Abstract: A system for ice storage provides time offset cooling to a cooling system. A tank defines a storage chamber including inlet and outlet ports through which circulates a heat transfer medium, and a plurality of buoyant panels filled with a storage medium are arranged for receiving, storing, and releasing thermal energy carried by the heat transfer medium. As the phase of the storage medium changes, flow paths past the panels are progressively occluded or widened. In a preferred embodiment, each panel is formed of a thin film material, such as a PVC plastic, with opposing sides of the panel fused together at discrete points to define a quilted bag-like structure having a defined maximum thickness and a generally undulating outer surface which allows passage of the transfer medium between adjacent bags. Granular ballast at the bottom of each bag maintains alignment and prevents thermal creeping. Bouyant spacers at the top define a self-aligning array.