Solar Water Heater Winterization: How to Prevent Frozen Pipes & Damage

Freeze Protection for Solar Water Heating Systems

The collectors and their outdoor piping in solar water heating systems are often damaged by the expansion of frozen water during the harsh winter months, especially in cold, high-latitude regions. Therefore, winterization measures for solar water heating systems must be considered. Currently, the following are some common freeze protection measures for solar water heating systems.

Choosing Freeze-Proof Solar Collectors

The collector is a critical component in a solar water heating system that must be exposed outdoors. Choosing a freeze-proof collector eliminates concerns about freezing damage during the harsh winter months.

The heat pipe vacuum tube collector and the all-glass vacuum tube collector with internally inserted tubes, introduced in Chapter 1 of this book, are both freeze-proof collectors. Because the heated water never directly enters the vacuum tubes, the glass cover tubes of the vacuum tubes do not come into contact with water. Furthermore, the heat pipes themselves have a very low working fluid capacity, so the vacuum tubes are protected from freezing even at temperatures of several tens of degrees Celsius below zero.

Another type of collector with antifreeze features is the heat pipe flat-plate collector. Unlike conventional flat-plate collectors, this one uses heat pipes instead of pipes in the absorber's tubing. These pipes use a low-boiling-point, low-freezing-point medium as the working fluid, preventing the absorber from freezing. However, because the technical and economic performance of heat pipe flat-plate collectors is inferior to the aforementioned vacuum tube collectors, they are not yet widely used.

Dual-Circulation System Using Antifreeze

A dual-circulation system (or dual-circuit system) incorporates a heat exchanger into the solar water heating system. The collector and the hot side of the heat exchanger form the first circuit (or primary loop). A low-freezing-point antifreeze fluid is used as the heat transfer medium, thus providing system antifreeze. Dual-circulation systems can be used in both natural and forced-circulation solar water heating systems.

In natural-circulation systems, although antifreeze is used in the primary loop, the water storage tank is located outdoors, and the system's cold water tank and hot water supply pipes are also partially installed outdoors. Even with insulation, these outdoor pipes cannot guarantee that the water in them will not freeze on cold winter nights. Therefore, when designing the system, it's important to consider some means to drain the hot water from the pipes after use. For example, a siphon-type hot water intake pipe could double as a cold water supply pipe, with an atmospheric valve installed at its top to control its opening and closing, thus draining the pipe.

Return system with automatic draining

In a single-loop forced circulation system, a temperature differential is typically used to control the operation of the circulating water pump, and the water storage tank is typically located indoors (on the ground floor or in the basement). During the daytime in winter, when there's sufficient solar radiation, the temperature differential controller activates the circulating water pump, allowing the collector to operate normally. At night or on cloudy days, when solar radiation is insufficient, the temperature differential controller shuts off the circulating water pump, allowing the water in the collector and pipes to flow back into the water storage tank by gravity, preventing damage caused by freezing. The next day, or when solar radiation returns to sufficient levels, the temperature differential controller restarts the circulating water pump, pumping the water from the water storage tank back into the deflector, allowing the system to continue operating. This antifreeze system is simple and reliable, requiring no additional equipment. However, the circulating water pump in the system must have a high head.

In recent years, overseas countries have begun applying reflux antifreeze measures to dual-circuit systems. In this system, the first circuit does not use antifreeze fluid, but still uses water as the heat transfer medium in the collector. At night or on cloudy days when solar radiation is insufficient, the circulating water pump automatically shuts down, and the water in the collector flows through a siphon effect into a small, specially designed storage tank. The water is then re-pumped into the collector the following day or when solar radiation is sufficient again, allowing the system to continue operating.

Using a drainage system to empty stored water

In a single-circuit system with natural or forced circulation, a temperature sensor is embedded in the pipework below the collector's absorber or at the lowest outdoor ambient temperature, connected to a controller. When the water temperature in the collector or outdoor pipe approaches freezing (3-4°C), the controller, based on a signal from the temperature sensor, opens the drain valve and the air vent valve. The water in the collector and outdoor pipes then drains by gravity out of the system, preventing reuse and achieving the desired antifreeze effect.

Automatic Nighttime Circulation of Hot Water from a Storage Tank

In a single-circuit system with forced circulation, a temperature-sensing element is embedded in the pipework below the collector's absorber or at the lowest outdoor ambient temperature, connected to a controller. When the water temperature in the collector or outdoor pipe approaches freezing (e.g., 3-4°C), the controller turns on the power, starting the circulation pump, which pumps hot water from the storage tank to the collector, raising the water temperature in the collector and pipes. When the water temperature in the collector or pipe reaches a set value (or when the pump has run for a set period), the controller turns off the power, stopping the circulation pump. Because this antifreeze method consumes a certain amount of power to operate the circulation pump, it is suitable for areas with occasional freezing, but not severe cold.

In a single-circuit system with natural or forced circulation, a self-regulating heating tape is installed in the outdoor pipework's most susceptible section to freezing. This method utilizes a thermistor located near the heating tape and connected to the heating tape's circuit. When the heating tape is energized, it heats the water in the pipes and simultaneously raises the temperature of the thermistor, which in turn increases its resistance. When the thermistor's resistance reaches a certain value, the circuit is interrupted, the heating tape is de-energized, and the temperature gradually decreases. This process is repeated countless times, preventing the water in the outdoor pipes from freezing while also preventing the heating tape from overheating and causing danger. This antifreeze method consumes a certain amount of electricity, but it is effective in very cold regions.