System efficiency for two-step metal oxide solar thermochemical hydrogen production - Part 3: Various methods for achieving low oxygen partial pressures in the reduction reaction

The solar-to-hydrogen (STH) efficiency is calculated for various operating conditions for a two-step metal oxide solar thermochemical hydrogen production redox cycle to compare the effects of three methods for achieving low oxygen partial pressures for reduction. The calculations examine the effect on system efficiency of vacuum pump efficiency and inert gas/oxygen separation efficiency for a variety of reduction partial pressures using ceria as the active metal oxide. Currently available vacuum pump technologies have very low efficiencies at low pressures, and are unlikely to provide efficient hydrogen production relative to other oxygen partial pressure lowering technologies. Using currently available pumps arranged in a cascade pressure reduction configuration increases the effective pump efficiency significantly, but by less than an order of magnitude and, therefore, still results in low STH efficiencies for the system. If vacuum pumps could operate at a low pressure with an efficiency of ∼10% or better, vacuum pumping (including cascade pressure reduction) has the potential to operate very efficiently for solar thermochemical hydrogen production. A novel recycled inert gas sweep with high temperature separation is suggested, and STH efficiency values vary significantly depending on the inert gas flowrate required, and will be reactor and reaction rate dependent. However, the use of an inert gas is likely able to take advantage of greater extents of reduction at very low oxygen partial pressures and produce high STH values if the inert gas/oxygen separation is ∼10% efficient.