The Equilibrium Solar Pond: A Laboratory Model for the Gradient Layer

Salts, whose solubility in water increases strongly with temperature, can be utilized as a solute in the equilibrium solar pond. In solutions of such salts, a temperature gradient usually gives rise to thermal diffusion of 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 non-convective fluid layer is stratified by a stable concentration graident and a destabilizing temperature profile, distributed in such a way that the regular and cross-diffusion fluxes of 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 the equilibrium solar pond is considered as a generalization of the saturated solar pond. Futher laboratory studies on practical aspects associated with the equilibrium solar pond are needed before the above ideas can be applied to a real solar pond.