2016-11-02T18:49:00

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Solar thermochemical energy storage has enormous potential for enabling cost-effective concentrated solar power (CSP). A thermochemical storage system based on a SrO/SrCO3 carbonation cycle offers the ability to store and release high temperature (≈1200 °C) heat. The energy density of SrCO3/SrO systems supported by zirconia-based sintering inhibitors was investigated for 15 cycles of exothermic carbonation at 1150 °C followed by decomposition at 1235 °C. A sample with 40 wt % of SrO supported by yttria-stabilized zirconia (YSZ) shows good energy storage stability at 1450 MJ m(-3) over fifteen cycles at the same cycling temperatures. After further testing over 45 cycles, a decrease in energy storage capacity to 1260 MJ m(-3) is observed during the final cycle. The decrease is due to slowing carbonation kinetics, and the original value of energy density may be obtained by lengthening the carbonation steps.

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Successful implementation of solar thermochemical metal oxide water splitting cycles is dependent upon the ability to reach low partial pressures of oxygen during the thermal reduction step. Low partial pressures of oxygen are required to provide a thermodynamic driving potential for the thermal reduction reaction and avoidance of recombination. Achieving low partial pressures of oxygen (e.g., < 10 −2 bar) may require a nontrivial energy input to the solar-to-fuel conversion process, negatively impacting the solar to fuel energy conversion efficiency. Three different strategies to reduce the partial pressure associated with oxygen generated during an iron oxide thermal reduction process were investigated using an open system thermodynamic analysis. These strategies include vacuum pumping, purging with an inert gas, and purging with steam. If the difficult to achieve solid-phase heat recuperation is neglected, open-system thermodynamic simulations show that vacuum pumping will have over twice the overall cycle energetic and exergetic efficiencies than those of inert purging; assuming oxygen separation is required every cycle in the case of inert or steam purging. To demonstrate the concept of vacuum pumping, thermal reduction of an iron−zirconia bed in a tubular reactor was performed at low pressures of approximately 10 −4 bar at a temperature of 1450 °C. The maximum extent of reduction (14.2 ± 1.7 mol %) was reached after approximately 1 h of reduction at 1450 °C, while the predicted theoretical extent of reduction ranges from 16.5 mol % at 10 −2 bar to 76.9 mol % at 10 −4 bar. In the present analysis, reaction kinetics are not considered, and its application is limited to the thermodynamically driven processes.

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