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Posts: 2,240
Reply with quote  #1 
Hi, All,

I found this interesting study, which FWIW I have attempted to digest...


In thermal systems when the available energy source is irregular or when a time lag exists between production and demand (such as in a solar hydronic heating installation…), a thermally stratified storage tank is an effective way of improving the overall efficiency of the installation. 

Substantial increases in the total efficiency of the thermal system may be achieved by assuring a good degree of thermal stratification, since this can improve the thermal performance of the storage tank.

  • In general, experiments and simulations on hot water heat stores have shown that an increase in the aspect ratio of the heat store (height over width…) leads to better thermal efficiency, optimal levels being attained for an aspect ratio of 3.

  • An increase in aspect ratio was also found to improve the charging-discharging process

  • At least two studies have demonstrated aspect ratio values of between 3 and 4 as optimal.

  • For higher aspect ratios, stratification is marked, and decays gradually over time.

The study investigated three tanks of differing aspect ratios:

� tank with AR=½ (“low cylinder”) : low level of stratification -- with cooling, top half of tank stratifies slightly more, while stratification in bottom half remains almost unchanged; steady temperature difference of around 5°C between top and bottom of tank;
� tank with AR=1 (“circumscribed cylinder”): weak stratification in a restricted upper layer of the tank; clearly visible stratification lower down, gradually degrading over time. Temperature difference between top and bottom of tank drops gradually over time, from around 20°C to 5°C;
� stratification in the tank with AR=2 (high tank) was marked throughout the whole tank. As tank progressively loses heat to surroundings, stratification in upper third degrades gradually while rest of tank becomes progressively less stratified;

Other findings indicate that a major factor in decay of thermal stratification in an uninsulated tank, is heat loss to ambient surroundings: as heat is lost, stratification is cancelled....

� Stratification was found to decrease continuously during the cooling period;

Improved thermal stratification is achieved by increasing the aspect ratio.

Modified with additions...

Another interesting study here concerning importance of stratification…


Additional points of interest :
  • Mixing of flow in the inlet opening is considered as one of the most important factors for degradation of thermal stratification.
  • Improved stratifiction can be obtained by increasing the diameter of the inlet port, and/or increasing the difference between the inlet and outlet water temperature.
  • Maximum thermal stratification can be achieved inside a cirular storage tank with a height-to-diameter ratio of four.
A few other thoughts here...


When a stratified hot water storage tank has closed water circuits, the water temperatures can be up to 90...95°C at the top - and e.g. 20...40°C at the bottom.

Calm and undisturbed water is a relatively bad heat conductor comparable with glass, bricks, soil. This is illustrated by a lake, where the surface water is warm but deeper layers are cooler. An arbitrary volume of hot water can be stored, as long as the stratification is kept intact. No vertical metal plates or tubes may be used inside the hot water storage tank, because this would conduct heat between the stratified layers. The hot water storage tank is insulated (…) energy can be stored a long time if the hot water is not diluted…

(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...


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Posts: 283
Reply with quote  #2 
Some good info G_H ,looks like rectangular shaped tanks will provide better performance, thanks for summarizing and sharing.
Regards Mattie

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Posts: 2,240
Reply with quote  #3 
cheers, Mattie,

reckon my yellow wheely bins just got the thumbs-up !


(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...

Posts: 4
Reply with quote  #4 
thanks G_H good info there. looks like tall is best. tall increases the pressure in the lower portions too, to keep in mind.
i have been looking at commercial tank of 80 Gal, but really cant justify the $2000 when i can build one larger myself.
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