Finding the optimum between volatility and cycle temperatures in solar thermochemical hydrogen production: Pb/PbO pair

Abstract

A pilot unit for a concentrated solar thermal reactor with solar tracking was constructed. A 70 cm diameter concentrator dish could provide temperatures around 800 °C at a fairly steady rate. In the search for a redox pair that can work at such temperatures, studies were conducted on the feasibility of the Pb/PbO cycle for the splitting of H2O for H2 production. Thermodynamics accounting for the vapor pressures of Pb and PbO indicated favorable water splitting until approximately 900 °C, at atmospheric steam pressures. After 1000 °C, the vapor pressure of PbO becomes greater than that of Pb, as a result, thermodynamic favorability in the gas phase begins to be suppressed. The thermodynamic estimations were tested experimentally both in lab scale, and in bench scale molten bed reactors. The process went through similar maxima in the hydrogen production rates. The field tests in solar concentrator using a Pb coated mullite system did not reveal much due to evaporative loss of Pb and PbO under high solar flux. In this paper, it was demonstrated that the relatively high vapor pressure of Pb and PbO can be exploited for designing efficient water splitting cycles at temperatures <1000 °C using a PbO vapor recovery and circulation system.