An experimental means of storing solar energy in either fresh- or saltwater. The “pond” is actually a large, suitably lined, open container of varying dimensions. In the freshwater type, the heat absorbed is retained at night by covering the pond with an inflated plastic film, supplemented by a layer of foam applied directly to the water. In the salt-water type, heat loss is prevented by a salt gradient, that minimizes convection currents that would cool the water. The sodium chloride or magnesium chloride used is not uniformly dispersed, but is most highly concentrated in the darker, bottom layers where most of the solar radiation is absorbed. Thus the water is warmest and densest at the bottom, where it remains, and no convection occurs. The salt concentration ranges from zero at the surface to 18% or more at the bottom. An experimental salt-gradient pond at Argonne National Laboratory 100 × 100 × 12 ft containing 2800 tons of water and 700 tons of sodium chloride has attained a bottom temperature approaching 180F.
