Jim Hinkley

Efficient ceria nanostructures for enhanced solar fuel production via high-temperature thermochemical redox cycles

Syngas synthesis by solar energy-driven two-step thermochemical redox cycles is a promising approach for large-scale industrial production of renewable fuels. A key challenge is developing durable materials capable of providing and sustaining high redox kinetics under harsh environmental conditions required for efficient operation. Here, we demonstrate that nanostructured ceria with a high surface area and porosity can significantly enhance the initial and long-term syngas production performance. Three types of ceria morphologies were synthesised and comparatively investigated against commercial powders in two-step thermochemical redox cycles, namely nanostructured flame-made and flower-like agglomerates and sol–gel sub-micro particles. Their syngas production performance was assessed in terms of redox kinetics, conversion stoichiometry and structural stability. The flame-made ceria nano-powders had up to 191%, 167% and 99% higher initial average production rates than the flower-like, commercial and sol–gel ceria powders, respectively. This resulted in the highest H2 (480 μmol min−1 g−1) and CO (230 and 340 μmol min−1 g−1) production rates and redox capacity (Δδ = 0.25) so far reported for ceria. Notably, the grain morphology played a key role in the long-term performance and while the redox kinetics of the flower-like ceria rapidly decreased below that of the commercial powders, the flame-made agglomerates maintained up to 57% higher average production rate until the last cycle. These findings show that the thermochemical stabilisation of nano-scale structural features, observed in the flame-made agglomerates, is key to engineering efficient materials for enhanced thermochemical solar fuel production.

HycycleS: a project on nuclear and solar hydrogen production by sulphur-based thermochemical cycles

The European FP7 project HycycleS focuses on providing detailed solutions for the design of specific key components for sulphur-based thermochemical cycles for hydrogen production. The key components necessary for the high temperature part of those processes, the thermal decomposition of H2SO4, are a compact heat exchanger for SO3 decomposition for operation by solar and nuclear heat, a receiver-reactor for solar H2SO4 decomposition, and membranes as product separator and as promoter of the SO3 decomposition. Silicon carbide has been identified as the preferred construction material. Its stability is tested at high temperature and in a highly corrosive atmosphere. Another focus is catalyst materials for the reduction of SO3. Requirement specifications were set up as basis for design and sizing of the intended prototypes. Rigs for corrosion tests, catalyst tests and selectivity of separation membranes have been designed, built and completed. Prototypes of the mentioned components have been designed and tested.

Investigation of lithium sulphate for high temperature thermal energy storage

Lithium sulphate (Li2SO4) was evaluated as a solid-solid PCM material to be coupled with concentrated solar power (CSP) technologies. The energy is stored in a cubic crystalline phase that is formed at temperatures above 576°C and can potentially be discharged at temperatures as low as 150°C, providing both sensible and latent thermal energy storage in a hybrid sensible-latent system. These operational conditions are appropriate for current CSP technologies based on subcritical steam Rankine power cycles. Results from thermal cycling experiments in air showed no change in energy storage capacity after 15 cycles. There was up to a 5% reduction in latent thermal capacity and 0.95% in total thermal capacity after 150 cycles in air. In our paper, we evaluate a hybrid sensible-latent thermal energy storage system based on lithium sulphate from an economic and technical performance point of view, demonstrating its potential as a high temperature thermal energy storage material.