Generation of hydrogen from high- temperature solar thermal energy. PROFIT Initiative
Victoria Gallardo;
PSA Contact: Alfonso Vidal,
Cooperation Project funded by the MEC (Spanish Ministry of Education and Science) PROFIT program. Total budget: 987 k€. CIEMAT Budget not including personnel: 286 k€.
January 1, 2004 – March 31, 2009

The most confidence is deposited in thermochemical cycles as a mid-to-long-range solution for mass production of clean H2 for solar energy.

Thermochemical cycles have the great advantage of allowing dissociation of the water molecule into several stages, generating H2/O2 in different stages. As an example, a thermochemical cycle based on the use of Ni-Mn ferrites, which is the subject of study in the SOLTERH Project, is summarized below:

The use of mixed oxides (ferrites) has some important advantages, as it is a simple cycle that considerably lowers the material regeneration temperature. Hydrogen generation is based on artificially creating defects in the oxide structure.

The main purpose of the SolterH Project is to demonstrate the usefulness of the binomial renewable energies-hydrogen vector, specifically, with solar thermal energy, for producing clean, renewable hydrogen from the unlimited solar resource. The final goal of the SOLTERH Project is to design, develop and evaluate a system able to produce hydrogen from high-temperature solar thermal energy, which would consist of testing a 5-kW solar reactor at the Plataforma Solar.

Testing was done in the PSA Solar Furnace using Ni ferrites supplied by Sigma-Aldrich. The first tests were directed at determining the amounts of O2 and H2 obtained from each stage based on our laboratory experience. The stages may be summarized along general lines as:

This operation has been performed for several cycles to confirm the repeatability of the cycle and durability of the material after cycling. Fig. 1 shows the molar flows of oxygen and hydrogen during the experiment, and the temperature on the surface of the ferrite bed.

The results of this first cycle, from the viewpoint of hydroproduction and the H2/O2 ratio are far from those found in the laboratory. However, in the second cycle, the contrary occurs. Although oxygen production in the second activation drops with respect to the first, hydrogen production increases noticeably in the second cycle over the first.

Similar results were found in laboratory cyclability studies with a NiFe2O4 sample, and may be explained by the second hydrolysis oxidizing part of the ferrite which had been reduced in the first cycle but not reoxidized. Laboratory tests combined with different physico-chemical characterization techniques for the study of this behavior are currently underway.

Evolution of O2 and H2 in the first thermochemical cycle with NiFe2O4
Hynergreen and CIEMAT.