Session: 06-03 Novel Reactors and Processes

Paper Number: 107385

107385 - Implementation of Thermochemical Oxygen Pumping for Improved Efficiency and Throughput of Solar Thermochemical Hydrogen Production 

Solar thermochemical hydrogen production (STCH) is a redox process that can use heat directly to split water or carbon dioxide to produce hydrogen and syngas. Such heat can be cheaper to source (e.g. via concentrated solar) and cheaper to store than electricity. STCH production using metal oxides have been demonstrated in the field at the 50kW scale¹. However, a multi-fold improvement in heat-to-fuel conversion efficiency and reduction in capital cost are needed before STCH can meet aggressive green hydrogen price targets like the US DOE Hydrogenshot.

The present authors have previously proposed² a Reactor Train System to address the dominant source of inefficiency in most STCH systems – sensible heat recovery from the redox solids between reduction and water-splitting temperatures (typically 1500 and 800 respectively). Another major source of efficiency loss is the removal of oxygen at low partial pressure during the reduction phase of STCH. In addition to low efficiency, vacuum pumps are too costly for reduction pressures below 1kPa because of high volumetric flowrates of oxygen at those pressures³.

The present authors have proposed using thermochemical oxygen pumping (TcOP) within the Reactor Train for improved performance⁴. TcOP involves a second redox cycle to absorb the low-pressure oxygen released during the STCH reduction step. The absorbed oxygen is then released at near-ambient pressure during TcOP material regeneration (typically at 700-1000). We reported that a preliminary Reactor Train + TcOP dual-train design can improve heat-to-fuel conversion efficiency by 10%-points and increase hydrogen productivity by about a factor of two compared to vacuum pumps. This can translate to significant reduction of green hydrogen cost. In the present talk we will report a more comprehensive study on STCH reactor systems using TcOP.

We will consider oxygen exchange between the STCH redox material (eg ceria) undergoing reduction, and the TcOP material (eg. strontium iron oxide). This exchange can be achieved by spontaneous oxygen flow from higher-pressure ceria reactors (1-10⁴Pa) and low-pressure TcOP reactors (<1 Pa). In a second alternative configuration, an inert gas can be circulated to sweep oxygen from the ceria reactor and deliver it to the TcOP material. We will investigate both configurations for a generic reactor system and assess their potential in terms of STCH efficiency and hydrogen productivity. We will then propose more concrete embodiments of these schemes by incorporating them into the Reactor Train. Practical challenges like the flow of oxygen under reduced pressure, and heat recovery between inert gas streams will be investigated. The talk will thus assess in a comprehensive way the potential of TcOP to advance STCH beyond existing oxygen removal technologies.

1.     Zoller et al., Joule 6, 1606–1616, 2022

2.     Patankar et al., Journal of Solar Energy Engineering, 144(6), 061014, 2022

3.     Falter et al., Sustainable Energy Fuels (4), 3992, 2020

4.     Patankar et al., ASME IMECE 2022, Columbus OH

Presenting Author: Aniket Patankar Massachusetts Institute of Technology

Presenting Author Biography: Aniket S. Patankar is a PhD candidate at the Department of Mechanical Engineering, Massachusetts Institute of Technology. His doctoral research relates to thermochemical solar fuel production, in particular, the design and optimization of reactor systems and redox materials. He obtained his B. Tech in Mechanical Engineering at IIT Bombay, where he was the class of 2017 valedictorian.