The document discusses solar ponds as a method for energy storage from renewable sources. - Solar ponds are shallow bodies of saltwater, about 1-2 meters deep, that generate stable density gradients from dissolved salts. This traps heat from sunlight absorption. - Without the salts, convection would mix the water and limit temperature rise, but the salts suppress convection, allowing heat to build up through conduction alone and reach over 90 degrees Celsius at the bottom. - The bottom of the pond has a plastic liner and salts like magnesium chloride, sodium chloride or sodium nitrate are dissolved creating a concentration gradient from 20-30% at the bottom to almost zero at the top.