Figure 15-4. - Typical configuration for solar water-heating systems.

If there is no danger of freezing and the collector loop consists of all copper flow passages, then ordinary water should be the choice for collector fluid. When encountering freezing conditions, there are several designs to consider before deciding to use a heat transfer oil or antifreeze mixture. For the purposes of this discussion, these freeze protection schemes are summarized using figure 15-4 to explain the basic open-loop type of collector circuit.

DRAIN-DOWN METHOD. - When the water in the collector approaches freezing, the water drains into the storage tank. This scheme requires automatic valves to dump the water and purge the air from the system. Often a large pump is required to overcome the system head and reprime the collectors. A way to avoid automatic (solenoid) valves is to drain the collectors whenever the pump shuts off. This still requires a larger pump. You may require heat exchangers to separate potable water from nonpotable water.

HEAT TAPES. - Electrical-resistant heat tapes are thermostatically started to heat the water. This scheme requires extra energy and is not completely reliable. Inserting heat tapes into preconstructed collectors may be difficult.

RECIRCULATING METHOD. - The control system shown in figure 15-4 merely turns on the pump when it approaches freezing. In this way, warm water from storage circulates through the collectors until the freezing condition is over. The only extra component needed is a freeze sensor on the collector, which is a minimum cost item. However, by circulating heated water, the capacity of storage decreases and less is available the following day.

HEAT TRANSFER FLUID

When the preceding methods are not acceptable or the choice of water is unacceptable because of concern about corrosion, you should use a heat transfer fluid. Use a heat transfer fluid with a heat exchanger in a "closed-loop" configuration.

There are two categories of heat transfer fluids nonaqueous and aqueous. Silicones and hydrocarbon oils make up the nonaqueous group, while the aqueous heat transfer fluids include untreated potable (tap) water, or inhibited distilled water, and an inhibited glycol/water mixture. The potable tap water and inhibited distilled water do not offer freeze protection.