Direct (open-loop) – Used in climates where freezing cannot occur, this is the simplest of the active systems. A standard electric tank is combined with a solar thermal collector. The electric tank serves as a storage tank for preheated water and secondary heating. A circulator pump moves water through the system.
Drainback (closed-loop) – This system relies on distilled water, and it’s extremely low-maintenance. The distilled water sits in a 10-gallon reservoir tank, and the solar collector is empty when the system is idle (it drains back), but everything is ready to go when you turn on your faucet and the pump starts operating!
Pressurized Glycol (closed-loop) – This system uses a heat transfer fluid (typically propylene glycol). The collector, warmed by direct sunlight, heats the propylene fluid that flows through it. The heated fluid is then directed to a tank, where it heats the home’s potable water by way of heat transfer. Put another way, the water resting in the storage tank is heated by the pipe, which has been warmed as a result of the fluid traveling through it.
Batch – In this very simple system, the water tank is contained in a glass-covered, insulated box and positioned in direct sunlight. Incoming water travels to the system, is heated appropriately, then flows when it is needed. The water pressure from your home directs cold water to the bottom of the system, where the cycle can begin again.
Thermosiphon – This system relies on natural convection (the principle that heat rises). Water stored in the collector heats, then travels to the tank above. And, naturally, cooler water flows to the bottom of the collector. Unlike batch systems, these systems use a highly insulated tank to avoid hot water loss during hours without sunlight.
Water Pumps – Required for the active systems described above. They power the flow that circulates (or pumps) water and heat transfer fluids between the collector and the solar storage tank.
Heat Exchanger – We explained how these work in a pressurized glycol system above. A heat exchanger simply facilitates the transfer of heat from a propylene solution to potable water. With a heat exchanger, your heating fluid is in no danger of contaminating your water.
Collectors – These are the pipes through which water and/or antifreeze solutions travel to be heated. There are three styles of collectors: