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Tank-in-Tank Storage Systems
Solar heating system
Solar heating system with "TERMO" combi system (yellow, 120 mm insulation; stabilised stratification by thermal flow funnel).

Solar Heating Systems often contain two storage tanks: the Buffer Storage Tank to provide storage capacity for Backup Heating (space heating) purposes and the Hot Water Tank to heat potable water used for bathes, showers and so on. To save space and to avoid the need for complex regulation strategies controlling the distribution of the thermal energy between both storage tanks, so called Combi Systems have been developed, which solve both problems by their design.

Among the most widely-used types of combi systems are tank-in-tank storage systems. They consist of a big buffer tank containing the heating-circuit water; inside of this big tank a second, smaller inner tank containing potable water is used to provide hot water. So the solar system heats heating-circuit and potable water simultaneously, the distribution of warmth between both circuits is governed automatically by the design of the tank. A Heat Exchanger tranfers heat from the Collector to the heating-circuit water in the outer shell; the surface of the inner tank transfers heat from the heating-circuit water to the potable water.

The most simple design is to hook a small potable water tank into the top section of the buffer tank, where the water is hottest (see Thermal Stratification). The main problem with this design is to avoid turbulences within the thermal strata caused by the inflow of cold potable water, which could damage greatly the efficiency of the solar system. To minimise this problem the inlet pipe will often enter the buffer tank at the bottom and lead through the whole buffer tank to the top, so preheating the potable water en-route.

A logical enhancement of this concept leads to inner tank which is formed like a mushroom with a wide head and a somewhat smaller shaft placed in the center of the outer tank. Passing the "shaft" the potable water is heated without disturbing the stratificiation in the buffer tank much; the head provides a sufficient reservoir of hot water. Other concepts use conical or plain cylindrical inner tanks, reaching from the bottom to the top of the outer tank; in the most applications the differences in efficiency between this concepts are not significant. As a rule of thumb it is recommended to dimension the inner tank to about a fourth or third of the total volumen of the tank-in-tank system.

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