What are you trying to accomplish with the thermal storage system? Are you looking to use solar for DHW only, space heating only, or both? I presume you're planning for this to be supplemental heat, not primary source, correct?

Yes I wanting to try and keep the garage at a somewhat stable temp. and since I was pouring a big chunk of concrete, way waste it.

Sand is rather crummy insulator, and 55 gallon plastic drums have to be the right polymer to be able to withstand the heat if it runs hot. Unless the slab is structurally self-sustaining you may run into othe issues as well as drums leak or collapse. You should DEFINITELY be concerned about the slab cracking & buckling should you lose all the water!

That said, if you used insulated concrete forms and poured yourself a EPDM lined 500-1000gallon tank up against one corner of the footing, with 2-4" of XPS on the floor of the tank (it's not as huge as you might think) to be topped over by the garage slab (with insulation between the garage slab and the tank if you want higher temp pre-heat on your DHW) you can probably get reasonable seasonal performance low-temp low cost solar pre-heat for your DHW. Design it with a maintenance access hatch, eh? The EPS concrete forms won't tolerate 150F temps forever, so you may have to design some temperature limiting controls. Run PEX loops through the slab to pump the heated water directly into the slab (either as a summer time heat dump, thermostatic control for the garage, or both.) Think & design carefully about the liklihood of the slab ever freezing though...(frozen water in PEX could crack the concrete.)

Design it to run as a drainback at atmospheric pressure, not a glycol loop and NOT your pressurized water system. The heat exchanger can be as simple as a large coil (100-200' of 1/2" or 3/4" PEX submersed in the tank.) Something along the lines of the tanks, heat exchangers & DIY panels in this guy's water pre-heat design should do you.

Alternatively, if the garage is reasonably insulated and air tight you could insulate the slab at R5 (1" XPS), forget the water storage and just use the slab itself as the storage medium & heat exchanger. Run the solar as a drainback through one set of PEX loops embedded in the floor, run the DHW preheat as another set of embedded PEX. It'll be lower temp preheat than if you went with an insulated tank, and it'll likely overheat the garage in the summer unless you shade the collectors.

If you go with the 55 gallon plastic drum storage concept, they tend to be far more structurally robust in the vertical, stacking direction. (Think of how much weight it takes to squash a toilet-paper tube on it's side, compared to standing on end. Geometry is your friend!) Putting 1-2" of XPS insulation under them. Dry sand is somewhat insulating, but it's not all that great. If you made a row of drums along the slab edge and embedded a vertical wall of 2" XPS insulation in the back fill such that it surrounded the drums & packed sand you'd have much better thermal preservation & control. Still go with a submerged coil in one or more of the barrels as the DHW heat exchanger though- it'll work fine and avoid all cross contamination & stagnation issues.

Using the slab as a storage medium is what I am going for. I was just trying to think of way to increase it's capacity (you know, the larger it is the longer the retention). I have to put in a 3' footing (that’s code) and would insulate the them, but what else could I do under the slab to increase the storage. Maybe just the sealed drums full of water?

I know sand is not a great insulator, would dirt be any better?

It sounds like what ever I build you would recommend having access to it?

I will insulated and make the garage air tight (as air tight as a garage can get).

I like the idea of a drainback system better then anything else.

[Something along the lines of the tanks, heat exchangers & DIY panels in this guy's water pre-heat design should do you.]
Did you forget something here? What DIY panels?

Thanks for your feedback.

Dry sand is a much better insulator than clay soils, but nothing like real insulation, eh?

A single tank is much easier to maintain, fill (re-fill) debug, potentially repair than a dozen 55 gallon drums with 24 plumbing connections. (If you had a leak, how would you know, and how would you fix it?)  If you make it out of concrete at the same time as you pour the footings it doesn't add much in the way of material cost.  Running it at atmospheric pressure (instead of at the water-system pressure) and backfilled around it with sand the wall/floor-thickness of the concrete tank isn't too critical- 4-6" should  do it, with pig-wire as reinforcement (no rebar required.)  See if you can't find some 6" insulated concrete forms to simplify the process.

These are the DIY panels he's using in his low-cost solar HW system:

http://www.builditsolar.com/Experimental/PEXCollector/PEXCollector.htm 

There are links to descriptions of the system components on the overview page:

http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm

Water stores roughly twice the amount of energy per cubic foot as concrete, and 3-5x as much as sand (depends on the type & density of the sand)- it's a great medium for both storing and transferring heat. Your inclination to use water storage is the right one.

How much storage you need depends both on the amount of collector area you have and the desired peak temperature, and the rate at which you lose/use that heat.  As the storage temperature goes up, the efficiency of the collector goes down.  If your goal is 130F hot water, reasonable systems can run in the ~40% efficiency range, but if your goal is 60F floor slab it's not too tough to hit 70%+ which means you'd need about a third less collector area to grab the same amount of heat.  But preheating your DHW to only 60F while significant, isn't likely to  provide even half the heat you need for your hot water needs.  Finding the right compromise of installation cost, efficiency isn't a simple process when you're looking for dual use minimal space heating (which can be very high performance with cheap collectors) vs DHW heating (mid to low-performance with even pretty-good commercal flat panels.)

The amount of collector area you'll need to heat the garage varies a lot with insulation values & infiltration levels, as well as the temps you hope to achieve.  It's generally a lot cheaper to insulate & seal better than to add more collector & storage to make up for the losses.  The rough rule-of thumb for well sealed reasonably insulated buildings is 7-10% of the floor area- err on the high side for a leaky garage.  Also, collector orientation matters. Since most of the heat requirment is for winter use,  vertically oriented collectors are preferable to the typical DHW installation that you see.  The performance hit is small in winter, but it keeps you from cooking the place to death in summer when the heat loads are low (the last thing you want is a 150F slab in July!).   If you store the water in an insulated tank as opposed to in an uncontrolled manner in the slab you'll be able to provide most or all of your DHW in the summer & shoulder seasons, if you have enough collector area to keep it 50-60F in the garage all winter.

Just an idea to kick around. If your footings are cast concrete, Insulated on the outside of the building, 3 feet high, why not increase the wall thickness of the footing by 20% to satisfy the building dept and increase mass and place the pipe in the footing utilizing the mass as storage, then insulate both sides before back fill, the top plat of wood will also function as insulation (r1 per inch). Pipe lay out might be several courses to load heat from colector and several courses to extract heat, aim to heat foundation to some design point loke 145 degrees, bank it n good days, in summer or when ever.
On the mechanical side take all solar panels to a drain back 75 gallon duel coil storage tank and pre-feed the domestic H20 now the panels (as many as yo want/need) and domestic side of the system labor to install are open to the 30% of total cost tax credit (new with the 7B bailout, probably the only part the serf population will see for a while), when the domestic tank goes over 145 it diverts to the foundation storage, also the additional pipe loops can be used to access tis heat for in floor heat in the house, thus loading th house when possible of the sun as well.
Just an idea, needs refining for sure.
Dan

Ok I'm confused. What is the term DHW? Maybe I've been using it for the wrong thing.

Dan, thats what I am looking to do. Some massive thermal storage that I could use to heat the garage a maybe supplement the house. But why just the foundation and the slab, why not utilize the 3276 cu ft under the slab? But then what ever it is would have to be simple, cheep, and permanent.

DHW = domestic hot water

You really need to have a handle on your heating load numbers for your design first. For example, if the entire 3276 cu ft is water, the best heat storage medium, that's = to 24,506 gals, or ~209,000 pounds. If you have the water at some temperature where you can draw it down 20 degrees in temp you can extract ~4.1 million Btus of heat from it. If your heat load is 50,000 Btuh it will take only 84 hours to draw all that heat.

What you will find is that with water even, it takes a massive amount cycled through a large temperature differential to provide any meaningful amount of heat storage/retrieval for space heating. It is feasible for DHW because the heat demand is much less.

True, all around!

But if freeze control is all you're about, the heat load isn't going to be anything like 50kbtu/hr in MO unless you leave the garage door open (and even then, a low-temp slab won't be able to DELIVER 50kbtu/hr.)  If I'm reading the guy right, he's looking to just scavenge & store some pre-heat for his DHW and keep the garage 10-15F warmer than the daily averaged outdoor temp.  (Or at least that's all I'd expect to get out of a cheap DIY solar project here.)  This isn't controlled for comfort space-heating in the normal ASHRAE or ACCA sense. 

This is (almost by definition) a hack.  But done reasonably (with low expectations for space heating performance) it doesn't have to be a low-efficiency hack.

WOW! I think I understand what your getting at. So how do I go about figuring my heating load? And how do you figure out the best way to achieve it?

Oh, and I thought DHW meant something else (boy do I feel dumb). :(

There is a variety of heat-loss freeware out there, some easier to use than others. The Slantfin one is OK and easy to use for some standard construction, but is less flexible if you're super-insulating, etc. since you can't assign an R-value. But for rough cut you might want to try to model the room.

http://www.slantfin.com/heat-loss-software.html

You have to select the closest description of the insulation levels length/height of the walls &ceiling have to insert a relevant outdoor temp, and an indoor temp (the slantfin tool won't let assign an indoor temp below 50F), and it'll tell you the instantaneous heat loss out of the structure. This tool tends to presume a fairly leaky structure- it overestimates very-tight houses by about 30-35%, but getting a perfect seal on a garage door isn't too likely. It'll give you the order-of magnitude, anyway. For something like this, since you're not really trying to maintain the temp at 4AM on one of the coldest nights of the year it's probably easier & more appropriate to design around the average daily temp for coldest month of the year, which is about 30F in January, for St. Louis:

http://www.wunderground.com/NORMS/DisplayNORMS.asp?AirportCode=KSTL&SafeCityName=Saint_Louis&StateCode=MO&Units=none&IATA=STL

As I've stated before, generally it's much more effective & cheaper to super-insulate the structure than it is to install enough solar to really heat the place. But if you have R19 24" oc wall construction, minimal window area on east/north/west facing walls and a tightly sealing door, and any amount of under-slab insulation you'll be able to get a useful amount of space-heating out of collectors in the 10% of floor area (120 square feet). But it's not cheap, if you're going all hydronic (water as heating fluid.) You may get better value out of integrating thermal air panels into the south-facing walls, and 50-80 square feet of drainback collector feeding a well-insulated sub-slab tank with a PEX coil as heat-exchanger for DHW pre-heat (like the system the guy at builditsolar.com cobbled together.)

Solar is cheap only in the operational costs- it can be quite expensive up-front. A true cheapskate will do the math, figure out how a which point doubling the insulation becomes more expensive than more solar panel area. Adding 2" of iso-board on the inside of the stud walls will likely have quicker payback than a full-on solar space heating system pumping water though slab embedded PEX. Make sure your garage door is insulated (most insulated garage door are only ~R3-R8 at best, but that's way better than 100 square feet of R0.5. For more money they're available in R10+, and (worth it if you hope to get much performance out of it.)

Check out more on the builditsolar site to see what makes the most sense for you:

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm

Cheap thermosiphoning air panels might make the most sense (and clearly the cheapest approach) for space-heating the garage:

http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

The thermal mass of the slab should help keep it from rapidly overheating on sunny days, but it'll still be considerably warmer 2-3 hours after sunrise than first thing in the AM.