In solar ponds constructed in israel, the sun’s energy is concentrated near the bottom of a salty pond. with the proper layering of salt in the water, convection, which would make the water temperature uniform, is prevented. below is a description of how it works: the pond is filled with a dense salt solution, which naturally separates into gradient levels: a weaker salt mix exists near the water's surface, and the higher—and thus denser —saline concentrations settle to the bottom. the topmost layer is, of course, relatively fresh water. as the sun beats down on the brine reservoir, its thermal energy passes through the "lighter" surface levels and heats the heavy saline mass below. the result is that the dense salt solution—held at the bottom of the pool and thermally protected by the surface layer—can reach temperatures between 158 and 176°f. the water at the surface of the pond, instead, stays at a warmth that's "normal" for the desert region, ranging from 68 to 90°f. the hot salt water is pumped through a heat exchanger, which is surrounded by a vessel filled with a substance similar to freon. this, in turn, is connected to a turbine that's specially designed to be driven by a much lower-temperature propellant than that used in a conventional steam turbine. since the medium changes from liquid to gas at a relatively low heat, the sun warmed water instantly flashes the fluid into a pressurized vapor which drives the turbine and its 150-kilowatt ac generator. what is the maximum efficiency with which useful mechanical work can be extracted from the pond?