Stratiﬁcation is the layering of water at different temperatures within the water tank, with hot water at the top and cold water at the bottom. Stratiﬁcation is caused by hot water (which is less dense) rising to the top of the tank.
Storage tanks are designed to minimise the mixing of hot water and cold water. Solar collectors are more efficient when heating cold water, thus excessive mixing of hot and cold water can reduce the efficiency of the solar system.
Some solar storage tanks are designed so that the inlet and outlet ports allow for a laminar ﬂow of water entering and leaving the tank, thus minimising turbulent mixing inside the tank. In some cases cold water may be fed into the bottom of the tank through a spreader pipe that slows the water velocity and spreads it along the bottom of the tank.
Water returning from the solar collector should be fed back into the tank at a higher position than the cold water outlet.