was used for optimization. Finally, the optimum design is simulated numerically to find the exact values of the output parameters. The CFD results for optimum design shows 44% less coolant mass flow rate while both optimization constraints are satisfied. Such a reduction in the coolant flow rate has a huge impact on the overall performance of a typical gas turbine, which is discussed in this paper.
    Keywords: CFD, Impingement Cooling, Film Cooling, Gas Turbine Cooling, Optimization
  • Maryam Moein Addini, Abdolreza Gandjalikhan Nassab * Pages 261-277
    This paper presents a numerical investigation of the laminar mixed convection flow of a radiating gas in an inclined lid-driven cavity. The fluid is treated as a gray, absorbing, emitting, and scattering medium. The governing differential equations including continuity, momentum and energy are solved numerically by the computational fluid dynamics techniques (CFD) to obtain the velocity and temperature fields. The discretized forms of these equations are obtained by the finite volume method and solved by using the SIMPLE algorithm. Since the gas is considered as a radiating medium, besides convection and conduction heat transfer, radiation also takes place in the gas flow. For computing the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinate method (DOM). The streamline and isotherm plots and the distributions of convective, radiative and total Nusselt numbers along the bottom wall of the cavity are presented. In this work, an attempt is made to investigate the hydrodynamic and thermal behavior of the mixed convection flow of a radiating gas at different values of the cavity inclination angle. The numerical results reveal that the variation of inclination angle causes a sweep behavior in the flow pattern inside the cavity. Besides, it is found that the value of radiative Nusselt number along the heated wall has a decreasing trend when the medium optical thickness is increases. Comparisons between the present numerical results with those obtained by other investigators in the cases of conduction-radiation and pure convection systems show good consistencies.
    Keywords: Laminar Mixed Convection Flow, Lid-Driven Cavity, Inclination Angle, Radiation, DOM
  • Ali Behbahani Nia *, Sadaf Nomani, Alireza Latifi Pages 279-292
    In this paper, an economic study of solar heat pumps and an investigation of differences between solar heat pumps and conventional heat pumps—based on their performances and energy consumption—are conducted for a residential apartment located in Austin, Texas, USA. Heating in the apartment is provided via a solar heat pump during the cold months of a year. Solar collectors are used to meet the domestic hot water requirement of the apartment during the other months. In order to carry out a rational comparison, the base system comes with an air-to-air domestic heat pump and a boiler to provide domestic hot water. The comparison includes both energy consumption (fuel and power) and economic aspects. The simulation is performed through the commercial software TRNSYS, in which optimization offers a collectors’ surface of 28 square meters. The proposed configuration requires almost half of the electrical energy that the conventional system consumes. Its fuel consumption is about a quarter of the non-solar system, which offers a double value of COP in comparison with the traditional system. The economic calculations reveal that the payback period is about two years.
    Keywords: Solar Heat Pump, Solar Heating, TRNSYS, Economic Analysis, Optimization
  • Milad Shakeri Bonab, Abolfazl Anarjani Khosroshahi, Mehdi Ashjaee, Seyed Farshid Chini * Pages 293-303
    The effect of utilizing superhydrophobic walls on improving the convective heat transfer in a rectangular heatsink has been studied numerically in this paper. The vertical walls were kept at isothermal hot-and-cold temperatures and horizontal walls were insulated. The boundary condition on the walls was: no-slip for regular, and slip (with slip length of 500 µm) for superhydrophobic walls. By changing the heatsink aspect ratio (AR, height/width) from 0.1 to 10, it was observed that regardless of the wall slip, the optimum AR is 1, i.e. square enclosure. For a square heatsink, using the nanofluid with = 3% could enhance the heat transfer (quantified by Nusselt number) by up to 9.8%. For the same enclosure filled with pure water, applying superhydrophobic horizontal walls could increase the heat transfer by 4.45%. The joint effect of using superhydrophobic walls and nano-particles enhanced the heat transfer by up to 14.75%. The results of this paper may open a new avenue for high performance cooling systems.
    Keywords: Natural Convection, Heatsink, Local Cooling, Nanofluid, Superhydrophobic, Slip Length
  • Marzieh Heidari, Afshin Ahmadi Nadooshan *, Morteza Bayareh Pages 305-315
    The design of a building can provide the highest thermal comfort in the interior without any mechanical equipment and save energy to a large extent. The roof of a building is an important part for thermal loss. This research studies the thermal performance of 14 conventional roof structures in Tehran city by using designbuilder 4.5. It is found that the polystyrene block performs best compared to other structures. Despite the time and cost required to implement the beam for building roofs, the use of the polystyrene block is recommended. The results indicate that the use of 5 cm of thermal insulation in the structure of the roof results in 5.85% decrease in heat loss during winter and 5.65% decrease during summer. A reverse roof has a more favorable performance on hot days of the year. Also, the performance of roofing with heat insulation is better than that of the reverse roof in cold days.
    Keywords: Thermal Performance, Building Roof, City of Tehran, DesignBuilder, Block Joint
  • Vahid Beygzadeh*, Shahram Khalil Arya, Iraj Mirzaee, Gholamreza Miri Pages 317-327
    This paper reports a plenary thermodynamic model of a novel solar system for water heating in buildings. Energy and exergy analyses are used to characterize the exergy destruction rate in any component and calculate system overall efficiency. The system consists of a solar evaporator, a heat exchanger to produce hot water, and an auxiliary pump. A computer simulation program using EES software is developed to model the solar water heating system. The system provides hot water during the hours of low sun radiation. Thermodynamic analysis involves the designation of effects of heat exchanger pinch point and ambient temperatures on the energetic and exergetic performance of the solar water heating system. The performance parameters computed are exergy destruction, and energetic and exergetic efficiencies. The result showed that the main source of exergy destruction is the solar evaporator. In the solar evaporator, 92.85% of the input exergy was destroyed. The other main source of exergy destruction is the heat exchanger, at 4.15%. The overall energetic and exergetic efficiencies of the solar water heating system were approximately 60.17% and 3.002% respectively.
    Keywords: Energy Efficiency, Exergy Efficiency, SLHPS, Solar Water Heating System, Pinch Point Temperature
  • Mojtaba Haratian, Majid Amidpour *, Aliasghar Hamidi Pages 329-336
    In this study, the relationship among the concepts of entropy generation rate, entransy theory, and generalized thermal resistance to the optimal design of thermal systems is discussed. The equations of entropy and entransy rates are compared and their implications for optimization of conductive heat transfer are analyzed. The theoretical analyses show that based on entropy generation minimization principle by decreasing irreversibility, thermodynamic optimization can be obtained. Significantly, the entransy concept merely describes the heat transfer ability and the minimum and maximum entransy dissipation principle can only lead to thermal optimization. However, due to decreasing thermal resistance both principles are considered as optimization tools for the optimal design of energy and thermal systems. Also, it is shown that the concept of entransy theory is more suitable than the concept of entropy generation for optimizing the performance of heat transfer processes.
    Keywords: Entropy Generation, Entransy Theory, Thermal Resistance, Optimization