Abstract: For collection, retrieval, and utilization of solar energy, there is taught a process and an apparatus wherein a photochemical fluid, containing an isomerizable compound, is passed through a collector for exposure to solar radiation to transform the isomerizable compound to a higher energy level isomer. The irradiated fluid leaving the collector passes in heat exchange relationship with photochemical fluid entering the collector and then proceeds into a trigger reactor means. In the trigger reactor means, higher energy level isomer in the irradiated fluid is triggered, such as by heat and/or catalyst contact, to revert to an isomerizable composition of a lower energy level isomer with exothermic release of heat in excess of that requisite for maintaining conversion of higher energy level isomer to lower energy level isomer.