Movable mirrors closely arranged over a large area capture sunlight, bundle it and direct it to a receiver, where the concentrated solar energy can be used e.g. for thermochemical reactions.

The concentrated solar energy is used to heat dark particles (e.g. sand-like bauxite ceramic solid particles) in a rotating solar particle receiver through direct irradiation in a solar tower. The moving solid particle streams act as an inexpensive heat transfer fluid (HTF) that can also be used as a heat storage medium. Downstream of the solar receiver, the hot solid particles are used for the allothermal performance of sulfur-based thermochemistry: the sulfur trioxide splitting reaction together with the preceding sulfuric acid decomposition. The obtained sulfur dioxide is then used in the SDE to re-generate sulfuric acid closing its cycle.

It is the electrochemical oxidation of sulfur dioxide (SO2) with water to yield hydrogen at the cathode (negatively polarized electrode) and sulfuric acid at the anode (positively polarized electrode). In this so-called electrolysis process, a certain potential difference has to be applied between the two electrodes, which are placed separately from each other through a polymer membrane in an electrolyte solution (sulfurous acid, formed by mixing water and SO2), while an external circuit ensures the transfer of electrons from the anode to the cathode.

The theoretical cell potential for the sulfur dioxide depolarized electrolysis (SDE) is only 0.17 V compared to 1.23 V for conventional water electrolysis. This means that the SDE offers a promising alternative to conventional electrolytical water splitting because of significantly reduced amounts of electrical power required.

Hydrogen is produced in the sulfur dioxide depolarized electrolysis, an electrochemical water splitting reaction, where electrons are transferred to protons from water molecules to form gaseous hydrogen. Its purity is mainly affected by SO2 carry-over from anode to cathode, where hydrogen is produced. Therefore, the suppression of SO2 carry-over is essential and represents a separate investigation task in the HySelect project in order to test new membrane materials and purification systems.

The final stage of the HySelect project foresees the operation of a pilot plant at the solar tower in Juelich with average daily solar-to-fuel energy conversion efficiency of >10 % and average hydrogen production rates higher than 0.75 kg/year per m2 land area used, equivalent of 2.16 kg/day/m2 (receiver area).

The overall process comprises the so-called hybrid sulfur cycle consisting of 2 reaction steps – a thermochemical and an electrochemical one – in order to recycle sulfur as the central element. In the thermochemical step, sulfuric acid is first decomposed to water (steam) and sulfur trioxide (SO3), which is then catalytically dissociated to sulphur dioxide (SO2) and oxygen (O2) at higher temperatures. Sulfur dioxide and water serve as educts for the electrochemical step, the second step, in the sulphur dioxide depolarized electrolysis, where hydrogen is formed and the initially used sulfuric acid is re-generated (recycled).

Sulfuric acid is the world´s most produced chemical. Hence, its safe use can build on a high degree of decades of industrial experience and operational safety and environmental impact have already been addressed to a large extent. Mature SOx emission capture/neutralization techniques can be applied in the HySelect process preventing environmental harm and ensuring that the sulphur containing compounds remain in the process cycle. Potential emissions of SO2 may be effectively captured through elaborated filters from other sectors (H2SO4 production).

Due to size limitations of the used CentRec® receiver, an up-scale of hydrogen production can be reached by coupling identical solar tower modules in a multi-tower solar plant arrangement, where the hot particles are diverted to a single central tube reactor for the thermochemical production of sulfur dioxide. According to a modelling experiment, 14 solar tower modules would be necessary to feed a 125 MWe sulfur dioxide depolarized electrolysis unit for hydrogen production.

The development and application of a heat recovery prototype unit is one of the targets within the HySelect project in order to exchange the heat from the hot outlet gas of the sulphur trioxide decomposition reactor that has to be cooled down before SO2 can be separated and fed to the downstream SDE.

The solar tower in Jülich, Germany, as part of the DLR-site has been selected for operational testing of the overall HySelect process. The centrifugal particle receiver has already been installed in the Juelich Multi Focus tower there in the frame of another project (HEHTRES).