HYBRID-stratification system providing hot water and heating backup.
HYBRID-stratification system providing hot water and heating backup.

Solar Heating Systems can fulfill two main purposes: Backup Heating (space heating) or heating potable water. In most cases backup heating systems should also provide hot water during the whole year, using a common array of solar Collectors.

Two different approaches can be used to achieve this objective: two separate storage tanks for the two separate water circulation systems (heating vs. potable water) or one tank combining both functions. Such systems are called combisystems respectively combi tanks.

While two-tank-systems require complex regulation strategies to secure an optimal allocation of the solar heat, the development of combisystems aims to accomplish such an optimal allocation only by an optimized tank layout.

The multitude of technical concepts in this field can be divided in two main classes:

- Tank-in-tank systems: a relatively small potable water tank is enclosed by the heating water in a big storage tank. The casing of the inner tank serves as heat exchanger between the tanks.

- Systems with integrated heat exchangers for potable water. In the most cases this will be an internal heat exchanger consisting of a potable-water pipe running through the storage tank.

Tank-in-Tank systems offer some advantages, especially regarding production costs, but are difficult to make compatible to today's highly sophisticated thermal stratification support concepts. Therefore most of such sophisticated tank concepts use internal heat exchanger pipes to provide hot water to shower or bath.

All solar storage tank systems including tank-in-tank combisystems, however, have to implement the main principle of thermal stratification (running from layers of cold water at the bottom to hot water at the top of the tank) and to avoid disturbing the stratification inside the tank to make efficient use of solar energy. One common characteristic of all solar tanks are therefore their proportions: relatively slim and tall - tanks containing about 1.000 litres have to be at least about two metres tall.

Smaller combisystem tanks can also be used in mere hot-water systems, replacing big hot-water storage tanks. Such big tanks containing potable water can cause hygienic problems in certain circumstances, especially if the daily consumption of hot water is highly vacillating, meaning high amounts of warmed water staying sometimes for days in the tank, slowly cooling down. By using combisystems instead, the volume of (half-)heated potable water will be minimized, while simultaneously providing a high storage capacity.

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