Hydrogen and Oxygen Production via Water Splitting in a Solar-Powered Membrane Reactor—A Conceptual Study

Among the processes for producing hydrogen and oxygen from water via the use of solar energy, water splitting has the advantage of being carried out in onestep. According to thermodynamics, this process exhibits conversions of practical interest at very high temperatures and needs efficient separation systems in order to separate the reaction products, hydrogen and oxygen. In this conceptual work, the behaviour of a membrane reactor that uses two membranes perm-selective to hydrogen and oxygen is investigated in the temperature range 2000–2500 °C of interest for coupling this device with solar receivers. The effect of the reaction pressure has been evaluated at 0.5 and 1 bar while the permeate pressure has been fixed at 100 Pa. As a first result, the use of the membrane perm-selective to oxygen in addition to the hydrogen one has improved significantly the reaction conversion that, for instance, at 0.5 bar and 2000 °C, moves from 9.8% up to 18.8%. Based on these critical data, a preliminary design of a membrane reactor consisting of a Ta tubular membrane separating the hydrogen and a hafnia camera separating the oxygen is presented: optimaloperating temperature of the reactor results in being around 2500 °C, a value making impracticable its coupling with solar receivers even in view of an optimistic development of this technology. The study has verified that at 2000 °C with a water feed flow rate of 1000 kg h−1 about 200 and 100 m3 h−1 of hydrogen and oxygen are produced. In this case, a surface of the hafnia membrane of the order of hundreds m2 is required: the design of such a membrane device may be feasible when considering special reactor configurations.