Date

2017

Author

Yılmaz, Arda

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Nowadays countries investigate to improve alternative energy technologies such as solar power, biomass, wind energy, hydrogen etc. Hydrogen gas is very useful energy carrier and fuel cells produce electricity through hydrogen gas. Hydrogen production technologies are also investigated by many researches due to its high cost production. Thermochemical production way is one of the hydrogen production methods. Solar energy is also clean, renewable and alternative energy source. It is used for heating reaction systems but modeling of solar system requires optimization in terms of heating need of reaction and operation temperature. Main purpose of this study is to model and design optimum reactor system in terms of heat, mass and momentum transport phenomena via statistical approach, JMP, COMSOL and MATLAB programs. In this reactor system, hydrogen gas is produced in monolith reactor from steam through solar energy and metal oxide catalyst. In front side of reactor, quartz glass takes place for solar irradiation. Backside of reactor is assumed well insulated because this side is closed and reactor channels connect to gas storage place via valve and vacuum system throughout this side. Reactor channel walls are coated with metal oxide catalyst. There is an insulation layer on the outside of reactor for decreasing energy loss. Artificial experiment (design of experiment-DOE) runs are set via JMP program to determine significant parameters for thermal and kinetic model. After that, thermal, mass-momentum transport simulation models, which are based on significant parameters, are configured on COMSOL. Hydrogen conversion value is obtained on MATLAB by using rate expressions of real experiment and temperature profiles of COMSOL results. Also, model validation studies are configured on COMSOL. In mass-momentum transport model, neglecting effects of mass transfer and momentum transfer on temperature profiles is verified due to low temperature differences for both reduction and oxidation reactions. Hydrogen conversion is found as 0.7. Hydrogen concentration toward end of the channel is higher because of high reaction rate. In kinetic model, when heating time is shorter than 3 min cordierite is the best material but when heating time is more than 3 min, silicon carbide is the best material in terms of oxygen conversion due to thermal conductivity. Surface area for solar flux and reactor length are very significant parameters for analysis of channel shape effect on oxygen conversion. In first and second simulations including main and second order effect except channel shape of thermal model and statistical approach, optimum conditions of reactor system are silicon carbide as reactor material, high CPSI (cell per square inch), averaged 300 sun solar flux, thin wall thickness for minimum temperature difference. According to final statistical analysis including all effects, optimum conditions of reactor system are high CPSI, high solar flux, square channel model, cordierite material, low wall thickness and optimum inner insulation thickness. In this optimum reactor model, oxygen production rate is 0.15-0.20 min-1, heating time is 1-2 mins and all temperature differences are 50-200 º C. Model validation is carried out for solar flux and temperature profiles of reactor at steady-state. Solar energy is determined 330 W and temperature profiles overlap each other by tuning some physical parameters.  

Subject Keywords

Solar energy., Stoichiometry., Thermochemistry., Hydrogen industry., Monolithic reactors.

URI

http://etd.lib.metu.edu.tr/upload/12620802/index.pdf
https://hdl.handle.net/11511/26287

Collections

Graduate School of Natural and Applied Sciences, Thesis