Different types of heat exchangers used in solar water heating systems

Solar hot water heating systems use a hot water heat exchanger for transferring collected energy by solar collectors to the air or fluid used to heat a given space or water.

Heat exchangers are typically designed from cast iron, aluminum, bronze, copper, and stainless steel. Solar hot water heater units use copper since it is a strong conductor of thermal energy and has a high degree of resistance to corrosion and rust.

Some of the most widely used hot water heat exchangers are discussed below:

Liquid-to-liquid

A liquid-to-liquid type heat exchanger deploys liquid that flows through the solar absorber, gathers heat, and then pushes it through a heat exchanger unit for transferring heat to water within a storage container.

Heat-transfer liquid, like antifreeze, safeguards the solar absorber unit from getting frozen in colder climates. Liquid to liquid type heat exchangers have one or two barriers between the original water supply and the heat-transfer fluid.

A heat exchanger that features a single-wall barrier is a tube or a pipe surrounded by the liquid. Either the liquid flows through the tube or the liquid surrounding the tube may be a heat-transfer fluid, the other liquid being portable water or another liquid such as pool water.

A double-wall solar heat exchanger has two barriers between the potable water and the heat-transfer fluid. Two barriers are used if the heat-transfer fluid is toxic.

Air-to-liquid heat exchangers

Solar hot water heater systems that have an air heat collector don’t require a heat exchanger between the air distribution system and the thermal collector. Such units that feature air heat collectors for water heating deploy an air-to-liquid heat exchanger that is more or less similar to a liquid-to-air heat exchanger.

Some of the most common heat exchanger designs include:

Coil-in-tank

The hot water heat exchanger in this type of design is a coil tube in the vessel. A single-barrier heat exchanger uses one tube, while the thickness can double in a dual-barrier heat exchanger. A comparatively lesser efficient option is to position the coil at the exterior of the vessel with an insulation covering

Shell-and-tube

The shell-and-tube heat exchanger is placed externally to the vessel. It has two different fluids looping within a shell or case. Both the fluids run in different directions through the heat exchanger unit, which results in maximising the heat transfer.

The liquid to be heated flows in one loop through the internally placed tubes. The second heat transfer fluid circulates between the water tubes and the outer shell. The shell and tube must be of the same material. When the heat-transfer liquid or the heat collector is toxic, dual-wall tubing is preferred, and an additional non-toxic intermediate transfer fluid is positioned between the inner and the outer walls of the tubes.

Tube-in-tube

The tube-in-tube is a highly efficient design for a heat exchanger. The tubes of heat-transfer fluid and water are in direct contact. The heat transfer fluid and the water flow in reverse direction to each other. The tube-in-tube heat exchanger has two loops that are quite identical to those discussed in shell-and-tube design.

Sizing

A heat exchanger should be properly sized to get maximum efficiency out of it. While deciding on the size of a heat exchanger, there are many factors to keep in mind, such as the inlet/outlet temperature of both the fluids, the flow rate, individual specifications of the heat-transfer fluid like density, viscosity and specific heat, and so on.

For optimal performance and durable lifespan, you must ideally consult a manufacturer for installation advice.