What is
a Solar Heat Exchanger

The purpose of a solar heat exchanger is to transfer the heat from the solar collector anti-freeze liquid closed loop to the hot water stored in the hot water tank. This is needed only in indirect solar hot water heaters

It is imperative to assure that the anti-freeze liquid (usually glycol, a toxin) will not leak into the water. Therefore, the two (water and anti-freeze liquid) are separated by a good heat conducting material (a metal, in most cases copper). The usual arrangement is to have an inner tube for the anti-freeze liquid and an outer tube for the water. They can run in parallel (same direction) or one flows in a counter direction to the other. The structure maximizes the surface area that separates the two flows, yet allows for minimal resistance to the flows.

In the context of solar water heating, heat exchange is necessary if the ambient temperatures ever drop below 32 degrees F (0 degrees C) – the freezing temperature. A direct system (water flows in the collector loop) can’t be used, because freezing water expand and the solar collector tubes crack. For the same reason, in these regions the car’s radiator uses an anti-freeze liquid and not water

Usually, the heat exchanger is built into the water tank, occasionally you might see an externally mounted heat exchanger. If the heat exchanger is inside the tank, it is a sure bet the manufacturer sized it correctly to the tank size and amount of liquid flows