Integral Collector Storage systems, additionally referred to as ICS, “batch” or “bread box” water heating systems, are similar in design and operation to the flat plate panel collector. You can find more about the flat plate panel collector at our previous article. (Learn about A Flat Plate Collector)

This time, however, the heat tubes inside the insulated glazed box are much, much larger in diameter. As their name suggests, in an integral collector storage (ICS) system, the collector and tank are combined into one single unit, therefore, circulation pumps and electronic controls aren't needed because the household tap water is heated and held in the combined heat storage and collection unit.

Integral collector storage units are one of the easiest solar hot water heating systems offered and can be simply installed into any standard water heating installation. ICS or “batch” systems, are made from some large diameter tubes of between 4 to 8 inches (100 – 200mm) in diameter with one or more black painted storage tanks with inside an insulated glazed weather-tight box. Coldwater flows under normal water pressure into the solar collector, which preheats and stores the water.

The transfer of the solar heat from the collector to the water is by natural convection, no outside energy is needed making it a passive system. Whenever hot water is needed, the solar-heated water kept in the batch collector flows out to buy the force of gravity or the pressure of the cold replacing it and then continues into the standard backup water heating plant within the house. this kind of predicament installation may be a direct system (open-loop) because the water being heated is the same water you drink.

The main advantage of an “integral collector storage” system is that they do not need controls, pumps, sensors, or any other mechanical or moving parts, therefore maintenance requirements are minimal. Most of the ICS units created nowadays are “progressive tube-type” heaters as against the older single tank in a box kind batch collectors creating them less costly than active solar systems.

One disadvantage of ICS batch systems is that they can lose their solar heat quickly at nighttime or during cloudy conditions once the sun's energy is at its lowest. The result is that the storage collectors are great for people who use hot water early at night, instead of first thing in the morning. An insulated cover placed over the collector at night can help retain heat in the storage tank however this can be impractical.

Also, the collector is a water storage device, some designs have double or triple glazing over the pipes or tanks to decrease heat loss however this adds to the overall weight and price of the unit. Then “Integral Collector Storage” units are more inefficient in cold climates, because of heat losses at night.