We design and implement several types of solar water heating systems described below
Active antifreezing systems are the most common type in the climates similar to ours where long term negative temperatures are observed during winter season. It is has no drainback tank, so the fluid in the collector cannot drain out of the outdoor plumbing. The freeze protection comes from polypropylene glycol antifreeze, so it can stay in the collector after the controller shuts the pump off.
Thermosyphon is the simplest system. These entirely passive (meaning no pumps) systems rely on the collector being below the tank, so that natural convection moves the hot water out of the collector and to the tank for household use. These systems are also inappropriate for freezing climes.
Direct forced-circulation is similar to the thermosyphon system, but adds a pump and controller. The pump makes it possible to pick up more heat out of the collector. The controller measures when the collector is hotter than the tank and turns the pump on or off as appropriate. These systems also have only basic freeze protection measures.
Drainback systems add a tank and a heat exchanger. The drainback tank is sized so that all the water in the collector and lines can drain down into it, when the pump turns off. That way there is no water left in the collector at times when it could otherwise freeze. The water in the system is separate from the household water, so it needs a heat exchanger to transfer the heat from the collector water to the domestic water.