Heat exchanger

The heat exchanger is normally inside the hot water cylinder. It transfers heat in the solar circuit (which has been water warmed by the sun) to the water inside the hot water cylinder/ tank.

The liquids are kept separate. The transfer works on the principle that the heat energy in the hotter material (solar heat transfer liquid) will be transferred to the cooler material (stored water inside the cylinder/tank).

The heat is transferred through the metal wall of the coil which is a pipe which runs in a loop inside the hot water cylinder. This loop or coil is surrounded by the water inside the cylinder and achieves effective heat transfer by having a large surface area in contact with both the solar heat transfer solution inside it, and the stored water (to be heated) which surrounds it.

The type of energy transfer occurring is both conduction through the coil to the surrounding water, and convection through the stored water in the cylinder, which moves via thermal currents, carrying the energy to the other areas within the cylinder.

A car radiator is a good example of this passive heat exchange: it transfers heat from the engine’s cooling system to the outside air, by exposing to each other two materials (water circulated around the engine; and the air temperature outside the radiator) of different temperatures while maintaining the separation of materials by an exchange surface - the exchange surface in this example being the external casing of the car radiator.

More information can be found here: Cylinders, Calorifiers & Heat Exchangers