Heat transfer fluid

As the solar collector heats up, the fluid in the collector increases in temperature. This fluid is then moved out of the collector so the heat can be extracted for some useful purpose. Larger solar systems either use the heat immediately, or use heat stored in a tank of heated fluid. When the solar system is located in a freezing climate, the fluid flowing through the collector is often an antifreeze solution. When storing heated fluid in a tank, water is often the preferred medium (to reduce cost). If this is the case, a second fluids is used and flow through a heat exchanger is required to transfer the heat from the collector to the storage tank. There also may be a third fluid, potable water, which is used directly for domestic purposes, in which case another heat exchanger would be required. The heat transfer fluids used in a solar hot water system are very important and must meet a number of requirements to ensure good performance:

• a high heat capacity and conductivity allowing efficient heat transportation from the collector
• anti-corrosive-protection, if mixed or corrosion prone materials are present in the collector
• non-toxicity and environmental-friendliness
• low viscosity for easy pumping of the fluid
• low cost and availability.

Except where freezing is a concern, water is the fluid of choice in solar energy hot water systems. It has a low cost, is plentiful, and is compatible with the materials used in these systems. Water also has high heat capacity, good conductivity, a low viscosity, and can withstand the hot temperatures that are experienced during stagnation periods of time. Passive solar systems normally use water in the solar collector. Water is also often used in evacuated tube collectors. In freezing climates an anti-freeze liquid must be used in flat plate collectors.

Water used for domestic hot water is a potable water source that must be kept safe to drink and not be contaminated by chemicals. Because of this, large systems must have at least two fluid circulating systems, one that flows through the solar collector, and another that is heated and dispensed as domestic hot water. A heat exchanger is placed between the two piping systems for the movement of captured heat from the collector heat transfer fluid to the domestic water. Figure 3.6 shows such a system. Separating the collector fluid from the domestic water allows for water treatment to prevent corrosion of the piping and collector materials. To protect the potable water a double wall heat exchanger must be used when the heated fluid is not-potable. This separation also allows for an anti-freeze solution to be used in the solar collectors if needed.