Review of the Two-Step H2O/CO2-Splitting Solar Thermochemical Cycle Based on Zn/ZnO Redox Reactions

Abstract

This article provides a comprehensive overview of the work to date on the two‑step solar H2O and/or CO2 splitting thermochemical cycles with Zn/ZnO redox reactions to produce H2 and/or CO, i.e., synthesis gas—the precursor to renewable liquid hydrocarbon fuels. The two-step cycle encompasses: (1) The endothermic dissociation of ZnO to Zn and O2 using concentrated solar energy as the source for high-temperature process heat; and (2) the non-solar exothermic oxidation of Zn with H2O/CO2 to generate H2/CO, respectively; the resulting ZnO is then recycled to the first step. An outline of the underlying science and the technological advances in solar reactor engineering is provided along with life cycle and economic analyses.

Solar Hydrogen Production via a Samarium Oxide-Based Thermochemical Water Splitting Cycle

“…Correspondingly, Irr quench is also lowered by 0.5 kW¨K´1 due to a similar decline in T H . These trends are in good agreement with the results reported in previous studies [9,10].…”

“…These include zinc oxide cycles [2][3][4][5][6][7][8][9][10][11], iron oxide cycles [12][13][14][15], tin oxide cycles [16][17][18], terbium oxide cycles [19], mixed ferrite cycles [20][21][22][23][24][25], ceria cycles [26][27][28][29], and perovskite cycles [30][31][32]. In the case of zinc and tin oxide cycles, due to the volatile nature of the MOs involved, the loss of reactive material due to high temperature operation is inevitable.…”

“…Due to operation at elevated temperatures, radiation heat losses from the solar reactor are inevitable. The radiation heat losses associated with the solar reactor installed for the Sm-WS cycle can be calculated as: (10) The values of reported in Table 1 indicate that radiation heat losses are greatest (2194 kW) when TH = 3000 K (oxygen partial pressure in the inert flushing gas = 10 −5 bar). A decrease in TH results in a significant reduction in .…”

“…As a first step in the exergy analysis, the absorption efficiency ( ), which is defined as the net rate at which energy is absorbed by the solar reactor divided by the solar energy input through the aperture, is determined as follows: The exergy analysis is performed following the methodology and governing equations derived and used previously for other MO based solar thermochemical cycles [6,9,10]. Thermodynamic properties are extracted from HSC software and databases and the analysis is normalized to a unit Sm 2 O 3 molar flow rate (1 mol¨s´1) entering the solar reactor.…”

Review of Heat Transfer Research for Solar Thermochemical Applications

“…Modeling the reactors for the solar thermochemical dissociation of ZnO has enabled a more comprehensive and in-depth understanding of the competing mechanisms and the identification of major sources of energy losses, which led to optimization and scaling up [99]. Various aspects of solar chemical reactors have been modeled at different levels of complexity.…”