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 Jülich, 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

Technical analysis and economic evaluation of solar reactors for sulfuric acid cracking for the thermochemical production of hydrogen

The Hybrid-Sulfur-Cycle (HyS), coupled with concentrated solar power, is a high- potential candidate for energy-efficient, renewable mass production of hydrogen. Sulfurous acid is electrochemically processed into sulfuric acid and hydrogen, which requires virtually one third of the electric energy in comparison to conventional water electrolysis. In a second step, sulfuric acid is decomposed thermally into sulfurous acid in order to be reprocessed. Two fundamentally differing approaches are conceivable to provide solar heat for the evaporation (at temperatures up to 500 °C) and decomposition (at temperatures up to 1000 °C) of sulfuric acid. In the first scenario, the corresponding reactors are directly irradiated and operated intermittently, thus avoiding the need for an additional heat carrier medium. In the second, the interposition of an efficient heat storage enables continuous operation of the entire chemical plant. Suitable reactor concepts for the solar sulfuric acid decomposition are under development by DLR (Germany) and SRNL (USA) and have been demonstrated in laboratory or even in a representative environment. This work undertakes a systematic comparison of both concepts in order to provide a guideline for the definition of further development towards the industrial scale. Thermodynamic models are developed at same level of detail, focused respectively on one-dimensional and temporal resolution, in order to determine the related mechanisms of loss, and to extrapolate achievable full load hours, yield, and hydrogen production costs. As a basis for this comparative analysis, the thesis addresses requirements concerning heat recovery, increasing sulfuric acid concentration level prior to its reprocessing, and the operating pressure, ensuring the efficient integration of both concepts into an overall process. It emerges from the analysis that the indirectly heated system promises significantly higher yields: 1. Assuming air as heat carrier, the fully integrated, indirectly heated reactor concept can be operated efficiently over the required part load range. 2. The thermal inertia of the relatively complex, directly irradiated system results in substantial losses, primarily caused by daily cold start-ups. Moreover, depending on local irradiation conditions and the extent to which the chemical subsystem will allow for operation with strong gradients, relative yields are likely to be yet further diminished. If ambitious development goals for the sulfur depolarized electrolyzer can be met fully, the HyS-process with the indirectly heated configuration reveals the potential to achieve hydrogen costs close to 4 €/kg, as long as thermal receiver efficiencies, utilization levels and economies of scale are fully exploited. In this case, the share on the hydrogen costs associated to the solar recycling of sulfuric acid is 2,2 €/kg. In the long term, if the process is to provide an economically more viable alternative to water electrolysis using renewable electricity, this value must be substantially lower

Modeling of a Solar Receiver for Superheating Sulfuric Acid

A volumetric solar receiver for superheating evaporated sulfuric acid is developed as part of a 100kW pilot plant for the Hybrid Sulfur Cycle. The receiver, which uses silicon carbide foam as a heat transfer medium, heats evaporated sulfuric acid using concentrated solar energy to temperatures up to 1000 °C, which are required for the downstream catalytic reaction to split sulfur trioxide into oxygen and sulfur dioxide. Multiple approaches to modeling and analysis of the receiver are performed 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

Application fo CAE Tools for design and scaling of a solar reactor and receiver for acid splitting for the HyS process at pilot plant scale

Thermochemical cycles for water splitting are considered as a promising emission free route of large scale hydrogen production. The direct conversion of thermal energy into chemical energy, potentially yields increased efficiencies and reduced costs compared to low temperature electrolysis of water. Feasibility and efficiency forecasts consider the hybrid-sulphur cycle (HyS) as one of the most promising candidates among other thermochemical cycles. Within the European research project SOL2HY2, the process key components are demonstrated at relevant scale. Coupling of concentrated solar power (CSP) into this process is a major res arch area at DLR. This paper shows the application of several CAE Tools within the scope of the SOL2HY2 project tasks in order to develop a solar receiver and reactor as part of a demonstration plant for sulphuric acid cracking on DLR´s solar tower in Juelich, Germany. Engineering approaches to address sizing, operational parameters and boundaries, flow homogeneity are illustrated, a plying Numerical Equation Solving, CFD Simulations, Raytracing Tools and CAD. Detailed models currently being developed will be validated with the experimental results to improve and extrapolate the receiver/reactor design to industrial scale

Solar Hydrogen Generation via the HyS-Cycle: Design and Demonstration of a Pilot Plant at the Solar Tower in Jülich

Solar Hydrogen Generation via the HyS-Cycle: Design and Demonstration of a Pilot Plant at the Solar Tower in Jülic

Solar thermochemical production of hydrogen: Steady-state and dynamic modeling of a Hybrid- Sulfur Process coupled to a solar tower

Thermo-chemical cycles for water splitting are considered as a promising alternative of emission-free routes of massive hydrogen production – with potentially higher efficiencies and lower costs compared to low temperature electrolysis of water. The hybrid-sulphur cycle was chosen as one of the most promising cycles from the ‘sulphur family’ of processes. Specific attention is put to the dynamics of the process. The process is separated into two different parts – one in steady-state, the other one imposed by transients. This allows considering concepts of coupling such a process to a concentrating solar system and analyse them with respect to energy and mass flows. Process efficiencies are calculated based on conservative assumptions revealing the most important development tasks for the future

MODELLING AND SCALING ANALYSIS OF A SOLAR REACTOR FOR SULPHURIC ACID CRACKING IN A HYBRID SULPHUR CYCLE PROCESS FOR THERMOCHEMICAL HYDROGEN PRODUCTION

Solar sulphuric acid cracking is a key step of the hybrid sulphur cycle (HyS) for thermochemical water splitting producing hydrogen free of CO2 emissions. In the European projects HYTHEC and HycycleS the concept of a receiver-reactor was developed by DLR and tested in its solar furnace in Cologne, Germany. A model of the high temperature chamber for SO3 decomposition is presented and validated with experimental results of the HycycleS test reactor. In a scaling analysis, this model is integrated into a previously published flowsheet of a solar HyS process predicting the performance of the system at industrial size. Applying stationary and dynamic simulation, an optimum reactor length of 1 meter can be identified. The results of the simulation are now used in the European project SOL2HY2 to develop and operate a demonstration plant for sulphuric acid cracking on DLR’s solar tower in Juelich, Germany