Posted by John on January 28, 2007, 1:44 am
I am in the process of increasing my solar water storage (used for space
heating) from 1000 gallons to 4000 gallons. The new tank is stainless steel
and has the proportions of a tin can. 8.5 ft diameter and 10.5 ft tall. The
collectors are 12 - 4 ft X 8 ft collectors (Grumman, Solar Shelter, and
Gobi). All are drain-back. I am looking for the most efficient design for
the storage tank. I am considering heating the entire tank consistently
(take cold water from the bottom and return heated water to the top) vs.
using stratification, taking cool water from 2/3 up the tank and returning
to the top. When top water reaches a set temp. hot water will then be pumped
to the bottom of the tank for reserve storage.
Anyone have any experience with either method? Or other suggestions?
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Posted by Robert Scott on January 28, 2007, 2:08 pm
I don't think stratification is best served by taking water from 2/3 up. For
maximum collector efficiency it is best to put your coldest water into the
collector. Stratification can be promoted by where you return the warmed water.
Ideally, you would want to return it to whatever layer is approximately the same
temperature as the heated water. Unless the heated water is hotter than
anything you currently have stored, in which case you might as well return it to
How about this: return the heated water through a moveable return tube inside
the tank. Put two temperature sensors on the tip of the return tube - one to
measure the temperature of the returning water and the other to measure the
temperature of the tank water at the current position in the tank. If the
returning water is warmer than the tank water, drive the outlet tube up within
the tank. If the returning water is colder than the tank water, drive the
outlet tube down. And how about this for driving the end of the return tube:
Put a small air bladder on the tip of the return tube with an air line leading
to the outside of the tank. There you put an aquarium pump and a slow leak and
some controls to either pump up the bladder or turn off the pump and let the air
slowly bleed out. Depending on the bouyancy, the return tube can be made to
move to any level in the tank. The control can be made a little easier to
stabilize if you attach a length of thin chain to the end of the return tube.
The chain just hangs down to the bottom of the tank. This way the higher the
tube is, the more chain is supported, providing negative feedback to the
position control system.
Posted by Gary on January 28, 2007, 4:53 pm
Here are a couple of schemes that attempt to keep a tank stratified or make
efficient use of stratification from William Shurcliff that might be of interest:
These are from this book:
Have you measured the temperature profile of your current tank to see if it
actually achieves any stratification?
I find on my tank that within a few minutes of when collection starts the whole
tank is the same temperature within about 1 F -- it gets totally mixed very
quickly. I have the outlet to the pump and collectors near the bottom, and the
return from the collectors to the air space just above the water surface on the
Some stratification develops overnight as heat is removed from the tank for
house heating, but this disappears as soon as collection starts in the morning.
I know practically nothing about this, but I am beginning to wonder if
stratification in thermal storage tanks is less common than people think?
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Posted by Steve Shantz on January 28, 2007, 10:13 pm
I have a small (120 gallon) tank for my DHW system, and it definitely
stratifies. Some ideas to help make it happen...
1. As you said, always draw from the bottom.
2. The return water must enter the tank at a velocity and/or geometry
that prevents it from mixing or being driven by momentum down to the
bottom of the tank. My DHW system has a very small ElSid pump (3
Watts) pumping the tank water through the HX and back, and it returns
to the side of the tank about 2/3 of the way up. I let the returning
water figure out if it needs to rise or fall to its proper level. It
seems to know how to do this very well in my system. Some recent
posts have pondered whether it is better to let the water sink to its
correct depth or rise to its correct depth, but IMHO, there might not
be a significant difference. I'd love to see some good hard data.
3. Robert Scott's idea of a variable height tube is an clever solution
to getting the water to the right height...if it works well. However,
even if it is delivered at the correct depth in the tank, if it is
blasting straight down, it will still mix with the cooler water below
4. Slow the returning water down by increasing the pipe diameter by 2
- 4x before it goes into the tank, and either use some sort of baffle
or discharge it sideways to prevent the momentum of the water from
making it mix with the cooler water below.
I hope this is helpful.