Solar Hot Water System Types

Closed Loop - Glycol System
Closed loop systems use a heat-transfer fluid to collect heat and a heat exchanger to transfer the heat to household water. Active closed loop systems use electric pumps, valves, and controllers to circulate the heat-transfer fluid, usually a glycol-water antifreeze mixture, through the collectors. This glycol-water antifreeze mixture makes closed-loop glycol systems effective in areas subject to freezing weather. For this reason, closed loop systems are preferred for year round use in Vermont.

Credit: DOE/NREL

Closed Loop - Drainback System
Drainback systems use water as the heat-transfer fluid within the collector loop. The water is forced through the collectors by a pump and then is drained by gravity to the storage tank and heat exchanger. These systems have no valves to fail and when the pumps are off, the collectors are empty, thereby assuring freeze-protection and auto shut-off if the water in the storage tank becomes too hot.

Open loop, seasonal, batch
Open loop systems heat and circulate household (potable) water directly in collectors prior to distribution in the household. One type of open loop system is a batch heater that is simply a black tank filled with water and placed inside a south-facing, insulated, glazed box, where it absorbs solar energy. The tank may incorporate a selective surface that that absorbs sun well but inhibits radiant loss. In climates where freezing occurs, batch heaters must either be protected from freezing or drained for the winter. Batch heaters are inexpensive and have few components, therefore they require less maintenance and experience fewer failures. These systems are good economical choices for seasonal applications such as summer camps.

Components

Solar hot water systems are made up of collectors, storage tanks, piping, controls, and in some cases pumps. Active systems use pumps to circulate water or other heat transfer fluid through the system. Passive systems have no pumps and rely on gravity or natural convection to circulate water depending on the system.

Collectors
The most commonly used collector is the flat-plate collector. It is an insulated, weatherproofed box, made of metal or plastic, containing a dark absorber plate beneath a translucent cover (typically tempered, low-iron glass). Copper piping carrying heat exchange fluid travels in an S-shaped pattern between the absorber plate and translucent cover. Typically, metal collectors are more sturdy, fire resistant, and expensive to manufacture and ship than plastic collectors. Contact your local solar contractor to learn which collector is best suited for your application.

Tanks
Most solar hot water systems require a well-insulated storage tank. Many systems use converted electric water heater tanks for storage or plumb the solar storage tank in series with a conventional water heater.

Controls, Circulator
A differential temperature controller monitors the temperatures at the solar collector outlet and at the storage tank. When the collectors are warmer than the tank, the control turns on a circulator which circulates a heat-transfer fluid, usually water or a water-glycol mixture, through the collectors and back to the heat exchanger located in or adjacent to the storage tank.

Heat Exchangers
A heat exchanger transfers heat from the heat-transfer fluid (usually either water or a water-glycol antifreeze mixture) to the household water supply. The heat exchanger itself is a series of copper coils submerged in the water storage tank or a compartment outside of the storage tank.