Abstract: A heat exchanger unit comprising a particulate heat exchanging mass or pack consisting of relatively small, mechanically immobilized particles having a thermal diffusivity constant of at least 0.5 cm.sup.2 /sec at 20.degree. C., and compressively retained in an enclosure in heat transfer relationship to each other and to a fluid directed therethrough. Preferred materials for the particles are crystalline carbon, copper and aluminum. The pack may be in cylindrical form or planar form and may be contained within metal conduits or, for solar radiation, within a transparent or translucent enclosure. Interconnected units may be disposed in an array or bank to provide the desired quantitive degree of thermal transfer.