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Heat Transfer For Indirect Systems

In an indirect water-based solar water heating system, the heat transfer fluid and the water in the storage tank should not mix. A Solar Heat Exchanger is needed to transfer heat from the loop to the storage unit.
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Solar Heat ExchangerImage: KoenB via Wikimedia CommonsThere are several different ways this transfer of heat can be done depending on limiting factors such as cost and need. A solar heat exchanger can be made of various metals. The use of a non-corrosive metal is important if any part of the solar heat exchanger uses water as a heat medium.

The first option is a liquid-to-liquid solar heat exchanger. In this design, the solar collector transfers its heat energy to a heat transfer fluid. There are many types of heat transfer fluids as detailed below. A liquid-to-liquid solar heat exchanger can be either single- or double-walled. Double-walled is preferred as it creates a secondary safety barrier between the usable water and the heat-transfer fluid to prevent contamination. However, a double-walled solar heat exchanger is less efficient.

Solar Heat Exchanger Designs

There are two possible ways to design the solar heat exchanger: either a tube is filled with the heat transfer fluid which is pumped through a tank filled with the water to be heated, or the water to be heated is pumped through a tube surrounded by the heat transfer fluid. The most efficient design for a solar heat exchanger is a combination of these principles and is commonly referred to as the “tube-in-tube” design. In this design, a tube of water to be heated is surrounded by a tube containing the heat transfer fluid. The two fluids are then pumped in opposite directions through a series of loops that connect to the solar collector and to the water storage tank.

There are a large variety of heat transfer fluids and it is important to select fluid and solar heat exchanger carefully according to need and sometimes local environmental or housing laws. It is also important to consider the risk of freezing or boiling the fluid. Common heat transfer fluids are water, water mixed with propylene glycol to act as an antifreeze, refrigerants, and silicones. It is important to discuss with a solar professional which liquid and solar heat exchanger is best for your system.

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Written by: Christian Märtel, Editor
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