An air-soil solar sub-basement heat battery

"C Johnson, Physicist, Univ of Chicago" writes at
http://mb-soft.com/solar/subbase.html

Then again, some houses have no basements. I seem to recall that simple concrete slabs cost about $3/ft^2, installed.

A window unit might handle dehumidification, for an "airtight" house.

The phrase "kind contractor" seems oxymoronic :-)

Pay no attention to all the empty dynamite cases in the yard :-) But how big a blower, and how many pipes, and will they collect water and dust and mold and mildew and varmints?

With significant thermal resistance, which limits the rate of heat storage and withdrawal...

How long would that take?

Assuming perfect heat exchange...

NREL says Chicago has 752 cooling degree days. Not much. A modest house with a 400 Btu/h-F thermal conductance might need 24hx752x400 = 7.2 million Btu/yr of cooling, or more, with some internal electrical use and unshaded windows.

How do we get the sun into the basement? :-)

Mirrors?
That works for Kachadorian "solar slabs" and Adirondack "solar houses."

NREL says Chicago has 6536 cooling degree days. Our 400 Btu/h-F house would need 24hx6536x400 = 63 million Btu/yr, or less, with some internal electrical use and sun into windows.

Is Chicago north of the arctic circle, with no sun for months at a time?

Like this?
The Lyckebo [Sweden] system is a cavern of 100,000 m^3 capacity, cut out of bedrock using standard mining methods, of cylindrical shape, with a central column of rock left to support the overhead rock. The cavern is about 30 m high and its top is about 30 m below ground level. It is water filled, and inlet and outlet pipes can be moved up and down to inject and remove water from controlled levels. The water is highly stratified with top to bottom temperatures of about 80 to 30 C. Figure 8.7.2 shows temperature profiles in the store at various dates in the second year of operation... No thermal insulation is used, and there is a degree of coupling with surrounding rock which adds some effective capacity to the system. Losses occur to a semi- infinite solid and can be estimated by standard methods. Observed losses from this system are higher than those calculated; this is attributed to small but significant thermal circulation of water through the tunnel used in cavern construction and back through fissures in the rock. It takes several years of cycling through the annual weather variations for a storage several years of cycling through the annual weather variations for a storage system of this size to reach a "steady periodic" operation. In the second year of its operation, while it was still in a "warm-up" stage, 74% of the energy added to the store was recovered.
from p 404 of section 8.7, "seasonal storage," of _Solar Engineering      of Thermal Processes_, by John A Duffie and William A Beckman, 2nd      edition, 1991, Wiley-Interscience ISBN 0-471-51056-4
But why do we need seasonal storage in Chicago?
And soil is not a good heat conductor and stores about 3X less heat by volume than water, and it's easier to transfer heat from water to water than soil to air, or move the warm air around very far. We might line a sub-basement with concrete block partitions to make tubs and line the tubs with single pieces of EPDM rubber folded up like Chinese takeout boxes (a 20'x20' piece of rubber could line a 12'x12'x4' deep tub) and fill the tubs with water, with a vapor barrier (eg foamboard) and a removable wood floor on top... 4 12'x12'x4' deep tubs would hold about 143.6K lb of water. With a 120 F temp on an average day and 80 F min temp, they could store 6 million Btu, enough to heat a modest house for 29 cloudy days in a row. But who would need that?

A water system might collect and distribute heat with some fin-tube pipes below a ceiling, with the help of a ceiling fan and a room temp thermostat and some simple solar air heaters and a low-e coating under the ceiling to avoid overheating the room.

This would be the second $250, or more?

As you say, we could use more details. More engineering than physics.
Nick
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Nick,
I've been lurking for too long now, and your comments here are related to an active interest of mine. The idea of using earth or water for heat storage is very interesting, but as your comments accurately indicate... it isn't all that easy. I have rolled my own numbers on a system such as this. My design would use parallel PEX loops to deposit and extract the heat. One loop for glycol, one loop for domestic hot water. Maybe a third loop for radiant floor heating. Unfortunately, when one adds up all of the costs, it gets very expensive.
One needs to insulate the sub-basement very well. My design uses lots and lots of PEX ($$$ - must be spaced quite close because of poor thermal conductivity of the ground). If you have sand, great. Easy to work with. Clay.... haul it away and truck in sand ($$$ - which is my situation). Also, the writer seems to ignore heat losses. More insulation helps, but more $$$. I understand that water flowing through the earth around the sub-basement might carry away a significant amount of heat.
I thought about placing some radiant subfloor aluminum heat plates in contact with the PEX to help distribute and collect the heat from the ground. This would give the system more 'power' by increasing the effective surface area of the PEX, but... $$$ - nobody gives these panels away for free.
Larger systems hold more heat for a given surface area. It seems that this system might work better for a large house. What about for larger buildings yet... or a community based project. There have been some designs along this line done in northern Europe. I think they used enormous tanks of water. I haven't seen any good evaluations on their performance.
One could make the sub-basement deeper than the standard 8' to help reduce the mass / surface area ratio. Now you have a big deep hole... more $$$$ How much mass can dense foam board hold before it compresses?
The use of water for heat storage has several big advantages, one being it's much higher heat capacity per cubic foot, but now the basement floor must span the pool in the sub-basement. Would the basement floor have to be concrete to resist the water just below? More $$$$. What about when you get a leak? You aren't going to send a diver into 120 F water to patch it! My current thinking is that sand has some advantages here in spite of it's lower specific heat. Does a large bed of sand expand when heated?
Near the end of 2002, there was a long thread at alt.solar.thermal on the solar cistern that Alan Stankavitz installed in his home. (www.daycreek.com) Alan's design is not at all like what you have dug up, but it is a related idea that has some merrit. The storage potential in Alan's system is much more short term, and the design is very different. Performance expectantions are very different too, subsequently, his cost is much less. I wonder what Alan does with the heat from all of those panels in the summer? Did he install a dump load?
A lot of the discussion in that thread was related to trying arrive at an understanding on how heat flows through Alan's system, (and trying to convert Mr. Pine). This is a very important point to consider. Failure to understand how heat flows in soil will likely doom a project, or at least make it a serious under-performer. Has there been any good research on this topic of heat flow in solar cisterns?
I have toured two buildings that use the solar cistern concept at the Midwest Regional Engery Fair 'Solar Tour'. Both systems were well thought out, and seem to be working very well. However, neither system design is full of the hype which you have correctly pointed out below, and both buildings use good passive design concepts.
I would like to point out some serious flaws in the assumption that one can heat and cool with the same mass. If one shuts off the heat in April, what will be the source of cooling for this insulated mass so that it is 55 F come the 4th of July? Sounds like horse feathers to me.
OK, so I've rambled long enough. I guess what I want to really say is that you are absolutely right to be highly sceptical of the claims made. However, I still think the idea has lots of merrit if correctly designed. Maybe one's house construction budget would be much better allocated to an extra 4-8" of insulation in the walls and ceiling, and good passive design. Under these conditions, a solar cistern wouldn't have to be so large... or expensive.
Steve
snipped-for-privacy@ece.villanova.edu wrote:

full
thousand
concrete.
simple
are
or
only,
of
without
and
costs
whom
anything new or

that they

complications
actually be

entirely
as a

buried in

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properly
and the

show
how
dust
feet by

then have

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materials.
storage is

of
storage
Chicago's,
temperature,
beginning of

76F, that

million
Chicago's,
actual
house
at 30,000

air
house with

million Btu/yr

windows.
the
sources
during
noticed that

summer day.

several
equipment.
similar.
houses."
of the

usual
million
winter.
reasonably
Btus.
would
electrical
load
furnace
time?
even
representing
for every

would be

cut out of

central
about 30 m

filled, and

remove water

bottom
profiles
thermal
surrounding rock

semi-
losses
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tunnel used

takes
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a storage

second
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Engineering
by volume

soil
sub-basement
single
piece
water,
top... 4

F temp

enough
need that?

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surfaces,
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might
this page,

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performance...
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From sand. Water seems better.

A lot... 30 psi?

That's one reason I suggested 12'x12'x4' tubs. You might fill a sub-basement with sand, then add some water for greater heat capacity and easier transfer (just pump water in an out, with no heat exchanger), leaving a layer of sand on top which could support a floor, but 12' isn't a large floor span. Water could increase fire safety.

I think it just needs a good vapor barrier. Plastic film and/or foamboard. Putting the tank in the ground removes the need to build strong sidewalls.

A tub shouldn't leak, if carefully lined with a single 20'x20' piece of EPDM rubber folded up like a Chinese takeout box. In the rare event that it does, it might be pumped out for repair, which might just consist of adding another layer of rubber. We could make 4'x19'x3' tubs with 10'x25' pieces of rubber.

Trickle water over a north roof at night, or through a pond, under some rocks for safety and shading. Or keep the tub hot to make DHW.
Nick
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wrote:> > If one shuts off the heat in April, what

Or use a heat pump to extract heat from it for DHW. But that defeats the object.
The Germans spiral a 6" plastic pipe around the outside of the foundations and use this as the inlet to a heat recovery and vent system. Pre-heats in winter and cools in winter. They don't need a/c in their climate.
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wrote:

full
thousand
concrete.
simple
are
or
only,
of
without
and
costs
whom
anything new or

that they

complications
actually be

entirely
as a

buried in

of the

properly
and the

show
how
dust
feet by

then have

cubic feet

pounds
materials.
storage is

of
storage
Chicago's,
temperature,
beginning of

76F, that

million
Chicago's,
actual
house
at 30,000

air
house with

million Btu/yr

windows.
the
sources
during
noticed that

summer day.

several
equipment.
similar.
houses."
of the

usual
million
winter.
reasonably
Btus.
would
electrical
load
furnace
time?
even
representing
for every

would be

cut out of

central
about 30 m

filled, and

remove water

bottom
profiles
thermal
surrounding rock

semi-
losses
attributed to

tunnel used

takes
a storage

a storage

second
of the

Engineering
by volume

soil
sub-basement
single
piece
water,
top... 4

F temp

enough
need that?

of the

surfaces,
inward or

must be

to the

economy
efficient
pipes
thermostat
ceiling to

might
this page,

of this

variables can

performance...
version of

might as

greatest
(2) the

of
areas, we

assure
and
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
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Upload

hit the key too early in the previous post - way too early :)
If I recall, the thread centred on the performance of the floor, which was good. He was using sand to store heat, which is not the best of materials for holding heat. The conclusion was that the sand was wet, and when dried out the performance would drop off.
I suggested this from ground up:
- insulation - concrete with plastic pipes embedded. - insulation - cement screed with underfloor heating pipes embedded. - floor covering.
Essentially a lower concrete layer in which to store heat. The upper cement layer is the normal underfloor heating thermal heater floor.
You need a floor anyhow, adding more concrete, pex pipes and insulation is no great expense. The control system would need to be set in such a way that solar heated water is sent directly to the cement screed and DHW. When there is available heat and not required for heating purposes, it can be stored in the bottom concrete floor section, and extracted at will. The incoming cold water mains pipe could also loop around the concrete floor and preheat the DHW.
OK that is for a slab on grade. With a basement you store under the basement in a well insulated tray of concrete in an insulated tray, with insulation over.
Water holds more heat, that is for certain, but as soon as you put it into a container it wants to get out and usually does in time. Using dense concrete under the house, and enough of it, a fare amount of heat can be stored, even at low temperature.
Or have an insulated semi sub-basement (crawlspace) with a large water thermal store made up of small off-the-shelf water cylinders. If one springs a leak, isolate it and replace quite cheaply.
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This all might look good on paper, I am still skeptical, I want to *see* actual installations and their respective costs for construction(compared to an identical home without this extra stuff), actual energy savings(utility bills compared with an identical home without this extra stuff) with all comfort levels being equal, and total repairs over 5 years, 10 years, 20 years.
As a contractor, I want to know what overall benifits and savings all these extra heat recovery systems will give *my* customers over and above a correctly sized, installed, and balanced super high efficiency HVAC comfort system.
My own home in south Mississippi, a 2400sqft split level built in '59 with a lot of single glazed glass in aluminium frames(over 100sgft of glass in the livingroom alone) with 4 tons of A/C had a high light bill of $109 last summer(august). My normal light bill without a/c or heat running is $61. I have been able to install comfort systems in 2000sqft homes where actual operating cost for the system in extreme months is less than $35.
How much more will the stuff that your playing with lower the utility bills, and what is the actual payback time?? an what additional savings can the customer expect over the normal design service life of your system?? BTW...What *is* the normal design service life of your system??
--

Steve @ Noon-Air Heating & A/C
snipped-for-privacy@comcast.net
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Nick,

I recall a girl from Chicago living in London. She said she liked England because at least you see sun in the winter. She said in Chicago they can go months on end without seeing any significant sun.
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News wrote:

England
can go

If you are trying to make a "rechargable thermal battery" for thermal storage, wouldn't a material that phase changes be a better bet, ie something like freon that evaporates and condenses or freezes and melts. These have a much higher heat capacity.
Mark
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Exactly right. http://www.eere.energy.gov/consumerinfo/factsheets/b103.html
But some of these have experienced troubles. The solutions tend to stratify while in the liquid form and then they lose effectiveness (liquid doesn't 'freeze' and solid in the bottom doesn't 'thaw'). Perhaps a storage tank with a very low-tech agitator?? Something like just a paddlewheel on a shaft that you 'crank' for a while once a week??
daestrom
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Nick,

How about digging out a part of the garden, installing foam insulation to the sides and bottom of the hole, spiralling these hollow tubes around the hole, backfilling and covering the whole garden area with foam insulation to prevent the air having an influence on the earth temperature and then soil on top. Must be an easier and cheaper way of doing it than having deep foundations for two basements.
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