The invention provides, in some aspects, a thermal energy storage and retrieval system with a cavity containing a working fluid and a heat transfer surface disposed in the cavity transverse to a gravity field and in thermal coupling with an ice that is formed from and floating in the working fluid. The heat transfer surface transfers to that ice heat from a heat transfer medium that is thermally coupled to the heat transfer surface. According to aspects of the invention in which the ice is less dense than the working fluid, the heat transfer surface is disposed above (with respect to the gravity field) a region of the cavity where that ice is formed or inlet. According to aspects of the invention in which the ice is more dense than the working fluid, the heat transfer surface is disposed below that region.