another storage idea

[GARYL]

I have spent the last several weeks going through all 39 pages of this site and several dozen posts, mostly dealing with storage. Great site with tons of info but I didn’t find anyone who has done what I have been planning. I will try to explain and invite your critique.

Basically it consists of a tank within a tank. I need 180* water to the house for the baseboard heat and 120* water to the barn for the yet to be installed radiant floor heat. I know I could use mixers but how is this for an idea:

An outer poured concrete wall approx 7w x 10L x 8d with an interior tank of layed up block approx. 5w x 8L x 8d. The inner tank of 180* water will be ringed by the outer tank of 120* water. Any lateral BTU migration from the inner tank will end up in the outer tank. The exterior walls will be insulated against the earth, with the inside of the outer tank will be epoxy painted. The interior of the inside tank will be insulated and a rubber liner installed. What I wind up with is an interior 2000 gallon tank with 180* water surrounded by 2000 gallons of 120* water. This creates a Delta T for the outside tank of about 70* and a Delta T for the inside tank of about 60*.
The interior tanks walls will have no pressure on them (as long as the two tanks are filled at the same time) and will only act as the barrier between the two temperatures. I plan to insulate everything in a like manner to what I have been reading in these posts.

A lady I know bought a very large Victorian home and ripped out the 4 year old hydronic system to put air back in. She asked me how much I would charge to remove the oil fired boiler with 6 zones and six circulators with a 100 gallon zoned DHW tank along with many, many copper valves and hundreds of feet of copper pipe and finned baseboard. I told her it had a lot of value and she said “ Well then you won’t charge me as much.” I felt so guilty that I never sent her a bill and she is still asking for one.

That was a long story but it will help to explain how I plan to set up the hx. On the bottom of each tank I will use finned baseboard pipe for heat in from the boiler and on the top of each tank I will use it for heat out. The reectangular shape with 8 ft depth should yield excellant stratification. To protect the finned pipe I plan on using a sacrificial anode.

There is a lot more I could explain but I hope you get the idea and offer some comments.

Gary

 

[Willman]

I have been checking out ICF's for the storage vessel wall use. HR mentions their use on his web site. Still trying to find out temp limits though. Would have to line all surfaces though.
Will

 

[Vtgent49]

I agree that you shouldn't temper the radiant water with pure cold water, but you can temper it withwater returning from the radiant slab. That way you aren't "losing" any heat. Then you could go back to a single tank.

Baseboards are designed around 180 degree water, but work fine with 140 or less, when the outside temps are moderate, which is 2/3 of the heating season. I often let 100 degree water thermosiphon thru mine when outside temps are 35+, and my circulators kick on at 140 for colder days.

 

[GARYL]

I know I could mix the return water but the interior tank would cost so little to make compared to the benefits. I would have two 2000 gallon tanks so I would have redundancy if one of them failed and I like the idea of having the lower deltas for each. I think at worst I will need to burn once a day and at best every other. I believe the cost will be below 50 cents a gallon doing it myself.
Thank you for your input. I may be crazy but right now I like my idea. I am still in the planning stages so everything could change by the time I get ready to do it.

 

[wdc1160]

Garyl,

I think you posted this for feed back so I will attempt to point out things some obvious, maybe some you didn’t think of.

I also went through a long planning stage for my tank. I didn’t require two different layers of heat, however, I believe many of the precepts for storage are applicable to your setup. So I will endeavor to apply some of the analysis, maybe to give an alternative view on your approach.

With any design there are inherent pros and cons. I will concentrate on the cons only because Its likely what your more interested in.

First. The inner and outer tanks will require all separate plumbing, HX’s and zone valves – not to mention materials. Heavy expense. PS HX's cost way to much for you
just to be only costing .50 a gallon.

It is simple math given the dimensions you have given your outer tank with absorb heat from the inner. You can calculate exactly how much, but I would posit that is not ideal. Your somewhat using the outer tank as a method to trap heat.

Your premise of lower your delta is simply a math trick your playing on yourself. Your delta is still 180f to the ambient temp. In this instance your just using a water tank on the outside as insulation. Can’t cheat thermodynamics brother, although I admire the attempt

What you may not be accounting for is your wasting a large number of Btu’s you could have otherwise stored. Your outer storage could potentially hold a great deal more btus’. The argument that mixing valves aren’t ideal is a hard sell. Mixing valves are purpose built for sending water to radiant floor. And, their just isn’t large amounts of waste involved. Clearly this is done with return water, not outside water sources, so simply little room to loose btu’s in this process – if done correctly.

All of the guys on board who have done engineering feats tangential to yours tout simplicity as a virtue to be maintained. Maintenance, construction, and even system component independence are being trashed in this approach.

In summary make one large tank insulate the hell out of it. Spend the extra dough you save on 2+ feet of insulation. Even with 90R value you could easily loose 40K btu a day.
If you don’t’ think this is true goto nofossils site and steal his calcs off of his scratch pad. Ultimately you will find that your still loosing btu’s at the same rate, only your leaving a large portion of your storage at a low temp. Meaning fewer BTU’s to loose.


Use a mixing valve. Make a 4000 gallon 180F tank. Save the money and keep the simplicity.

 

[Nofossil]

Very clever idea. I'll agree with previous posts about the thermodynamics and lost heat storage capacity but there's another dimension to this. Water by nature wants to be thermally stratified vertically, not horizontally. as your approach does. The two tank approach would make sense in one specific application: if you needed a separate water system (potable, for instance, or glycol mix), you could immerse a small tank inside your large tank. Given today's copper prices, an immersed stainless tank would be a great way to preheat your DHW, for instance. The immersed tank could be vertical, or horizontal at the top or bottom, depending on the temperature needs.

 

[BrownianHeatingTech]

Very clever idea. I'll agree with previous posts about the thermodynamics and lost heat storage capacity but there's another dimension to this. Water by nature wants to be thermally stratified vertically, not horizontally. as your approach does. The two tank approach would make sense in one specific application: if you needed a separate water system (potable, for instance, or glycol mix), you could immerse a small tank inside your large tank. Given today's copper prices, an immersed stainless tank would be a great way to preheat your DHW, for instance. The immersed tank could be vertical, or horizontal at the top or bottom, depending on the temperature needs.

An immersed horizontal tank could interfere with convection within the water, slowing down heat transfer in the same way that baffles in a gas tank help reduce "sloshing" in the gasoline.

The tank design would be rather important, in that regard, and a few small tanks would be better than a large tank (both for that reason, and for increased surface area).

Joe

 

[Nofossil]

I think it might work well to have a 5 or 10 gallon tank mounted near the surface of the larger tank to supply preheated water for DHW. The nature of DHW demand is many short bursts - high flow rate, but short duration. My HX coil gets depleted and doesn't have enough surface area, so I get less than optimum preheat performance. An immersed tank would heat up gradually, and eventually reach tank top temperature. Any demand less than the 5 or 10 gallons would get full preheat performance. Just a thought.....

 

[BrownianHeatingTech]

I think it might work well to have a 5 or 10 gallon tank mounted near the surface of the larger tank to supply preheated water for DHW. The nature of DHW demand is many short bursts - high flow rate, but short duration. My HX coil gets depleted and doesn't have enough surface area, so I get less than optimum preheat performance. An immersed tank would heat up gradually, and eventually reach tank top temperature. Any demand less than the 5 or 10 gallons would get full preheat performance. Just a thought.....

Definitely. Immersed tanks offer a lot of performance gains in many cases. Traditionally, they haven't been used because of the cost, but with the way copper heat exchangers are going, I think we'll see more immersed-tank options on the market...

Joe

 

[tom in maine]

I would be careful using ICF's for a tank shell. Most polystyrenes will start to melt over 140F. If you use ICF's, you should probably use 2" of Polyisocyanurate foam to protect it. That's like Tuff-R, Energy Shield, etc. The foil faced yellow foam.

 

[antknee2]

Check out this tank system , is it kind of what you would like to do but on a much larger scale ??


(broken link removed to http://www.triangletube.com/PDF/Ind_Fir_WH/PIIICommercial%20IWH.pdf)

 

[Nofossil]

Gotta love that 'duplex stainless steel' - almost as good as my two favorite materials - unobtanium and impervium.

 

[Telco]

Gotta love that 'duplex stainless steel' - almost as good as my two favorite materials - unobtanium and impervium.

"Now available in a Duplex model, the TR-80 and TR-120
can be used for a variety of different process applications.
Duplex contains a higher chromium content (18-26%) and
a lower nickel content (4-7%) than 316 stainless steel.
Duplex stainless steel has a structure that is a combination
of both Ferritic and Austenitic, hence the name Duplex."

I dunno, sounds pretty good but then I'm not a metallurgist. Is this some new sort of alloy or are they just fluffing an existing alloy for sales purposes? If it's a new alloy, will it do what they claim?

 

[Bob Rohr]

I like some of the ideas in-corporated in this tank. There seems to be a trend away from large amounts of stored DHW. I've noticed the "legionella' word popping up more and more in this regard, even in Europe. This works along the lines on a reverse indirect tank with the DHW in the coil, but unlike the Ergomax or Turbomax style the put the solar or boiler input in a coil also.

I'd borrow their idea of a "phase change" basically a food grade wax, to eliminate the evaporation, odor potential, and energy storage medium. Two of those issues troubled me when I had an unpressurized tank in my shop.

As you can imagine a tank like this doesn't come cheap, and I'd wonder about a coil failure requiring a complete tank replacement.

hr

 

[Nofossil]

I did the math for phase change wax. Might make sense if space were a real constraint and cost wasn't. It doesn't evaporate, but it is pretty flammable, and I had some concern about how it would affect serviceability. How would you drain and refill the tank if it's filled with wax that has a 170 degree melting point?

We need to be careful about just how much of the European model we emulate. A good percentage of European applications are tiny living spaces by our standards. Wood is not so widely and cheaply available, and people tend to be packed in tighter.

A modulating boiler can allow you to heat with less storage, but you can buy a lot of storage for the cost of the additional complexity that allows the boiler to modulate well. My hunch is that you want as much simple (but stratified) storage as you can fit. If that's not enough, then you want to look at more exotic storage and / or more exotic boiler and control technology.

 

[GARYL]

[quote author="ABGWD4U" date="1208041820"]

The inner and outer tanks will require all separate plumbing, HX’s and zone valves – not to mention materials. Heavy expense. PS HX’s cost way to much for you just to be only costing .50 a gallon.

I probably wouldn't consider this option if I didn't have a supply for the HX and circulators and zones. As I stated in my post, I got them for free. The major cost will be cement, blocks, insulation, liner, wood and of course sweat!

It is simple math given the dimensions you have given your outer tank with absorb heat from the inner. You can calculate exactly how much, but I would posit that is not ideal. Your somewhat using the outer tank as a method to trap heat.
Your premise of lower your delta is simply a math trick your playing on yourself. Your delta is still 180f to the ambient temp. In this instance your just using a water tank on the outside as insulation. Can’t cheat thermodynamics brother, although I admire the attempt

If I were to construct this tank I would insulate the inner walls of the inside tank just as if it were to the outside, so as to keep my 180* as long as possible. My thought was that since I will lose BTU's anyway why not lose it to my outter tank at 120*. I also thought the stratification would be excellant when you consider my design is just like a "box within a box".

All of the guys on board who have done engineering feats tangential to yours tout simplicity as a virtue to be maintained

I won't argue with that. I like simple....so does my wife!

Gary