Heat exchange apparatus (24, 80) includes first heat transfer tubes (50), contained within an enclosure (43) and coupled to receive a first fluid heated by an energy source (22, 36). Second heat transfer tubes (52) are interleaved with the first heat transfer tubes within the enclosure so as to heat a second fluid contained in the second heat transfer tubes by transfer of heat from the first fluid, and are coupled to output the heated second fluid to drive target equipment (30). A heat storage medium (48) fills the enclosure.