Session: 17-04: Symposium Steinfeld - New solar chemical processes and cycles

Paper Number: 138329

138329 - Modeling of a Photoelectrochemical Reactor and Solar Cavity-Receiver for Co-Production of Steam and Hydrogen 

Abstract: 

The conversion of solar light and reactants, such as water or CO₂, via photo-electrochemical (PEC) conversion is a promising route for renewable energy fuel generation¹. A kW-scale plant on the EPFL campus has been designed, implemented, characterized and successfully tested under real sun conditions². The reported device-level solar-to-hydrogen (STH) efficiency of the PEC reactor was 22.6% (solar light falling on photoactive area vs. produced H₂). However, the system-level STH efficiency of the plant showed a value of 6.5% only (solar light incident on solar dish mirror vs. produced H₂). The main loss on the system level was attributed to the PEC reactor aperture covering only a small part of the concentrated solar flux spot (~33%). The efficient use of the entire solar flux spot is highly challenging given the requirement for a uniform flux distribution falling on the PEC reactor.

We propose a design combining a solar cavity-receiver and a PEC reactor for the co-production of hydrogen and steam. The solar cavity-receiver is used for absorbing the highly non-uniform concentrated solar radiation. The cavity’s aperture is described by two concentric squares where the inner square surrounds the PEC aperture. The cavity’s cross-section is a square, resulting in an extruded square annulus. The cavity (i.e. the enclosure) comprises 4 surfaces where i) the upper surface forms the cavity’s aperture, ii) the inner and lower surface represent water-cooled surfaces for protecting sensible components (e.g. photoabsorber) from overheating, and iii) the outer surface is the actual solar absorber. An analytical heat transfer model considering radiative heat transfer in the cavity was implemented. The radiative heat transfer equations were solved by means of the radiosity method. Heat conduction and heat convection in the absorber were neglected.

The incident solar input on the secondary solar cavity-receiver was 6.37 kW with a mean concentration of 351.5 suns. For a targeted absorber temperature of 700 K (i.e. steam at 700 K) and assumed emissivities of 0.2 and 0.7 for the water-cooled, diffusely reflecting surfaces and the absorber surface, respectively, we determined an absorbed thermal power of 2.86 kW. This resulted in a steam production rate of 53 g min^-1 (refer to the produced H₂ of 0.8 g min^-1 by the primary PEC reactor²). The calculated thermal efficiency of the solar cavity-receiver (i.e. absorbed thermal power vs. incident solar input) was 44.9%. The main losses occurred due to re-radiation (2.21 kW) and absorbed power on the inner and lower water-cooled surfaces (1.3 kW). The determined overall efficiency of the plant (i.e. solar-to-hydrogen+steam efficiency) was 13.7%, which shows the potential for doubling the system-level efficiency (compared to the PEC-plant without secondary solar cavity-receiver with 6.5% efficiency).

We proposed the extension of primary PEC production of H₂ using concentrated solar radiation by a secondary solar cavity-receiver for steam generation utilizing the non-uniform part of the flux spot. An analytical radiative heat transfer model was implemented for investigating the potential of increasing the overall system-level efficiency of a co-generation plant. This co-generation ability and efficiency increase improves the economic viability of PEC-based solar plants, and thus will facilitate the production of solar fuels.

1.           Dumortier, M., Tembhurne, S. & Haussener, S. Holistic design guidelines for solar hydrogen production by photo-electrochemical routes. Energy Environ Sci 8, 3614–3628 (2015).

2.           Holmes-Gentle, I., Tembhurne, S., Suter, C. & Haussener, S. Kilowatt-scale solar hydrogen production system using a concentrated integrated photoelectrochemical device. Nat Energy 8, 586–596 (2023).

Presenting Author: Clemens Suter EPFL

Presenting Author Biography: 2016 – today: Post-doc at Laboratory of Renewable Energy Science and Engineering (LRESE), School of Engineering, EPF Lausanne, Switzerland, https://lrese.epfl.ch/:
• Lab responsible
• Education: lectures, exercises, development and lead of practical laboratory course on temperature measurements
• Organization and execution of experiments in high flux solar simulator and solar dish
• Inverse analysis of flux maps for the characterization of high-flux sources, such as solar simulators or solar dishes
• Integrated high-temperature electrolysis solar reactor operating under concentrated irradiation
• Implementation of thermal energy storage testbed and testing of advanced heat storage configurations combining sensible and latent media
• Characterization of porous materials in extreme radiative environments
2021 – 2022: Soft Power, Start-up from EPFL, Switzerland
https://softpower2020.com/
• Project management
• Solar-driven electrolysis plants for H2 and O2 production
• H2 for cooking fuel in development countries
• O2 processing for medical centers in development countries
2012 - 2016: industry (AFC Air flow consulting, www.afc.ch)
2008 – 2012:PhD thesis at the Professorship of Renewable Energy Carriers (PREC), Institute of Energy Technology, ETH Zürich, Switzerland, www.pre.ethz.ch:
• Elaboration of the PhD thesis entitled “Thermoelectric Conversion of Concentrated Solar Radiation and Geothermal Energy”
• Organization and execution of experiments in high flux solar simulators
• Development of numerical heat transfer models considering conduction, convection and radiation in thermoelectric converters
• CFD simulations (ANSYS CFX) considering heat transfer and water flows in thermoelectric converters
• Synthesis of thermoelectric material based on perovskite-type oxides and assembly of thermoelectric modules
• Simulation and optimization of thermoelectric systems for conversion of concentrated solar radiation and geothermal energy

Authors:

Clemens Suter EPFL
Mahendra Patel EPFL
Sophia Haussener EPFL