Solar water heater at Samye Ling

There was a display about the solar water heaters at the Buddhist centre near Eskdalemuir.If you follow the pipework on the left hand side of the diagram there is a countercurrent heat exchanger. Cold water flows into the bottom of the panel. This is because the solar heat radiation raises the temperature of the water making it expand and become less dense. Hence it tends to rise by convection. Best to have the water moving in the direction of the natural current. But notice that the hot water pipe then goes into the middle of the hot water tank and goes downwards. This seems odd because a kettle heats water from the bottom. Given that colder water in the tank will tend to sink, it is best to heat water in the middle first. That lowers the temperature of the solar heated water. If it comes into contact with colder tank water at the bottom of the tank, there will still be a temperature gradient to deliver more energy to the tank. The flow of the solar-heated water is opposite to the direction of any warm convection currents in the water.