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How It Works — Solar Water Heaters

Solar water heaters come in a wide variety of designs, all including a collector and storage tank, and all using the sun's thermal energy to heat water.

Solar water heaters are typically described according to the type of collector and the circulation system.

Collector Types

Solar Batch Collector Water Heaters

Batch collectors, also called Integrated Collector-Storage (ICS) systems, heat water in dark tanks or tubes within an insulated box, storing water until drawn. Water can remain in the collector for long periods of time if household demand is low, making it very hot. A tempering valve is your protection from scalding at the tap. The tempering valve mixes in cold water to decrease the water's temperature before it's delivered to the tap.

Batch collectors are incompatible with closed-loop circulation systems. Thus, they are generally not recommended for cold climates.

Solar Flat-Plate Collector Water Heaters

Flat-plate collectors typically consist of copper tubes fitted to flat absorber plates. The most common configuration is a series of parallel tubes connected at each end by two pipes, the inlet and outlet manifolds. The flat plate assembly is contained within an insulated box, and covered with tempered glass.

Flat plate collectors are typically sized to contain 40 gallons of water. Two collectors provide roughly half of the hot water needed to serve a family of four.

Solar Evacuated Tube Collector Water Heaters

Evacuated tube collectors are the most efficient collectors available. Each evacuated tube is similar to a thermos in principle. A glass or metal tube containing the water or heat transfer fluid is surrounded by a larger glass tube. The space between them is a vacuum, so very little heat is lost from the fluid.

These collectors can even work well in overcast conditions and operate in temperatures as low as -40°F. Individual tubes are replaced as needed. Evacuated tube collectors can cost twice as much per square foot as flat plate collectors.

Circulation Systems

Direct systems circulate water through solar collectors where it is heated by the sun. The heated water is then stored in a tank, sent to a tankless water heater, or used directly. These systems are preferable in climates where it rarely freezes. Freeze protection is necessary in cold climates.
Closed-loop, or indirect, systems use a non-freezing liquid to transfer heat from the sun to water in a storage tank. The sun's thermal energy heats the fluid in the solar collectors. Then, this fluid passes through a heat exchanger in the storage tank, transferring the heat to the water. The non-freezing fluid then cycles back to the collectors. These systems make sense in freezing climates.
Active, or forced-circulation, systems use electric pumps, valves and controllers to move water from the collectors to the storage tank. These are common in the U.S.
Passive systems require no pumps. Natural convection moves water from the collectors to the storage tank as it heats up.