Hi, newbe here. I'm building a house with a 24' X 30' X 6' deep, insulated block of earth
under the house. (10" concrete footing walls down 6')
I have searched the internet looking for ideas for using this for heat storage
and keep finding passive  heat storage type designs. I want to use Pex tube and solar
to put the heat in, and recover the heat for use in radiant slab on grade heat.

Looking for advice.

Does anyone know where I can find any data for storing heat in earth?

Cheers,
Scott

http://earthshelters.com/

good luck

The napkin-math doesn't really work on this if your goal is seasonal heat storage. But there are methods of arriving at the theoretical numbers.

See some of the napkin-math in these threads:

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/72961/afv/topic/Default.aspx

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/75617/afv/topic/Default.aspx

Bottom line, by the time you've put enough insulation PEX & pumps in place to store up a relevant amount of summertime BTUs for wintertime use, you could have air-sealed and super-insulated the house enough that you don't NEED an active heating system 3x over. The PassiveHouse approach is more methodical, cheaper to implement, and more likely to work- you'd end up expending fewer kwh to deal with peak heating loads using resistance-heaters than you would have spent with pumps getting solar heat into your soil thermal-mass.

People have been doing the math on this and experimenting at least as far back as the 1970s- the physics hasn't changed. The most successful implementations use stable aquifers and multiple wells for a community or campus-wide district-heating solutions for a large number of buildings/houses, eg: http://www.dlsc.ca/ But for a single building and using dirt at near-grade for heat storage fuggedaboudit. You'll need more insulation & higher temps than would be even close to reasonable.

By the time you're up to R50 walls and R75 roof, with R30 under the house to get the heat requirements low enough that you COULD store enough BTUs in a block of dirt long enough to coast through a winter on summertime solar, the whole cush-factor of radiant floors is gone, since they'll be barely above room temp even under design-day conditions (the coldest hours of the year), and 3/4 of the year you'll be in cooling-mode, moderating the indoor temp by raising/lowering the ventilation rates. And even then, just the cost of insulation for storing that much heat for months in a buried box of dirt would be daunting, and putting that insulation on the house itself instead of on the heat-store would still be more effective.

If you live in an area with sub-soil temps in the 65-75F range you can get there completely passively with earth-coupling. What's your sub-soil temp? Where is this installation?

The earthshelters.com approach is to put the house INSIDE the thermal-mass storage and relies on a well designed high wintertime passive solar gain. The temperatures of their "heat storage" is essentially room temp, and the radiating surfaces for heating are the walls, floor AND ceilings. If you dig into the design particulars you'll find there is more insulation involved than would be used in an above grade PassiveHouse, eg:

http://earthshelters.com/green-blog/umbrella-homes/

It's a more complicated way of building a house, but still less complicated than an active-solar/earth storage system would be.

Such thermal storage would be useful for storing heat for some number of days - say replacing a water tank for active solar energy storage.

No idea on the cost effectiveness of it.

Posted By jonr on 25 Oct 2010 07:41 PM
Such thermal storage would be useful for storing heat for some number of days - say replacing a water tank for active solar energy storage.

Yeah- that's how the earth-shelters work- they have enough thermal mass and radation-surface to the conditioned space that they can coast through maybe even a week of sunless cold weather. They tend to oversize the passive-gain aspect too, relative to what you could get away with on a lower-mass house with less surface area presented to the mass.  They call it "seasonal storage", but that's not 'xactly what's happening, once you look at it close-up.

Hi, sorry I'm slow to get back, thank you for the input!!

I think this dirt space could store, or moderate enough heat to get
through the cold spells here, maybe a weekor more? Mostly I want to stabilize or
moderate the house heat swings. I was curious about how much PEX to bury
in layers of dirt to effectively exchange heat.

This heat would just be the big warm spot under the house.

2" insulation is going against all the stub walls, down 5'.

I live in the PNW, our weather is pretty mild.

This block of dirt is all above original grade. You can see the concrete retaining wall 8' in front of the dirt block.
There is drainage in the original grade.
This is why the footings are so tall. The dirt will be dry, above grade, and insulated, like if you insulated your basement,
filled it with dirt, and heated that dirt in the winter......maybe even cool it in the summer!

How would you put heat from hot water into this basement?

This building is 1/2 house, 1/2 shop, on a 72' X 72' foot print.
The "cold" north side is the shop, a sort of envelope for the
cold side. The other 3 sides of the house have a 8' covered porch, which
will be enclosed later to totally envelope the house. In front, south side will be
a green house, heat from this can flow into the porch envelope through
louvers / doors.

The new shop "envelope" will be heated...a little by PEX in the floor slab.

This heat comes from waste process heat from our plastic injection molding shop.
This "old shop" is 72' X 40' and just 15 feet behind the new shop.

Now we cool the machines with water / oil heat exchangers, and dump the heat outside
with a cooling tower.

I put modulating heat control valves on all the heat exchangers for the hydraulics on all the machines, the
water gets to 90 degrees pretty fast if I turn the fan off on the cooling tower.

We run the presses with 110 degree temps on the oil. no water flows through the heat exchangers
until they get to 110 degrees. This heat is available 1 to 3 days a week, depending on how
busy the shop is.

So in the winter I will just heat the new shop floor, also I will have a plumbing loop in the house heat control room, so I could
run the process heat into the dirt block.

I would like to use a solar hot water system for the house hot water and house radiant floor heat, as
well as be able to put some "extra" heat under the house.

One of my customers has a solar hot water heater, he heats two 500 gallon !copper! tanks
that are in a super insulated room attached to his house, he gets them smoking hot. The heat
is passively moved through the house through louvers. The water pump runs off a solar electric panel
using a DC motor and a recycled auto water pump. Pretty cool setup. He got the copper tanks from a brewery
going out of business.

I have seen, somewhere on the internet, where a company was selling a system of
resistance heaters, which were to be buried under the house, in a similar way as to what I want to
do with PEX. Their system used solar electric panels to run the heaters.


Just looking for ideas for effectively exchanging the water heat into the insulated dirt.

Cheers,
Scott

Show me the math that sez you can store a weeks worth of heat in dirt for less insulation cost that what it would take to turn your place into a PassiveHouse that NEEDs no heating system.

If you have access to copius quantities of 110F water, and heating radiation capable of delivering heat as-needed with 90F water (such as a radiant slab), build yourself a well-insulated tank, and MAYBE you can get more than cool-weather-day's worth of heat out of it. The amount of heat you can store in a volume of water is many times what you can store in dirt, the heat exchangers can be smaller, and the amount of insulation you'd need is much less (due to the much reduced surface area of the smaller volume.) At temps that low you can build big tanks out of plywood with EPDM liners. With a couple thousand gallons of 110F water and 90F requirements on the radiation you can store ~334,000 BTUs. If the 72x72' building is well insulated, not too air-leaky and not too many windows you can probably geta heat loss of say, 15,000BTU/hour when it's 40F out, that gives you a bit less than a day's worth of 40F weather heating if you have something like R25 in foam on all sides of the tank to keep the standby losses bounded. If the heat loss is more like 30KBTU/hr you're looking at a half-day's worth.

A 10' cube is about 7400 gallons, so with a tank that size you might get as much as 3 days of storage out of that much water, as long as it's standby losses are well-controlled with a high-R, and that's probably the same order of magnitude of thermal storage capacity of your 30 x 24 x6' block of dirt. Two measly inches of EPS is only about R8. If it's XPS, figure R10. Your standby losses with 110F water (or dirt) would be as high or higher than the building's overall heat load with R values that low. Without insulation UNDER the dirt, fuggedaboudit. Subsoil temps in the PNW are in the 48-54F range and even though dirt is both insulative (~R1 per foot- don't count on more, and when humidity is high it'll be less), and has thermal mass, standby losses to mother Gaia with 100F+ storage temps will be huge without some real insulation.

And even with substantial insulation, you won't be hanging onto this heat for more than a few days at usable temps. Taking the 10' cube insulated 6 sides with R25, assuming an average temp of 50F on all sides of the cube that's a 50F+ delta from the 110F water temps, and it'll be losing 1200BTU/hr in standby. At the end of 7 days 2/3 of your stored heat will have dissipated. With only R8 you'd be looking at only a couple of days before the water is too cool to use for heat.

If you put R25 under the siding & under the slab and over the roof of the building, air-sealed it well, you wouldn't need to heat the building.

Then consider, just how much 110F water is available, REALLY? Is it going to be 7400 gallons a day/week/month? Rather than trying to store low-grade heat by expensive dirt-block or tank methods, it may be better to use it IMMEDIATELY, storing it in the radiant slab when there's a heat load for the building(s) and the heat is available. A few hundred gallons of tank with a PEX heat exchanger for domestic hot-water pre heat may be worth it, but unless you know just how many BTUs are available, it's all just idle speculation on how much storage would make sense.

scottstool: Using electric solar panels to power a resistance heater doesn't make sense to me - better to collect the heat directly.

The numbers on water tank storage (say 3000 gallons) can work well in areas where there are off-peak power rates. In the normal case, the heating system (resistance or heat pump) runs only at night. A heat pump system might have no extra capacity on the coldest days and have to run all day too - but this is rare and oversizing the system should not be necessary.

Dana is right--as is the norm. All I would say in addition is that with micro-encapsulated phase change material all of his calculations for heat capacity could have two more zeros added as the specific heat capacity of this product has up to 250 calories per gram as opposed to waters 0ne calorie per gram. Yes it cost more than tap water--about the cost of bottled water.

Peops a lot smarter 'n me have looked hard and long at how to make thermal storage in dirt work, but it turns out to not be dirt cheap. It seems like a popular subject amongst those who refuse to apply crayon to wall (lipstick to mirror?) for even the most rudimentary rough cut.

With appropriate temp phase-change candy you could sure save a lot on insulation the storage eh? ;-)

PV to resistance heat may make sense in some places, but none of those places are in the PNW. When going for a moderately high-temp applications in a very cold place, (say 300-500F in Siberia mid-winter) it can be more efficient and cheaper to implement than evacuated tube thermal. But in a place like the PNW, going for domestic hot water temps or cooler you'll get 3-4x the collection efficiency out of evacuated tube or flat-plate thermal than PV+ resistance heater, and at a much lower installed cost per unit energy bagged. With commercial PV you're pretty much stuck at 15-18% collection efficiency, somewhat independent of the ambient temp, and independent of the storage temp, but for the same area flat panel thermal would average better than 50% collection efficiency in most of the PNW, but the actual performance is very sensitive to the ambient & panel temp difference, unlike PV.

We put 2 ft of stone on top of the insulation to create a weekly thermal store within the fabric of the building, excess summer heat will be stored in soil beneath the house. This is for a Passive House we will build in Ireland. We've built a number of Passive Houses already and they all require a 1-2kW heating system as back-up, if this can be supplied by solar using a cheap heat store it could be the answer. The efficiency of solar panels doubles when used for heating in winter.

If there is now a phase change thermal storage medium that is cost effective and practical, I'd like to hear more details. (other than ice, which is practical in some off-peak cooling situations).

I like your pictures here that show the thermal effect of a pressure differential. Remember, both leaks and a pressure differential are needed for air to leak in or out.

Hi,

Thank you for beating this idea down!!

seriously!

I did not think it would be more cost effective to insulate and lower heat loss
than to invest in storage!

I want to learn more about this, most cost effective investments to save energy.
I'm building this -out-of-pocket, and with the slow economy I surely don't need
to waste money.....

I can't wander too far from the engineering drawings, but I asked my building
inspector if I could make thicker walls without engineering re-approval.
My walls are supposed to be 2x6 studs, and he said I could go to 2x8 since my stub walls are 8"
and he would still sign off on it.
He suggested putting 2" foam, sealed with spray foam between the studs, then the standard 6" glass insulation
if I do go with a 8" stud outside wall.

Any suggestions for building a 8" wall, being stuck with minimum 2X6 studs attached to "engineered"
shear panels? Would be nice to have a break between outside wall studs, and inside wall....is it worth the trouble?
The shear panels are all on the outside of the house walls.

The 2" bigger window sills sounds nice!

I don't know what "super insulated" is.
Can you still have outside air exchange into the house, maybe with air to air heat exchangers?
I don't want to live in a sealed box!!

The house floor is concrete slab on grade, about 80 yards of concrete counting the load
bearing thickened areas.
That is a little over 2000 cubic feet of concrete.
Just wondering, does it make any sense (on a napkin with lipstick) to invest in thicker concrete
to moderate temp swings? The floor is radiant heated with PEX tube. It would seem to me, the
more mass inside the house, the smaller the inside temps will swing, but that darn concrete is expensive!

Another question, the engineer only has 2"X 2' foam insulation around the perimeter under the
slab, as well as 2"X2' down the sides of the footings.


The question is: the engineer told me that no insulation was needed in the center of the slab, the floor
heat had nowhere to go, and the dry dirt would stabilize. Does this make sense to you guys?

This 2" insulation goes up the concrete stub wall inside the house by 2' or so (1' past the top of the
concrete stub wall with a "chair ledge". The floor slab edges end 2" before they reach the stub walls because of
the footing foam board. But that is "code" here. I could run that 2" insulation to the ceiling inside, instead of just 2',
making the whole wall 10" thick....hummmm

Is there a benefit, or an investment loss to insulate under the whole slab rather than just around the outside?


Cheers,
Scott

If this was my house I would wrap it in 1Ft of insulation all around, eliminate all Cold Bridges at the wall/floor junctions and the wall/roof junctions. Then I would fit triple glazed windows, wrap them in insulation and try to achieve the highest levels of airtightness possible. After that I would worry about how to heat the house. Insulation and airtightness will reduce your heatloss by 95%, a heatstore can be a simple way to supply the final 5%.
A simple pipe-coil heat exchanger buried 5Ft under your house is sufficient to supply heat into your heat store, you don't need layers and layers of pex pipe. I would use FiWi HRV to supply fresh air without needing 100's of feet of ducting.

One of the most important and inexpensive energy saving building techniques is eliminating "thermal bridging". Wood studs, concrete foundations and floors all transfer interior heat to the outside. By separating these lelments from direct contact with the exterior we can inexpensively keep much of the heat we have paid for in the house. There are several ways to do this with your wall: by building two exterior2x4 walls with a space between them you create a "thermal break" in the conductivity of the wood studs. Adding a layer of 2" extruded foam to the outside of the sheathing accomplishes the same goal. ICF s and SIPS are effective for the same reason. Your concrete floor must be seperated from the footings and walls and the underlying ground by 2" foam so that the heat you put in the slab with your PEX tubing stays in the slab rather than being transfered into the adjoining cold concrete, the ground or outside air. The fewer thermal bidges your house has the more efficient it becomes.

The second techniques is to understand and eliminate air leakage. There are numerous ways to accomplish this which are addressed on most of the threads of this website so I won't go into them now.

You will need an air to air heat exchanger which brings fresh air into the house. They are pretty simple devices consisting of one small fan each for incoming and outgoing air and a core through which the streams of air pass through without mixing and where the heat frm the outgoing air is drawn to the colder incoming air. The problem with living in a sealed box is the poor air quality. The air to air heat exchanger is the solution.

The cost effective solution is to understand and deal with thermal bridging and air tightness first, then increase your insulation levels as much as your can.

Scott- rather than bumping up to 2x8 studs with 2" of spray polyurethane & glass batts, , for the same wall thickness you could add 2" of foil faced polyisocyanurate rigid foam to the exterior of the sheathing of a 2x6 stud frame, tape the seams with FSK tape, and seal it with spray foam where it meets your foundation's exterior foam etc. Seal each stud & plate to the sheathing on the inside wiith foam (corners only), and insulate it with wet-spray cellulose or a super-fine wet spray fiberglass like JM-Spider blown in blanket like Certainteed Optima, not batts. With the exterior foam thermally breaking the studs and gap/void/compression-free blown or sprayed insulation, you end up with an R-value in the ~R30 range. With 2" closed cell foam + R 19 batts that would only be your center-cavity R value, and the R-7-ish short circuits of the framing timber would cut you down to the R26-27-ish range. This is a more than 10% boost in performance, for similar or less installed cost. see: http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

With foil-faced iso on the exterior do NOT put a vapor retarder on the interior side of the studs. In most of the PNW (what's your zip code?), with at least 25% of the total R on the exterior you won't have wintertime moisture accumulation in the studs or sheathing, so no interior vapor retarder is necessary. Even if you've cheated the exterior R fraction a bit, if you use cellulose rather than glass or mineral fiber the cellulose will buffer the moisture, protecting the structural wood.

Putting foam as a thermal break on the interior of a studwall is less desirable, since it lowers the temp of the structural wood and is harder to air-seal, increasing the potential for mold & rot conditions in the studs.

Unless your engineer did the full test on the soil and measured the distance to the water table put in at least R15 (using low density Type-I EPS) in the center slab. That heat has plenty of places to go- DOWN to the sub-soil. Yes, if uninsulated the temps will eventually stabilize reducing (but not eliminating) the heat loss. In old-school 2x4 fiberglass batt construction that heat loss is "in the noise", overwhelmed by the high loss of the above grade losses, but for R30+ walls it's enough to matter. Going with R20-25 on the stem walls and ~R50 in the attic would also be about right. (Trusses with an "energy heel" designed in to allow 15-18" of blown insulation to extend all the way out to over the framing top plate maximizes performance.)

No matter what color your crayon or lipstick the numbers are the same- adding a lot more mass to the slab doesn't buy you much except in places that regularly see 40F+ daily swings in temperature (maybe at-2500'+ altitudes on the eastern side of the Cascades that might be the case, but not often on the west side or lower altitude.) For typical Seattle or Portland or coastal B.C. weather you'd be better off spending the money on more insulation or better windows than more thermal mass.

It seems everyone is missing the point. His heat is FREE. It can be stored. it is just a question of how long. As to the amount of pex, you need to determine the amount of heat available and then put in enough pipe to disperse the heat. Also the "dirt" needs to be wet or it will be insulation. You also need to be thinking in terms of more than 4" of foam and that's where the equation needs to figure out is the heat is really FREE.

Posted By FBBP on 04 Dec 2010 12:24 AM
It seems everyone is missing the point. His heat is FREE. It can be stored. it is just a question of how long. As to the amount of pex, you need to determine the amount of heat available and then put in enough pipe to disperse the heat. Also the "dirt" needs to be wet or it will be insulation. You also need to be thinking in terms of more than 4" of foam and that's where the equation needs to figure out is the heat is really FREE.

The heat is free, but the quantity hasn't been calculated. Every  home gets a quite a bit of "free" solar energy every week, but is it "enough"?  Without a number for the quantity of waste heat is even available there's no basis on which we can calculate how much heat storage would even be appropriate (at any cost.)


But...

The amount of insulation required to store the weekly heat energy of TYPICAL home for a week using soil for a medium is more insulation than what it would take to go PassiveHouse on the structure, at which point storing a few days worth in the thermal mass of the house itself is dead-easy.