Session: 08-01: Solar Chemistry: Thermochemical Fuel Production I

Paper Number: 130525

130525 - Impact of Oxygen Removal and Hydrogen Separation Approaches on Stch Efficiency Using Different Redox Materials 

Abstract: 

Solar Thermochemical Hydrogen (STCH) production using two-step redox cycles is a promising route for green fuel generation utilizing water and/or CO₂ splitting. STCH’s advantages over alternatives, e.g., electrolysis and photo-electrochemical processes, include using the entire solar spectrum, simultaneous H₂O and CO₂ splitting, and the possibility of round-the-clock operation when integrated with high-temperature thermal energy storage.

While STCH research has largely focused on novel redox materials, less attention has been devoted to gas-handling “auxiliary” processes such as oxygen removal and hydrogen separation from unconverted steam. While these processes are routinely used in industry, the operating conditions of STCH are “extreme” by comparison, and a direct adoption of existing technology can lead to prohibitively low efficiencies.

While ceria has been the state-of-the-art STCH redox material for about a decade, it has limited fuel production per cycle and needs high reduction temperatures. Other redox materials including perovskites, doped ceria, and ferrites can reduce more easily and at lower temperatures but need excess steam of the order of r_steam = 10²-10⁴ [2][3], where r_steam denotes the steam to hydrogen ratio. Many STCH system-level analyses assume that such dilute steam-hydrogen mixtures can be separated at no energetic cost by condensing steam [1][2][4]. However, this results in the loss of the latent heat of vaporization (h_vap) of water during condensation: this heat cannot be used to boil water because both processes happen at the same temperature. Furthermore, STCH demonstrations largely use vacuum pumps or a once-through inert sweep for oxygen removal. These approaches are inefficient and expensive: mechanical pumps are prohibitively costly for pressures below 1kPa because of the high volumetric flow rates at those pressures. Meanwhile, our recent work shows that inert gas sweep is not practical for reaching < 10Pa because of the high inert volumetric flow rates (approaching supersonic flow conditions). There has been work on thermochemical and electrochemical oxygen pumping, but a robust system-level analysis for operating them with different redox materials is missing.

In this work, we perform system-level thermodynamic analysis and optimization for novel STCH redox materials coupled with different O₂ removal and H₂ separation approaches. We consider ceria as the baseline material and include alternatives such as metal-substitute ferrites and perovskites (CTM55 [6], LSMA [7], etc.) proposed in the literature. For H₂-separation we consider high-temperature and intermediate-temperature proton-conducting membranes (operating between 200-800) and mechanical vapor recompression to recover the latent heat of water condensation. For O₂-removel we consider thermochemical oxygen pumping (TcOP) and a pressure cascade of mechanical vacuum pumps [5]. This work will provide a comprehensive study of the impact of efficient O₂ removal and H₂ separation on STCH efficiency for a variety of redox materials.

 

1.     Muhich et al., Int. J. Hydrogen Energy 43, 18814–18831, 2018

2.     Li, S. et al., Energy Technol. 2000925, 1–18, 2021

3.     Bayon et al., Energy Technol. 10, 1–11, 2022

4.     Lou et al., Solar Energy. 241, 504-514, 2022

5.     Patankar., Solar Energy 264, 0038-092X, 2023

6.     Ezbiri et al., Journal of Materials Chemistry. A, 5(8), 4172–4182, 2017

7.     Qian et al., Matter,4, 688-708, 202

Presenting Author: Ziyao Wu Massachusetts Institute of Technology

Presenting Author Biography: I am a Ph.D.student at the Department of Mechanical Engineering at MIT. UMich MS ’22, UCSD BS ’20. I am passionate about making an impact in the renewable energy and sustainability space with breakthrough technical solutions.

During my PhD I participated in developing a reactor for producing zero-emission hydrogen and SAF. This “Reactor Train” system is 5 times more efficient than state-of-the-art thermochemical systems. I am now participating in the development of a first prototype of this reactor as part of a US Department of Energy-funded project.

Authors:

Ziyao Wu Massachusetts Institute of Technology
Aniket Patankar Massachusetts Institute of Technology
Xiao-Yu Wu University of Waterloo
Wonjae Choi Ewha Womans University
Harry Tuller Massachusetts Institute of Technology
Ahmed Ghoniem Massachusetts Institute of Technology