Thermal Engineering

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Browsing Thermal Engineering by Author "Abebayehu, Assefa (PhD)"

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    Experimental Study of Biomass Gasification in a Downdraft Gasifier Using Different Feedstocks
    (Addis Ababa University, 2012-02-19) Henok, Mekonnen; Abebayehu, Assefa (PhD)
    The primary interest of this research work is to study the gasification properties of different biomass feedstock materials which are easily available in Ethiopia. An overview of biomass is discussed in this paper so as to have a clue what chemical composition does a biomass material can have and its implication of energy conversion. The different types of conversion processes are also covered in order to show the possible ways that energy can be extracted from biomass and to compare their merits and demerits relative to gasification. A precise theoretical overview of gasification is dealt which has laid a very starting ground for what is done in the simulation and experimental works. Experimental investigation is an irreplaceable tool to get a very meaningful finding in this research even though simulation can give a good guidance and a pre insight. Simulation work alone can be misleading since many simplifying assumptions were taken in order to make it possible to simulate the very complicated thermo-chemical process inside the gasifier and also the still unclearly phenomenon of tar formation. Both simulation and experimental work are undertaken in this research. A Matlab program is used to simulate the effects of moisture content and equivalence ratio on the constituent gas composition of the produced gas. An algorithm based on a thermodynamic model with certain simplifying assumptions is used so that the Matlab program will not miss the physical outlook of the modeled gasification process. Tables and figures are generated which are suitable to see the effects of different parameters on the final output of the gasification process. An experimental investigation is undertaken using different feedstock materials. The experiments are being carried out for some selected biomass materials which are charcoal, sawdust, coffee husk and a mixture of sawdust and coffee husk. The effects of moisture content and equivalence ratio on the final syngas composition are studied. The amount and nature of tar produced while gasifying the different feedstock is also examined. A comparison of the simulation and experimental works is done and it was reasoned out what effects the deviation of the experimental result from that of the simulation work. It was also attempted to compare the simulation and experimental results with those in literatures. At the end recommendations and suggestion of future works are presented concerning the work of this research.
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    Modeling Simulation and Performance Evaluation of Parabolic Dish Solar Power Plant
    (Addis Ababa University, 2011-01) Aklilu, Tesfaye; Abebayehu, Assefa (PhD)
    A solar parabolic dish electric power generation system is one option for a high temperature solar concentrator that is capable to achieve a high system performance. This results from the fact that it combines an excellent concentrator, a very efficient cavity receiver and a high performance heat engine. The Stirling dish system produces electricity using concentrated solar thermal energy to drive a Stirling engine. The system utilizes a parabolic mirror equipped with dual-axis tracking to concentrate solar radiation onto a thermal receiver integrated in the Stirling engine. The receiver consists of a heat exchanger designed to transfer the absorbed solar energy to the working fluid, typically, hydrogen. The Stirling engine then converts the absorbed thermal energy to mechanical power by expanding the gas in a piston-cylinder in a manner similar to a gas or diesel engine. The linear motion is converted to a rotary motion to turn a generator to produce electricity. The electrical output of the system is proportional to the size of the reflector, its optical losses and the efficiencies of the Stirling engine and the generator. This thesis outlines the theory and models for the collector, receiver, and Stirling engine, the parasitic power. An energy prediction model was created for solar Stirling dish systems to predict the location dependent long term performance of these systems. The model analyzes the performance of the parabolic mirror, receiver, Stirling engine, and the parasitic power consumption to predict the net power produced. The power plant analyzed in this paper has a capacity of generating 10MW electric power. The performance prediction models were implemented in EES and TRNSYS and include location dependent properties that affect the performance based on the direct normal insolation, ambient temperature, density of air (altitude), sun elevation angle, and the wind speed. The cost and financial analysis is made for the Dish System. Solar Advisor Model is used to make this analysis under Kombolcha weather condition. This analysis is used to determine the different costs associated with the power plant. The cash flow for the 30 years of operation of the power plant is also shown. I
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    Thermal Analysis, Design and Experimental Investigation of Parabolic Trough Solar Collector
    (Addis Ababa University, 2012-02) Yidnekachew, Messele; Abebayehu, Assefa (PhD)
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    TRNSYS Modeling and Simulation of Solar Water Heating System for Addis Ababa University Technology Faculty Students Residence Hall
    (Addis Ababa University, 2011-01) Tsegaye Seyoum; Abebayehu, Assefa (PhD)
    In this study, the transient performance of direct open-loop circulation active solar water heating system is computed using TRNSYS modeling and simulation software for flat-plate collector from the input of average annual climatic data of Addis Ababa and collector parameters. Typical daily hot water consumption for about 1000 students of Addis Ababa University, Technology Faculty was considered. The hot-water demand temperature (60 °C) is controlled by a conventional electrical/fuel auxiliary heater. The thesis focuses on simulation of solar water heating system and its economic analysis. The various design parameters are taken from the manufacturers and dealer companies of solar water heaters in Ethiopia and also from the world experience. The meteorological data for simulation was taken from SEWERA website found in a text and TMY format. The TRNSYS modeling and simulation are used to determine the outlet temperature of the solar collector and the daily contribution of the solar and the auxiliary heating system to the heating load. From the annual contribution of solar energy to the heating load and the estimated investment cost, the payback period of the heating system using two backup systems (i.e. fossil fuel or electricity auxiliary heating) was compared. The results obtained indicate that the payback period of the solar water heating system using furnace oil as backup heater is less than that of electricity, almost by half.