long term storage of solar heat

Hmmm... I don’t think I would trust anything important to a $150 generator from mallwart. I was envisioning something like the Honda suitcase 2kw.
Good point about the circulation, I hadn’t thought of it that way. So... the slab always being 57 degrees, if you were to have a super insulated house, how much heat loss is there? Would the house ever get below freezing? I’d also think that you could add another 10-15 degrees for much of the year with passive solar?

I’ll ask another question, one that an ‘alternative energy’ engineer couldn’t answer. What if, before you build that super insulated house on the heated slab, you dig a full basement hole, say, 10’ deep and the footprint of the house. You insulate the sides, top, and bottom of that hole to R50ish and fill the entire area with sand. You have horizontal tubing loops running in the sand with 6” spacing (both vertically and horizontally). The tubing is connected to a coil of black pipe on the roof, and all summer the hot water is circulated into the sand mass by a 12v solar powered pump. The sand is insulated on top so you control when the heat gets to the slab above. You place probes at several locations to monitor the temperature of the mass. At the point freezing temps begin, or as soon as monitors show diminishing returns, you drain the roof tubing, and switch the circulator pump to moving the heated water from the sand mass into the slab. The question is, how long would this provide heat? How much mass is needed to get heat all winter? There has to be a way to calculate this. Even in places like Maine- short summer long winter- you are pumping 150+ degree water into the ground for at least 8 hours most days from mid May to mid September. Maybe it’s a crazy idea, maybe it’s already been tried, I don’t know. It SEEMS logical, because it’s free solar heat, with the pump and pv panel to power it (and maybe a deep cycle battery to smooth things out between sunny/cloudy days) being the only components which can wear out. Cost would be about the same as a quality boiler installation, the one thing I do know how to calculate.

This is an interesting one.  If you have the space and easy digging to be able to do it, of course.  

My inclination would be that you want a big mass of water, rather than just sand with pipes, as water is very good at storing heat, so maybe a big tank, perhaps surrounded by sand on all sides.  You'd heat the tank up (and you want a control system that stops the tanks trying to heat up the world at night in the summer) and use the heat later to heat the house.  I kinda doubt that you could really keep it hot, even with super insulation and underground, for all that long.  Also, of course, if you use the heat it's gone, and so your heat store gradually works less well as its temperature drops, although sunny days in winter could help top it up again.

How would you get the heat out of it?  Could be via some kind of radiator or could maybe just be passive under the house, although that would be hard to control.

Water would be better, but a tank that large (80,000+ gallons) would probably be expensive. A cheaper liner could be made with pvc or tpo, but then you risk leaks. As I said, maybe it’s a crazy idea, or maybe it has even been tried and disproven. As you draw out heat you are cooling the mass, though you only draw a small fraction of the total volume per day. But it gets into calculations like Delta T, which I don’t know how to even begin to do!

A look on Wiki shows this is done on a much larger scale

estimates a 11.6 GWh capacity and 120 MW thermal output for its 260,000 m³ water cistern

the heat capacity of water is  4.2 J/gC and sand if it is pure dry quartz is 0.83 J/gC (that's Joules per gram per degree centigrade) (wet sand is higher of course)

So your hypothetical 10ft (say 3m) by 1000 sqr ft? house (92m2) has a volume of 276m3 conveniently close to the figure in the quote lets lose the 16m3 on the grounds of a smaller system being less efficient so your system would be 1000th of the size and because it's using sand instead of water could hold approximately 20% of the energy per unit volume. Which comes out at 2320KWh if the system used water it could store 11600kWh


I Read here that a passive house requires 15kwh per m2 per year which is 1380KWh for the hypothetical house in the example, much less than the figure you could store in that amount of sand.  So I would say it is possible, whether these numbers hold up in the real world and whether you can move heat around as fast in sand as in water is quite another matter.

Check out the Groundswell greenhouse in Invermere BC (I think I got the name/location right).  They store heat in a sand mass under the greenhouse.  I think they calculated a thickness of insulation that they put between the sand and the floor to delay the transfer of the heat to the building.  So the heat stored in summer bleeds through into the greenhouse in the winter.

Here is a tour of a house built in 1994 that uses an insulated sand bed as a heat store.

 

It's a prety good example of active solar done right.