Abstract: A medium for energy storage includes a plurality of capsules. Each capsule contains a phase changing material (PCM) configured to undergo a liquid-solid phase transition at a solidification temperature, TS. The PCM undergoes a relative volume change due to the phase transition. A shell is filled with the PCM. The shell contains a first heat-conducting material, and is configured to comply to the relative volume change. The relative volume change is configured to cause a buoyancy force, which acts on the capsule when the capsule is disposed in water at a water temperature, TW, to be larger than the capsule's weight for Tw>Ts, and equal to or smaller than the capsule's weight for Tw<Ts. The Ts can be within ±5° F. of a design water temperature To at the outlet of a water tank. The capsule can be neutrally buoyant in water at To.