Solar Fuels offer a promising route for deep-decarbonization of the energy sector. They can be used for affordable, long-term storage of renewable electricity, in long-distance transportation, and for providing high-temperature heat in industry. Thermochemical redox cycles are a promising pathway for converting solar heat directly to energy carriers like syngas and liquid fuels. In state-of-the-art thermochemical cycles, metal oxides like ceria undergo endothermic thermal reduction using solar thermal energy, typically at 1500°C and oxygen partial pressures lower than 1mbar. The metal oxide is subsequently regenerated in a water-splitting or CO2-splitting step (gas-splitting), typically around 800°C and atmospheric pressure. The temperature and pressure swings are necessary for each step to be thermodynamically favored.Significant efficiency improvements are needed before this technology can produce hydrogen at less than $3/kg-H2 – a key price target for making green fuels viable. The dominant cause of low efficiency is the lack of solid heat recovery between the reduction and oxidation temperatures. Other factors include inefficient gas exchange processes like vacuum pumping, low fuel production per cycle, intermittent operation, and the lack of “waste" heat recovery&nbsp;at the lower temperature. Additionally, many reactor designs include significant technical challenges like sealing between reduction and oxidation zones, and transporting materials at high temperatures. Although several reactor designs have been proposed, it has not been possible to demonstrate even 50% solid heat recovery in a realistic thermochemical system.We present a novel “Reactor Train” design, comprising of several identical reactors arranged in a closed loop and cycling between reduction and oxidation steps. Between the reduction and oxidation zones, the reactors form a Counterflow Radiative Heat Exchanger (CfRHx), where reactors that are being cooled exchange heat with reactors that are being heated, significantly improving heat recuperation. The use of multiple reactors enables continuous solar energy utilization and highly scalable fuel production. Moreover, this design significantly reduces the technical difficulties of high-temperature material conveying and sealing, because each reactor is heavily insulated and can easily be sealed. In the CfRHx, reactors exchange heat radiatively through face-to-face windowed openings, without the need for heat transfer fluids. The reduction zone contains a radiative heat source to supply thermal energy to heat up the reactors, and for the endothermic reduction reactions. This heat source could be a coil of graphite tubes carrying molten metal coupled to the high-temperature solar receiver. Such an indirect coupling between solar collection and the thermochemical system allows for thermal energy storage, guarding against intra-day transients in solar irradiation and potentially enabling continuous fuel production through the night. Similarly, the oxidation zone contains a radiative heat sink for harvesting unrecovered reactors thermal energy (around 800oC) and from the exothermic gas-splitting reactions, for electricity production in a bottoming cycle.In this paper we describe the Reactor Train design in detail, and a preliminary design for the individual reactors. We have identified window and insulation materials for the reactors, and constructed a heat transfer model to simulate the radiative exchange through semi-transparent windows. This model was coupled with conductive heat losses through the insulation material, and models for redox reactions throughout the operation. We have also developed a computationally efficient numerical model to predict the performance of the Reactor Train. Solid heat recovery effectiveness was estimated to be between 75-80%, leading to a heat-to-fuel cycle efficiency of upto 30%. The Reactor Train design thus achieves significant efficiency improvement over existing reactor designs, while addressing some of their most pressing technical challenges.

A Reactor Train System for Efficient Solar Thermochemical Fuel Production

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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