Experimental study of a double-diffusive system: application to solar ponds

A salt gradient solar pond is an artificial device used to collect and store solar thermal energy. A non-convective zone, in the middle of the solar pond, reduces thermal losses and allows a significant rise of temperature in the saltier lower zone where the solar thermal energy is stored. The non-convective zone, also named gradient zone, is characterized by a salinity gradient that increase the density with depth promoting the stability of this layer. The absorption of solar radiation in the pond creates a destabilizing temperature gradient in the non-convective zone that contradicts the density gradient. The different molecular diffusivities of heat and salt and the opposing effects on the vertical density distribution of the two gradients can lead to double-diffusive convection phenomena. In this context, a double-diffusive system has been studied experimentally in laboratory by heating a stratified salt layer from below. The instabilities caused by the destabilizing temperature gradients lead to the formation of convective zones separated from purely diffusive zone by thin interfaces. The main goals of this work are the study of the evolution of the double-diffusive layer and the analysis of the behaviour of the diffusive interface near conditions for which instabilities appear