Tuesday, February 8, 2011

"passive annual heat storage," by john hait SOLAR.KnowledgePublications.com

"passive annual heat storage," by john hait SOLAR.KnowledgePublications.com

9 sep 1997
i just interlibrary-borrowed and finished reading the first edition, second
printing of john hait's 152 page out-of-print (?) 1983 "passive annual heat
storage" book, which references and seems to take off where mike oehler's
"$50 and up underground house" book leaves off, thermally-speaking.

hait is/has/had a non-profit organization called the rocky mountain research
center/pobox 4694/missoula mt 59806, not to be confused with amory lovins'
rocky mountain institute. hait seems less well-funded and sure of himself than
lovins, and less survivalist and outspoken than oehler. i wonder what he's
doing these days. he studied mazria's book and the underground space center's
earth sheltered housing books and built an underground "geodome" home in
missoula with 48 temp sensors and 5 moisture sensors to try out his ideas.

he's done a lot of good basic thinking. the book has novel and interesting
concepts, and some very clear explanations, eg this one on page 83:

figure 43 shows two types of heat exchangers, parallel flow and counterflow.
in the parallel flow, as the name suggests, the air is moving in the same
direction in both pipes. heat will pass through the walls of the interior
pipe from the hot air to the cold air. the cold air is warmed up, and the
hot air is cooled off, and their temperatures meet in the middle coming out
the other end, warm.

the counterflow heat exchanger, on the other hand, has its fluids (gas or
liquid in either or both tubes) traveling in opposite directions. once again,
the hot one cools, and the cold one warms up. but, when the cold one reaches
the other end of the pipe it sees, not a warm fluid that has already been
cooled off, but a hot one that hasn't had a chance to cool yet, and the hot
one sees a cold fluid at its other end... when the fluid which is supplying
heat is hot, the one receiving heat can get hot. but if the source fluid
were to lose its temperature in the process, as with the parallel flow heat
exchanger, how could it make the destination fluid anything but warm?
this way, the hot one comes out cold, and the cold one comes out hot.

hait may have been the first person to suggest making earth tubes into
counterflow heat exchangers. he goes on to describe the camel's nose :-)
a heat exchanger with flow that periodically reverses direction.

the book is short on arithmetic: at one point hait carefully explains how to
calculate the area of a circle. on the other hand the book seems full of good
physical insights, experience, and numerical clues, like "it takes six months
to conduct heat 20 feet through the earth." he says "plain old dirt is the
ideal heat storage medium," and suggests covering the dirt around and over
an underground house with an insulating "umbrella/watershed" made with several
layers of foamboard and plastic film, to keep rain from washing stored heat
down and out of the dirt. the umbrella slopes downward to shed water and
extends some 20' outwards from the house, about 2' underground, and it
contains about 4" of foamboard at the thickest point.

deep earth normally has a temperature close to the average annual air temp, but
in this case, the house with uninsulated underground walls is used as a central
heater to slowly warm ("it takes three years to fully climatize the soil around
the home") and bias the earth to a higher temperature (eg from 45 to 68 f)
under the umbrella. passive solar heat is one way to do this, allowing sun to
shine into the house through windows ("whatever you wish the average indoor
temperature to be, adjust the inside temperature to be about 3 or 4 f (1.5-2 c)
higher in the summertime. the whole structure should swing only about 6 to 8 f
(3.3-4.4 c) all year.") another way is to make use of seasonal air temperature
differences. he describes a possible origin of his system:

baked dry in august... frozen stiff all winter, a montana sodbuster and his
neighbors battled the elements. they shivered through the frigid northern
winters, gathering buffalo chips for fuel, to ward off the frostbite.

our field farming friend noticed that the vegetables in his root cellar
never got hot and never got cold. they were always comfortable. he wasn't!
so he installed a window in this root cellar and moved in.

within the first year, the unheated indoor temperature rose from its natural
45 f (7 c) to 55 (13 c), all by itself. this drastically reduced the amount
of fuel he needed, but his neighbors just laughed at him and continued
gathering buffalo chips.

this rise in temperature was a surprise improvement, since everyone had
told him that it would always be 45 (7 c) no matter what. mulling this over
in his mind he tohought: "if i could only raise the termperature another
10 or 15 degrees (6-8 c)... i wouldn't need any buffalo chips at all."

but how can you intentionally raise the constant temperature that occurs
naturally in the earth? well, he had already raised that average temperature
about ten degrees by installing the window. he reasoned, "it must be like
raising the natural level of a lake. you let more water in and less water
out. that's it!"

he grabbed his hat and dashed into town. soon he returned with a pickup load
of styrofoam insulation and several rolls of plastic sheeting. he put the
insulation and plastic over the top of his home, dirt and all, and covered
the whole thing with another layer of earth.

all summer long, the heat which collected inside soaked into the ground to
keep his home cool and comfortable. just as he had suspected, the newly
insulated earth began warming up from 55 to 65 (13-18 c) and, finally by
fall, all the way up to 71 degrees (22 c.) when cold weather arrived, the
earth remained warm and kept his new earth sheltered abode cozy all winter.
our subterranean sodbuster was at last _continuously comfortable._ he had
invented passive annual heat storage!

and his neighbors? well, times have changed. now a big monopoly collects
and distributes all the buffalo chips... and goes to the public service
commission each month to ask for a rate hike.

hait says the us r-value of earth is often assumed to be about 0.08 per inch
(about r1 per foot) vs 3 to 7 per inch for many commercial insulations. hence,
we have something like an r20 wall with lots of thermal mass (20 btu/f-ft^3
would make rc roughly 4,000 hours, ie 6 months, with the capacitor in the
center of the r20 resistor and both ends of it at a low-impedance) if heat
has to travel 20' horizontally before escaping upwards. hait also points out
that a wall thicker than 20' doesn't add much effective thermal mass, since
the winter cold front that travels into the wall becomes a warm front after
6 months, and heatflow reverses. i guess most people would say it's easier
to build a wall with 6" of fiberglass insulation than to arrange this sort
of earth umbrella. we might cover a house with polyethylene film and surround
it with a gigantic earth berm, with a ring of tires to make the outer walls
steeper, and 2' of some sort of fluffy compost on top, over plastic film...

hait has an interesting way of providing fresh air and conducting and storing
heat in the earth around the house, like this:

earth earth earth earth
solar gain --------------------- <-> uuuuuuuuuuuuuuuuuuu
uu \ | | \ upper earth tube uuu
uu | house | \ uu
lower earth tube<-->| | \

the lower earth tube slopes up to the house (to make an igloo-like heat trap)
and enters the living space at floor level. the upper earth tube enters at
ceiling level, and slopes down to exit at the same outdoor level as the lower.
if the house becomes slightly warmer than the surrounding earth, outside air
naturally enters the lower earth tube and warm air flows out of the upper one.
if the house becomes cooler than the surrounding earth, flow reverses. in
either case, heat is exchanged between earth tube air and surrounding earth,
storing excess house heat in the earth or removing it to heat the house.

he suggests eliminating the windows and turning all this upside down, with
upward sloping earth tubes (making a cold trap), to make this into a permanent
cold storage place in a cold climate.

the book ends with design guidelines:

insulation/watershed umbrella:

1. extend umbrella out and around the entire home and above also, if
the home is fully earth sheltered.
2. extend out 20 feet (6 m) wherever possible.
3. taper insulation from 4 inches (10 cm) down to one (the first inch
is the most important.)
4. insulate the backs of retaining walls and other items that will be
backfilled before the main umbrella goes into place.
5. plastic: (0.006 inch (0.15 mm), largest sheets practical.)
a. 3 layers min.
b. separate layers with soft insulation or dirt that will drain well.
c. provide adequate drainage out the end of the umbrella.
d. do not stretch, but allow for settling with folds and slipping overlaps.
e. lay like shingles.
f. prevent future ponding after settling by allowing sufficient
drainage angles.
g. pay particular attention to possible extreme settling and the problems
that might occur given its new confuguration.
h. make underground gutters to guide water off the front and away from
the building.
i. cover it with flashing if it exits the ground.

earth tubes:

1. size... 4 to 18 inches (4-46 cm) in diameter.
2. length...60 feet (18 m) min. more like 100-200 ft.
3. must go downhill from the house at least a foot plus the tube diameter.
4. must be kept relatively level (1/4 inch to the foot for drainage in
those places where heat exchange is to be minimized.
5. the greatest angle of grade must be in those areas where heat
exchange is preferred.
6. at least two tubes must be used.
a. one enters the home at the highest point where air can be taken.
b. the other enters the home at the lowest point where air can enter.
7. provide for condensation removal.
8. provide for backfill settling so that the tubes will not be sheared off.
(backfull with gravel under the tubes.)
9. provide bug screens.
10. a small umbrella should be provided over the tube if it is not
under the main umbrella (about 8' wide, full length.)
11. do not "short out" the storage zone of an earth tube by placing it
too close to an interior wall so that conduction becomes a more
prominent factor than the convection through the tube.
12. plastic tubes should work quite well; their r-factor is small (given their
thickness) and they can withstand the earth environment for a long time.

internal design:

1. light colored walls and ceilings to spread the heat around.
2. medium colored floors so they will be slightly warmer than the ceiling,
to help avoid stagnation.
3. carpet should make little difference [hait likes mike oehler's carpet
over poly film over earth.]
4. allow for free flow of air between places where sun comes into the home and
conductive surfaces near storage, so heat can be transferred by convection.
5. provide a place for the warm air to go.
6. provide a place for the cool air to go.

window layout:

1. use moderately sized windows. greater than the 10% required by law [?],
and less than the usual passive solar recommendation. probably about
25 to 30% of the equivalent floor area in glazing.
2. do not localize the windows all on the south, as if it were a regular
passive solar building. spread the windows out so the heat input is
spread over the whole day.
3. at the same time (be careful) do not severely reduce the conductive
surface area, and thus the storage mass accessibility.


1. external shading devices that are adjustable!
2. earth tube shut-offs and one way doors.
3. windows, floor vents, skylights.
4. provide for cross ventilation and high and low vents also with earth tubes.


at least, monitor critical temperatures inside, outside, and in the
storage mass.


insulation/watershed umbrella.
gravity drainage.
do not allow hydrostatic pressure from water table.
at least one layer of plastic as a vapor barrier.
use the complete water control program.


No comments:

Post a Comment