Martina Neises

Materials-Related Aspects of Thermochemical Water and Carbon Dioxide Splitting: A Review

Thermochemical multistep water- and CO2-splitting processes are promising options to face future energy problems. Particularly, the possible incorporation of solar power makes these processes sustainable and environmentally attractive since only water, CO2 and solar power are used; the concentrated solar energy is converted into storable and transportable fuels. One of the major barriers to technological success is the identification of suitable active materials like catalysts and redox materials exhibiting satisfactory durability, reactivity and efficiencies. Moreover, materials play an important role in the construction of key components and for the implementation in commercial solar plants. The most promising thermochemical water- and CO2-splitting processes are being described and discussed with respect to further development and future potential. The main materials-related challenges of those processes are being analyzed. Technical approaches and development progress in terms of solving them are addressed and assessed in this review.

Technologies and trends in solar power and fuels

Solar radiation is the largest indigenous energy resource worldwide. It will gain a significantly more relevant role in covering the energy demand of many countries when national fuel reserves fall short and when demand increases as is expected within the next 10 years. If solar energy is transformed into heat by concentrating and absorbing the radiation, energy can be stored easily. Thermal energy from mirror fields that focus solar radiation not only is able to generate electricity but also can be used to generate storable heat, to desalinate salt water or to synthesise fuels from water and carbon dioxide to store, transport or use them on-site. The application of concentrated solar radiation as a primary energy source can help to decarbonise electricity generation and many other sectors to keep the chance of staying within the 2 °C goal for limiting the effects of global warming. The aim of the present contribution is to give an overview on the state-of-the art of technologies for solar thermal power production and fuel production and to describe the status and outlook of commercial projects and perspectives of market development.

Simulation of a Solar Receiver-Reactor for Hydrogen Production

The transient thermal behavior of two solar receiver-reactors for hydrogen production has been modeled using Modelica/Dymola. The simulated reactors are dedicated to carry out the same chemical reactions but represent two different development stages of the project HYDROSOL and two different orders of magnitude concerning reactor size and hydrogen production capacity. The process itself is a two step thermochemical cycle, which uses mixed iron-oxides as a redox-system. The iron-oxide is coated on a ceramic substrate, which is placed inside the receiver-reactor and serves on the one hand as an absorber for solar radiation and on the other hand as the reaction zone for the chemical reaction. The process consists of a water splitting step in which hydrogen is produced and a regeneration step during which the used redox-material is being reduced. The reactor is operated between these two reaction conditions in regular intervals with alternating temperature levels of about 800 °C for the water splitting step and 1200 °C for the regeneration step. Because of this highly dynamic process and because of fluctuating solar radiation during the day, a mathematical tool was necessary to model the transient behavior of the reactor for theoretical studies. Two models have been developed for two existing receiver-reactors. One model has been set up to simulate the behavior of a small scale test reactor, which has been built and tested at the solar furnace of DLR in Cologne. Results are very promising and show that the model is able to reflect the thermal behavior of the reactor. Another model has been developed for a 100 kWth pilot reactor which was set up at the Plataforma Solar de Almeria in Spain. This model is based on the first model but special geometrical features had to be adapted. With this model temperatures and hydrogen production rates could be predicted.Copyright

Investigations of the Regeneration Step of a Thermochemical Cycle Using Mixed Iron Oxides Coated on SiSiC Substrates

A two-step thermochemical cycle for hydrogen production using mixed iron oxides coated on silicon carbide substrates has been investigated. The water-splitting step proceeds at temperatures between 800 and 1000 °C while for the regeneration step temperatures around 1200 °C are needed. A deactivation of the material resulting in a decrease of the hydrogen production within the first couple of cycles was observed in preceding tests. For detailed investigations of the system composed of the redox-material and the substrate small scale samples were tested in a laboratory test-rig. For identification of material changes the samples were investigated via XRD and SEM-EDS analysis. The analysis revealed the reasons for the deactivation of the redox-material. Through parametric studies the influence of the regeneration parameters, namely regeneration temperature and time on the hydrogen production was analysed. A model for the regeneration step was developed describing the performance of the regeneration step as a function of temperature and time and additionally as a function of total regeneration time, i.e. the cumulated time the sample has been regenerated.Copyright

Kinetic Investigations of a Two-Step Thermochemical Water-Splitting Cycle Using Mixed Iron Oxides Fixed on Ceramic Substrates

A two-step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. A reactor concept has been developed in which the metal oxide is fixed on multi-channelled honeycomb ceramic supports capable of adsorbing solar irradiation. In the solar furnace of DLR in Cologne coated honeycomb structures were tested in a solar receiver-reactor with respect to their water splitting capability and their long term stability. The concept of this new reactor design has proven feasible and constant hydrogen production during repeated cycles has been shown. For a further optimization of the process and in order to gain reliable performance predictions more information about the process especially concerning the kinetics of the oxidation and the reduction step are essential. To examine the kinetics of the water splitting and the regeneration step a test rig has been built up on a laboratory scale. In this test rig small coated honeycombs are heated by an electric furnace. The honeycomb is placed inside a tube reactor and can be flushed with water vapour or with an inert gas. A homogeneous temperature within the sample is reached and testing conditions are reproducible. Through analysis of the product gas the hydrogen production is monitored and a reaction rate describing the hydrogen production rate per gram ferrite can be formulated. Using this test set-up, SiC honeycombs coated with a zinc-ferrite have been tested. The influences of the water splitting temperature and the water concentration on the kinetics of the water splitting step have been investigated. A mathematical approach for the reaction rate was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ/mole was found.Copyright

EXAMPLES OF SOLAR THERMAL FUEL PRODUCTION

The conversion of renewable energy especially solar energy into versatile fuels is a key technology for an innovative and sustainable energy economy. To finally benefit from solar fuels they have to be produced with high efficiencies and low to no greenhouse gas emissions in large quantities. The final goal will most probably be the carbon free fuel hydrogen. But the main challenge is its market introduction. Therefore a strategy incorporating transition steps has to be developed. Solar thermal processes have the potential to be amongst the most efficient alternatives for large scale solar fuel production in the future. Therefore high temperature solar technologies are under development for the different development steps up to the final goal of carbon free hydrogen. This paper discusses the strategy based on the efficiencies of the chosen solar processes incorporating carbonaceous materials for a fast market introduction and processes based on water splitting for long term solar hydrogen generation. A comparison with the most common industrial processes shall demonstrate which endeavors have to be done to establish solar fuels.

Analysis of Thermodynamics of Two-Step Solar Water Splitting

A two-step thermo-chemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After proof-of-principle, successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, and elaboration of process strategies presented in previous contributions, the present work describes a thermodynamic study aiming at the fine tuning of the redox system, at the improvement of process conditions, and at the evaluation of the potential of the process. For the redox material the oxygen uptake capability is an essential characteristic, because it is directly connected to the amount of hydrogen which can be produced. In order to evaluate the maximum oxygen uptake potential of a coating material and to be able to find new redox materials theoretical considerations based on thermodynamic laws and properties are helpful and faster than actual tests. Through thermodynamic calculations it is possible to predict the theoretical maximum output of H2 from a specific redox-material under certain conditions. Calculations were focussed on the two mixed iron oxides nickel-iron oxide and zinc-iron oxide. In the simulation the amount of oxygen in the redox-material is calculated before and after the splitting step on the basis of laws of thermodynamics and available material properties for the mixed-iron oxides used. For the simulation the commercial Software FactSage and available databases for the necessary material properties were used. The analysis showed that a maximum hydrogen yield is achieved if the regeneration temperature is raised to the limits of the operation range, if the temperature for the water splitting is lowered below 800 °C and if the partial pressure of oxygen during regeneration is decreased to the lower limits of the operational range. The increased hydrogen yield at lower splitting temperature of about 800 °C could not be confirmed in experimental results, where a higher splitting temperature led to a higher hydrogen yield. As a consequence it can be stated that kinetics must play an important role especially in the splitting step.Copyright