Thermal stratification (solar heating)
Solar tank with optimised stratification, 2000 litres volume.
Solar tank with optimised stratification, 2000 litres volume.

The principle of thermal stratification in insulation of the storage tank – of very high importance to the efficiency of a solar heating system.

The thermal stratification is based on a natural process: Since warm water is lighter than cold water, it will ascend until it reaches a layer of warmer water or the top of the tank. This process facilitates the efficient utilization of solar heat: The higher the temperature difference between Collector and Solar Storage and the longer such a difference exists, the higher the Efficiency of solar heating. Therefore the solar Heat Exchanger will be mounted near the bottom of the tank, where the water is relatively cold, so even small amounts of solar heat can be "harvested". And while the outlet will be near the top of the tank, where the temperatures are highest, the inlet feeding fresh cold water will be positioned near the bottom. The more stable the thermal stratification, the higher the efficiency of the solar heating system and the comfort for the user by providing reliably sufficient amounts of hot water.

While all state-of-the-art solar storage tanks make use of thermal stratification, there are significant discrepancies in design and efficiency.

The most basic variant is represented by an so-called bivalent, upright-standing hot water tank for solar systems. Such a tank should be tall und have a relatively small diameter to allow significant stratification. The term "bivalent" refers to the number of heat exchangers integrated in the tank: The "solar" heat exchanger transfers the solar heat to the cold water at the bottom of the tank. A second heat exchanger, mounted at the top of the tank, serves as backup heater (electrical or connected to a central heating system), if e.g. in winter the energy provided by the sun is insufficient to produce as much hot water as needed. A thick insulation minimises the cooling-down near the walls of the tank and thereby thermal turbulence in the tank caused by such local heat losses.

More sophisticated design concepts include different additional measures to further the thermal stratification and to avoid thermal turbulence. Some systems add a third heat exchanger in the middle part of the tank ("trivalent tanks"); an electronical regulator then uses pre-programmed strategies to distribute the solar heat between the different heat exchangers. Other concepts try to achieve similar effects solitary by the design of the interior of the tank; a wide-spread implementation of this idea are the so-called Low Flow Systems.

Tanks with optimised thermal stratification boost not only the efficiency of the solar heating system, but also the comfort provided to the users: Starting with a totally cooled-down (unloaded) storage tank they will be able to supply solar heated water at significantly shorter notice than non-optimised systems.

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