Martin Roeb

SOLAR HYDROGEN PRODUCTION BY A TWO-STEP CYCLE BASED ON MIXED IRON OXIDES

A very promising method for the conversion and storage of solar energy into a fuel is the dissociation of water to oxygen and hydrogen, carried out via a two-step process using metal oxide redox systems such as mixed iron oxides, coated upon multi-channeled honeycomb ceramic supports capable of absorbing solar irradiation, in a configuration similar to that encountered in automobile exhaust catalytic converters. With this configuration, the whole process can be carried out in a single solar energy converter, the process temperature can be significantly lowered compared to other thermo-chemical cycles and the re-combination of oxygen and hydrogen is prevented by fixing the oxygen in the metal oxide. For the realization of the integrated concept, research work proceeded in three parallel directions: synthesis of active redox systems, manufacture of ceramic honeycomb supports and manufacture, testing and optimization of operating conditions of a thermochemical solar receiver-reactor. The receiver-reactor has been developed and installed in the solar furnace in Cologne, Germany. It was proven that solar hydrogen production is feasible by this process demonstrating that multi cycling of the process was possible in principle.

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.

Concentrating on Solar Electricity and Fuels

Thermal energy from mirror arrays that focus solar radiation not only generates electricity but also could be used to synthesize fuels from water and carbon dioxide. Intermittent sources of renewable energy, such as solar and wind power, pose a “storage problem.” They must generate more energy than the immediate demand, so that stored energy can be used when the resource is unavailable. Aside from pumped hydropower, large-scale storage of electricity is still technically and economically challenging. For solar energy, concentrating solar power (CSP) plants in regions in the sunbelt of Earth offer ways to store this energy on a large scale, either thermally or as chemical fuels.

Isothermal Water Splitting

Process operation at constant temperature may enhance the solar-thermal production of hydrogen from water. [Also see Report by Muhich et al.] The potential of solar radiation as an infinite resource of renewable energy is widely recognized. The challenge is to convert it as efficiently as possible into useful energy forms such as hydrogen or liquid carbonaceous fuels. Thermochemical cycles are a number of consecutive chemical reactions (≥2) that lower the maximum temperature compared to a single chemical reaction. Typical temperatures for these cycles to reach full conversion range from 800°C to 2000°C. The necessary solar heat can be generated by concentrating optics that direct the solar radiation on a single point—in solar towers or central receiver systems. To use the solar power efficiently and economically, losses must be minimized, with major factors being the minimization of re-radiation and the reduction of gases that are used to transport the reactant. As pointed out by Muhich et al. (1) on page 540 of this issue, a particularly important factor in this respect is the minimization of temperature differences between reaction steps. However, the necessary temperatures are high, and it has yet to be proven that the materials and components are stable over a long time to make the processes economically attractive.

Solar Thermochemical Generation of Hydrogen: Development of a Receiver Reactor for the Decomposition of Sulfuric Acid

A key step in the sulfur based thermochemical cycles for hydrogen production is the highly endothermic decomposition of sulfuric acid at temperatures between 800 °C and 1200 °C. This reaction can be carried out in a receiver-reactor which is irradiated with concentrated solar radiation from a heliostat field. To investigate this process a test reactor was developed and built. The reaction takes place on the surface of a catalytically coated porous absorber irradiated through a quartz pane of the receiver-reactor. This concept has the advantage of a minimum number of heat transferring steps. Experiments with the test reactor were performed in the DLR solar furnace in Cologne. Firstly the feasibility of a solar decomposition of sulfuric acid in a receiver-reactor containing volumetric absorbers was investigated and proven. Then the reactor was qualified at different operating points. Finally the receiver-reactor and strategy of operation was iteratively optimized with respect to chemical conversion and reactor efficiency. Several test series were performed with variation of the absorber temperature, the mass flow and the dilution rate. Partial pressure of SO3 , residence time, absorber temperature, and the kind of catalyst applied were identified and quantified as parameters with the most relevant influence on chemical conversion and reactor efficiency. The operation behavior observed and the detailed knowledge of dependencies of different operation parameters assist in evaluating the potential of scaling up the described technology.Copyright

Aluminum Remelting using Directly Solar-Heated Rotary Kilns

To check the feasibility of solar thermal remelting of aluminum scrap a directly absorbing rotary kiln receiver-reactor was constructed for experimentation in a mini-plant scale in the DLR high flux solar furnace. Conventionally the high energy demand for heating rotary kilns is met by the combustion of fossil fuels. This procedure generates a big exhaust gas volume which is contaminated by volatiles if the technology is applied to treat waste materials. Application of concentrated solar radiation to provide the high temperature heat enables to substitute the fossil fuel. Thus smaller off-gas streams are generated and lower investment and O&M cost are expected for the off-gas purification. In this paper market and environmental issues are discussed and pre-designs both for solar pilot and industrial scale applications are presented.

Perovskite oxides for application in thermochemical air separation and oxygen storage

Perovskites AMO3−δ are ideal for thermochemical air separation due to their oxygen nonstoichiometry δ, which can be varied by changing the temperature and oxygen partial pressure. We show in this work how materials can be selected for chemical looping air separation from thermodynamic considerations and present thermogravimetric experiments carried out on (Ca,Sr) ferrites and manganites, and doped variants, all synthesized via a citric acid auto-combustion method. SrFe0.95Cu0.05O3−δ and Ca0.8Sr0.2MnO3−δ show the best gravimetric oxygen storage capacity of all tested materials at T < 1200 °C. The redox reactions are completed in <1 min in air and are highly reversible. A significant re-oxidation reaction of reduced samples was observed at temperatures as low as 250 °C at an oxygen partial pressure of 0.16 bar. We studied phase formation via XRD and lattice expansion during reduction via in situ XRD experiments. The objective is to validate the potential and boundary conditions of such materials to pave the way for competitive air separation based on thermochemical cycling.

Redox chemistry of CaMnO3 and Ca0.8Sr0.2MnO3 oxygen storage perovskites

Perovskite oxides CaMnO3 and Ca0.8Sr0.2MnO3 show continuous non-stoichiometry over a range of temperatures and oxygen partial pressures. In this work a thermobalance equipped with an oxygen pump was used to measure the equilibrium non-stoichiometry of both materials for temperatures in the range 400–1200 °C and oxygen partial pressures in the range 1–10−5 bar. Analysis of the data showed that Ca0.8Sr0.2MnO3 has a lower enthalpy of reduction and thus can be more easily reduced. The strontium added sample was also robust against a phase transition that was seen in CaMnO3 at high temperatures. A statistical thermodynamic model of the system suggests that the defects form clusters of the form . The oxidation kinetics were also investigated with Ca0.8Sr0.2MnO3 showing faster kinetics and maintaining activity at lower temperatures. Overall, Ca0.8Sr0.2MnO3 shows very promising properties for redox applications, including gravimetric oxygen storage up to 4% by mass, high stability and rapid reversibility, with re-oxidation in less than 1 min at 400 °C. Finally, the redox chemistry of Ca0.8Sr0.2MnO3 was also investigated using in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption measurements at near ambient pressure in oxygen atmospheres.

HycycleS: a project on nuclear and solar hydrogen production by sulphur-based thermochemical cycles

The European FP7 project HycycleS focuses on providing detailed solutions for the design of specific key components for sulphur-based thermochemical cycles for hydrogen production. The key components necessary for the high temperature part of those processes, the thermal decomposition of H2SO4, are a compact heat exchanger for SO3 decomposition for operation by solar and nuclear heat, a receiver-reactor for solar H2SO4 decomposition, and membranes as product separator and as promoter of the SO3 decomposition. Silicon carbide has been identified as the preferred construction material. Its stability is tested at high temperature and in a highly corrosive atmosphere. Another focus is catalyst materials for the reduction of SO3. Requirement specifications were set up as basis for design and sizing of the intended prototypes. Rigs for corrosion tests, catalyst tests and selectivity of separation membranes have been designed, built and completed. Prototypes of the mentioned components have been designed and tested.