Freeze protection

When water freezes, it expands. Frozen water in pipes is likely to rupture them. This is a major cause of breakdown of solar heating systems. Water thus must be prevented from freezing inside the collector loop. The following options will protect against freezing of the piping system:

• Provide drain-back pipe design.
• Use an antifreeze solution in the outdoor collector and piping, a water-glycol mixture.
• Add heat from the storage tank to the outside piping and collector field.

Each solution has its benefits and drawbacks. Some general aspects are discussed here. The freeze protection implemented is usually not a free choice, but depends on the selected collector technology and supplier; it is usually based on environmental conditions, operating temperature, maintenance, costs, and local availability.

Please note: Common practice of solar system designers in Europe is to use a water glycol mixture in drain back systems nevertheless. This way it is ensured that heat transfer fluid does not freeze if draining does not work perfectly. The remaining advantage of drainback systems is: Overheating of oversized collector fields does not necessarily lead to problems during stagnation.

When using a water-glycol mixture the freezing temperature of the mixture is lowered (lower than water alone). The mixing ratio determines the lowest operating temperature. It is important to follow the manufacturer’s or installation engineer’s recommendations. The different types of glycol that could be used vary in their properties. See Section 3.4.1 (p 33) for more information. The pipes still need to be pitched to drains so that the collector field can be emptied for maintenance even when a anti-freeze collector fluid is used.

The main drawbacks of using a glycol antifreeze water mixture are that:

• The heat capacity of the fluid is reduced while the viscosity is increased. This causes a reduction in efficiency of the collector field. In addition, for a comparable mass flow more pump energy is needed.
• Glycol deteriorates when heated up to common collector stagnation temperatures that occur in evacuated tube collectors. Deteriorated glycol forms solid particles that can block and even destroy the collector loop. The glycol mixture then must be replaced.
• Care must be taken when an antifreeze solution is used to heat domestic water (which is a potable water source). The two fluids must be kept separate with a leak detection device at the point of heat exchange. This affects the efficiency of the heat transfer.

The third option is to use water in a closed, always filled loop, and to heat up the collector loop by turning on the pump when near freezing temperatures are sensed. This means that heat from the storage tank is used to prevent the collector and piping from freezing. This has the obvious drawback of wasting captured heat and using pump power. Therefore, the viability strongly depends on the site with the specific occurring ambient temperatures in winter. It would probably be the logical choice at locations where freezing temperatures are not normally experienced. This option also requires a reliable power supply because if no power is available, this option will fail to protect the collector loop.