High Temp Limit on Concrete

I was going to say that it shouldn't be a problem, since concrete gets very hot in the sun.

Now I just don't know. Are you thinking of using water? If so, the boiling point is 212F. I can imagine that there could be a lot of problems. Like the water boiling off as steam, or, if enclosed, the pressure cracking the concrete. Sorry I couldn't help. I want to be subscribed to see what comes up though.

I will use water only in the initial testing of the concept. After that I will be using some more esoteric heat transfer and storage fluids. The temp of the storage unit will be brought up incrementally during the ramp up period.

I've contemplated doing something similar during the summer months on a smaller scale so I could shut off the propane to my water heater. For storage I was going to look into old electric water heaters, where the element had burned out, but the tank was still good.

Septic tanks are pretty big, is this for household hot water? You must have something bigger in mind.

What I have in mind is radiant heat 24/7 for the house as well as hot water. I need to have heat available for those rainy days as well.

I would expect that concrete will easily handle these temperatures providing that you gradually achieve this temperature to not produce expansion cracking. However, I really doubt that you will actually achieve these temperatures in the concrete because of the amount of mass a concrete septic tank brings into your solar collector system. Now I don't doubt that parts of your solar collector will easily reach 350°F, just not the concrete itself.

That temperature differential might need special attention, around here, the underground temperature is in the fifty to sixty degree range. The tank would also need room to expand.

You might be best off in several ways if you had a thin wall tank within an underground tank and compressible insulating fill in the space between.

Your concrete heat tank will be a very leaky tank. While I think concrete can handle 350~400 F, it is not a very good insulator. You have to add better insulating liner.My suggestion, build an inner wall using steel sheet/plate, allow ample air gap between steel plate and concrete wall.

Forget it! Too many problem with heat leaks. But, your concrete will be eaten by acidity or salinity.

The OP did not tell you what fluid medium he is using, how do you know it will be eaten by acidity or salinity?

The obvious heat transfer medium to use is thermal mineral oil. It is Ph neutral, will not corrode the copper coil or a steel tank, cheap, readily available, and designed for heat transfer at these temperatures. With a vented tank, oil expansion will not be a problem.

Why pre-test with water?

I agree that if he does not insulate a tank within the concrete liner, heat will rapidly be lost to the ground. Assuming a 1000litre cubic tank, the surface area excluding the top (we will assume that the tank is not quite full) would be 5m². Under steady state the ground temperature would be higher than normal, so say 75°F and taking the average for oil at 375°F, the Δt is 300°F. Concrete has a thermal conductivity of 0.8 and a tank that big would need to be 100mm thick. Without any insulation the heat loss would be approximately 11Kw. 100mm of polyurethane foam would reduce the heat loss to about 280Watts.

You are correct, I did not mention what heat transfer or storage medium I will be using because evaluations are still being done, including potential corrosion and tank degredation from the fluid. I will be testing different fluid combinations and measuring the result against a base line. That base line will water and that is why I am starting with that. I am looking at different means of insulating the tank. Closed cell foam is preferred but the quote I got was out of sight. I may have to buy my own closed cell system and do it myself.

I think if you used a reflective barrier on the outside of the tank to reduce the radiant heat loss and then a six inch or greater layer of insulating foam outside of that, the concrete tank will perform exactly as you wish without experiencing any of the problems some are suggesting. You should probably see about getting the tank cast with concrete that has Xypex, Hycrete, or some similar type of densifier and water-proofing added to the mix. Let us know how it works.

http://www.scientificamerican.com/article.cfm?id=how-to-use-solar-energy-at-night

Hello Mike,

Assuming that you're burying this septic tank, you are going to have to heavily insulate the outside of the tank on the top, bottom and sides. Expanded foam core board is a good choice as long as it's compatible with soil contact.

You may want to look into purchasing a polypropylene plastic solar water storage tank that comes with factory installed R22 foam insulation (or you can install the insulation yourself to save money) instead of using a concrete septic tank. A 500gallon poly tank will cost you around $1,775, depending on the manufacturer. See below why the higher cost......In the end you'll be spending about the same amount of money and avoid having to resort to installing air blowers to help with heat transfer to the rock mass. It'd be more efficient too going with a large poly tank & a couple of multi-flat plate SS heat exchangers (can be found on Ebay for cheap...I have a few here myself for the solar heat system that I'm building). Just make sure your heat source circuit is a closed one (w/ circulation pumps, valves and controls) and full of a water/RV anti-freeze solution. the tank, depending on it's size, can be installed in the basement (or heated garage) and full of good old plain tap water and a separate closed circuit piping w/ pump and the heat exchangers.

Most vertically oriented poly tanks come with a 24-inch diameter screw-on manway & cap located on the top. Most manufacturers will cast pipe holes in the sides of the tank per you specifications.

Ohhhh, and one more thing about poly tanks: due to maximum allowable working temperature of most poly tanks you will need to buy a tank that is specifically manufactured with a specially modified polymer polypropylene plastic to handle the heat up to around 200 degrees F, give or take a small amount.

Please check out the following poly tank manufacturer (they're reasonable in terms of cost):

www.plastic-mart.com

Look under "solar hot water tanks" at that website. Also, depending on how much hot water you want to store, you can always install 2 or 3 tanks in parallel. They're fairly light and easy to move around vs. a concrete or steel tank.

I think I have looked at all the 'plastic' tanks made in America and the problem is that they will not with stand the temperatures I am looking for. The ultimate problem is cost per gallon of fluid stored. I have a 325 gal polyethylene tank that will go to 100 F or above in a pinch for a short period of time. That is fine for as 'assist' to my instant heat hot water in the house. As a primary source of hot water it is not acceptable.

the insulated concrete forms are the right kind of idea

the foam won't take the heat, custom formulation would drive up the price

there are precast concrete culverts & even chemical tanks that are designed to be buried, but theyaren't going to have the insulation factor you need

I sense some of the design criteria you are trying to fulfill are aesthetic [form] rather than functional [spherical collectors]

burying the storage would reduce the visible footprint

maybe build a nice sauna/greenhouse/smoke house over the thermal storage area

There is a 'new' construction technique i need to explore. They use foam blocks that look like concrete block and that are concrete filled for strength. If that foam used is able to be formulated in a high temp variety, that might work. More work to build tho.

Mike, the tank manufacturer's link that I provided you does indeed include tanks made to withstand up to 200 degrees F. I highly doubt you're going to get above that temp with your solar collectors, unless your installing some sort of oil-filled or brake system fluid in your piping circuit.....and definitely will not be reaching that temp with a water-filled system unless you want to be making a lot of steam. Yes, those tanks are somewhat costly, but they're easy to move around, flexible design-wise and easy to empty, etc.

If you do any research into rock heat storage designs you will find that they're not too efficient and you'll need a hell of a lot more rock storage volume than what your planning on using.....heating a house 31 (24/7).

Have you calculated your winter heat losses of your dwelling vs. design heat supply, and compared them? Then you must calculate the required minimum heat storage volume of your "rock pile", with the proper heat transfer coefficients and properly sized and carefully chosen heat exchanger(s). You need to know how your house was built with also knowing its wall, window & door areas, ceiling areas, and floor areas, as well as the materials installed therein with their respective U-values & R-values. I hope you know how to calculate heat losses and gains....

I seriously doubt that a concrete septic tank is going to give you enough heat storage volume if you suffer from a hard winter freeze (say down to 0 F or lower) in the northern USA states that lasts a few days or more....even a 2,000 gallon septic tank will not be sufficient in my estimation....as depending on many factors involved, you, may need several (or more) septic tanks installed in parallel to achieve the correct rock heat storage volume. And how are you planning on the heat exchange to and fro inside the tank(s)? What mechanisms are to be employed. I just don't see how a series of copper pipe (or circuits) or coiled copper tubing heat exchanger will suffice, as they're just not all that efficient.

I strongly suggest that you run some very serious number crunching and play in conservatively with the numbers and assumptions that you include in your analysis. Seriously, or your system may fall flat on it's face.

Where on God's green earth are you located, BTW?

[Garthh has a very good idea about using those insulated concrete forms...they work very well, but unfortunately are somewhat limited in regard to increasing he insulation thickness....all depends on the desired design thickness required to properly retain heat and minimize heat dispersion to the surrounding soil mass....and they're not cheap to buy either + hard to find, sometimes.].

Capt.

My reasons for reaching such high temps are as yet beyond reach, but suffice to say that when you consider that any fluid at 200 F only gives you 100 degrees of usable heat. If I want to put radiant heat in a structure on a 24/7 basis, I need a LOT of heat. . If I can get those temps, then my goals are reachable.

My market is across the southern US. My system could be used anywhere, but the southern section of the US and likewise other areas of the world in the same latitude(either northern or southern hemisphere) are best suited because of the number of sunny days in the year.

I will not be using water, that is only to calculate a 'base' from which to measure other fluids. I have not ruled out using pcm's although they present other problems I am not ready to deal with at this time.

There's a couple of things in this thread that have been bothering me that I finally figured out what has been bothering.

First, one of the primary advantages of working with the ancient synthetic stone, concrete, is that it can be cast in place to any desired shape and depending on the additives to a variety of different densities and porosity. So I do not understand why one would choose a pre-cast form (septic tank) designed to be highly porous and go through great efforts to make it water tight. Also by having the septic pool buried deep enough into the earth the very large heat sinking capability of the concrete and earth surrounding the vessel will keep the temperature at a fairly constant temperature. A very desired condition to promote bacterial growth to safely dispose the sceptic refuse. Instead of using this heat sink characteristic you wish to add insulation to reduce heat transfer. Next, this form is designed to prevent soil from filling the void that will contain the waste materials by utilizing the high compression strength of unreinforced concrete. By putting this above ground to isolate from the heat-sinking to the earth you will be using the tensile strength to contain your hot fluid instead of compression.

You seem to systematically take every attribute of a pre-cast sceptic tank and turning it into a disadvantage that needs to be mitigated for your use. Casting a form to suit your needs will require carpenters and the correct concrete mix to be poured. The casted stone can be a mix designed to contain fluid and contain re-bar reinforcement to permit improved tensile strength. You may even be able to use the added mass of the poured concrete in this fashion as part of your heat sink reservoir.

What you say is correct. Stone has been used in this context before and successfully. I am looking for a way to do what I want to do and that is to store heat in some way. I wanted to know if concrete and stone was a good way of doing that. They are very dense and as such should hold a great deal of heat.

I have it below ground because of the asethics of the process. My solar thermal collector sits on the ground and above the heat storage. Part of my objective is to keep everything looking 'acceptable' to the neighbors and neighborhood.

I also started looking at alternative means of storing heat because of frustration with my inability of finding suitable plastic tanks to do the job. Nothing I have seen is able to withstand the kind of heat I want to store. Now I can either increase the size of the tank or find one that handles more heat at one time. I am looking for the latter. I also am lookking at cost as a function of cost per gallon of stored fluid.
Too high a cost makes the solution impractical and undoable.

I have identified a couple of resins that will handle the job but the cost of producting them may be prohibitive because the end product needs to be 'baked' for a few hours at a high temp to "cure" the plastic. That entails including another manufacturing process in my plans, which I may have to do.

Thank you for your input

Just curious. Why are you looking for such high temps? Do you want to store oil?

Current methodologies for radiant heat use the suns energy during the day and electric hot water heaters at night. Now as far as I am concerned, If you spend that kind of money for radiant heat you should get it all day and night! You don't need much in the day as you have the sun to begin with coming in your windows. You really need it at night when it is colder. I am looking for the 'long' term solution. Short term is easy. I never take the easy way!

I thought that the phase change materials already did what you want to do, at lower temperature, too.

CPM's do store heat and lots of it in a small space. 50 to 100 times what water will. the only problem is that the temperature is limited to between 90 and 145 F. Just not hot enough for what I want to do. I have considered doing a two stage heating process, using the glaubers salt at 90 F and then reheat it to the temps I am looking for. That is still a possibility.

Hi Mike,

Everyone seems to be talking about using foam for insulation. This cost money! As my earlier suggestion, why not use just air, a space of trapped air is a very good insulator, as good as any foam. Foam is good insulator becuase of air space within . You can use as much air gap as you require for free! Just make sure the air space is sealed properly.

My idea is, you can use a concrete tank as outer shell of the tank , build an inner tank with suitable material , I think steel or stainless steel sheet is good , make the inner surface of the steel sheet as shinny , just like thermo flask.

You should also make sure the tank's support on all sides , has as small diameter as possible and are well insulated .

a thermo flask has a partial vacuum between the layers

the question changed I think the updated title reflects this

good title change.

Yes, vaccuum is best, but it would be very difficult and expensive to realise on a big tank.

I have not issue with your response except to point out that my market is the abundance of mobile homes with insufficient insulation and who draw an inordinate amount of electricity to heat. I concur with your observations, I just differ in my market and objectives.

Ahhh, I think I see. I sense that you are looking at maybe single-wide and/or significantly older double-wide mobile homes. Many of these types of mobile homes came with electric forced air furnaces, the most inefficient furnaces you can possibly get.

If you are experiencing just a handful of rainy or cloudy days I would look at solar hot water system (yes, they work on cold sunny days too). This could be augmented by efficient electric zonal heating on those rainy days. If you will experience significant rainy days I would look at a geothermal augmented heat pump. Even where it gets freezing, properly installed geothermal can still deliver around 50°F water temp.

With a high SEER heat pump (+ geothermal) the electrical demand should be cut significantly. Of course with any upgrades the biggest bang for the buck will reducing the hourly air changes, use double pane vinyl windows, weather striping that seals the doors, foam pads behind outlet and switch plates, R-30 (min.) under the floors, as much R in the ceilings and walls as possible.

This kind of set up is much more predictable and proven then what you are contemplating, and I believe much more cost effective. The above scenario can also be done in stages as resources and time allow. I hope this helps.

What you fail to understand about this market is that these people who live in these older mobile homes have no money for geothermal. It is not a cost effective heat source imho. It must be cost effective and cheap at the same time. I have designed an inexpensive solar thermal collector and am now trying to add the other element and that is thermal storage at a cost effective price. By cost effective I am anticipating a 2 year payback. Anything longer in this market will not be accepted. By and large they are not the segement of the population that does long range planning. It must be a 2 year of less payback to be accepted.

underground heat storage, won't be allowed in most parks

a 2 year payback is maybe $2k worth of electricity?