Harel, Z., Tel-Aviv Univ, Tel-Aviv, Israel
Tanny, J., Tel-Aviv Univ, Tel-Aviv, Israel
Tsinober, A., Tel-Aviv Univ, Tel-Aviv, Israel

Salts whose solubility in water increases strongly with temperature can be utilized as a solute in an equilibrium solar pond. In solutions of such salts, a temperature gradient usually gives rise to thermal diffusion of the salt from lower to higher temperature zones in the fluid (negative Soret effect). If the direction of this molecular cross flux is opposite to that of the regular molecular salt flux (down its own gradient), the two fluxes can balance so that the net salt flux is zero and the system is in equilibrium. In the equilibrium solar pond suggested here, the insulating nonconvective fluid layer is stratified by a stable concentration gradient and a destabilizing temperature profile, distributed in such a way that the regular and cross-diffusion fluxes of the salt are exactly balanced and the net salt flux is zero. An experimental study was carried out to investigate such a double-diffusive system. A simple method is suggested to build up the concentration and temperature gradients: a two-layer stratified fluid consisting of KNO3 solution and contained within a large tank is heated at its bottom up to a prescribed temperature above the ambient, while the upper free surface is exposed to steady room conditions. The resulting double diffusive system is found to be highly stable; observations and measurements indicate that the net salt flux is zero, and as long as the temperature gradient across the layer is maintained, the system is at equilibrium and basically remains unchanged (for a period of at least one week). The concept of an equilibrium solar pond is considered as a generalization of a saturated solar pond. Further laboratory studies on practical aspects associated with an equilibrium solar pond are needed before the above ideas can be applied to a real solar pond.