System planned is solar hot water panels for DHW (usually 3 people using hot water)as well as radiant in floor heat (slab on grade in new addition) AND garage/workshop with pex in 2 ft. of sand heat sink under garage, slab on grade(garage part is already done).  Need a 3rd opinion as 2 pros are disagreeing.  Planning on having a 200gal to 300gal tempering tank.  One pro says to have all panels directly dumping to tempering tank, one says some straight to garage sand heat sink.  This is in central WI.  Have 6 flat plate, 4 x 8 ft panels so far to install, perhaps more in the future.  Want warm toes in addition ( about 600sf) and garage(1200 sf) temp wish to be 35 to 55 degrees in our sub zero winters. 

Tx,
KB

you mean, when you are trying to dump heat?

You definitely want to heat the tempering tank before you dump to the heat sink. Otherwise it doesn't really matter whether you dump direct or from the tank. Just make sure solar temp water is limited before it hits anything plastic.

No, not as a heat dump but as heat sink, another form of storing heat (the idea is storing some of the summer heat for a couple months).

Kelly

I doubt that will work at all. it certainly shouldn't take the lead over a tank of water.

long term low level heat storage hasn't, as far as I know, been shown to work anywhere. and the math on it isn't very hopeful.

Long term storage can work on a scale 3 orders of magnitude larger than a single family dwelling by charging large stable aquifers. The kinds of water/soil volumes & typical storage-tank R-values required to do a 1-off storage of late-summer heat are beyond ridiculous. (It would be at least 5x more effective to put that money into a PassiveHouse upgrade to the building than to try to supply November's heat load for a typically-insulated home with September's sunshine stored in a big tank.

Using the soil under the slab as a heat sink/heat-dump would provide only modest heating performance improvements, negligible unless the slab-edge is well insulated down to several feet below the frost line. But if the garage is atypically tight & well insulated and has minimal glazed area, merely earth-coupling it with insulated dirt (no heat dumping) to the mid-40sF subsoil would likely be sufficient to meet your 35-55F requirements:

http://mb-soft.com/solar/soilmap.gif

Still without at least an R10 garage door and very low air- infiltration rates it could still drop below 35F in there during cold snaps. And without insulating down to well below frost line, the effect of September sun on December slab temps won't even be measurable.

With sufficient slab insulation, the mid-winter efficiency of the system will be slightly higher by going direct to sand under the otherwise unheated(?) garage, since that would provide the lowest possible operating temps at the panels, but depending on how you're operating the tank it could be academic. If the tempering tank is full of potable water, care needs to be taken that those temps not be allowed to stay between ~80F & 120F, or you'll be in Legionella-farming territory, which mayalso tip the argument toward dumping heat to the sand first. If the tempering tank has only a smalll volume of potable water inside an internal heat exchanger the bio-risk is negligible, since the tepid-water would then fully purged on almost every hot water draw rather than stagnating for days/weeks/months in the hazard zone.

Dana, remember that 90% of domestic tanks in this country are set for 120 degrees F. Solar is not special in that regard. Tanks dilute on every single use of hot water and offer a fairly low area to volume ratio... I don't think tanks are really the problem, typically.

I know the euros do sanitary settings, but honestly... seems a little overkill to me.

Good Day All,
On the same topic I have been pondering an idea, I have several 80 gallon insulated (2 inch foam) stainless tanks,They are a waste with 2 inch holes on the top. It is well possible to cut a slot length wise on top, weave a 100 foot zig zag of 2 copper 1/2 L in to the tank and fill with concrete at the time of casting foundation. One loop to the drain down, one to the heat system.
A solar storage tank of insulated stone, permanently placed in the crawl space, no maintenance. the copper pipe if isolated should last 50 years +
stone and copper storage at 200+ degrees,
Capacity? Fools dream?
Dan

I did the math on seasonal storage once.

You would need to basically be able to fill an eight foot basement with water and insulate the heck out of it to do anything worth considering, really.

You might be able to use your tanks for regular usage, but for large scale, medium term offset storage... I think it's a pipe dream.

Tanks are where there's plenty of surface area to let stuff grow. At a maintained 120F setpoint the risk of something getting started is low- it doesn't kill it, but it doesn't grow very fast. But at 100-110F it's a large multiplier (orders of magnitude) higher risk. Regular dilution doesn't change much to the risk factor but regular full purges knocks the risk back by a similarly large factor. The bigger the tank, the lower the fractional displacement. We're talking a tank 4-5x larger in volume than a typical tank water heater, far more likely to be spending real time in the sweet spot, with only 20-25% of the dilution factor.

Overall the risk of legionella is low even for tepid 50 gallons tanks, but more orders-of-magnitude of risk factor that get added, the less low "low" really is. If it's just as easy to get rid of a risk factor as keep it, with essentially neutral effects on performance, why keep it?

Pulling some numbers out of thin air...

If it's 100x more likely to get colonization at the expected operating temp rather than at 120F, and 100x more likely to get colonization since it's only dilution only rather than full purges, and 4x more likely with a 200 gallon tank than a 50 gallon tank due to the lower dilution rate, it goes from being ridiculously low a one in a million chance for a 120F full-purging heat exchanger type system or pretty-low one in 10,000 for a 120F 50 gallon tank, to a not-exactly-risk-free one in 250 chance in a 100F tempering tank. I don't know if the real risk factors are higher or lower than the examples, but the risk factors multiply. Lowering (or raising) the tempering tank's operation so that it doesn't run "in the zone" much of the time shouldn't be too big a deal.

Posted By NRT.Rob on 13 Sep 2010 01:58 PM
I did the math on seasonal storage once.

You would need to basically be able to fill an eight foot basement with water and insulate the heck out of it to do anything worth considering, really.

You might be able to use your tanks for regular usage, but for large scale, medium term offset storage... I think it's a pipe dream.

There have been a couple of threads on other forums on this website on the subject earlier this year.  Even a back-of-the-napkin or lipstick-on-mirror calc is enough to dismiss it for the engineers, but some people simply refuse to pull out a crayon (or other writing implement of choice), no matter how much encouragement they get to at least find out the order of magnitude for the problem:

http://www.greenbuildingtalk.com/Fo...fault.aspx

http://www.greenbuildingtalk.co...fault.aspx

The only remotely credible versions of true seasonal heat storage I've read about are the district heating concept at the housing development in Alberta, and a couple of  university campus setups using geothermal heat pumps for heat extraction, and very low-temp solar for heat-charging deep stable aquifers.

There was an experimental solar house done by Harvard University back in the 1930s taking the massive high-R tank in the basement approach that sorta did it.  The volume of the tank truly gia-normous though- with about as much volume in the tank as there was air in the conditioned space.

Seriously- spend the money on envelope & end-use efficiencies, reducing the load down by at least 75% first, at which point the amount of solar panel & thermal storage that can actually support that load

A: fits within the footprint of the house, and

B: becomes at least remotely affordable.

In WI if you don't have at least R35 clear wall values, minimal glazing, and an ACH/50 under 1.0 you'll get better return spending the money elsewhere.