Question # 2 underground storage tanks

[Richardin52]

OK

I want to put in 2000 gal of storage and I want to have it underground. Right now I'm thinking of two concrete 1000 gal septic tanks side by side and insulated with two layers of two inch foam board then sealed with maybe 4 inches of spray foam by the guy that will spray foam the pex.

Why concrete septic tanks? cheap, $700. each and look at all the themal mass. My question is # 1 will the concrete hold up to heats of high temp water and # 2 will that be enough insulation

Thanks for your thoughts

 

[leaddog]

The other problem you have to cover is make sure you seal the concrete. It has to be waterproofed as water will seap through. Not a problem with septic use but for storage it will ruin the insulation value. A better plan is to bury propane tanks in a concrete vault with a drain in the bottem to drain any surface water and insulate the inside. Closed system, no heat ex, cheaper tanks.
leaddog

 

[Richardin52]

If I use septic tanks why couldn't I line the inside with some type of liner so they would not leak? You say I could bury propane tanks in a concrete vault, but wouldn't they rust into the water? and eventually leak?

I like the idea of having a system under pressure but isn't that one more thing that could go wrong.

What I think your saying is I to get some round propane tanks, spray foam the outside then set the whole thing on a bed of concrete and then pour concrete around the rest of it when it is in the ground, right? If I'm wrong please tell me what I'm missing


One thing I had not thought about is expansion. With all that water heating up wouldn't anything I use expand enough to crack the insulation or concrete?

What have other people used as tanks? Has anyone else ever had some type of tank they tried fail?

 

[Bob Rohr]

I'd put underground storage at the bottom of my list. It hard to insulate and be 100% sure it stays insulated and DRY.

Here is one I inherited from a contractor that "chickened out

I used 2" foam inside with a thick roofing EPDM liner. Then 2" of foam around the outside. A group of 3 copper coils for solar input, radiant out, and DHW pre-heat. this is a solar storage so i don't worry about excessive temperatures against the EPDM and the glued seam. I'd worry about anything over 150- 160F against that type of liner.

To really leverage the concrete mass you would use a liner and all the foam on the outside. That is a hard detail to do.

Mother nature hates an imbalance and will do her best to grab all that heat back. In the end she will get it all. Keeping 180F inside a tank with 55F or lower around it is putting most of the cards on ma nature's side. No amount of insulation will stop that transfer 100%. With some time you could calc the loss with various r values, although it is a very tough calc as the water temperature varies from top to bottom, and is ever changing. The hottest tank temperature will be at the coolest point of the ground temperature, unless you stay well below frost line.

The question becomes how much is enough insulation 4, 6, 12" or more??

If you can't be talked out of it, do all you can to insulate and assure the earth stays dry. Any moving water around the tank will carry BTUs quickly away. Maybe a french drain to a lower point to monitor the water table around the tank.

Are you sure no inside storage space is available or create-able? It may be money well spent.

Stick come copper tubes around the tank to the surface to drop some sensors to monitor temperature loss to the ground, just for science sake.

hr

 

[Richardin52]

The problem with going above ground is 20 years ago when I built no one was doing insulated slabs on grade. I had seen them done in Canada and didn't see why they wouldn't work in Maine. So I did a slab on grade and built a large two car garage at a 45% angle and placed my boiler room in the triaglualr area between the garage and house then put a wood shed on the end of the garage. It has worked out great but as you can see I have no room for a 2000 gal tank. My wife would veto the whole project before she would let me put it in a garage bay.

Unless--- I could get a tank made that was 3 feet wide by 4.5 feet long by 7'6" high. That would hold 757 gallons

I have another spot where I could put a tank 3 x 7'6" by 6' that would be 1010 gallons.

That would give me 1767 gallons.

Next question what would I make the tanks out of and how much would they cost?

 

[Bob Rohr]

I'm of the opinion that 500 gallons should be enough storage for a wood boiler that is sized close to the design load. Wood boilers need pretty much daily attention. They don't WANT to operate like a gas fired appliance. Trying to "make" them do something they were not designed to do, gets silly after awhile.

Maybe that is why we have seen only one company offer a wood fired boiler that tried to incorporate large large water content.

It's really hard to keep track of those BTUs stored in water, for extended periods.

Keep in mind hundreds of companies manufacture wood and pellet gasification boilers in Europe. If there was a need, or even an advantage to large storage I bet they would have offered it. Too many folks trying to American-ize a proven well designed Euro appliance, in my opinion.

An exception might be a solar application where you could drive large tanks with fairly inexpensively harvested and distributed energy. Maybe one small 100 watt pump, or better yet a PV powered pump to move the energy from the sun to the storage. Check out this Swiss approach to mega insulated storage with well insulated tanks of the 104,000 liter size! www.jenni.ch

Storing enough energy to handle near 100% of your DHW and heating load may make good sense if you have the $$ to want to be that green, I suppose. Or a gallon of heating oil costing $5.00 or more and flows at the whim of the Russians that control it

hr

 

[jebatty]

A steel tank (LP for example), if in a closed-loop pressurized system, and with appropriate boiler chemical, should not be subject to much, if any, rust. Note that your boiler is steel. Even if open loop, and boiler chemical monitored, you can keep corrosion to be quite minimal. The OWB systems are like this.

Scale, oxygen and acid are the three culprits of steel. Scale relates to hard water, oxygen relates to ordinary rust corrosion, and acid relates to ph. Boiler chemical with monitoring can control all of these quite well. See: (broken link removed to http://www.p2pays.org/ref/32/31321.pdf)

 

[wdc1160]

There has to be more than 1 way to skin this cat. I thought that the problems with steel and septic tanks were too overwhelming. I am sumerging protected wood and waterproof/rodent proof insulation. This is more expensive but I am trying to design for 20 years of use. There is simple math to figure heat loss. And, the reason for large amounts of storage are very simple.
There are times when you have abundant amounts of fuel and heat via solar and wood heat. Storage can buffer or smooth out the highs and lows. This is especially true for those of use who like the new solar water heat solutions.

Thanks Bill

 

[reaperman]

I'd put underground storage at the bottom of my list. It hard to insulate and be 100% sure it stays insulated and DRY.

Here is one I inherited from a contractor that "chickened out

I used 2" foam inside with a thick roofing EPDM liner. Then 2" of foam around the outside. A group of 3 copper coils for solar input, radiant out, and DHW pre-heat. this is a solar storage so i don't worry about excessive temperatures against the EPDM and the glued seam. I'd worry about anything over 150- 160F against that type of liner.

To really leverage the concrete mass you would use a liner and all the foam on the outside. That is a hard detail to do.

Mother nature hates an imbalance and will do her best to grab all that heat back. In the end she will get it all. Keeping 180F inside a tank with 55F or lower around it is putting most of the cards on ma nature's side. No amount of insulation will stop that transfer 100%. With some time you could calc the loss with various r values, although it is a very tough calc as the water temperature varies from top to bottom, and is ever changing. The hottest tank temperature will be at the coolest point of the ground temperature, unless you stay well below frost line.

The question becomes how much is enough insulation 4, 6, 12" or more??

If you can't be talked out of it, do all you can to insulate and assure the earth stays dry. Any moving water around the tank will carry BTUs quickly away. Maybe a french drain to a lower point to monitor the water table around the tank.

Are you sure no inside storage space is available or create-able? It may be money well spent.

Stick come copper tubes around the tank to the surface to drop some sensors to monitor temperature loss to the ground, just for science sake.

hr

I'm not so sure you couldnt insulate concrete enough to keep mother nature from stealing back the heat. Look at any pre-cast concrete building. The insulstion on the huge pre-cast wall sections are layered between the outside and inside concrete slabs, otherwise "sandwiched" between. These buildings are very common here in Mn and withstand much more than 55 degree ground-temp. Four inches of insulation is the standard insulation used.

Also I believe the tank could be sealed up very easily with a product that is used for sealing cement block on homes. I dont know what the product is called, but it is a rubber-based, tar like sealer that never full sets up and is suppose to be 100% water-proof. Its normally applied by a commercial sprayer that is equipped to handle the thick gooey material. You dont want to get this stuff on your skin or cloths. It doesnt come off. Check with a water-proofing company who specializes in waterproofing basement walls.

 

[Bob Rohr]

I'd put underground storage at the bottom of my list. It hard to insulate and be 100% sure it stays insulated and DRY.

Here is one I inherited from a contractor that "chickened out

I used 2" foam inside with a thick roofing EPDM liner. Then 2" of foam around the outside. A group of 3 copper coils for solar input, radiant out, and DHW pre-heat. this is a solar storage so i don't worry about excessive temperatures against the EPDM and the glued seam. I'd worry about anything over 150- 160F against that type of liner.

To really leverage the concrete mass you would use a liner and all the foam on the outside. That is a hard detail to do.

Mother nature hates an imbalance and will do her best to grab all that heat back. In the end she will get it all. Keeping 180F inside a tank with 55F or lower around it is putting most of the cards on ma nature's side. No amount of insulation will stop that transfer 100%. With some time you could calc the loss with various r values, although it is a very tough calc as the water temperature varies from top to bottom, and is ever changing. The hottest tank temperature will be at the coolest point of the ground temperature, unless you stay well below frost line.

The question becomes how much is enough insulation 4, 6, 12" or more??

If you can't be talked out of it, do all you can to insulate and assure the earth stays dry. Any moving water around the tank will carry BTUs quickly away. Maybe a french drain to a lower point to monitor the water table around the tank.

Are you sure no inside storage space is available or create-able? It may be money well spent.

Stick come copper tubes around the tank to the surface to drop some sensors to monitor temperature loss to the ground, just for science sake.

hr

I'm not so sure you couldnt insulate concrete enough to keep mother nature from stealing back the heat. Look at any pre-cast concrete building. The insulstion on the huge pre-cast wall sections are layered between the outside and inside concrete slabs, otherwise "sandwiched" between. These buildings are very common here in Mn and withstand much more than 55 degree ground-temp. Four inches of insulation is the standard insulation used.

Also I believe the tank could be sealed up very easily with a product that is used for sealing cement block on homes. I dont know what the product is called, but it is a rubber-based, tar like sealer that never full sets up and is suppose to be 100% water-proof. Its normally applied by a commercial sprayer that is equipped to handle the thick gooey material. You dont want to get this stuff on your skin or cloths. It doesnt come off. Check with a water-proofing company who specializes in waterproofing basement walls.

But the delta T is the driver. Keeping a building warm, lets say 70F when it is 30F outside is a delta t of 40 degrees.

Keeping 190F inside a tank with 55F or maybe 30F against the insulation is a delta t of 125 - 150.

Heat travels to cold, always, and the rate of heat transfer is related to the temperature difference, or delta t between them. Double the temperature difference (delta t) and you pretty much double the rate of heat transfer through it.

Insulation only slows the transfer, it cannot be stopped completely. Even vacuum insulated tubes lose some energy to the surrounding air, like your coffee in an old vacuum thermos bottle. How long does it stay warm?.

Which is why it takes much more fuel to warm your home at 30F outside temperature, compared to 55f outside temperature.

Some quick calcs show an 800 gallon tank, r 7, at 190F in a 50F ambient air space would lose about 110,000 btu per day mounted indoors.

In the ground with a stratified tank with cooler conduction transfer potential, from ground contact???? Possibly two hours a day of run time to cover shell loss. If it is well insulated and sealed.

Also factor the loss between the stove to tank and tank to building in any underground piping.

Which is why I advocate storing the least amount possible, you can't stop it's escape.

hr

 

[Willman]

An exception might be a solar application where you could drive large tanks with fairly inexpensively harvested and distributed energy. Maybe one small 100 watt pump, or better yet a PV powered pump to move the energy from the sun to the storage. Check out this Swiss approach to mega insulated storage with well insulated tanks of the 104,000 liter size! http://www.jenni.ch

HR, Thanks for the great explanation on btu's ,heat loss, delta-t etc. As always a pleasure to read. The way I am reading this is its too expensive to fire up a large storage tank with wood but solar is ok due to free energy source. I will rethink my storage design to incorporate solar and use it to take any excess btu's from wood. I checked out the link but didn't understand anything i was looking at except the pics.

Will

 

[Richardin52]

HR

What you say makes very good sence. But, (there always a but) When it comes to burning chunk wood it is always better to burn it quick and hot and as far as heat exchanging it is alway better to have a large delta-T between the heat produced and the water thats taking away the heat.

So unless I want to get up to fix the fire in the middle of the night or I want to heat a greenhouse at night plus my house and don't want to buy a huge unit or two units the next best thing is storage that I can put extra heat in during the day.

So using your great example the next best thing would be to have stoage that is # 1 very well insulated # 2 To put the storage somewhere where the delta-T is as small as posible and # 3 Put it where you can use the any heat that escapes if possible.

So we are looking at

a lot of insulation (Sytrofoam, Fiberglass, cellulose, treated sawdust?)

around a lot of thermal mass (water, sand, some type of gel, concrete, rocks?)

somewhere that you want to heat (in a living area, under a slab, under a greenhouse, in a wall?).

Can you think or anything I have missed?