Distributing hot water into a heat store to maintain thermal stratification in a system driven by natural convection.

History, and a proposal.

David M. Delaney, December 13, 2006

Fig-1-2-US-patent-1425174.gif An early solar water heater patented in 1922 by Cartter et al, US Patent 1,425,174. Water warmed by heat exchanger attached to the solar collector enters the main storage tank at three levels according to temperature, creating and maintaining thermal stratification.
Fig-1-US-patent-4146087.gif In 1979, Johansson patented a way of distributing warm water entering a tank from a branching pipe rising within the tank.  US. Patent 4,146,087.
Sheet-1-US-patent-4510922.gif In 1985, Roussos et al patented a vertical perforated pipe, 77,  and a momentum reducing device, 84, to distribute warm water  within the tank.  US. Patent 4,510,922. The momentum reducing device will not usually be required in thermosyphon systems.

shurcliff-horn.gif strat-tank-1.GIFIn his 1979 book, New Inventions in Low-Cost Solar Heating,William Shurcliff proposed a scheme to distribute hot water into a storage tank in such a way as to create and maintain thermal stratification.  See the article on page 177 of New Inventions in Low-Cost Solar Heating..

From that article: "The returning water enters the tank via a horizontal, wide-mouthed, "horn" or cone that slows it almost to a stop, without turbulence, with the consequence that, by virtue of its natural buoyancy, (whch depends solely on its temperature) it rises or falls to join the temperature matching stratum."

Where circulation is powered entirely by natural convection, special attention to slowing the already slow incoming water is not required.  It remains to minimize  mixing as the incoming water rises or falls to its temperature stratum.  In this circumstance a simplified narrow version of Shurcliff's  horn might provide considerable  protection for the rising or falling incoming water.


The version shown here is simpler than any of the above schemes. It should be adequate for thermosyphon systems. It  is to be constructed from two CPVC  pipes and a Y connector. The pipes and the Y are slotted and assembled as shown in Fig. 2 to provide a rising and a descending guide for the incoming water. Both guides are sloped at 45 degrees. The slot of the rising guide faces down. The slot of the descending guide faces up.

As with Shurcliff's horn, the slope of the pipes ensures that buoyancy forces will keep the incoming water pressed against the pipe away from the slot until it reaches its temperature stratum, reducing mixing.

The rising guide is shorter than the descending guide 1) because hot water is more valuable than tepid water, which says it should be exposed to less mixing hazard, and 2)  becauses the dynamics of cool water falling through warm water are different  from the dynamics of warm water rising through cool water.  In the case  of cool water falling through warm water, the force producing the movement is the force of gravity pulling the more dense cool water down through the warmer water. The cool water forces its way through the warmer water.   In the case of warm water rising through cool water, the force causing the upward movement is produced by the heavier surrounding cool water squeezing into and under the warm water at every level of its rise.  assembly-slotted.gifAs a result, the maintenance of a compact and steady rising stream of warm water supplied from below in a tank of cool water is less likely than the maintenance of a compact and steady descending stream of cool water supplied from above in a tank of warm water.  Rising warm water is impelled by forces that tend to mix the ascending stream. Descending cool water is impelled by a force that tends to keep the descending stream compact and unmixed.