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
=B0F 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 =B0F 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




wrote:
"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 =3D 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=B0F 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=B0F - 70=B0F) * 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 =3D 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=B0F!

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

If the storage could be warmed to 140=B0F, 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.
=20
Nick