Design of a Concentrator with a Rectangular Flat Focus and Operation with a Suspension Reactor for Experiments in the Field of Photocatalytic Water Splitting

Abstract

Photocatalytic water splitting is a potential route for future carbon-free production of hydrogen. However catalysts still need to be enhanced in order to reach acceptable solar efficiencies. In the context of the project HyCats funded by the Federal Ministry of Education and Research of Germany a high performance test facility for the evaluation of the activity of photocatalysts under practical conditions was established. It mainly consists of a solar concentrator and a planar receiver reactor. A modified linear Fresnel concentrator configuration was chosen based on ray tracing simulation results and improved concerning the number of different facets and the tolerance of tracking errors. It meets the major demand of a homogeneous intensity distribution on the surface of the reactor. The SoCRatus (Solar Concentrator with a Rectangular Flat Focus) is a 2-axes solar concentrator with a geometrical concentration factor of 20.2 and an aperture area of 8.8 m2. The tracking accuracy is better than 0.1° respecting both the solar azimuth and altitude angle. Its 22 highly UV/Vis-reflective flat aluminum mirror facets reflect the sunlight resulting in a rectangular focus with a nominal width of 100 mm and a nominal length of 2500 mm. The reactor is placed in the focal plane at a distance of 2500 mm from the mounting plane of the facets and allows concentrated solar radiation to penetrate suspensions of water, electrolytes and photocatalyst particles flowing through it. Corresponding to a maximum angel of incidence of 36.6° the Quartz window reflects not more than 5% of the incoming radiation and assures only marginal absorption, in particular of the UV-part of the sun’s spectrum. The material of the receiver body is PTFE (polytetrafluoroethylene) providing reflection coefficients above 90% concerning wavelengths of UV-A and UV-B. The special design of the reactor featuring two parallel reaction chambers offers the possibility to test two separate suspensions at the same irradiation conditions. A pump transports the temperate suspension to the reactor. The geometry of the reactor inlet and outlet minimizes critical volumes with inadequate flow. Any evolved gases are separated from the suspension in a tank together with nitrogen introduced in the piping upstream and are analyzed by micro chromatographs. Numerous devices are installed in order to control and monitor the reaction conditions. First experiments have been carried out using methanol as a sacrificial reagent.