Research on concentrated solar technologies for fuel production in Germany was started in the 1980s based on the extensive work done by nuclear research first by solar heating of reactors for the steam reforming of natural gas [1] and then by adapting processes originally designed for the use with heat from very high temperature nuclear reactors to concentrating solar facilities [2].
At that time no specific German research program for solar fuels existed. Therefore, since then the projects were carried out either in the European Research Framework Programmes, in regional programs like the AG Solar of the Federal Sate of North Rhine Westphalia or in specific research projects funded by the German Ministries for Education and Research and for Economic Affairs and Energy.
In 2015, a new renewable energy program of the German Helmholtz Association (HGF) started [3]. In this program, solar fuels appears as an own topic for the first time equal to other renewable energy technologies like photovoltaics, solar thermal electricity, bio-energy, geothermal energy, and wind energy. It combines the work on solar thermochemical, photo-electrochemical, and artificial biochemical processes. The reason to start this topic was the recognition of its importance for our future energy economy. The follow-up HGF program is presently in preparation and shall start in 2021. Solar fuels will have at least an equally strong role in it. In September 2019, the seventh Energy Research Programme of the German Government was published, and it also contains the topic of solar thermal fuel production for the first time [4].
The global research on thermochemical cycles for hydrogen production started in the 1960s. Over the last 40 years, the interest in these technologies strongly varied. Since the end of the 1990s, the focus shifted from nuclear to solar heat for water splitting and since a few years also the splitting of CO2 is a major topic as it opens up the possibility to produce liquid fuels. However, out of the 3000 possible thermochemical cycles only very few are identified to be relevant for bulk production of solar fuels. In Germany, mainly two groups of cycles are worked on: the solid metal-oxide cycles and the sulfur cycles.
The metal-oxide concept was proven experimentally at the German Aerospace Center (DLR) in its solar furnaces in Cologne for the first time in a batch reactor in 2002 within the European HYDROSOL project. A quasi-continuous operating monolithic solar receiver reactor was the second step in the project in 2005. From 2008 until 2015, the 100 kWth HYDROSOL 2 pilot plant was installed on a solar tower of the Plataforma Solar de Almería (PSA), Spain. A further scale up was prepared by the design study HYDROSOL 3D and realized in the HYDROSOL-Plant project [5]. In 2017, a three chamber reactor with a maximum power input of 750 kWth was inaugurated on the PSA. The development continues in the HYDROSOL Beyond project starting in January 2019. In parallel projects, more sophisticated reactor technologies were developed as well as new and more efficient redox materials, and in the European Sun-to-Liquid project, the splitting of water and CO2 will be demonstrated on a solar tower at IMDEA Energia, Mostoles, Spain [6].
Originally, sulfur-based thermochemical cycles were developed in the 1970s to produce hydrogen by coupling them with heat from very high temperature nuclear reactors. Most of the sulfur cycles are based on the thermal splitting of sulfuric acid into sulfur dioxide and oxygen. Although metal oxide-based cycles are presently more in the focus, sulfur cycles have the big advantage of 500 K lower temperature for performing the endothermic process step than metal-oxide-based systems. Therefore, Germany continues to work on sulfur cycles as it seems to be likely to get to competitive efficiencies earlier.
In the frame of the EU projects HYTHEC and HycycleS, the reliability and the potential hybridization of the hybrid sulfur cycle with solar energy [7] were analyzed and materials were identified for a technical realization of the solar thermal decomposition of sulfuric acid [8]. For this step, a test reactor was developed and tested in the solar furnace of DLR in Cologne, Germany. In the European SOL2HY2 project, a 100 kWth reactor was demonstrated on DLR's solar tower in Jülich, Germany. It includes the possible coupling to an open thermochemical cycle. This Outotec™ Open Cycle uses SO2 from roasting ores for hydrogen production. In the ongoing European PEGASUS project, the solar thermal splitting of sulfuric acid will be demonstrated in the MW range. The necessary heat will be provided by solar-heated catalytic particles [9].
The next goals in Germany are to further increase the efficiency and reliability of the technologies and to further scale-up the processes for a fast market deployment.
