I picked up a 10,000 gallon fuel tank this week, that was removed from a local gas station because they were removing their gasoline service. I intend to excavate a forty foot hole in the field behind my house, spray the outside of the tank with spray insulation, wrap in plastic, refill the hole with the tank at the bottom with clean earth, (not rocks) and insert 1 inch plastic tubing in several sites throughout the tank. Some going to the bottom, some feeding from the tank top. The tank will be filled with a dense stone to collect the heat of the fluid to be added lator. The lines installed will connect to a solar intensifier, (Mag glass) which will be aimed directly at the Sun by computer tracking. I will start with about 100 sq feet, but I expect I will need more. Temp monitering will determine the circulator run times. I will be using salt water as the transfer fluid, considering the corrision factor whith all materials used. Plastic and fiberglass will not be effected by salt, and salt will never change in property. I do not intend to dig this container up ever, and all installations will be done in a manor eliminating this possibility. We hope. The mission is to heat this tank up to above 250 degrees during the summer months. Then using simple circulators, pulling this heat out of the ground in the winter and running through standard hot water heat exchanger. Yes, I will have to build my own exchanger. I will have to build all of it. Because earth is such a poor transfer element of heat, it should hold these elevated temps for a season once the surrounding earth is heated. Of course this is all theory. I have not heard of anyone doing this. Can anyone here give me any info / suggestions on problems I might face? I am interested in fact, I already have enough theory to choke a pony ... well maybe a little more theory would not hurt ... ;-P

Dragmit,
What are the approximate dimensions of the tank?

Do you plan to set the tank on its end in a 40' deep hole or lay it flat?

Do you plan to let the salt water flow over the rock?  If so, then the rock must be clean.

You will need to determine the melting point for the fuel tank and the plastic lines.  Also make sure that the chlorine in the salt water does not corrode the heat exchanger.

Keep us posted on your plans and actual implemenation.

This tank is about ten feet high, cylinderical, and approx 30 feet long. I will bury it flat. That will give me 30 feet to ground level from tank top surface. The salt water will flow through the rock in this tank so the heat can be deposited into the rock. I planned on washing this ballast to prevent sediment pick up by the pumps. I.m thinking granite off the top of my head. More research necessary here also. The material operational temps will dictate the temps I will aim for. I believe fiberglass operating temps are fairly high, but plastic piping can be questionable. Easy to find out when I am ready. I would have to use stainless if the plastic could not hold the temps without breaking down. The heat exchanger will have to be stainless. But the price of stainless would be viable if it would double system life. I am shooting for a 50 year lifespan.

My guess is that a dragline excavator will have to be used to dig a hole that deep.  Digging a hole that deep could be costly.  You might want to consider a hole less deep since you plan to insulate the tank anyhow.

That is a pit, isn't it? But the people I rent excavation equipment from say it can be done with an excavator. Dragline usually can not be rented by untrained operators where small excavators can. I have about ten feet of dirt and clay mix, then shale. My brother has a back hoe excavator, and we will use that to remove the material once we reach the excavators limit. Drive into the pit, drive it out with the bucket loader. Thats how the big boys do it. I'm hoping a week if all goes well. That puts the cost of the hole at about $650,00 plus fuel. Plus beer for the horses. So, a grand.
That depth is what is required due to the penetration of the cold into the ground. That would be different elsewhere. But earth is the best insulator, and the cheepest, so the savings by going more shallow might not work out. Once the unit is up and running I plan to record data regarding the install. I will install test devices to track multiple depths and width temps, as well as temperature dispersment every couple of feet in all directions to see if any heat 'hides' because of pipe layout. I also plan to do some borings in the yard in March to see what depth the average lower ground temp starts at. then modify the datum based on how cold the season was. This season I expect to reach a constant temp well before 30 feet, but I am looking for a worst case scenero. R&D you know ...

Another question. Exterior insulating foam. I understand that the insulating foam used on interior apps is an open cell foam which will not work underground due to moisture. I have used a two part foam on boat interiors with closed foam characteristics. Closed foam will not absorb water. Not cheep stuff, and applied by putting in mouth and spraying on ... well sort of. Does anyone know of a machine available that can mix and apply closed cell foam or can I do it with a standard residential foam applicator? How about 'cheep closed cell foam? By the barrel? This is two part stuff. I intend to put a couple of inches of this foam on the interior of this tank to protect the fiberglass from sharp rock edges as well as a heavier layer on the exterior. Any thoughts?

OK, rethought in my sleep. I am going to change from rock surrounded by liquid with oil impregnated sand, with copper pipe fitted with radiation fins buried in the sand within the insulated tank. The sand will greatly increase the stone heat storage as well as reduce the fluid necessary for the unit. As suggested by an email reply by you all, salt water might be a PIA. By reducing the fluid amount, we can use antifreeze or the like as a heat transfer medium. The heated fluid will be inserted at the base, and the heated fluid used in the house radiators will remove heat from the top of the underground storage unit. In this benign oil / sand atmosphere the copper pipe should last indefinitely, and if the fluid needed to be replaced due to breakdown it would not be a huge expense or problem. Inert, safe, and inexpensive.

If I recall from reading other threads, the specific heat of water is higher than rock/sand and is really the ideal (cheap and effective) heat storage media.  Phase change salts have better specific heat but I don't know if they are DIY materials.

Even if I am wrong about the specific heat, it would still seem that using water would still be MUCH simpler than other solutions and would also mitigate any possible environmental consequences.

You shold be able to do the excavation as you describe, but you will need a lot of area to ramp down, provide slope stability and for spoil storage.  Be safe.  That is a big hole.  Think through how you will backfill, too.

Bruce

Bruce is right. The specific heat capacity of a material is a measure of how much energy is required to change its temperature by one degree, with weight and time given as constants. By that measure, water is five times more effective than stone for heat storage. But stone is 2 1/2 times heavier than water, so the relative advantage by volume works out to 2 to 1 in favor of water.

Just curious. Does anyone sell small-scale concentrating collectors? You'd need a collector that routinely puts out 250-degree heat to maintain efficient storage at that temperature. If your system put out 180 degrees on an overcast day, for example, and the tank was 205 degrees, its net efficiency for that day would be zero.

Now, with an outdoor wood boiler and 10,000 gallons of storage...

I hear that Bruce. The excavation is not a simple one, but we have experience in the construction industry, and you are correct. This is not a DIY project.

On the specific heat of water, consider this. If you take a cubic foot of air and insert X joules of energy, then take a cubic foot of rock and put the same X joules of energy, which will have the higher temperature? You will find the rock is cold while the air is warm. It takes energy to increase the temp of higher density materials, so higher density materials can store more heat. Your specific heat question "the specific heat of water is higher than rock/sand" is an inverse issue ... I can store more energy in a block of gold than I can a block of rock, but it upsets my wife when I use her jewelry in the yard!!

Hi Todd, Here is your error in your own words (not the statement but the variables)... >with weight and time given as constants I noted one cubic foot of air vs one cubic foot of rock. The weight (mass)of a cubic foot of rock is not the same as a cubic foot of air, do you concur? As a result, your conclusion is incorrect. The same error Bruce made. Mass is the huge difference between rock and air in this scenero. And mass is what I am using to store heat. >Just curious. Does anyone sell small-scale >concentrating collectors? I'm looking at a Chinese manufacturer. They have a Fresnal lens for about $8. And you are right about the 180 degree / 205 degree heat transfer issue. Also, the closer you get to the same temps the slower the transfer becomes. However the transfer will always (minus Supreme intervention) be warmer to cooler. However the focused output of a magnifying glass (a Fresnal lens) can reach well over 450 degrees F, the ignition temp of dry paper. If I expose 205 degree water to a 450 degree temp, will the waters temp not increase? You bet your Bippy!

I did adjust for weight, but let me try again. Water has a specific heat of 1.0. Stone has a specific heat of 0.2 The energy necessary to raise a pound of water by 10 degrees would raise a pound of stone by 2 degrees. Now stone is 2.5 times heavier than water, so we have to multiply by that amount to come up with equivalent heat storage by volume. 2.5 times 2 is 5 degrees, or half the storage of an equal volume of water.

As for the relative ease of doubling the heat output of collectors, I remember when the DOE installed a demonstration Fresnel lens PV system at DFW Airport in the mid 1980s. I am guessing that if an $8 lens is all you need, some enterprising collector company would be offering it by now.

, if memory serves. ut PVFrenI'd want a manufacturer telling me that the Fresnel lens won't cook what's under it, and the system will handle extreme temperatures on the brightest days. But that's just me.

Let me see if I understand you. If I put 10 units of heat, whatever a unit might be, into a cubic foot of water, and the same 10 units of heat into a cubic foot of rock, the rock will have a higher temp?

Here is the formula for calculating storage. The temperature change in fahrenheit equals additional btus divided by pounds times the specific heat of the material. Assuming your 10 units equal 100 btus (to reduce decimal places), a cubic foot of water would be 1.67 degrees warmer. (100/60*1=1.67) A cubic foot of stone would be 0.133 degrees warmer. (100/150*0.2=0.133)
In the traditional uses of storage, extending solar through the night, or stretching out a wood boiler firing over several days, water is best. There are two arguments for stone or sand over longer periods. Nothing bad happens when you heat it past 212 degrees, assuming you skip the water, and you aren't constrained by how much tank you can afford. Here is a fellow who has 200 tons of sand under his concrete slab: http://www.ases.org/images/stories/ST/pdfs/Radiant-Comfort-Ramlow-STND07.pdf
You have a huge tank, so an old grouch like me wonders why you want to turn free into "free."
Here is a web page with a number of garage built and experimental concentrating collectors http://builditsolar.com/Projects/Concentrating/concentrating.htm I'd start there and wait to soup up your tank when and if you need it.

Your math is riddled with variables irrevelant to the question, and as a result, contrary to the answer to my simple question Todd. But this question is paramount to understanding the process. So, I will ask it again. If I put 10 units of heat, whatever a unit might be, into a cubic foot of water, and the same 10 units of heat into a cubic foot of rock, the rock will have a higher temp? This question is important to follow if you really want to see what the point is.

Beyond that, your formulas which seem to be correct, point out the very aspect of using sand to store energy rather than water. I can store much more energy in sand than I can in water. Its mass makes that possible. Once I reach 112 degree in water my material changes state and requires pressure to continue holding the energy inserted. Sand will remain stable at higher temps. Now I just have to find a way to deliver that energy to the sand. But beyond that, because of the energy leaks of ground level tanks at varing temp atmospheres, what everyone else is doing in the site you pointed out does not apply. Your sand in your floor could not hold heat for any extended length of time. It would leak quickly into your basement. In the device we are building, sand is not the only element, any more than a spark plug in an internal combustion engine moves a car. Its only part of the story. I do not want to burn wood because of its expense and the fact that my wife would not cut, split, store, or regularly feed a fire unless there were no other choice. As with 98% of the rest of the American population. (Plus she had an Aunt whose health was ruined by a bad wood stove installation.) She wants to set a thermostat and read a book. The other 95% just wants to set a thermostat then flip on a TV. You and I would split wood, but we are a bit eccentric. I am not looking to follow the path of others, but explore unseen lands. This is simular to heat pump science except I am not using a large amount of electricity to squeeze temp from cold climates, I am putting heat down to a level where it has trouble moving around, then holding it till needed. The 40 foot depth is not for show. The best experiment I have ever heard of is without the inevitable variable, but it shows the general lack of understanding of this science. It goes like this. If you take a ten cubic foot room, insulate it so radient heat can neither enter or leave, then add a receptical and a refridgerator, set the refridgerators temp at 33 degrees C and run it for two hours, will the room temp be below 33 degrees, at 33 degrees, or above 33 degrees?

I know this is counterintuitive. I am not a physicist, but you'll probably find the explanation in the relative excitability of electrons in liquid state vs solid.
Anyway, one more calculation.
Total storage capacity is the same math as above using the temperature difference between the tank's maximum heat and the minimum heat required by the HVAC system. Let's say the minimum for radiators is 150 degrees, allowing for line loss. So 10,000 gallons of water at 200 degrees would store 4 million btus. (50 degree temperature difference times 80,000 pounds times a specific heat of 1.) An equal volume of stone at 250 degrees would also store 4 million btus.(100 degree temperature difference times 200,000 pounds times a specific heat of .2) A blend of oil/sand/stone would have a specific heat and weight averaging the materials in the proportions present. Using vegetable oil and roughing out equal parts, let's say specific heat is .27 and the weight is 135,000 pounds. So an equal volume of this mix at 250 degrees would hold 3.65 million btus. (100 degree difference times 135,000 pounds times a specific heat of 0.27.)

Hey Todd,

Well, I have to admit, you can not insert more energy into water than sand. I can. Because when my sand sits there emitting energy, your water will be floating away as a gas.

>So 10,000 gallons of water at 200 degrees would store 4 million btus

This is one of the reasons for using sand as the primary energy storage component. I will be increasing the temp of this mass as high as possible. If the system can carry 300 degrees then that is the goal. If we can increase this to 400, then so be it. We can do that with sand, but not water. As you have pointed out, simple water has too much limitation. Next is the transfer of collected heat. Can ordinary antifreeze function at 300 degrees?, probably. 400? Not likely. Unless we modify it ... maybe. With salt? Commercial heat generation uses salt. More research to be done. However in any case, if one of these tanks can not heat our home for a full season, then we will simply install two.

You see, as long as you put defeating limitations on your abilities, your abilities will always be limited. You keep doing math with water, and water has limitations in max temp as well as safty issues. (pressure) Sand allows me to move beyond those limitations. Everything you have said is true ... for water. Now, switch gears and do sand ...

I understand your theoretical point about sand at 400F holding more overall energy than water at 200F. BUT in real life the higher the temperature of the stored heat, no matter in what, the quicker it will be lost. It is just like a heat loss calc on a home. If it is insulated with R30 and its 70F inside and -30F outside you have a delta of 100 and the house will loose heat at a much faster rate than if it were 20F outside or a delta of 50. It could be that taking the loss in to consideration you would likely be better off storing 200F water compared to 250F sand given the same insulation. But again if you can get the sand to 400F it will certainly store more energy, but loose it or be less efficient at storing it as well.

Also as noted above you looking at approximately 4 million BTU's. Even with some exotic mix you might get say, 10 million. That isn't going to get you very far in winter, well I guess it depends on how tight your house is and the location, but in the grand scheme of things it's not a whole lot.

And again the whole issue of depth shouldn’t be an issue if it is well insulated. The whole reason geothermal works is because anything buried tends to want to sit at what the "earth" is at for that location. Going another 10 feet or 20 feet isn’t going to change that baseline temperature more than a couple of degrees and again when your talking a 200+ storage tank, if it is surrounded by 55F earth or 50F earth isn't going to change much delta wise.

You can't put more energy into sand in an FRP tank. You should check with the manufacturer, but 250 degrees is probably pushing it.
Nor would you escape specific heat. The tank would be the buffer in a system with considerable potential for state changing events. To strip heat efficiently in oil/sand/stone you'd need an exchanger with three to four times the surface area. It would probably be steel at those temperatures.
The folks who superheat sand and stone put it in beds and blow hot air through it. Stone works well, and it got fairly popular in the 70s until some beds turned up with mold problems.

Ahhh, thank you guys ...Info I can get my teeth into.

I had asked a question earlier. Want to take a crack at it Brock? Todd chickened out! <;-P If you take a ten cubic foot room, insulate it so radient heat can neither enter or leave, then add a receptacle and a refrigerator, set the refrigerators temp at 33 degrees C and run it for two hours, will the room temp be below 33 degrees, at 33 degrees, or above 33 degrees?

>>I understand your theoretical point about sand at 400F holding more overall energy than water at 200F. BUT in real life the higher the temperature of the stored heat, no matter in what, the quicker it will be lost.<< Absolutely, the delta heat transfer issue. I had originally not planned to insulate the tank for this very reason, and insulate the trench walls 20 or so feet from the tank, expanding the field size. I think I will go back to that partly because of your point. I want as much ground around this thing heating up as much as possible. Take advantage of the whole field. That way the tank becomes simply the purveyor of energy, and the entire field becomes the storage container. And Todd, <> Also the operating temp for epoxy based fiberglass is up to 300 degrees, depending on the manufacture of the tank. This is probably an open (unburied) pressurized limitation, and as with all test figures, worst case scenario is implemented. But there is no warrenty in any case. However there is a good possibility the tank will become unnessary altogether, or just an element to keep the sand field together until its burried. More research. If the losses can be reduced by insulation well away from the tank, the problem is solved. Earth does have higher R values than most materials, but holes can be punched into its barrier. But lets not forget, if we can stop all heat loss from this field, then all heat installed will remain for use. Zero heat loss makes any Delta figures irrelevant. Zero is the mission. Or as close as feasible. A 10,000 sq foot vacuum around a sand field? I think not. Mold issues ... yes, more research, but not an insurmountable problem.