The oldest design for a medium-temperature collector is the batch water heater. At its simplest, it is an insulated box with a black-painted tank inside and a glass cover that faces the sun. The use of batch heaters in mild climates transcends more than a century of fairly wide popularity. In places where freezing temperatures are common, batch heaters can be used seasonally, and bypassed and drained when winter weather comes.
Progressive-tube batch water heaters are a more recent development. Instead of a single, large tank, the water is contained in several 4-inch-diameter tubes. The tubes are piped in series, with the cold water entering at the bottom of the collector and the hot exiting at the top. The progressive-tube design allows the water to stratify more, limiting the mixing of the incoming cold water with the exiting hot water. Both the tank-type and progressive-tube batch heaters are classified as integrated collector/storage (ICS) units.
Flat-plate and twin-tube evacuated collectors that are designed with the tank attached are also classified as ICS units. These designs depend upon thermosyphoning, where hotter liquids “rise” and colder “fall” as long as the storage tank is higher than the collector. ICS units are completely passive and don’t depend on any moving parts or electricity to operate, though some contain electric water-heating elements for backup.
The flat-plate ICS designs use a classic flat-plate collector. The hot exit pipe of the collector is piped to the top port of the storage tank. The cold (or bottom) port of the storage tank is piped to the cold inlet at the bottom of the collector. As long as the sun is shining, water circulates through the collector and tank.
The twin-tube evacuated tube design is similar, although these heaters are only for use with unpressurized water systems. The inner tube of the twin tube is painted black and acts as an absorber. Each tube is “plugged in” at the bottom of a special stainless steel tank with gasketed ports the same diameter of the twin tube, one port for each tube. When the tank is filled with water, the inner tube is also filled with water. When the sun heats the inner tube through the black-painted absorber, the water moves by thermosyphon into the tank. The evacuated tube ICS has a freeze-tolerance advantage over the flat-plate thermosyphon unit due to the high insulation value of the vacuum.