Thermochemical cycle

Thermochemical cycles combine solely heat sources (thermo) with chemical reactions to split water into its hydrogen and oxygen components. The term cycle is used because aside of water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled.

If work is partially used as an input, the resulting thermochemical cycle is defined as a hybrid one.

This concept was first postulated by Funk and Reinstrom (1966) as a reflexion about the most efficient way to produce fuels (e.g. hydrogen, ammonia) from stable and abundant species (e.g. water, nitrogen) and heat sources. Although fuel availability was scarcely considered before the oil crisis era, these researches were justified by niche markets. As an example, in the military logistics field, providing fuels for vehicles in remote battlefields is a key task. Hence, a mobile production system based on a portable heat source (a specific nuclear reactor was strikingly considered) was being investigated with the uttermost interest. Following the crisis, many programs (Europe,Japan,USA) were set up to design, test and qualify such processes for more peaceful purposes such as energy independence. High temperature (1000K) nuclear reactors were still considered as the heat sources. However, the optimistic expectations of the first thermodynamics studies were quickly moderated by more pragmatic analysis based on fair comparisons with standard technologies (thermodynamic cycles for electricity generation, coupled with the electrolysis of water) and by numerous practical issues (not high enough temperatures with nuclear reactors, slow reactivities, reactor corrosion, significant losses of intermediate compounds with time...). Hence, the interest for this technology was fading away during the next decades, or at least some tradeoffs (hybrid versions) were being considered with the use of electricity as a fractional energy input instead of only heat for the reactions (e.g. Hybrid sulfur cycle). A rebirth in the year 2000 can be explained by both new energy crisis and the rapid pace of development of concentrated solar power technologies whose potentially very high temperatures are ideal for thermochemical processes, while the environmentally friendly side of these researches attracts funding in a period with the peak oil shadow.

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