Non pressurized storage a compromise?

[mcdonals]

Nofossil/BrownianHeatingTech:

I'm reading the thread "Read Before Burning", posted 17 September 2008.
A lot of information summarized in one spot - it's excellent.

I'm confused about pressurized vs non-pressurized storage though:


"9) If you can do it, pressurized storage is preferable."

"I’d amend 9 to say that pressurized storage is preferable on smaller systems, and unpressurized on larger systems. I wouldn’t realistically consider pressurized storage above 1000 gallons, except for certain commercial applications."


I'm soon to get an Eko 60, and will be getting some type of water storage with it. I've located an ~1800 gallon stainless steel open tank for a reasonable price, or I can search out a few/couple propane tanks to get pressurized tanks adding up to the same volume.
Is the unpressurized tank a compromise ? I understand that heat exchange efficiency will be reduced every time I transfer from one medium to another, but can I make up for this somehow ?
For example, more insulation to minimize standby heat loss, or large capacity plate type heat exchangers - one for boiler to storage, and another for storage to load.

 

[kabbott]

If I needed 1800 gal. and had a line on a stainless tank I would go for it.

My tank is about that size, EDPM liner and ~750 feet of 1/2 inch PEX in 4 spiral coils, works OK.
If I were starting over I think I would go with a HUGE flat plate with some way to reverse flow for charging/draining storage.
Much has to do with what temp or range of temps you need. With pressure system you will get storage charged higher and draw down lower
than through heat exchanger but may not matter if you only need 100 degree water for radiant.

 

[sdrobertson]

There is several factors in not having pressurized storage over 1000 gallons. I think the biggest is dealing with the expansion tank sizing and cost. I went with 4-500 gallon propane tanks for storage and then a 330 gallon tank that came off a field spayer for expansion (or you could use a smaller propane tank) for my system. I like mine because I have a air handler in my furnace so I run my storage up to just short of 200 degrees. With the open system you don't have the cost of the expansion tank but you have the expense of the heat exchangers and extra circulators. Depending on the cost factors of what is available in your area as afar as tanks, it probably would come out to a even trade once your all said and done.

 

[Piker]

I used an Extrol sx-160v diaphragm type expansion tank (86 gallons) for the 1000 gallons of pressurized thermal storage that I now have. Those run around $500. Add that to the $200 I paid for the tanks, the $60 the welder charged me to put attach new fittings, and the $300 worth of copper to pipe everything up, and I'm just over a grand. Not to shabby. Of course the tanks are not new... had I purchased new tanks I would have added about $2500 to the price... but that still would be justifiable in my opinion for a closed loop storage system that can offer higher delta t than an atmospheric system, and essentially zero maintenance.

One drawback of pressurized storage is not having a DHW coil in the tanks. Zoning domestic hot water off of the storage can cost a little more than just dropping a coil into an atmospheric tank.


cheers

 

[DaveBP]

I used an Extrol sx-160v diaphragm type expansion tank (86 gallons) for the 1000 gallons of pressurized thermal storage that I now have.

Piker, what pressure range does your system go through from the lowest to the highest temperatures?

 

[DaveBP]

I used an Extrol sx-160v diaphragm type expansion tank (86 gallons) for the 1000 gallons of pressurized thermal storage that I now have.

Piker, what pressure range does your system go through from the lowest to the highest temperatures? I will have about the same volume and was thinking about using the same expansion tank.

 

[hkobus]

"9) If you can do it, pressurized storage is preferable."

"I’d amend 9 to say that pressurized storage is preferable on smaller systems, and unpressurized on larger systems. I wouldn’t realistically consider pressurized storage above 1000 gallons, except for certain commercial applications."


I'm soon to get an Eko 60, and will be getting some type of water storage with it. I've located an ~1800 gallon stainless steel open tank for a reasonable price, or I can search out a few/couple propane tanks to get pressurized tanks adding up to the same volume.
Is the unpressurized tank a compromise ? I understand that heat exchange efficiency will be reduced every time I transfer from one medium to another, but can I make up for this somehow ?
For example, more insulation to minimize standby heat loss, or large capacity plate type heat exchangers - one for boiler to storage, and another for storage to load.

I would say that they both have compromises to consider:

Presurized:
Large volume of water may be an issue when it needs to be treated and for expansion.
Propane tanks are heavy and hard to place in a basement. More turbulence in the stored water, reducing thermal stratification.

Non-presurized:
More flexible for glycol or treatment because of lower volume in the boiler loop.
Can be build in place, and in large sizes.
Easier to maintain stratification as the water in the tank does not get pumped around. This may help with the efficiency difference between the two.
Can have solar coils added at later stage.

These are just my thoughts, there will be more to add to these lists. I am partial to non-presurized at this time and will start my tank shortly and set my deadline for the 15 of Oct. ( putting it down here may help to keep me on track .

 

[Nofossil]

My personal thoughts on advantages and disadvantages of pressurized...

Advantages:

1) Higher effective storage capacity and heat transfer - no HX to limit temps and transfer rates.
2) No headaches trying to minimize evaporative heat loss and resulting condensation in insulation.
3) Simpler plumbing, fewer components.

Disadvantages:

1) Harder to add extra heat exchangers (DHW preheat, solar)
2) Need large expansion tank

 

[tom in maine]

Everyone has made really good points as to the pluses and minuses of both pressurized and non-pressurized tanks.

Since we manufacture non-pressurized tanks, I have a bias, but let me explain why in some minor detail.

Necessity being the mother of invention, many years ago, I started out in the solar business (1979). Before starting on my own, I worked for someone who used Vaughn stone-lined tanks for solar storage. When we needed a 120 gallon tank, we needed three men and a small boy to get the tank into the basement and even then it was a real hassle.
When I started out on my own, I was working by myself and being tall and skinny and not burly like my former co-workers, a stone lined tank was not going to make it.
I started building my own tanks. The big thing about unpressurized tanks is the ease of installation--at least, hopefully, with our tanks!
You can sneak a big tank into a basement that has access as small as a window that is 25"x6". That would get all of our tank into the basement (not heat exchangers, but even there you could use plate hx).

Of course, heat exchangers can make or break the thermal performance of the tank. I think we have dealt with that fairly well.

Serviceability is the other thing. An unpressurized tank can be rebuilt if it was ever necessary.

Our tanks seem to seal up well. You do need to follow our simple directions (this does not always happen), but they can be sealed well.

I would rather deal with rebuilding one of our tanks--or even our competitors (which we have done) over trying to repair a pressure tank, any day of the week!