Abstract: Light weight, compact and efficient thermal energy reservoirs are described in which thin layers of a heat storage material, such as an inorganic salt having a high latent heat of fusion, are heated or cooled by an intermittent primary energy source, such as solar or off-peak electric power. The stored energy is utilized during periods when the primary energy source is not available. The heat storage material is contained in flat fluid tight packages, such as a heat sealed polyethylene film bag, which can readily be removed and replaced through removable panels or doors in the insulated reservoir enclosure.