The European research project SOL2HY2 (Solar to Hydrogen Hybrid Cycles) investigates the Hybrid Sulphur Cycle (HyS) for solar hydrogen production.
This process - also known as Westighouseprozess or "Ispra Mark 11" - is considered one of the most promising approaches to generate hydrogen by use of thermal energy.
Hydrogen, as a potential substitute for fossil fuels, is free of CO² emissions if generated from water using renewable energy sources. Conventional water electrolysis, as the state-of-the-art technology, has a low overall efficiency as it demands the generation of electricity.
On the other hand, direct thermal splitting of water would require temperatures in excess of 2000 °C in order to yield sufficient amounts of hydrogen. Therefore, thermochemical cycles are introduced to lower the process temperature to a technical manageable level of about 1000 °C.
The project consortium is led by the Italian scientific consultant company EnginSoft with participation of the DLR Institute of Solar Research, the Finnish university Aalto, the Italian research center ENEA, the Finnish technology company Outotec, the Swiss ceramics company Erbicol and the Finnish gas company Woikosky.
The HyS presented in the figure below is a two-step cycle producing hydrogen and oxygen out of water using solar energy.
Figure: Hys - Hybrider Schwefelsäureprozess
In the first reaction, sulphuric acid is decomposed at high temperature forming sulphur dioxide and oxygen; the latter being separated from the product gas as a by-product. Sulphur dioxide is electrolysed at about 80 °C together with water in the second reaction generating hydrogen and fresh sulphuric acid, which is recycled back to the first reaction. The electrical power required for this electrolysis is only about a tenth of that needed for conventional water electrolysis so that the energy demand for hydrogen production is significantly reduced. This is crucial for an industrial realization of the hydrogen technology.
In the project SOL2HY2 all key components of the HyS process (e.g. solar sulphuric acid cracker, sulphur dioxide depolarized electrolysis, gas separation, heat storage) will be developed and demonstrated at relevant scale. DLR will develop and operate a pilot plant for solar decomposition of sulphuric acid in the 100 kW range on the Jülich Solar Tower.
The demonstration work is accompanied by a detailed process analysis and multi-objective optimization aiming at a future commercialization of green technology hydrogen production.