"C Johnson, Physicist, Univ of Chicago" writes at

http://mb-soft.com/solar/subbase.html

Do you plan a basement for this new house? If so, consider making a full
sub-basement underneath it! That's essentially it! Maybe an extra thousand
dollars of excavation cost, and an extra thousand dollars of concrete.

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

There are some additional costs, but they tend to be minor. There are
actually two nearly identical storage versions, one for heating only or
heating and cooling, and the other for cooling only. For the cooling only,
for around (possibly) $2,000 differential in the construction cost of
a new house, you could have completely normal air conditioning without
having to dig up the whole yard (as in our Free Air Conditioning) and
without paying for a conventional central A/C unit (which generally costs
AT LEAST $2,000 that you would save!)...

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

(These costs are only if the contractors involved are "friendly" and whom
will be kind in their cost additions. Many contractors dread anything new or
unusual, and they might charge ten times that much, to make sure that they
would still make a profit even if unexpected construction complications
occurred!)

The phrase "kind contractor" seems oxymoronic :-)

This sub-basement room would not be used as a room. It would actually be
painted with a waterproof sealer, insulated on all sides, and entirely
filled with relatively standard backfill earth materials, and used as a
"heat storage" area. A complex arrangement of hollow tubes gets buried in
the backfill material to provide the method to get heat into and out of the
storage. Once this sub-basement was made and filled in (and then properly
compacted), a standard basement floor would be poured on top of it, and the
house built normally above it. In other words, the final house would show
absolutely no evidence of the sub-basement even existing!

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?

What would be the point of this? Imagine a modest-sized house, of 40 feet by
25 feet, or 1,000 square feet floor area. This sub-basement would then have
1,000 square feet of area and 8 feet in height, or around 8,000 cubic feet
of the earth materials trapped inside it. At a density of around 100 pounds
per cubic foot, that's around 800,000 pounds of heat storage materials.
Using some standard engineering information, this huge amount of storage is
able to store around 500,000 Btu of heat (or cool) for each degree of
temperature change of the material.

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

First, consider cooling. If this house is in a climate similar to Chicago's,
the natural ground temperature is around 53F. If, in the Spring, the
storage is permitted to revert to its natural deep earth temperature,

How long would that take?

then all of the 800,000 pounds of storage would be at 53F at the beginning of
the Summer. If the desired summer house temperature is the common 76F, that
means that the storage contains around (76F-53F)*500,000 or 11.5 million
Btus of cooling!

Assuming perfect heat exchange...

What is the expected house use of cooling? In a climate like Chicago's,
there are commonly around 20 days each summer where a central air
conditioner is used for the six hours of the afternoon. That's 120 actual
hours of air conditioning that is needed in a summer. A modest-sized house
such as the above would often have a central air conditioner rated at 30,000
Btu/hr (2.5 tons). Multiplying these numbers gives a full summer's air
conditioning usage of around 3.6 million Btus of cooling...

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.

There are even wonderful heating benefits from this system! If, in the
Autumn, either solar heating or any of a variety of other heat sources
is used to warm up the storage, that stored heat could be used during
the winter for heating the house.

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

This is meant as a very "low-tech" system. You have certainly noticed that
the interior of a closed car can soon get over 140F on a sunny summer day.
The point is, getting this massive storage up to, say 120F, over several
weeks is quite easy without having to resort to any exotic equipment.

Mirrors?

Many other heat sources are possible, too, like a woodstove or similar.

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

Using this very conservative value of 120°F for the top temperature of the
storage, let's do the math. If a desired house temperature is the usual
70F, then the storage would have (120°F - 70°F) * 500,000 or 25 million
Btus of heating available for the house at the beginning of the winter.
In Chicago's fairly nasty climate, our modest-sized house, if reasonably
well insulated, might have an annual heating load of 45 million Btus.

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.

That means that more than half of the entire house's winter heating load
could be provided by our massive storage, without turning on any furnace
or using any heating fuel, EVEN IF IT IS ONLY AT 120°F!

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

If the storage could be warmed to 140°F, then the benefits would be even
better. The storage would then contain around 35 million Btus, representing
MOST of the house's heating load for each winter. That means that, for every
following winter, only a small portion of the usual heating bills would be
necessary! Forever!

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?

There ARE some Engineering considerations. All of the inner surfaces of the
(concrete) chamber would have to be painted / sealed to seal those surfaces,
such that moisture did not permeate through the concrete, either inward or
outward. Effective, crush-resistant, durable, thermal insulation must be
provided, so the storage does not lose too much of the stored heat to the
surrounding ground (such as what traditionally had been called blue
Styrofoam). Proper selection of the best storage materials (for both economy
and thermal characteristics) is very desirable. The system needs efficient
ways to both get heat into and out of the storage, for efficient
performance...

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.

As to specifics for a particular application, well, that's where we might
earn our keep. We would have liked to include those specifics in this page,
but there are quite a few variables that can affect the performance of this
system. The size and shape of a house, the climate, and other variables can
affect how this system should be engineered for optimum performance...

This would be the second $250, or more?

The discussion above should have convinced you that almost any version of
this concept will be of benefit, but if you're going to do it, you might as
well get maximum benefit from it! The variables that have the greatest
effects on performance are three: (1) the insulation R-factor used; (2) the
type and condition of the storage medium itself; and (3) the method of
efficiently getting heat into and out of the storage. In these areas, we
have extensive understanding, and we are confident that we can assure
maximum performance of either version of this system for any house and
climate.

As you say, we could use more details. More engineering than physics.

Nick