An Integral Collector Storage (ICS) system could also be called "a big, black water tank". They're the simplest form of solar thermal system because the storage tank is also the collector. They're also extremely efficient, though they are climatically limited.In the same way as a collector box, an ICS box is insulated on three sides and has one glazed side that faces the sun. However, the hot water tank itself sits in the box. The tank is painted black or with a selective paint for heat transfer. The sun's radiation passes through the glazed side of the box and warms the tank directly, which warms the water inside it. Simple.

Some systems use a single, large water tank, often containing 30 or 50 gallons and usually made of steel. Others use a series of smaller metal tubes plumbed together, which contain a similar amount of water when considered as a whole, and which are usually made of copper.The tubes outperform a single tank because they have a larger surface area, but they cool off quicker for the same reason. They're also more aesthetically pleasing as they generally have a much lower profile than a single, large tank.

Both types of ICS should be mounted at an angle so they drain properly. They should be plumbed in series with your backup water heater used as a pre-heater.

It is very important to note that, because all the water in the system is stored directly in the collector box, an ICS is comparatively very heavy: most of these collectors weigh between 275 and 400 pounds (125 to 180 kilograms). Wherever you intend to install your ICS, you should check that the structure is strong enough to hold that much weight!The main downside to using an ICS is that they can freeze and burst. They are extremely successful in climates where below-zero temperatures are unknown and in seasonal applications (holiday homes, summer camps, etc.) where they are used in the hot months and drained before winter hits. In such situations, they can provide 100% of a family's daily hot water needs.

Thermosiphon systems

A slight modification to the basic ICS setup is to add a collector (either a flat plate or evacuated tube) to the system. The collector works in the normal way, gathering heat and passing it directly to the storage tank by means of a closed loop or a heat pipe.Although they cannot be used where freezes occur, thermosiphon systems are more efficient than a basic ICS because the tank can be better insulated, thus retaining more heat during cooler hours.