Is co2 heat storage tank possible?

An opportunity just came up where I can buy a ~1,500 gallon co2 tank that was apparently used by the miitary. I think it must be over 1,000 gallons, but am not sure because the thing just says it is 5 tons. I was planning on using a 1,000 gallon propane tank, but this will be about half as much and is already insulated and jacketed. It has a release valve at the top accessed with an attached ladder and a few pipes coming out one end. You can't actally see the tank as it is insulated and then covered with a steel metal jacket . The whole thing is shaped like half an oval. It must be about 7 feet tall and about 14 feet long. It looks like it is in great shape as it was stored indoors. Apparently there is some new code and they are replacing some of their older tanks. I understand that propane tanks are welded and designed to take pressures in the hundreds whereas co2 tanks are seemless and can tank pressures in the possible 1,000's. Would there be any problems with this type of setup if I use it for heat storage for my EKO60? The guy is thinking about scrapping it, but he wants it out soon, so I need to have an answer for him before the snow starts falling as he puts it. Any help would be appreciated.

I di the calculations before, and it is anywhere from 1,000 gallons to 1,500. Not sure as I can't really see the tank. There must be about 10 inches of insulationg surrounding the tank and then steel covering the insulation. From what I understand, these co2 tanks are like big thermoses. They seem to be perfectly suited for this applicatoin. They are much thicker than propane tanks as well. I am just worried that I am missing something. I haven't heard of one being used for this application. Structurally would there be any problems with converting a large co2 tank to holding water at temperatures around 200 degrees. I don't want this thing exploding on me or cracking. I also wonder about having the piping all on one end of the tank. I know that the large propane tanks used for storage have multiple fittings on the top. Will having all the fittings on one end restrict the heating of the water? It seems like it would work. The thing is enormous though. It weighs about 5 tons compared to your average 1,000 gallon propane tank at 1 ton. I was planning on putting on an attached garage within a few years. Would keeping this insulated tank for heat storage outside piped underground make sense until then? Not sure of the heat loss on a well insulated co2 tank. Any thoughts.

I know it is co2 as the guy who has it now was there when they let the gas out. The pressure gauge on the side goes up to 2000 psi as I recall. Not sure what it is rated for though. I honestly didn't see anything showing that. I talked with some people who thought that it was probably used to extinguish fires, as it was used by the military and they said that they would use large quantities of co2 all at once.

I talked with Mark at AHONA and he seemed to think it would be perfect. I think I might just bite the bullet on this one. For the price, size, insulation, and the thing has a cool history, good conversation piece. I just don't want the thing to blow up. I just wasn't sure if they would fit this application.

Thanks Mike. I am worried about the pressure for heat as well. I am positive the insulation was to keep it cold as is the case for co2. I worry about that expansion. I haven't found much information on this. This is an all new thread on heat storage. We are crossing a new frontier here. I know the new co2 tanks are rated for temperatures ranging from -45 degrees celcius to about +45 degrees celcius with pressures much higher than propane. I know with co2, if the refrigeration goes bad that they bleed off, and one would think they build in some room for extra pressure as a fail safe. I am wondering if there is anyone out there who has a co2 tank used for heat storage.

First, let me just say I love this site. It seems we all have a common interest in finding new and imaginative ways to save money on heating. I am cheap by nature, so I love this.

Free75, that is exactly what Mark at AHONA mentioned in regards to the piping. Great idea!

Jim, if I end up getting it, I promise to post a picture of this beast. A picture is worth a thousand words right. The insulation, at least the visible stuff that you can see around the release valve at the top of the tank is actually a type of blanket insulation. Of course, not actually seeing the tank, the tank itself may be coated with a foam type insulation. Not really sure. There must be about 10 inches of this insulation around the tank. I have heard of just taking batt insulation and wrapping it around a propane tank to insulate it, so I don't see how this will be a problem. Though I have never done it myself. I do know that the tank was used for co2 until it was decommissioned recently. I guess if it wasn't for a change in some code of some sort they would still be using this monster. It looks clean on the outside, and like I said, the guy who currently has it was there when they emptied it. From there it was brought to his place and that is where it has sat since, unused.

The guy had another co2 tank taken from the same base that had no steel shell and the foam insulation scraped off the outside, that is about 2,500 gallons that he is giving to his brother to be used as a septic tank at a golf course. That one I could see inside, and it was very clean. It also was extremely thick. Seriously thick. Close to an inch thick. As the guy said, you could stick this thing in the ocean for a few years and it still would hold pressure.

I probably should have from what I have been gathering. Oh well, what is meant to be will be. Right.

Just to let everyone know in case people were wondering, the deal fell through. Haven't heard from the guy. Its probably a good thing too, because we were initially going to put it into a garage. We have decided we aren't going to build the garage anytime soon, so the tank would have been an eyesore in our yard indefinately. Also, from what I have been learning, for DHW, pressurized tanks seem to be a little trickier. We have actually decided to go with unpressurized due to the problem of having to fit a heat exchanger in a tank. I liked the idea of pressurized, but it gets tricky when you try to heat your DHW. So, I have been converted from pressurized to unpressurized.

Is it possible to have a hybrid system whereby you can use both, say a 500 gallon pressurized tank and an open tank together. Of course the open tank shouldn't be allowed to have water temps over 175 degrees. I was wondering if it would be at all possible. I am thinking it wouldn't be since the tank temp can only get as hot as the boiler. If the open system can only allow temps of up to 175 degrees, then how would the boiler know the difference? I am thinking you would need to have some serious controller on there. Has anyone heard of using them together, pressurized and unpressurized?

I guess harder in the sense of getting enough surface area in the tank with a heat exchanger, coil or other means. I want to make sure hot water isn't a problem and I worry about getting enough heat transfer by means of stuffing something in a small opening in the tank. I am not saying that it can't be done, I am just saying that for DHW, the open system seems a lot easier and straightforward. My biggest fear would be not having enough hot water for showers etc. and the wife going ballistic. I have seen other people switch from the idea of pressurized to unpressurized when it comes down to the DHW situation. Beleive me, I went kicking and screaming as I had my heart set on a pressurized type system. I think it is a lot cleaner, and the direct tranfer for heating the house appealed to me, not to mention the price. What type of heat exchange did you use in your pressurized system? How did it fit inside the tank. What kind of surface area do you have? The opening on say a 500 gallon propane tank can't be that much, and that is what we were considering, two 500 gallon propane tanks. (Actually we were considering a 1200 to 1500 gallon Co2 tank that probably had a fairly sizeable opening for getting a heat exchanger in. The problem with it is that it would need a home and the beast weighed in at 5 tons.)

Boilerman,

How much coil can you fit in a pressurized tank using a 1.5 inch hole. How much length? How much surface area?
I just worry about getting enough heat from the heat exchanger in the tank. I know with the open system, you can easily put in lots of coil that encompasses most of the tank to get as much heat out of the water as possible.

Leaddog,

I considered that option as well. I know the tank we have has very little piping in it. It is a 40 gallon tank. Not exactly sure of the coil length, I know it is steel. I have heard that they are about 5 to 15 sq feet of surface area generally. 120 feet is more in the 30 to 35 foot range and am told should be plenty to pull heat out of the tank. I just worry about the complexities of adding another tank on top of the two 500 gallon tanks we would add. How many feet of coil are in your 80 gallon tank? How would you pipe two tanks together, for instance, an 80 gallon tank and the existing 40 gallon tank? Is it as simple as I think it is, just putting the outflow into the inflow on the second tank?

The cost difference for me isn't that great to make me really want to go through the headache of getting the tanks and getting them in the house and the headache of finding someone other than me to hook them up, if there is a qualified installer willing.

There are a lot of ifs in there for me to go with the pressurized storage. The unpressurized tanks take that out since they are ready made. They are light enough to carry into the house. I do worry about the complexities and the DHW. Not many installers are familiar with the pressurized storage as well. The unpressurized systems seem to be more common and so the installers are more familiar with them, and long term if anything goes wrong, a plumber or the installer will be able to help me out.

I wish there were kits for the pressurized systems utilizing old propane tanks for cost savings. It really sucks, but just finding an installer in my area that seems to understand these systems has been a headache in and of itself. I just have been beaten down so much by this process that the prospect of having to get two large propane tanks into my basement and the headache of finding a qualified installer willing to hook them up does not appeal to me. I am shocked at how few plumbing/heating professionals in Maine there are that are licnensed for solid fuels, and how few of them are familiar with wood gasifiers and willing to hook them up. This is technology that is not new, and really should be embraced. Its quite sad that Europe is kicking our butts on the prevalence of these systems. I'll get off my soap box now. This has turned out to be quite the message.

Leaddog you almost had me back on the pressurized bandwagon, but I just don't want to deal with the headaches. If two 500 gallon tanks fell on my lap, well I guess I might try harder to go that route. I am curious how you did it though with the 80 gallon tank. Any issues of DHW when the boiler isn't running? What temp of storage do you need to make the temp for hot water sufficient?

We don't have a problem with our 40 gallon tank with temps at 180 with the oil boiler running. The problem we are foreseeing is during the summer, with storage, we want to make sure we aren't having to charge it every day. I am told that with 120 feet of coil in the unpressurized tank, that the water coming out is almost the same as the temp at the top of the tank. So, conceivably, temps in the tank could get pretty low before you lose your hot water.

I took your advice and read a little about the sidearms, but how much heat exchange do you get from them? It acts as basically a shell over the pipe for your hot water heater. Am I right? Basically a small storage tank with the hot water pipe acting as a heat exchanger. The diagrams I have seen are basically a larger pipe running the DHW to the tank and the sidearm running heated storage/boiler water through it. Is that the principle? How much heat can that sidearm pull from the storage water? It just doesn't seem like enough surface area to pick up enough heat for DHW. Most coils they recommend quite a few feet to effectively get the heat out. That sidearm just doesn't seem like enough.

Our problem for storage is mostly during the summer for DHW. We want more time between burnings. I should say I want more space between burnings since I will be the one doing it not the wife. The unpressurized system can store water at 175 degrees safely for the liner. That gives us 1200 gallons of about 55 degrees to work with down to the 120 degree water temp. That should buy us lots of time during the summer. Also during the spring and fall. If a sidearm can be had for fairly cheap and is as effective as 120 feet of coil, then I can see the benefits of pressurized over the unpressurized in that you can store water at even higher temps. Then we might be looking at 80 degrees of play with 1000 gallons.

The problem I see is an "easy" way of heat exchanging the storage tank for DHW.

We use a lot of hot water. Not sure how long 40 gallons would last. Not sure how long 80 gallons would last. I don't think 40 or even 80 gallons would last much more than a day. I see what you are doing though, having it circulate every so often to ensure the temp in the hot water heater is about the same as your storage tank. How hot can a hot water tank get? I know pressurized storage tanks are about 200 degrees, but say a DHW tank, is that too high? Would you need some sort of regulator to make sure the temps entering the hot water tank are not above a certain max? I see what you are doing now though. Interesting. So the sidearm is just circulating storage tank water through the hot water tank to get it up to temp? Is that an oversimplification or what.

Your method of cycling definately takes care of the issue of running out of stored DHW. I like your method better than running an exchanger in the tank itself. How do you have the cycles timed. I didn't know you could do that. Could you have a circulator kick on if the temps got down to a low amount?

Damn I like it because you can store higher temps. I still have the problem and headache of finding someone to actually hook it up. Around here, if you want to hook up the pressured storage tank, you pretty much have to do it yourself.

Thats what I thought. Thanks for the explanation leaddog.

I guess there is no way of knowing if your water usage will be greater than the time it takes to heat it up. Unless you are well versed in thermodynamics. I worry that our usage, although in a given day may not be huge, may be very high at times. Both my wife and I take long showers, we have a dishwasher and are guilty of rinsing dishes with hot water without shutting it off between dishes. I also have a tendency to run hot water nonstop when shaving. We really suck at hot water conservation, and that obviously would be the easy answer, but I don't want to have to worry. Wood is cheap. So my biggest fear is that the DHW will be under too much stress to be compensated by the exchange surface of the sidearm.

The thing is I personally am the type of person that can stand it even if it isn't and add another hot water tank or whatever until it is right at my liesure. My wife, god bless her, is the type that will give me holy hell if her shower gets cold. I'll never hear the end of it.

So I thank you for the info leaddog, maybe I can convince my wife to wait on the storage until I can find cheap tanks and an installer willing to hook them up.

Brialin,

That is what we are considering, and I have heard good things about it. I would suggest keeping to the maximum suggested temp of 175, since I have heard many stories of deteriated EPDM at even 185. It sounds like you are one of them. Although honestly, at about only $200 for the liner, that extra 10 degrees gives you a lot of stored BTU's with a 1000 gallon tank, but I have never changed a liner, and I am sure it isn't exactly quick and easy, so wasting liners every 7 years although not too much money, might be a pain in the @ss. So a days burn, is that one burn or a few burns in the day? A weeks worth is awesome, that is what I am looking for, and no more than one burn a day in the winter. The open tanks seem simple and straightforward, and more do it yourself since you can self assemble due to lighter weight, but they are pricey especially after all the coils. I like the idea, just not the price. The tank we are looking at would cost us about $4500 for everything but the install. I think most of the people with pressurized systems paid little or nothing for their tanks, so price wise, there is no comparison.

We just got a quote from a place in Portland ME this morning. I worry that they are charging too much since the quote for the materials alone for the boiler and storage installation was about $8500. To me that is crazy since they quoted their tank package at about $4500 for materials with the self assemble, that is the tank with liner, and 4 120' coils. That leaves about $4000 for a few valves a circulator a few odds and ends and some piping. That seems crazy to me and I am seriously questioning the integrity of the installer. To me you pad labor, not materials and I already feel they are padding labor.