NOVELTY - Method for reducing metal oxide particles (I) comprises providing a solar-thermal fluid-wall aerosol transport reactor comprising at least partially transparent outer protection shell and an inner reaction shell having an inlet and an outlet; flowing a first gas stream comprising entrained metal oxide particles from the inlet to the outlet of the reaction shell; flowing a second gas stream radially inward through the porous section of the reaction shell; and heating the metal oxide particles in the reactor through indirect solar thermal heating to cause reduction of the particles.    USE - For the production of hydrogen (claimed) from water splitting using a high temperature solar aerosol reactor.    ADVANTAGE - The method can be run at moderate residence times. The method increases the yield of the reduction reaction.    DETAILED DESCRIPTION - Method for reducing metal oxide particles comprises providing a solar-thermal fluid-wall aerosol transport reactor comprising an at least partially transparent outer protection shell and an inner reaction shell having an inlet and an outlet, where the reaction shell is partially porous and has a porous section located at the outlet end of the shell; flowing a first gas stream comprising entrained metal oxide particles from the inlet to the outlet of the reaction shell; flowing a second gas stream radially inward through the porous section of the reaction shell, for generating a fluid wall along the inside of the reaction shell; and heating the metal oxide particles in the reactor at least in part with a source of concentrated sunlight through indirect solar thermal heating to a temperature at which the metal oxide particles undergo a reduction reaction, for producing a reduced metal oxide product, which is a metal and/or a metal oxide of a lower valence state.    INDEPENDENT CLAIMS are included for:    (1) a method for producing hydrogen comprising reducing metal oxide particles by the above method; and reacting the reduced product with water vapor to form hydrogen; or reducing particles of a first metal oxide by the above method, for producing a second metal oxide of a lower valence state; reacting the second metal oxide of a lower valence state with sodium hydroxide to produce hydrogen and a sodium metal oxide; and reacting the sodium metal oxide with water vapor to produce the first metal oxide and sodium hydroxide;    (2) a solar-thermal reactor system comprising a solar-thermal aerosol transport reactor comprising a partially porous/nonporous first inner shell having an inlet and an outlet and a porous section located at the outlet end of the first inner shell, a second inner shell substantially enclosing the first inner shell, an at least partially transparent outer shell substantially enclosing the second inner shell, a first gas plenum located substantially between the first and second inner shell, where the first gas plenum has an inlet and an outlet, and a second gas plenum located substantially between the second inner shell and the outer shell; and the second gas plenum has an inlet and an outlet; and a cooling device comprising a cooling chamber having an inlet and an outlet, where the inlet of the cooling chamber is connected to the outlet of the solar thermal reactor first inner shell and the inner diameter of the cooling chamber being greater than the inner diameter of the first inner shell.