Citation
Shaner, Matthew Reed (2016) An Experimental and Technoeconomic Study of Silicon Microwire Arrays for Fuel Production Using Solar Energy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z98C9T7Z. https://resolver.caltech.edu/CaltechTHESIS:05312016-151045544
Abstract
Direct solar energy conversion is one of few sustainable energy resources able to wholly satisfy global energy demand; however, utility scale adoption and reliance are currently limited by the lack of a cost effective energy storage technology. The production of fuel from sunlight (solar fuels) enables solar energy storage in chemical bonds, a volumetrically and gravimetrically dense form compatible with current infrastructure worldwide. Hydrogen production via water splitting is a first generation solar fuel targeted herein that is currently used for hydrocarbon up-grading and fertilizer production and could further be utilized in combustion cycles and/or fuel cells for electricity and heat production and transportation.
This thesis presents achievements that form the foundation for Si microwire array based solar water splitting devices beginning with a tandem junction device design using Si microwire arrays as the architectural motif and one of many active components. Si microwire arrays have potential advantages over two dimensional planar device architectures such as minimized resistance losses, lower semiconductor material usage, and embedment in a polymeric membrane enabling a flexible device.
Experimental fabrication and characterization of this tandem junction device design was realized in the form of a np+-Si microwire array coated by either tungsten oxide (WO3) or titanium dioxide (TiO2) as the second tandem semiconductor. The Si/TiO2 device demonstrated the highest performance with an expected solar-to-hydrogen efficiency of 0.39%. To achieve these demonstrations new processing methods were needed and developed for formation of the np+-Si microwire array homojunction and formation of a low resistance contact between the p+-Si and second semiconductor using sputtered tin- doped indium oxide (ITO) and spray pyrolyzed fluorine-doped tin oxide (FTO).
Another achievement includes demonstration of the longest known (>2200 hours) photoanode stability for water oxidation using a np+-Si microwire array coated with an in-house developed amorphous TiO2 protection layer and NiCrOx electrocatalyst. Additionally, the Si microwire array architecture was used to enable decoupling of semiconductor light absorption and catalytic activity, two performance metrics that ideally are maximized simultaneously. However, all previous demonstrations have shown anti-correlation between these performance metrics because planar architectures are subject to a trade-off where adding electrocatalyst increases catalytic activity, but decreases semiconductor light absorption and vice versa.
Finally, a techno-economic analysis of solar water splitting production facilities was performed to assess economic competitiveness because this is the ultimate metric by which all energy production technologies are currently evaluated. This analysis suggests that a hydrogen production facility that is cosmetically similar to current solar panel installations with hydrogen collection from distributed tilted panels is unlikely to achieve cost competitiveness with fossil fuel derived hydrogen due to the balance of systems costs alone. A cost of CO2 greater than ~$800 (ton CO2)-1 was estimated to be necessary for the least expensive base-case solar-to-hydrogen system to reach price parity with hydrogen derived from steam reforming of methane priced at $3 (MM BTU)-1 ($1.39 (kg H2)-1). Direct CO2 reduction systems were also explored and resulted in even larger challenges than hydrogen production. Accordingly, major facility wide breakthroughs are required to obtain viable economic costs for solar hydrogen production, but the barriers to achieve cost-competitiveness with existing large-scale thermochemical processes for CO2 reduction are even greater.
Item Type: | Thesis (Dissertation (Ph.D.)) | ||||||||||||||||||||||||
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Subject Keywords: | Artificial Photosynthesis, Solar Fuels, Solar, Hydrogen, Microwires, Silicon, Energy, Energy Storage | ||||||||||||||||||||||||
Degree Grantor: | California Institute of Technology | ||||||||||||||||||||||||
Division: | Chemistry and Chemical Engineering | ||||||||||||||||||||||||
Major Option: | Chemical Engineering | ||||||||||||||||||||||||
Awards: | Constantin G. Economou Memorial Prize, 2012 | ||||||||||||||||||||||||
Thesis Availability: | Public (worldwide access) | ||||||||||||||||||||||||
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Group: | Resnick Sustainability Institute, JCAP | ||||||||||||||||||||||||
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Defense Date: | 23 November 2015 | ||||||||||||||||||||||||
Record Number: | CaltechTHESIS:05312016-151045544 | ||||||||||||||||||||||||
Persistent URL: | https://resolver.caltech.edu/CaltechTHESIS:05312016-151045544 | ||||||||||||||||||||||||
DOI: | 10.7907/Z98C9T7Z | ||||||||||||||||||||||||
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Default Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. | ||||||||||||||||||||||||
ID Code: | 9817 | ||||||||||||||||||||||||
Collection: | CaltechTHESIS | ||||||||||||||||||||||||
Deposited By: | Matthew Shaner | ||||||||||||||||||||||||
Deposited On: | 06 Jun 2016 23:04 | ||||||||||||||||||||||||
Last Modified: | 08 Nov 2023 18:46 |
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