Solar Thermal Hydrogen Production: A Dynamic Reactor Model for the Hercynite Cycle
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The focal point of this thesis is the production of hydrogen. Hydrogen isclassified as a secondary energy source and must, analogous to electricity,be efficiently produced from some primary energy source. This primaryenergy source has traditionally been fossil fuels. Steam methane reformingand gasification of coal are the most common routes for hydrogen productiontoday. This thesis models and evaluates an alternative to fossil basedhydrogen production, namely a solar thermal water splitting cycle. A solar thermal water splitting cycle is a processes where a metal oxide isalternately reduced and oxidized in a cyclic fashion using solar energy andwater. In the reduction step the metal oxide is thermally reduced by solarradiation at a temperature TR and oxygen gas is evolved. A re-oxidation ofthe reduced material is performed at a temperature TOX in a subsequentstep where water is introduced to the system. Water will decompose on themetal surface and deposit oxygen while hydrogen gas is evolved. The goal of this thesis was to model a proposed solar thermal reactor designfor the so-called doped hercynite cycle and to evaluate the energy efficiencyof the modelled system. There has recently been discussions in the solarthermal hydrogen production community regarding the optimal temperatureof separation between the oxidation and reduction steps. The optimaltemperature of separation can be defined as Topt = TR - TOX, where TR TOX and represents the most energetically efficient way of producing hydrogen.Solar thermal processes have traditionally been driven at constantpressure. However, it has recently been claimed that the optimal temperaturecan in fact be zero, i.e. isothermal operation, if a pressure swing isalso imposed on the reaction system. It was desirable to contribute to thisdebate using modelling results. The proposed solar thermal hydrogen reactor can either be a stand-alonereactor or it can, as current research at NTNU suggest, be implemented ina Fischer-Tropsch (FT) plant. The effect and feasibility of a solar thermalhercynite reactor implementation in an FT plant was also considered. A dynamic, one-dimensional, pseudo-homogeneous reactor model was constructedand evaluated for the thermochemical production of hydrogen viathe hercynite cycle. It was found that isothermal pressure swing operationwas less energy efficient than combined temperature and pressure swing operation.The optimal temperature of separation based on maximum cavityreceiver temperature (1623.15 K) was Topt = 63.15 K with a reductionsystem pressure of 0.21 bar and a oxidation system pressure of 5 bar. Theoxidation temperature was Tox = 1560 K. Based on energy efficiency requirements found in relevant literature, a targetvalue for solar thermal reactor efficiency was a 20% solar energy-tohydrogenconversion efficiency. The highest calculated energy efficiency for astand-alone hydrogen reactor was 9.7%. For a solar thermal Fischer-Tropsch(FT) plant coupled reactor the efficiency was estimated to be 11.7%. For everyevaluated temperature reactor implementation in an FT plant increasedtotal energy efficiency by 2 - 4%. It was concluded that the designed hercynite cycle reactor would probablynot be able to reach the 20% energy target based on the provided values.However, a rigorous optimization was not within the scope of the thesis;system optimization is a multivariable problem that is worthy of a thesison its own. Better efficiencies and more optimistic predictions might resultfrom a more rigorous system optimization. The conversion of water peaked at XH2O = 7.1% for the cycles evaluatedwithin the modelling framework. Reactor implementation in an FT plantrequire a relatively dry feed and the coupling of the two concepts seemsimprobable at this stage. The biggest uncertainties tied to the results is the aforementioned lack of atrue system optimization and uncertainties tied to the validity of the kineticmodel at elevated pressure (5 bar).