|dc.description.abstract||The focal point of this thesis is the production of hydrogen. Hydrogen is
classified as a secondary energy source and must, analogous to electricity,
be efficiently produced from some primary energy source. This primary
energy source has traditionally been fossil fuels. Steam methane reforming
and gasification of coal are the most common routes for hydrogen production
today. This thesis models and evaluates an alternative to fossil based
hydrogen production, namely a solar thermal water splitting cycle.
A solar thermal water splitting cycle is a processes where a metal oxide is
alternately reduced and oxidized in a cyclic fashion using solar energy and
water. In the reduction step the metal oxide is thermally reduced by solar
radiation at a temperature TR and oxygen gas is evolved. A re-oxidation of
the reduced material is performed at a temperature TOX in a subsequent
step where water is introduced to the system. Water will decompose on the
metal surface and deposit oxygen while hydrogen gas is evolved.
The goal of this thesis was to model a proposed solar thermal reactor design
for the so-called doped hercynite cycle and to evaluate the energy efficiency
of the modelled system. There has recently been discussions in the solar
thermal hydrogen production community regarding the optimal temperature
of separation between the oxidation and reduction steps. The optimal
temperature 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 constant
pressure. However, it has recently been claimed that the optimal temperature
can in fact be zero, i.e. isothermal operation, if a pressure swing is
also imposed on the reaction system. It was desirable to contribute to this
debate using modelling results.
The proposed solar thermal hydrogen reactor can either be a stand-alone
reactor or it can, as current research at NTNU suggest, be implemented in
a Fischer-Tropsch (FT) plant. The effect and feasibility of a solar thermal
hercynite reactor implementation in an FT plant was also considered.
A dynamic, one-dimensional, pseudo-homogeneous reactor model was constructed
and evaluated for the thermochemical production of hydrogen via
the hercynite cycle. It was found that isothermal pressure swing operation
was less energy efficient than combined temperature and pressure swing operation.
The optimal temperature of separation based on maximum cavity
receiver temperature (1623.15 K) was Topt = 63.15 K with a reduction
system pressure of 0.21 bar and a oxidation system pressure of 5 bar. The
oxidation temperature was Tox = 1560 K.
Based on energy efficiency requirements found in relevant literature, a target
value for solar thermal reactor efficiency was a 20% solar energy-tohydrogen
conversion efficiency. The highest calculated energy efficiency for a
stand-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 every
evaluated temperature reactor implementation in an FT plant increased
total energy efficiency by 2 - 4%.
It was concluded that the designed hercynite cycle reactor would probably
not 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 thesis
on its own. Better efficiencies and more optimistic predictions might result
from a more rigorous system optimization.
The conversion of water peaked at XH2O = 7.1% for the cycles evaluated
within the modelling framework. Reactor implementation in an FT plant
require a relatively dry feed and the coupling of the two concepts seems
improbable at this stage.
The biggest uncertainties tied to the results is the aforementioned lack of a
true system optimization and uncertainties tied to the validity of the kinetic
model at elevated pressure (5 bar).||en