High-temperature heat recovery from a solar reactor for the thermochemical redox splitting of H2O and CO2

The solar splitting of H2O and CO2 via a thermochemical redox cycle offers a viable pathway for producing sustainable drop-in fuels for the transportation sectors. The key performance metric is its solar-to-fuel energy efficiency, which is strongly dependent on the ability to recover heat during the temperature swing between the reduction and oxidation steps. Here we report on the experimental investigation of a novel heat recovery method based on coupling the solar reactor with two thermocline energy storage units made of a packed-bed of alumina spheres. Using N2 as an inert heat transfer fluid, the heat rejected during cooling from the reduction to the oxidation temperature is stored and, following the oxidation step, delivered back to preheat the solar reactor towards the reduction temperature, thus reducing the required solar input and consequently increasing the efficiency. With a first experimental prototype, a heat extraction effectiveness of up to 70% from a 4 kW solar reactor is obtained with measured N2 outlet temperatures exceeding 1250 degrees C. Energy flow modeling of a 50 kW solar reactor predicts a theoretical upper limit value of the energy efficiency of 42% for perfect heat recovery without transient losses, and 14.7% with such losses included. Several improvements and insights into high-temperature heat recovery are detailed.

Automated summary

The solar splitting of H2O and CO2 through a thermochemical redox cycle is a viable way to produce sustainable fuels for transportation. The ability to recover and reuse heat during the process is crucial for achieving high solar-to-fuel energy efficiency.