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Steve Baer on Air Loop Rock Storage Systems

>From  his 1979 Cloudburst Press book, ISBN 0-88930-062-3, pp 62-66,
_Sunspots_ "An Exploration of Solar Energy Through Fact and Fiction."

Air Loop Rock Storage Systems

Much work still needs to be done on the behavior of convective air loop
rock storage systems. The way they work flies in the face of the typical air
conditioning engineer who can't believe such systems could operate without
fans.

At a solar energy conference in 1968, Farrington Daniels mentioned the letters
he had received from someone in New Mexico building solar chimneys that pushed
air through rock storage bins. He was told by an engineer in the audience that
the chimneys would have to be as tall as the Empire State Building. I was very
pleased to report on the performance of the just-completed Drop City heater
where the chimney was only 14 feet high.

More recently, Paul Davis and I were told by engineers at the Los Alamos Lab,
who had been studying fan-driven air loop rock storage systems, that
convective systems were "way down in the mud," with little to recommend them.
Paul Davis' house is heated by such a system and functions satisfactorily
with no fans. After talking to the Los Alamos group we found that they had
never actually experimented with any rocks, but instead were using only
computer simulations.

I can't help dwelling on these petty slights at the hands of the engineers,
probably because there is an element of truth in what they say. Designing
a convective air loop system is a somewhat tricky and difficult task. If
you aren't very respectful of the will of the air, the system won't work.

The collector should have a great deal of surface area through which to
transfer heat to the slow flowing air stream. We have found that multiple
layers of expanded metal lath work well for this purpose. The flow channel
within the collector should have a large cross section so that even at
slow velocities enough air moves to transport the heat. The rocks should
be arranged so that they place little resistance in the path of the air.

During experiments at Zomeworks in 1969 we were able to demonstrate that,
with a collector and storage bin at the same elevation, it is possible to
store an average of about 750 Btu/square foot/day during December weather
in Albuquerque.

It is very easy to measure collection efficiency and quantity of heat stored
in water systems: a tank of water can be mixed and its temperature measured
with one thermometer; with air and rock systems you need numerous temperature
probes that can be placed within the rock bin...

Solar Chimney

Air heated by the sun in a sloping, glass-covered channel acts like air in
any chimney--it rises. But solar chimneys differe from other chimneys. The
heat is added to the air as it travels along the chimney rather than at
the bottom. This complicates chimney design. In a normal chimney you have
a round channel with smooth sides so the air may flow quickly to the top.
A solar chimney would like to be this same shape, but it must also collect
sunlight and transfer the heat to the flowing air.

There are contradictions that must be balanced. You want to put surfaces
in the path of the moving air in order to transfer heat, but then you are
sorry to have to do this because the added surfaces in the flow channel
slow the air down.

The air has a limited budget of energy that it can spend circulating itself
>from  one place to another. It is fairly easy to calculated what this is.
The air is warmed as it rises in the solar chimney and is cooled as it
descends through a storage bin. The difference in the average absolute
temperatures on the two sides of this loop creates the pressure difference
that drives it.

The pressure drops through ventilating ducts and other resistances to
atmospheric air are normally given in inches of water. Inches of air become
the natural units for the solar chimney designer. It is very convenient
that water weighs lbs. per cubic foot and air weighs about 1/1000 this
amount (at sea level, air at 175 F weighs 0.625 lbs/ft^2; at 5,000 ft
air at 62 F weighs 0.625 lbs/cu ft.)

Example: If a solar chimney is 8 feet high,              ffffffffffffffffff
the average temperature of the air rising in             .    ===>        f
the chimney is 130 F, and the average temperature        .      f thermal f
of the air descending on the storage side is 90 F,       .      f  mass   f
then the pressure difference driving the loop is         . 130F f.........f
40/550 x 96 inches = 7 inches of air, or about           .      f         f
0.005 inches of water. Unfortunately, engineering        .      f     fffff
tables rarely give figures for flow rates with           .      f     f
such low resistances.                                    .      f 90F f
                                                         .      f     f
Convective systems balance themselves. The chimney side  .      f     f
will continue to increase in temperature until the air   .      f     f
flow increases enough to carry away the heat from the    .      f     f
collector. If it is Self-Balancing, What's the Problem?  .    <===    f
                                                         ffffffffffffff
A poorly designed convective air loop system may have
to get so hot in order to transport heat to storage that the collector
losses become large and the whole system becomes inefficient. The problem
is how to have low resistance to air flow and also to have rapid heat
transfer from the collector to the air and from the air to the storage.

Chimney Design

We have found that multiple layers of expanded metal lath work well as
heat exchangers in convective systems. The sunlight filters through the
layers and warms up each of them. Heat is transferred from the sunny
exposed spots in the matrix to the shaded parts, so the entire surface
transfers heat.

Each square foot of lath (counting both sides) has about 3/4 square foot
of transfer surface. Se have successfully used 5 layers of mesh. Counting
the channel back and sides, the total transfer area to the air stream
is about 5 square feet per square foot of glass.

The U factor between the slow moving air and the mesh is probably only
about 1.5. The delta T between the mesh and the air when the solar flux
equals 240 Btu/square foot/hour is then equal to 32 degrees F.

The mesh should be placed diagonally across the            hot air
collector so that the rising air must flow through       .   out    f
it before it leaves. The mesh can also be placed on      .        . f
repeated diagonals. I do not know which arrangement      .          f
is best.                                                 .       .  f
                                                         .          f
Design tips                                              .      .   f
                                                         .          f
1.  Make the width of the flow path at least L/15.       .     .    f
2.  Make rocks (h) 2 feet deep if small gravel (1"),     .          f
    and up to 4 feet deep if large rock (6").            .    .     f
3.  Make collector slant at least 45 degrees.            .          f
4.  Insulate storage with at least 6 inch batt.          .   .      f
5.  Make collector at least 6 feet long [tall].          .          f
6.  Keep all flow channels at least 1/15 of the          .  .     . f
    collector area.                                      .       .  f
7.  Avoid corners in flow channels.                      . .    .   f
8.  Make storage cross section at least 1/3 of           .     .    f
    collector area.                                      ..   .     f
9.  Insulate divide between down flow and up flow        .   .      f
    with at least 1 inch duct board.                     .  .       f
10. Double glaze collectors if 7,000 degree day          . .        f
    climate or more.                                     ..         f
11. Hand place rock if possible to avoid layers          .          f
    of dirt in bin.                                      .       .  f
12. Place all of storage rocks above collector,          .     .    f
    or use damper.                                       .   .      f
13. Build house above storage bin.                       . .      . f
14. Build vent flap at top of collector to open          .     .    f
    in summer to prevent overheating.                    .  .       f
15. Heat house with trap door to rock bin and duct       .          f
    to cold underneath for return air.
                                                            cool
(Most of these rules are probably too strict,               air in
while some of them may not be strict enough.)

Steve also mentions further work on such systems in UN Solar Energy
publications, volume 5, as well as work by Dunkel in Australia and
Scott Morris in Santa Fe, NM.

Nick