Janna Martinek

Efficient generation of H2 by splitting water with an isothermal redox cycle.

Isothermal Water Splitting Solar concentrators can create extremely high temperatures that can drive chemical reactions, including the thermal splitting of water to provide hydrogen. A metal oxide catalyst is needed that is usually cycled between hotter conditions where it is reduced and cooler conditions where it is reoxidized by water. This cycling can limit catalyst lifetime, which can be costly. Muhich et al. (p. 540; see the Perspective by Roeb and Sattler) developed an approach that allowed the redox cycle to be driven isothermally, using pressure swings. A thermal process for generating H2 from water uses pressure changes to recycle between catalyst redox states. [Also see Perspective by Roeb and Sattler] Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide–based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS’s overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the “hercynite cycle” exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Rapid Solar-thermal Decarbonization of Methane in a Fluid-wall Aerosol Flow Reactor -- Fundamentals and Application

A graphite fluid-wall aerosol flow reactor heated with concentrated sunlight has been developed over the past five years for the solar-thermal decarbonization of methane. The fluid-wall is provided by an inert or compatible gas that prevents contact of reactants and products of reaction with a graphite reaction tube. The reactor provides for a low thermal mass that is compatible with intermittent sunlight and the graphite construction allows rapid heating/cooling rates and ultra-high temperatures. The decarbonization of methane has been demonstrated at over 90% for residence times on the order of 10 milliseconds at a reactor wall temperature near 2000 K. The carbon black resulting from the dissociation of methane is nanosized, amorphous, and ash-free and can be used for industrial rubber production. The hydrogen can be supplied to a pipeline and used for chemical processing or to supply fuel cell vehicles.

Thermodynamic Considerations for the Design of Solar-Thermal Chemical Processes

The maximum efficiency of a solar reactor/receiver is limited not by the Carnot efficiency, but rather by the product of the Carnot efficiency and a factor involving both the enthalpy and entropy changes occurring as a result of the chemical reaction. This limiting efficiency can be greater than the Carnot efficiency when the entropy change is positive, and is a function of several parameters including the standard property changes of the reaction, inlet and outlet stream conditions, reactor temperature, conversion, and relative amounts of inert gas. By including this factor in the overall limitation on the reactor efficiency, both the optimal reactor temperature for a given concentration ratio, and the concentration ratio required to achieve a given temperature and efficiency can, in many cases, be found to shift to lower values.Copyright