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Flat-plate collector



Thermosyphon systems



Batch system




solar collector



PDF of Direct Open Loop Circulation system



PDF of Indirect Closed Loop Circulation system



Drainback system diagram














Types of Solar Heaters for Domestic Water Heating


Solar water heating systems can be either active or passive, but the most common are active systems. Active systems rely on pumps to move the liquid between the collector and the storage tank, while passive systems rely on gravity and the tendency for water to naturally circulate as it is heated. prefers the active system powered by a small PV (solar electric) panel and 12 volt pump. This way the system is 100% powered by the sun, like a passive system, but has the advantages of much easier installation and attractive appearance by not having a tank on the roof.

Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Solar Systems may also use a modulating type demand (tankless or instantaneous) water heater for backup.


Types of Solar Collectors for Residential Applications


1) Flat-plate collector

Glazed flat-plate collectors are insulated, weather-proofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors - typically used for solar pool heating - have a dark absorber plate, made of metal or polymer, without a cover or enclosure.

2) Thermosyphon systems

Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank which is located on the roof above the collectors. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. Because the water storage is on the roof, these systems are less attractive and typically weigh over 800 pounds when filled with water. They are usually more expensive than integral collector-storage passive systems.


3) ICS or Batch Systems

ICS (also known as batch) systems, feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. These systems should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather. Because the water storage is on the roof, these systems typically weigh over 800 pounds when filled with water.


4) Evacuated-tube solar collectors

These collectors feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These high-temperature collectors are used more frequently for commercial applications as their high temperatures are inappropriate for the majority of mid-temp solar water heating applications.

Active Solar Water Heating Systems

1) Direct Open Loop Circulation Systems

In these systems a pump circulates household water through the collectors and into the home. Direct Open Loop Circulation Systems work well in climates where it rarely freezes.'s systems 1, 2 and 3 are direct open loop systems.

2) Indirect Closed Loop Circulation Systems

Pumps circulate a heat-transfer fluid through the collectors and a heat exchanger. This process heats the water that then flows into the home. These systems are popular in climates prone to freezing temperatures.
The heat transfer fluid (antifreeze) is usually a glycol-water mixture with the glycol concentration depending on the expected minimum temperature. The glycol is usually food-grade propylene-based so that it is non-toxic. SolarRoofs' systems 4 and 5 are indirect closed loop systems using non-toxic polypropylene antifreeze.

Drainback Systems


Drainback systems, a type of indirect system, use pumps to circulate water through the collectors. The water in the collector loop drains into a reservoir tank when the pumps stop. Drainback systems must be carefully installed to assure that the piping always slopes downward, so that the water will completely drain from the piping. This can be difficult to achieve in some circumstances so sometimes glycol is used. Drainback systems also require larger pumps to pump the water up to the roof.




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