At night or whenever the collector is cooler than the water in the tank, the direction of the thermosiphon flow reverses, thus cooling the stored water. It should be noted that thermosiphon collector loop circulation is driven by thermal stratification in the collector loop and the section of the tank below the collector flow return level. The major problem in thermosiphon system design is to minimize heat loss due to reverse thermosiphon circulation at night, when the sky temperature is low. Norton and Probert (1983) recommend that, to avoid reverse flow, the tank-to-collector separation distance should be between 200-2000 mm. A practical way to prevent reverse flow is to place the top of the collector about 300 mm below the bottom of the storage tank.
Nighttime heat loss from a collector is a function of ambient air temperature and sky temperature. If the sky temperature is significantly below the ambient temperature, cooling of the collector will cause fluid to thermosiphon in the reverse direction through the collector, and the fluid may be cooled below the ambient temperature. When the reverse flow enters the return pipe to the bottom of the tank, it is mixed with the warmer water contained in the storage tank. The combination of cooling below the ambient temperature in the collector and heating in the return pipe causes reverse flow in all thermosiphon configurations, irrespective of the vertical separation between the top of the collector and the bottom of the tank (Morrison, 2001).
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