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S

S curve Graphic representation of population growth that levels out once the population reaches its carrying capacity. See also: J curve

Salt gradient solar pond An efficient, lowcost solar energy collection and long-range storage system for low-temperature heat. Although current research is primarily geared to space heating (which varies seasonally) and industrial process heating (which poses a constant demand), crop-drying, water desalination, cooling, and electricity production are possible applications of the solar pond. In a constructed salt gradient solar pond, there are three main layers. The top layer is near ambient temperature and has low salt content. The bottom layer is hot, typically 160-212 °F (71-100 °C), and is very salty. The important gradient zone separates these zones. This zone acts as a transparent insulator, permitting sunlight to be trapped in the hot bottom layer from which useful heat is withdrawn. This is because the salt gradient, which increases the brine density with depth, counteracts the buoyancy effect of the warmer water below, which would otherwise rise to the surface and lose its heat to the air. A Rankine cycle engine is used to convert the thermal energy to electricity.