1. The Flat-Plate system comprises an insulated, weatherproofed box containing a dark absorber plate under one or more transparent or translucent covers. Water or conducting fluid pass through pipes located below the absorber plate. As the fluid flows through the pipes it is heated. This collector, although inferior in many ways to evacuated tube collectors, is still the most common type of collector in many countries.

2. Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes. (Glass-Glass) tubes consists of a glass outer and glass inner tube. The inner tube is covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions as well as low temperatures. Because the tube is 100% glass, there prob- lem with loss of vacuum due to a broken seal is greatly minimized. Glass-glass tubes may be used in a number of different ways, including open water flow, heat pipe, or U pipe configuration.

3. Concentrating collectors for residential applications are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid, or the water itself. This type of solar collector is virtually antiquated as it compares poorly with evacuated tube solar collectors in terms of reliability and efficiency.

Solar water heating systems (SWHS) can be either active or passive. An active system uses an electric pump to circulate the fluid through the collector; a passive system has no pump and relies on thermosiphoning to circulate water. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors. SWHS are also characterized as open loop (also called "direct") or closed loop (also called "indirect"). An open-loop system circulates household (potable) water through the collector. A closed-loop system uses a heat-transfer fluid (water or diluted antifreeze) to collect heat and a heat exchanger to transfer the heat to the household water.

Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but generally more efficient. Active systems are generally easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. If installed using a PV panel to operate the pump, they can operate even during a power outage.

Open-loop active systems use pumps to circulate household water through the collectors. This design is efficient and lowers operating costs but is not appropriate if your water is hard or acidic because scale and corrosion will quickly disable the system. Open-loop active systems are popular in regions that do not experience subzero temperatures. Flat plate open-loop systems should never be installed in climates that experience sustained periods of subzero temperatures. They may be installed in mild climates that experience occasion subzero temperatures, but freeze protection must be implemented.

These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through the collector. Heat exchangers transfer the heat from the fluid to the household water that is stored in tanks. Double-walled heat exchangers prevent contamination of household water. Some standards require double walls when the heat-transfer fluid is anything other than household water. Closed-loop glycol systems are popular in areas subject to extended subzero temperatures because they offer good freeze protection. However, glycol antifreeze systems are more expensive to purchase and install and the glycol must be checked each year and changed every 3 to 10 years, depending on glycol quality and system temperatures.

The Drainback systems use water as the heat-transfer fluid in the collector loop. A pump circulates the water through the collectors. The water drains by gravity to the storage tank and heat exchanger; therefore there are no valves to fail. When the pumps are turned off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.

STOPER Evacuated Tube Collectors can be used in both closed or open-loop configuration, although we advise the use of a closed loop system wherever possible as this will prevent corrosion or scale build up in the collector manifold. Using a closed system will therefore extend the life of the collector considerably.

Passive systems move household water or a heat-transfer fluid through the system without pumps. Passive systems have the advantage that electricity outage and electric pump breakdown are not issues. This makes passive systems generally more reliable, easier to maintain, and possibly longer lasting than active systems. Passive systems are often less expensive than active systems, but are also generally less efficient due to slower water flow rates through the system.

A thermosiphon system relies on warm water rising, a phenomenon known as natural convection, to circulate water through the collectors and to the tank. In this type of installation, the tank must be located above the collector. As water in the collector heats, it becomes lighter and naturally rises into the tank above. Meanwhile, cooler water in the tank flows downwards into the collector, thus causing circulation throughout the system. For small systems, the tank is actually incorporated into the collector. Large tanks must be located next the collector. These systems are reliable and relatively inexpensive but require careful planning during installation because the water tanks are heavy, and they are not as efficient as active systems. STOPER manufactures high quality Thermosiphon solar collectors. These systems are not automated like active systems, but are much cheaper as no pump or controller is required. They are ideal where a low cost solar heater is required such as holiday houses and cabins, or countries where low cost solar heating is required.