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Constant variations in the amount of sunlight available on Earth at any given location make energy storage a necessary design feature of terrestrial solar-energy systems. For systems transforming solar to thermal energy, the thermal energy may be stored in matter as either latent heat or sensible heat. Latent heat is absorbed or released whenever matter changes phase, as when matter changes from liquid to gas, for example, or from gas to liquid. Large heat capacities are associated with certain materials, like salts, but in any substance this storage is available only at the unique fixed temperature at which the particular phase transition occurs in that substance. Moreover, materials that have transitions at the temperatures that terrestrial solar-energy systems are likely to encounter are usually destructively corrosive at those temperatures. The storage of sensible heat, on the other hand, allows flexibility as to temperature, in addition, safe substances like water and most rocks have large sensible heat capacities.