Solar concentration systems typically entail optics (mirrors or lenses) to focus a large area of sunlight, or solar thermal energy, onto a small area. The solar thermal energy may drive a heat engine, such as a steam turbine, which may be further coupled to an electrical power generator to convert a portion of the solar thermal energy into electricity. Solar concentration systems may also drive a thermochemical reaction to generate a fuel that chemically stores a portion of the solar thermal energy. Water splitting, gasification of coal, and reforming of methane are all under investigation as potential solar thermochemical fuel production techniques. Solar concentration systems may drive other important reactions on an industrial scale as well, such as CO2 reduction into CO, for example.
Many solar thermochemical reactions entail a redox cycle. In a water splitting reaction to produce hydrogen from water, a metal-oxide redox pair is thermally reduced and the reduced reactive media then drives decomposition of water. The metal oxide is then reduced again to repeat the cycle. While identifying advantageous metal-oxides is currently a subject of research, thermodynamic considerations dictate the thermal reduction portion of the cycle generally requires a high temperature, typically between 1000-2000° C., depending on the reactive oxide chosen and other conditions in the system.
Solar thermochemical reactors can take many forms, affording more or less efficient fuel production, scalability, etc. One conventional system utilizes a honeycomb substrate that is coated with the reactive oxide. The honeycomb substrate is alternately exposed to collected solar energy to heat the system and reduce the reactive oxide, and to a reactant gas, such as H2O in the case of water splitting, to generate fuel. Such a reactor is essentially a fixed bed, and as such, suffers temperature non-uniformities and low thermal efficiency because much of the solar energy 105 is expended on heating non-reactive portions of the bed (e.g., honeycomb substrate) and is ultimately rejected from the system as waste heat, rather than utilized for fuel production. Also, with each redox cycle, the entire system undergoes extreme thermal cycling, leading to component fatigue.
FIG. 1 illustrates a conventional solar thermochemical reactor design including a fluidized bed of reactant particles. As shown, the solar thermochemical reactor 100 includes a reactor vessel 110 having a window 115 through which a solar flux 105 is received into the reactor vessel 110. Reactive oxide particles 150 and a gas 140 form a fluid-solid mixture 120 contained within the reactor vessel 110. The reactive oxide particles 150 are internally circulated within the vessel 110 (up through draft 130 and down the external annular region), to improve temperature uniformity relative to a fixed bed. However, similar to the honeycomb reactor, the thermochemical reactor 100 operates in two discrete modes, depending on the composition of the gas 140. During a thermal-reduction mode, the gas 140 is an inert (e.g., N2) and the reactor vessel 110 is heated by the solar flux 105. During a water-decomposition mode, the gas 140 is steam and the solar flux 105 is discontinued to cool the reactor vessel 110. As such, much of the solar energy 105 is again rejected from the reactor 100 as waste heat, rather than utilized for fuel production and components of the reactor 110 are repeatedly temperature cycled.
A system which avoids many of the difficulties and efficiency limitations associated with existing reactors would advantageously advance the art of solar thermochemical fuel production.