I am installing a solar hot water heating system into an existing oil fired system. the collectors are ground mounted and I want to bury a 1000-1500gal concrete tank  for heat storage for space and cost reasons. All the information I've been able to gleen so far indicate that indoor tanks can't take sustained temps higher than 170 degF or so but the collectors can max out near 300 degF. A system with 8 or 9 collectors is recommended to have 500 to 600 gals of storage inside,but then dump excess heat over 170 deg or so. My question is ; is it feasible to use a larger concrete tank insulated on the out side to prevent thermal breakdown from letting the heat storage medium(water?) reach higher temps ,and can this setup retain the heat longer to maximize the collector output?  I am in upstate NY  and don't know if a concrete tank with 2" (or more?) of insulation buried  in the ground  can retain a high enough percentage of its stored heat to be practical and what would be the best tank size to collector quantity combination. Anyone in the know about these issues ? I  would  greatly appreciate  any feedback as soon as possible. Thanks.

Concrete may work, but 2" insulation is not nearly enough. Also be sure that insulation will not absorb water over time, as water in the insulation is a great thermal conductor. Your best bet may be a used LP gas tank installed indoors so any escaping heat can be used. Insulate well with fiberglass or whatever. But be sure to clean the tank well and do it outside away from any place where the lingering mercaptan smell will be long remembered.

If you go with open storage, be sure that corrosion in your heating system is not the result. Need to balance pH, treat with sulfites to eliminate O2, and monitor for proper chemical balance. If a closed system, once you have these in balance should not have to do much else unless you add new water.

Thanks for the quick response. I plan on running 2 closed loops, 1 from the collectors to the tank and the other taking the heat off the tank and tying it into existing system. The tank is unsealed heat storage media,most likely water . Would sand/water slurry be advantageous? Also does anyone have any info on useing water/methanol mix in collector loop as opposed to glycol? 

RJ, I'm nobody's expert, but I've been reading a bit, and contemplating a solar system for the house I'll be building next year. Here are a few observations, based upon what I've been reading:

Theoretical and actual amount of heat collected can be significantly different, especially in upstate NY.
Solar collectors are less efficient at lower ambient temps. When you need it the most, it works the least.
Solar collectors are less efficient at higher water temps. Your boiler system probably needs 160 F or 170 F to work. This is WAY hot.
Your mean deep ground temp is probably 46 F or so, down deep--colder near the surface. To hold a buried tank at 170 F in that environment, you are going to need HUGE amounts of insulation.
Sand (rock) has a much lower heat storage capacity than water, so adding sand will make your system less efficient.

Where you live, I'm sure you're taking it on the chin with heating costs. You shouldn't let desperation lead you to do something that you have not completely thought through.

Sometime late last year, there was an article in Mother Earth News about a guy named Gary, up in Montana. He had built a shed with the south-facing surface covered by a solar collector. He learned a lot about solar, and generously, posted his thoughts and his numbers on a web site, along with a wealth of other information. It would probably be a worthwhile read, given what you're contemplating. Gary's website is: http://www.builditsolar.com/

Good luck with your project.