Image: KoenB via Wikimedia CommonsA solar heat exchanger is a device in which a working fluid is heated in the collector and air or water which awaits heating will both flow through their own tubes or a tank. Due to the close proximity of the hot fluid with the cooler fluid, heat is transferred from one to the other.
A solar heat exchanger can be made of almost any material that readily conducts heat. The preferred material is copper due to its high conductivity, but solar heat exchangers may also be constructed of steel, aluminum, or other materials.
Liquid-to-liquid & air-to-liquid heat exchanger
The two basic types of solar heat exchanger are liquid-to-liquid and air-to-liquid. In a liquid-to-liquid solar heat exchanger, some liquid is circulated through the solar collector to absorb energy from the sun and then through the heat exchanger to transfer that heat to the water for use. This liquid can simply be water, but is usually some other working fluid such as ethylene glycol which acts as an anti-freeze to protect the solar collector from freezing.
An air-to-liquid solar heat exchanger is similar, except that the working fluid which absorbs heat in the solar collector is air. The air is then circulated through the heat exchanger to heat water. If the objective is heating a house, no solar heat exchanger is needed. Air is circulated through the solar collector and then used to heat the room.
The most simple solar heat exchanger to construct is the coil-in-tank design. In this heat exchanger, a coil of tubing is placed inside a tank. The hot working fluid from the solar collector is circulated through the tubing. Water fills the tank around the coil and is heated by the hot coil. In a tube-in-tube heat exchanger, one tube is inside the other. One fluid flows through the inner tube and heats the other fluid which surrounds it inside the outer tube.