Dennis Thomey

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.

Sulfur Based Thermochemical Energy Storage for Concentrated Solar Power

A sulfur based thermochemical energy storage cycle for baseload power generation is being developed under the support of US DOE Sunshot program. Solar heat is stored in elemental sulfur via thermal decomposition of sulfuric acid and disproportionation of sulfur dioxide into elemental sulfur and sulfuric acid. Heat energy is recovered upon sulfur combustion. On-sun decomposition of sulfuric acid in a solar furnace has been demonstrated between 650 and 850°C. Near equilibrium conversion was obtained at high temperature but conversion was reduced due to catalyst poisoning at the lower temperatures. Sulfur dioxide disproportionation modeling showed the reaction driving force is maximized at the high system pressure and low system temperature. The effect of system pressure was validated experimentally. However, the disproportionation rate was found to increase with system temperature as a result of increased reaction kinetics. Homogenous iodide catalysts were used to further enhance the degree of disproportionation and the reaction rate. The process steps required to recover the catalyst for reuse have been verified.Copyright

Solar Thermal Water Splitting

Abstract Hydrogen produced from water by concentrated solar power (CSP) is a clean and renewable energy carrier, free of greenhouse emission offering the potential to fully replace fossil fuels. Today’s benchmark water-splitting process is the alkaline electrolysis, which, however, has a relatively low overall efficiency limited by the efficiency of power generation. In comparison, solar thermal processes have the potential to significantly increase the hydrogen yield by about 50%. Direct water splitting can theoretically be driven by CSP but requires technically unfeasible temperatures. Thermochemical cycles separate the water-splitting step into two or more reactions significantly reduce the temperature level and, by this, achieve high thermal efficiencies. High-temperature electrolysis of water is a promising technology for solar thermal water splitting due to its reduced electricity demand and, therefore, higher efficiency compared to room-temperature alkaline or PEM electrolysis. Such processes promise a huge potential for carbon-free and sustainable hydrogen production on large scale, but still major effort is needed to enhance the stability and thus suitability of the materials involved and used at high operating temperature.

Modeling of a Solar Receiver for Superheating Sulfuric Acid

A volumetric solar receiver for superheating evaporated sulfuric acid is developed as part of a 100 kW pilot plant for the hybrid sulfur (HyS) cycle. The receiver, which uses silicon carbide foam as a heat transfer medium, heats evaporated sulfuric acid using concentrated solar energy to temperatures of 1000 °C or greater, which are required for the downstream catalytic reaction to split sulfur trioxide into oxygen and sulfur dioxide. Multiple parallel approaches for modeling and analysis of the receiver are used to design the prototype. Focused numerical modeling and thermodynamic analysis are applied to answer individual design and performance questions. Numerical simulations focused on fluid flow are used to determine the best arrangement of inlets, while thermodynamic analysis is used to evaluate the optimal dimensions and operating parameters. Finally, a numerical fluid mechanics and heat transfer model is used to predict the temperature field within the receiver. Important lessons from the modeling efforts are given, and their impacts on the design of a prototype are discussed.

Multi-scale modelling of a solar reactor for the high-temperature step of a sulphur-iodine-based water splitting cycle

The 3-step sulphur-iodine-based thermochemical cycle for splitting water is considered. The high-temperature step consists of the evaporation, decomposition, and reduction of H2SO4 to SO2 using concentrated solar process heat. This step is followed by the Bunsen reaction and HI decomposition. The solar reactor concepts proposed are based on a shell-and-tube heat exchanger filled with catalytic packed beds and on a porous ceramic foam to directly absorb solar radiation and act as reaction site. The design, modeling, and optimization of the solar reactor using complex porous structures relies on the accurate determination of their effective heat and mass transport properties. Accordingly, a multi-scale approach is applied. Ceramic foam samples are scanned using highresolution X-ray tomography to obtain their exact 3D geometrical configuration, which in turn is used in direct porelevel simulations for the determination of the morphological and effective heat/mass transport properties. These are incorporated in a volume-averaged (continuum) model of the solar reactor. Model validation is accomplished by comparing numerically simulated and experimentally measured temperatures in a 1 kW reactor prototype tested in a solar furnace. The model is further applied to analyze the influence of foam properties, reactor geometry, and operational conditions on the reactor performance.

Thermodynamic Model of a Solar Receiver for Superheating of Sulfur Trioxide and Steam at Pilot Plant Scale

Within the European research project SOL2HY2, key components for a solar hybrid sulfur cycle are being developed and demonstrated at pilot scale in a real environment. Regarding the thermal portion, a plant for solar sulfuric acid decomposition is set up and initially operated at the research platform of the DLR Solar Tower in Julich, Germany. One major component is the directly irradiated volumetric receiver, superheating steam and SO3 coming from a tube-type evaporator to above 1000 °C. At the design flow rate of sulfuric acid (50%-wt.) of 1 l/min, a nominal solar power of 57 kW is required at the receiver. With a flat ceramic absorber made from SiC and a flat quartz glass window, the design is based on lab scale reactors successfully demonstrated at the solar furnace of the German Aerospace Centre (DLR) in Cologne, Germany. A flexible lumped thermodynamic tool representing the receiver, compiled to assess different configurations, is presented in detail. An additional raytracing model has been established to provide the irradiation boundaries and support the design of a conical secondary concentrator with an aperture diameter of 0.6 m. A comparison with first experimental data (up to 65% nominal power), obtained during initial operation, indicates the models to be viable tools for design and operational forecast of such systems. With a provisional method to account for the efficiency of the secondary concentrator, measured fluid outlet temperatures (up to 1000 °C) are predicted with deviations of ±60 °C. Respective absorber front temperatures (up to 1200 °C) are under-predicted by 100-200 °C, with lower deviations at higher mass flows. The measured window temperature (up to 700 °C) mainly depends on the absorber front temperature level, which is well predicted by the model.

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.