(281a) Development and Testing of a High-Temperature Heat Recovery System for a Solar Redox Reactor

We consider the production of solar fuels via a thermochemical redox cycle for splitting H₂O and CO₂, driven by concentrated sunlight. The product is a specific mixture of H₂ and CO – syngas – which can be further processed downstream by the established Fischer-Tropsch synthesis to liquid hydrocarbons such as kerosene (jet fuel). The entire solar fuel process chain has been successfully demonstrated with a solar reactor concept based on a cavity-receiver containing reticulated porous ceramic (RPC) ceria structures. The key performance metric of the solar reactor is its solar-to-fuel energy efficiency, which is strongly dependent on the ability to recover heat during the temperature-swing redox cycle. We report on the experimental investigation of a novel heat recovery method based on coupling the solar reactor with two thermocline energy storage (TES) units made of a packed-bed of alumina spheres and using an inert heat transfer fluid (HTF). With this arrangement, the heat rejected from the solar reactor during the switching from the reduction to the oxidation step can be recovered and stored in the TES units, and recuperated back to the solar reactor for the switching from the oxidation to the reduction step. By recovering half of the sensible heat in the RPC, the solar-to-fuel energy efficiency can reach values exceeding 20%.

The first generation setup has been developed and tested in the ETH’s high-flux solar simulator. The measured heat extraction effectiveness, defined as the ratio between the energy transferred to the HTF over the total amount of energy rejected or lost from the reactor between reduction end and oxidation start, was up to 70%, and the maximum HTF temperature obtained was over 1300°C. However, due to severe heat losses in the piping manifold between the reactor and the TES units, heat recuperation could only be performed at low temperatures. Based on the measured effectiveness of the TES units, a heat transfer model predicts an increase of up to 18% in the solar-to-fuel energy efficiency compared to the values without heat recovery.

A second-generation system has been designed, incorporating several improvements aimed at reducing the heat losses and increase the overall performance. Results of the experimental campaign with both first and second-generation systems will be presented.