A system for drying or heating product using solar energy including an enclosure defining a kiln chamber in which a charge of the product is placed, the enclosure having at each side of the chamber at least one passage through which air from the kiln chamber can pass to be heated by solar energy impinging on the enclosure, and an air flow system having at least one fan for generating a circulating air flow within the kiln chamber whereby air passes from one side of the charge to the other, and an air supply for drawing air into each passage from the bottom part of the kiln chamber at opposite sides of the charge to move through the passage into an upper portion thereof for discharge into an upper part of the kiln chamber through one or more outlets leading into the upper part of the kiln chamber.