(257c) Functional Ceramics for Solar Thermochemical Storage, Hydrogen and Liquid Fuel Production

normal">Functional ceramics for solar thermochemical storage,
hydrogen and liquid fuel production

The
intermittent nature of renewable sources is one of the biggest challenges to
overcome in their widespread utilization. In case of solar energy a promising
method is to store it in the form of chemical energy in compounds such as
hydrogen or hydrocarbons. These products are useful as storage medium, as
energy carrier, as a “solar fuel” or as an intermediate for chemical
commodities. Among the fuels, hydrogen is of specific interest, since the only
product of hydrogen combustion is water, which in turn is the feed to produce
hydrogen with solar energy.

Thermochemical
cycles utilize a combination of chemical reactions driven by heat to thermochemically decompose water; therefore they are a
viable option to produce hydrogen at acceptable temperature levels. The
two-step cycles can achieve high efficiencies, and the materials are noncorrosive.
The high temperature, therefore thermodynamically limiting step is the
reduction of the oxidized form of the redox material, while the water splitting
step takes place at lower temperatures as shown in Fig. 1. The most prominent
examples are redox cycles applying metal/metal oxide pairs (such as ZnO/Zn), multi-valent metal oxides (e.g. Fe₃O₄/FeO, CeO₂/Ce₂O₃), or a
metal oxide with oxygen non-stoichiometry (perovskites), which are capable to
reversibly incorporate oxygen in their lattice.

Fig. 1. normal"> Scheme of metal/metal oxide cycles for
thermochemical hydrogen production.

Thermodynamic
studies showed that ceria-based as well as perovskite materials are viable
candidates as redox materials for thermochemical water splitting cycles. Perovskites
are of special interest since they undergo a partial reduction without phase change;
therefore they are more durable for long-time applications. The AMO₃
perovskite structure can be achieved by using different A and M ions, which
their thermodynamic properties tuneable. In the scope of this work a library of
redox materials with different compositions was established via large-scale
material screening. The most promising compositions were subjected to equilibrium
reduction and oxidation experiments under varying conditions to study the
thermodynamic properties. Further experiments were conducted to achieve a basic
understanding of interrelations between atomic structure, transport mechanisms,
microstructure, reactivity and life time of the materials. The chosen
functional ceramics were further developed to define and create suitable
designs for solar receivers, chemical reactors and process schemes.

In parallel
to material development optimizing the performance of the equipment needed to
run such a process in a solar tower facility is essential. The use of
particulate redox materials has been proposed to decouple the reduction
reaction from the oxidation reaction, to decouple the absorption of solar
radiation from the reduction reaction by using inert heat transfer particles
and to allow a counter current heat exchanger design by using a combination of
both particles. This concept can be implemented into any particle process and
offers potentially high heat recovery rates and thus it can improve the overall
system efficiencies. A detailed model to calculate the performance of the
concept in consideration of temperature dependent material data and several
other influencing factors is developed and validated by experiments.

Summary

The present work
aims to give an overview on recent developments and the state-of-the art of
high temperature fuels solar processes, in particular hydrogen production via
solar thermochemical cycles. It will have a look on the most important
performance parameters involved and will give an outlook on further potential
and necessary developments.