Experimental study of direct solar photocatalytic water splitting for hydrogen production under natural circulation conditions

Photocatalytic water splitting for hydrogen production provides a promising route for the future hydrogen economy, being operational in the visible light domain with a potential use of solar radiation. An outdoor pilot demonstration of CPC-based photoreactors has been designed, installed and tested at the State Key Laboratory of Multiphase Flow in Power Engineering to assess its effectiveness in solar photocatalytic hydrogen production. Nine sets of CPC-based photoreactors, each of which is 3.6 m² in area and 23 L in volume, are connected, controlled and operated in parallel. The high efficiency photocatalyst (Cd_1-xZn_xS), low concentration sacrifice agents (Na₂S and Na₂SO₃) and deionized water are the raw materials of the pilot system. Two operation models, viz. the natural circulation model and the gas disturbance model, are proposed considering the expense and the efficiency. From our observations, the slurry temperature inside the tubes rises by 20–30 °C from the ambient. The slurry velocity can reach 1.2 m/s in the gas disturbance model, but is as low as 3.5 cm/s in the natural circulation model. The average hydrogen productivity is 184.30 mL/min and accumulated to be 10.321 L/h in the natural circulation model, with the average solar radiation, photocatalyst concentration and sacrifice agents’ concentration being 803.8 W/m², 2.77 g/L and 0.1 mol/L, respectively.