(712f) Solar High-Temperature Processes for Solar Fuels Production and Solar Energy Storage Applications

When solar energy is employed as the energy source for the production of the raw materials for the synthesis of fuels, the latter are characterized with the term â??solar fuelsâ?. In the broad sense this term can contain in addition to â??solar hydrogenâ?, synthetic liquid hydrocarbons and alcohols that are produced from reactions between H₂ and CO (syngas) that have originated from solar-aided dissociation processes as well as metal powders obtained by solar thermal reduction of metal oxides. Among the various solar energy variances that can be employed to produce such fuels, the thermochemical route uses solar heat at high temperatures supplied by Concentrated Solar Power (CSP) systems â?? i.e. special mirror assemblies that track the sun - for performing various high-temperature reactions that produce hydrogen or syngas from transformation of various fossil and non-fossil fuels. Such chemical reactions are steam methane (natural gas) reforming (SMR) or CO₂ (or dry) methane reforming (DMR), (reactions (1) and (2) respectively), gasification of solid carbonaceous materials like coal or biomass, or Water Splitting (WS) to hydrogen and oxygen; this last reaction often in combination with Carbon Dioxide Splitting (CDS) to CO and O₂, to produce syngas. In addition, CSP systems can be employed alternatively to photovoltaics for hydrogen/syngas synthesis indirectly, e.g. by supplying the (solar thermal) electricity for high-temperature electrolysis of steam or of steam/CO₂ mixtures.

The so-called â??closed-loopâ? WS thermochemical cycles (TCs) are a series of consecutive chemical reactions (â?¥ 2), their â??netâ? sum being the splitting of H₂O to H₂ and O₂, where the maximum-temperature (endothermic) step takes place at a temperature lower than that of the single-step water decomposition chemical reaction. Nevertheless, since they involve a highly endothermic step, they need the input of external energy. The most prominent examples of such solar-aided thermochemical cycles are cycles from the so-called sulphur family (the hybrid sulphur cycle described in reactions (3)-(5) and the sulphur-iodine process) and two-step redox cycles via multivalent metal oxide pairs, as in reactions (6) and (7). Each particular route exhibits advantages as well as drawbacks. An advantage of the latter route vs. the Sulphur-based ones is that the same thermochemical principle can be exploited for CDS as well for the production of CO (reaction (8)) that can be further combined with the H₂ produced via reaction (7) to syngas. On the other hand the high-temperature endothermic step of the sulphur cycles, SO₃ splitting to SO₂ and O₂ requires much lower temperatures than WS/CDS.

CH4 + H2O + (Î?H) â?? 3 H2 + CO	â?¦(1)
CH4 + CO2 + (Î?H) â?? 2 H2 + 2CO	â?¦(2)
2 H­2SO4­ (l)­+ (Î?H) â?? 2 SO3 (g) + 2 H2O (g)	â?¦(3)
2 SO3 (g)+ (Î?H) â?? 2 SO2 (g) + O2 (g)	â?¦(4)
2 SO2 (g) + 4 H2O (l) â?? 2 H2­SO4 (l)+ 2 H2 (g) (electrolysis)	â?¦(5)
MeOoxidized + (Î?H) â?? MeOreduced + ½ O2 (g)	â?¦(6)
MeOreduced + H2O (g) â?? MeOoxidized + H2 (g) + (Î?H)	...(7)
MeOreduced + CO2 (g) â?? MeOoxidised + CO (g)+ (Î?H)	...(8)

Similar chemistry schemes can be employed in the same solar thermal power plant for the so-called thermochemical storage (TCS) of solar energy. For instance, Sulphur-based cycles can be modified to produce solid Sulphur in next steps of the cycle (disproportionation reaction (9)), which can be then combusted in air to produce high-temperature heat and SO₂ (reaction (10)). Similarly, the heat produced by e.g. a solar receiver during on-sun operation can be used to power an endothermic redox oxide reduction like (6); should this reaction be completely reversible the thermal energy can be recovered completely by the reverse reaction (11) taking place during off-sun operation. The â??netâ? result of such cycles (reactions (3), (4), (5), (9), (10) or (6), (11)) is not the production of a particular chemical but the exploitation of the heat effects of the reactions for the â??storageâ? of solar heat.

2H2O(l)+3SO2(g)â?? 2H2SO4(aq)+ S(s) (disproportionation)	...(9)
S(l) + O2(g) â?? SO2(g)	...(10)
MeOreduced + O2 (g) + (N2 (g)) â?? MeOox + (N2 (g)) + (Î?H)	...(11)

Large scale production of solar fuels will require the economies of scale offered by heliostat fields with central tower receivers that can comfortably generate solar thermal fluxes in the MW capacity, although the deployment of such fields to date has been limited to electricity generation. For the efficient design and operation of solar aided-reactors for such processes, concepts from â??traditionalâ? chemical reactor engineering should be combined with ways to achieve efficient heating of the reactor via concentrated solar irradiation. Among the endothermic steps of the reaction schemes above, reforming and SO₃ splitting are catalytic reactions of gases on a solid catalyst surface. In contrast redox oxide-based cycles involve gas-solid reactions where the solid reactant (metal oxide) is not a â??catalystâ? present in much smaller quantities than those of the gaseous reactants, but a reactant itself, with non-negligible mass, that not only has to be heated to the reaction temperature but gets progressively depleted during the course of the reaction, having to be replenished. Therefore it either has to be fed constantly into the reactor if the reactions are to be performed in a continuous-mode, or alternatively, practical ways have to be â??inventedâ? for itsâ?? in-situ regeneration imposing rather challenging reactor operation conditions.

All processes described above, reforming, splitting and TCS, involve not only gas-solid chemical contact and reaction but heat exchange between either the solar receiver and the heat transfer fluid (SMR/DMR, WS/CDS) or the heat transfer fluid and the storage medium (TCS). The major technical challenge for these applications lies in the proper design and operation of reforming/splitting/storage reactors that will operate simultaneously and efficiently as heat exchangers. The two main research tasks are the improvement of solar interfaces and integrated heat recovery schemes in the one hand and solving the main materials-related issues and providing the right functional materials at reasonable costs in the other hand. The present analysis gives an overview on the recent developments and state-of-the art of those concepts at DLR as well as at international level, emphasizing on the most important performance parameters involved and the commonalities that can lead to convergence of reactor concepts employed up until now in different applications. Finally, the current outlook on further potential and necessary developments in the areas of solar fuels production and the use of chemical reactions to store solar heat is discussed.