stratification thoughts

[free75degrees]

Why is stratification good? It seems like a uniform tank temp would be effectivelty the same as stratified. Rememer the 1st law of thermo - you can't get nothing for free.

One way that I can think of to leverage stratification would be to have 2 separate HXs. Heat would enter the tank through a HX that is located entirely at the bottom and heat would leave through a HX that is entirely at the top. As the heat enters via 180*F water at the bottom, the natural tendency would be for the hot tank water to rise to the top. For example yYou might get to a point where the water at the top is 180*F and 170*F at the bottom due to the heat rising. But the input temp is still 180* and so it still heats the water at the bottom. The heat continues to rise so the water at the top could actually exceed 180*F. Then we draw from the top, so we are actually able to draw water that is above 180*F.

Does this sound like it would work? I know HX are expensive so adding an extra one might not make sense. Maybe it could be accomplished by making the HX movable within the tank, or by using a flat plate HX with the input/output switchable between the top and bottom using a valve.

 

[JrCRXHF]

I think i am going to use truck radiators for HX and put a path in the tank so that water has to flow though the Radiator then just pump my closed loops though the radiator. It seems cheaper then plate HX

 

[Nofossil]

Here's why you want stratification - there's a difference between heat energy and usable heat. Take this example:

Two tanks, one stratified with 50 degrees on the bottom and 110 on the top. The second one is uniform 80 degrees - same energy in both. Your wife wants to take a shower. You can draw water from anywhere on either tank. What's your choice?

The point of stratification is that higher temperatures are more useful than lower temperatures, and temperature differences can be exploited. If you're extracting water from a solar panel or a boiler, you can extract more faster if you can supply lower temperature water.

An ideal storage system would be sharply stratified with a layer of 180 degree water sitting above a layer of much colder water - perhaps 90 degree water if that's the return from a radiant slab, for instance. When charging, the 90 degree water would be drawn out the bottom, heated to 180 degrees, and returned to the top, pushing the thermocline downwards. The exact opposite would happen when withdrawing heat.

I can heat my house with baseboards for quite a while if I have a tank where the top half is 140 or more, even if the bottom half is 100. Once the top is below 140, I don't have much usable heat - same like the shower example.

 

[free75degrees]

That makes sense, thx. I still wonder if one could get the top of the tank above the boiler water temp by only heating the bottom. If the bottom is at 170 and the top is at 180 and the boiler water is 180, then the boiler water would still heat the bottom. However, if it flows from bottom to top it will heat the bottom then it will cool down a bit and will actually draw heat away from the top, since it is will be slightly below 180. If you could avoid the top during the tank charging phase then you would be sure that you always have the largest delta in temp between the boiler water and tank water.

 

[Nofossil]

If you're using a HX coil, you want to flow top-to-bottom when heating the tank, and bottom-to-top when extracting heat. Check out my plumbing diagram on my site - link in my signature below.