A heat exchanger will be needed for most solar water heating systems, other than direct systems, to separate the fluid in the solar loop from the potable water being heated and used. Heat exchangers introduce losses in system efficiency so they must be sized and specified properly to minimize these effects.
Most heat exchangers used for solar water heating applications are the same ones used in traditional boiler systems and have a rating based on this type of heating. In a traditional boiler system 180° F water is used to heat 100° F water in the storage tank. The temperature difference between the heating fluid and the fluid being heated is known as the “approach”. In the case of boilers systems this would be around 80° F, but for solar thermal systems the approach should be much lower, closer to 10-20° F. This is because solar thermal collectors are most efficient when the water entering the collectors is as close to the storage temperature as possible. There are many types of heat exchangers (brazed plate, tube-in-shell, immersed coil, etc.) but the steps below give a generic method for how to size any heat exchanger. Once the basic aspects of the heat exchanger are sized they can be matched to manufacturers’ published data and sizing tools.
Heat exchangers should always be piped in counterflow for the best efficiency. This means that hot and cold fluids are flowing in opposing directions to one another. See Figure 1 for a diagram of a plate heat exchanger in counterflow.
Figure 1: Schematic Diagram of Plate Heat Exchanger in Counterflow