Hydrogen gas is produced by reacting a carbon-hydrogen-containing species with water provided as a water reforming inflow at a reforming temperature to produce a primary reacted gas flow containing hydrogen gas. The carbon-hydrogen-containing species and water are heated to the reforming temperature with solar energy. The heating is preferably performed by heating a molten metal to at least the reforming temperature with solar energy, and using the molten metal to heat the carbon-hydrogen-containing species and water to at least the reforming temperature. The water reforming inflow is preheated by heat exchange from the primary reacted gas flow. The primary reacted gas flow is reacted with water to produce additional hydrogen gas in a secondary reacted gas flow.