A Thermochemical Study of Iron Aluminate-Based Materials: A Preferred Class for Isothermal Water Splitting

The use of hydrogen as a renewable fuel has been stymied by our inability to produce it cleanly and economically. The conventional solar thermochemical approach considers a two-step redox cycle...

Application of Porous Materials for CO2 Reutilization: A Review

CO2 reutilization processes contribute to the mitigation of CO2 as a potent greenhouse gas (GHG) through reusing and converting it into economically valuable chemical products including methanol, dimethyl ether, and methane. Solar thermochemical conversion and photochemical and electrochemical CO2 reduction processes are emerging technologies in which solar energy is utilized to provide the energy required for the endothermic dissociation of CO2. Owing to the surface-dependent nature of these technologies, their performance is significantly reliant on the solid reactant/catalyst accessible surface area. Solid porous structures either entirely made from the catalyst or used as a support for coating the catalyst/solid reactants can increase the number of active reaction sites and, thus, the kinetics of CO2 reutilization reactions. This paper reviews the principles and application of porous materials for CO2 reutilization pathways in solar thermochemical, photochemical, and electrochemical reduction technologies. Then, the state of the development of each technology is critically reviewed and evaluated with the focus on the use of porous materials. Finally, the research needs and challenges are presented to further advance the implementation of porous materials in the CO2 reutilization processes and the commercialization of the aforementioned technologies.

Efficiency Enhancement of an Ammonia-Based Solar Thermochemical Energy Storage System Implemented with Hydrogen Permeation Membrane

The ammonia-based solar thermochemical energy storage (TCES) is one of the most promising solar TCESs. However, the solar-to-electric efficiency is still not high enough for further commercialization. The efficiency is limited by the high ammonia decomposition reaction temperature, which does not only increase the exergy loss through the heat recuperation but also causes a large re-radiation loss. Nonetheless, lowering the reaction temperature would impact the conversion and the energy storage capacity. Thanks to the recent development of the membrane technology, the hydrogen permeation membrane has the potential to enhance the conversion of ammonia decomposition under the moderate operating temperature. In this paper, an ammonia-based solar thermochemical energy storage system implemented with hydrogen permeation membrane is proposed for the first time. The system model has been developed using the Aspen Plus software implemented with user-defined Fortran subroutines. The model is validated by comparing model-generated reactor temperatures and conversions profiles with data from references. With the validated model, an exergy analysis is performed to investigate the main exergy losses of the system. Furthermore, the effects of the membrane on system efficiency improvement are studied. The results show that exergy loss in the charging loop is dominant, among which the exergy losses of Heat Exchanger Eh,A, together with that of the re-radiation Er, play important roles. Compared with the conventional system, i.e., the system without the membrane, the Eh,A and Er of the proposed system are more than 30% lower because the hydrogen permeation membrane can improve ammonia conversion at a lower endothermic reaction outlet temperature. Consequently, the proposed system, presumably realized by the parabolic trough collector at ~400 °C, has a theoretical solar-to-electric efficiency of ηste, which is 4.4% higher than the conventional ammonia-based solar thermochemical energy storage system. Last but not least, the efficiency is 3.7% higher than that of a typical parabolic trough solar power plant, which verifies the thermodynamic feasibility of further commercialization.

Non-uniform porous structures and cycling control for optimized fixed-bed solar thermochemical water splitting

Solar thermochemical redox cycles provide a sustainable pathway for solar fuel processing. If done in porous (ceria) structures, they can profit from faster reaction rates owned to the enhanced heat and mass transport characteristics. However, the exact porous structure and operating conditions significantly affect the performance. We present a transient volume-averaged fixed-bed model of a thermochemical redox reactor utilizing macroporous ceria. We studied the porosity-dependent (ε=0.4-0.9) and operating condition-dependent (solar concentration ratio, ratio of oxygen partial pressure to total pressure, gas flow rate) performance of the fixed-bed ceria redox cycle. Structures with large porosity (ε=0.9) showed better performance than low-porosity structures, owning to the enhanced heat absorption and resulting higher temperatures. We show that the cycle duration requires optimization according to the porosity of the structure. Two hours of operation for a structure with ε=0.75 resulted in the largest hydrogen production (115.78) if the single cycle duration was 240 s (i.e. 30 cycles in 2 hours), while nearly five times less was produced for a 15 times longer single cycle duration (i.e. 2 cycles in 2 hours). We subsequently introduced porous structures with different types of non-uniform porosity distributions. For an average porosity of ε=0.75, the most favorable non-uniform porosity media exhibited higher porosity at the boundaries and a denser core. The fuel production of the best non-uniform porous structure was six times larger compared to a uniform porous structure. Adjusting on top of this the cycling conditions, a 14.6 times production gain was achieved. This work suggests that under non-isothermal operation condition for macroporous ceria redox fixed-bed cycling, non-uniform porous structure with higher porosity boundaries and a dense core benefit fuel production and porosity-dependent cycle duration modulation can be used to increase performance.