Solar-Driven High-Temperature Electrolysis Reactor for Fuel Processing

Solar-driven high-temperature electrolysis (HTE) and thermochemical cycles (TCC) are two promising pathways for fuel processing and energy storage, which are considered in this thesis. The goals of this thesis are: i) offering engineering guidelines at system level via thermodynamic modeling frameworks for both technologies to evaluate and maximize their solar-to-fuel (STF) efficiencies and economic competitiveness, ii) the modeling, optimization, and experimental demonstration of a tubular solar reactor for efficient STF generation, and iii) investigation of a fully integrated solar reactor, which uses a tubular high-temperature electrolyzer as the solar absorber to reduce transmission heat losses and hence to improve the STF efficiency.

The techno-economic analysis of HTE systems is performed leading to the identification of the hybrid approach (utilizing concentrated solar heating and photovoltaics) as the optimal choice generating hydrogen at a high efficiency and low costs. This supports the competitiveness of the hybrid approach for scaled solar hydrogen generation.

A thermodynamic modeling framework for a two-step ceria-based TCC is developed analyzing different routes for oxygen partial pressure reduction. This allows for performance comparison between HTE and TCC. Compared with TCC systems, HTE systems work at significantly lower temperatures and the less stringent requirement for heat recovery and low oxygen partial pressure, while equivalent STF efficiency can be achieved indicating that HTE can be a more promising technology for scaled-up solar fuel plants.

The design and optimization of a tubular solar receiver, a key component in HTE system, are conducted based on an experimentally validated coupled heat and mass transfer model for the concurrent direct steam and CO2 generation. This integrated numerical model is composed of a detailed 1D two-phase flow model in the receiver tubes, which is then incorporated into a coupled 3D heat transfer model for tubular reactor cavity. Based on the model, design and demonstration of a 1 kWth solar reactor are presented. The demonstrator employs a direct steam generation solar absorber (two parallel helical tubes) directly connected to a 250 Wel solid oxide electrolyzer stack forming a compact design to reduce the transmission heat losses of high-temperature fluids. A solar-to-thermal efficiency of 77.8% (at a fluid temperature of 700 K) and STF efficiency of 5.3% (with a 15% solar-to-electricity efficiency) are achieved. The proof-of-concept demonstration leads to a promising pathway for highly efficient STF generation.

Further, a fully integrated solar reactor where the tubular solid oxide electrolyzer cell acts as the absorber as well as the reactor is investigated based on a 2D multi-physics model to further reduce the transmission heat losses and hence improving the STF efficiency.

A comprehensive investigation of HTE for solar fuel processing is presented in this thesis. The hybrid coupling strategy between solar energy and HTE using concentrated solar heating and photovoltaics is proven to have a high techno-economic competitiveness. This thesis provides a detailed and powerful tool for analysis, optimization, design, and prototyping of solar driven HTE systems and reactors. This thesis opens a new pathway toward compact solar reactor design and engineering for highly efficient solar fuel generation.