Hamidreza Najafi

Sensitivity analysis of a hybrid photovoltaic thermal solar collector

In the present paper, a hybrid photovoltaic thermal (PVT) solar collector is considered. Due to the high complexity and non-linearity of the equations, investigation of the effect of each parameter on the performance of the system is an important task. In this work, the effects of four types of effective parameters including the climate conditions, the mass flow rate of the coolant water, the inlet water temperature and geometric variables are investigated. For investigating the effect of climate conditions on the PVT system, three cities in Iran with different climates including Tehran, Hamedan and Kerman are considered and the performance of the system during sunny hours in each city is presented. The considered geometric variables include PVT thickness, tube spacing and collectors width. Results are shown by several graphs and discussed extensively.

Developing an Inverse Heat Conduction Analysis Tool for Real Time Heat Flux Estimation in Directional Flame Thermometer Application

Accurate measurement of heat flux and temperature can significantly affect the energy usage in several industrial applications, including furnace operation, metal processing, fire safety tests and more. Directional Flame Thermometers, or DFTs, offer the ability to use both temperature and heat flux measurements for furnace control. Currently, analysis of dynamic temperature data from the DFTs to compute heat flux information must be performed off-line at the conclusion of data-gathering by using software tools such as IHCP1D. Availability of a near real-time algorithm for accurate reduction of the data will allow for continual monitoring of the furnace during operation. This will result in better control over the process and significant savings in energy and cost. In this paper, a filter form of the inverse heat conduction algorithm is developed for utilization in DFTs. The algorithm is based on linearized solutions of the direct heat equation, and non-linear effects introduced by temperature dependent thermal properties are accounted for by interpolating of the resulting filter coefficients. The developed method is tested through several numerical experiments and also ANSYS model. A graphical user interface is developed in LabVIEW to provide a friendly interface for the end user. The temperature data measured by thermocouples on the DFT are transmitted to the computer through data acquisition card and the developed tool in LabView display the heat flux in a near real time fashion.© 2014 ASME

Optimal design of gas turbine-solid oxide fuel cell hybrid plant

In the present study, a hybrid solid oxide fuel cell-gas turbine power plant consisting of a compressor, SOFC stack, heat exchangers, combustor and turbines is considered. Individual models are developed for each component through applications of the first law of thermodynamics and the corresponding cost of each component is also presented. Two objective functions including the total thermal efficiency of the system and the capital cost of the plant are defined. Since any effort to decrease the total cost of the plant leads to a less efficient system, the considered objective functions are conflicting. Therefore, multi-objective optimization using genetic algorithm is utilized in order to achieve a set of optimal solutions, each of which is a trade-off between objective functions. The main advantage of this work is providing a wide range of optimal results each of which can be selected by the designer considering available investment and the required efficiency of the system.

Multi-objective optimization of a fire-tube heat recovery steam generator system

In the present paper, a fire-tube heat recovery steam generator (HRSG) system (including evaporator, superheater and economizer) have been considered and multi-objective optimization using genetic algorithm is applied in order to obtain optimum design parameters which yield the maximum Second law efficiency and the least capital cost. Geometric variables including number of tubes in a row, tube length and diameter, transverse pitch of each section and pinch point difference as the thermodynamic variable are considered as optimization parameters. The study was performed via MATLAB to develop model equations and implementing optimization.

Photovoltaic-Thermoelectric Systems for Building Cooling Applications: A Preliminary Study

Thermal comfort in buildings using energy efficient methods has attracted lots of attentions during last few decades. In the present paper, the feasibility of using thermoelectric (TE) modules to provide cooling for a residential building located in Melbourne, FL is studied. It is assumed that photovoltaic (PV) panels are used to provide necessary power for TE operation. The building roof is covered with PV panels and an air channel is installed on the bottom of the panels. The TE modules are located inside the air channel and are attached to a metal sheet from the bottom which constructs the ceiling of the building. The metal sheet on the ceiling cools down in summer to provide space cooling through natural convection and radiation. A mathematical model is developed to simulate the system in MATLAB. The simulation results include transient temperature values through each layer of the system and heat removal/rejection rates from thermoelectric modules. A specific temperature is defined for thermal comfort and the number of TE modules that is required to maintain the temperature below the limit is determined. The results suggest that TE modules can be successfully used for building cooling applications. Further investigation and experiments are required for a comprehensive design of the system.Copyright

Modeling and Fuzzy Control of a Crude Oil Preheating Furnace

This paper presents the development of mathematical model and designing a temperature control system for an industrial preheating furnace. In the first part of the paper, the simulation model was developed based on thermodynamics principles, energy-mass balance and semi-empirical relations. The parameters of developed models were defined with respect to available operational and geometrical data from real system. In the second part, an appropriate control system was designed for regulating the preheating furnace temperature. A fuzzy logic controller and a feedback/feedforward controller were employed for operating in coordination with each other to maintain the process outlet temperature around 360 oC. Simulation results show the capability of the designed control system to regulate the furnace outlet temperature at different operating conditions and in the presence of disturbances.

Feasibility Study of Using Thermoelectric Cooling Modules for Active Cooling of Photovoltaic Panels

Temperature increment is one of the main challenges for solar concentrating photovoltaic (CPV) systems which cause cell degradation and significant efficiency loss. To overcome this issue, a novel cooling method by using Peltier effect is proposed and investigated. In this approach, thermoelectric cooling (TEC) modules are considered to be installed on the back side of the photovoltaic (PV) module. The required power to run the TEC module is provided by the PV panel itself. A comprehensive model is developed and simulated via MATLAB in order to determine the values of temperatures in different sections of the system and calculate the required power to run the TEC module and the extra generated power by PV panels due to the cooling effect. The result shows that using TEC modules can successfully keep the PV cell temperature within the desired temperature range during a hot day when limited temperature reduction is needed.Copyright

Application of Artificial Neural Network as Digital Filter for Online Heat Flux Estimation

Online heat flux measurement can greatly enhance the controllability in several industrial processes. Using heat flux estimation techniques based on temperature measurements is the best approach in many cases. Estimating the unknown heat flux (boundary condition) at the surface when temperature measurements are available in the interior points of the medium is an inverse heat conduction problem (IHCP). Several IHCP solution methods need the whole time domain data for the analysis and cannot be applied for real-time applications. Digital filter representation is one of the methods which can be used for near real-time heat flux estimation by using available temperature measurements. The idea of the filter algorithm is that the solution for the heat flux at any time is only affected by the recent temperature history and a few future time steps. Artificial Neural Network (ANN) is utilized in this study as a digital filter, for near real-time heat flux estimation by using temperature measurements. The performance of the ANN is compared with the digital filter coefficient method. ANN consists of a set of interconnected neurons that can evaluate outputs from inputs by feeding information through the network and adjusting the weights. Considering temperatures as the inputs and heat flux as the output, the weights can be interpreted as the filter coefficients. In using ANN, calculation of sensitivity coefficients is not needed which can lead to less computational cost. It is showed that the ANN method can estimate the heat flux closer to real-time comparing with digital filter approach. The developed method is tested through several numerical test cases using exact solutions.Copyright

Performance Analysis of Photovoltaic Panels Using Tracking, Concentration and Cooling in Alabama Area

Using photovoltaic panels in the southeast Unites States has been judged less attractive than in other parts of the country owing to the humid climate and numerous cloudy days. Nowadays, use of lenses or mirrors to concentrate sunlight on PV cells can multiply the direct incident sunlight on the cells leading to higher power output and better conversion efficiency. In the present paper, the annual performance of a photovoltaic panel located in Alabama is investigated. A computer simulation via MATLAB is carried out by using typical meteorological data for Alabama in order to accurately model the behavior of the system. The effect of using fixed optimal tilt and single-or double-axis tracking methods on incident solar irradiation on the PV panel is presented. The power generated by the system under different levels of concentration during the year is discussed and the effect of cooling is also investigated. Comparison graphs are presented in order to demonstrate the effect of tracking, concentration and cooling on the output power and conversion efficiency of the PV panel. The result of this paper can be utilized to select the most appropriate system regarding the application and available funding of the project in Alabama area.Copyright

Modeling and Analysis of a Combined Photovoltaic-Thermoelectric Power Generation System

In the present paper, the possibility of using thermoelectric power generator modules (TEGs) to convert the heat generated by the photovoltaic/thermal (PVT) collector into electricity is investigated. A comprehensive heat transfer model for the combined PVT-TEG system is developed via MATLAB and simulated under different conditions. The hot side of the TEG module is considered connected to the top of the air channel which is attached to the backside of the solar panel and the cold side is cooled down by air flow through the air channel under the PV panel. The TEG modules convert the temperature gradient to electricity and generates extra power from the excess heat which results in higher total efficiency. The effect of using combined PVT-TEG on total generated power under different levels of irradiation is presented and discussed.Copyright