To PAHS or not to PAHS, earth moving question!
PAHS (Passive Annual Heat Storage) is essentially placing an insulating and
waterproof "umbrella" over and about 15-20 ft. beyond a typical
earth sheltered home. 20 ft. is the nominal distance that heat migrates through
typical dry soil in half a year. With the PAHS insulated umbrella keeping
your earth dry, thermal energy will travel most slowly through the earth,
and thus allowing you to store the maximum amount of thermal energy under
and around your foundation." The insulated umbrella also serves as a
thermal break between the cold and wet top soil (top 2-3 feet), and the warm
and dry soil below, thus increasing the amount of thermal mass available to
you to store warmth.
The umbrella is made of 3 alternating layers of plastic sheeting and , eg,
Polystyrene insulating board (many alternatives here). The insulation umbrella
has the effect of raising the temperature of the earth, in which the dwelling
is embedded, up significantly from the average (approx. 50 degrees). The insulation
is, of course, covered with more earth. Because of the huge amount of mass
involved, it takes a season or two to accomplish this. The temperature swings
now become seasonal rather than diurnal. And the interior temperature continues
to stabilize at the desired constant temperature with minimal passive charge,
summer and winter.
In principle - and John Hait gets the credit for being first with this - PAHS
is universal in that it can be designed to heat, cool or maintain thermostasis.
It's easier to heat, and you could probably do that in the Arctic or Antarctic,
because you get clear skies. PAHS is fundamentally an exercise in thermodynamics.
You have to have heat which is above ambient (or high grade energy which you
can convert to it) AND/OR a heat sink below ambient. No temperature difference
= no work performed. PAHS succeeds by using the thermal mass to seasonally
phase shift things to construct effective temperature differences. Secondarily,
Hait uses diurnal phase shifting to (markedly) improve engineering and transfer
efficiency.
Though radical, the concept is fairly simple and intuitive. It's amazing that
more earth sheltered structures and tire houses don't implement it more often.
For combined passive heating and cooling in seasonal regions, it's hard to
imagine a better design principle. Perimeter, horizontal insulation has been
used effectively in the Scandinavian countries for years and is gaining popularity
in the US. Passive annual heat storage exploits the concept to the maximum.
One owner-builder just finished a hybrid PAHS home that is stabilizing (first
season) at 64 degrees with no auxiliary heat. It is a partial (60%+) earth
berm with conventional super insulated roof and walk out basement on the south.
It will have insulation on south glazing at night which will probably boost
the interior temperature a few degrees. It will still need a very small amount
of auxiliary heat to attain a comfortable 70-74 degrees (for spoiled Americans).
PAHS is a great idea, but does require close attention to backfilling/berming,
thermal breaks, and insulation placement. In short, it's great if you have
your own backhoe. It?s also somewhat site selective, though the hybrid just
completed is mostly on flat grade. It would also work exceptionally well with
a ferrocement domed home - strong enough to be backfilled and covered over.
There would be no worrying about insulating directly against the shell because
with annual passive heat storage all the insulation goes on much like a large
tapered umbrella after the walls have been backfilled.
We strongly encourage
anyone considering coupling their internal space to the earth to read Hait's
book "Passive Annual Heat Storage, Improving the Design of Earth Shelters
: http://www.rmrc.org/order.htm
It will probably be the most important $47 you spend on your house. This book
really shows you how to think about thermal energy movement and storage, both
for earth shelters and passive solar homes.
Cautions:
Water is one of the best conductors of thermal energy, and is therefore the
enemy of PAHS." If you expose the earth surrounding your foundation to
rain gravitating through the soil, it will rob your earth of any heat it has
been storing: the thermal energy in the soil conducts to the passing cold
water, warming up the water, which then either moves deeper into the earth
or conducts the warmth further from your foundation, leaving the earth under
your foundation cooler as a result." Likewise, if the earth under or
near (within 20' I suspect Hait would say) your foundation has ground water
present, it will quickly conduct away any stored heat, leaving your thermal
mass?? cooler. How deep your groundwater is, and how close it is to your foundation
will definitely affect your ability to store warmth in the earth around your
house." It may be that your situation will not have sufficient dry thermal
mass to do a passive annual heat storage system.
In addition, concern has been expressed about condensation and mold in the
Earth Tubes (see below) in humid climates. Various solutions have been suggested,
by Hait and others.
Earth Tubes
Another important (although not essential) aspect of PAHS is Earth Tubes.
These are pipes going from the hottest (Top Tube) and coldest (Bottom Tube)
areas of the house, down through the earth mass under the umbrella, to emerge
outside. The Tubes are quite long, 100 to 200 ft; the total cross-section
needed will vary with the size of the house, but a typical design would involve
4 4-inch pipes for each Tube (Top and Bottom). Air flows through the Tubes
powered by convection (though in some cases a small fan might be needed),
and this supplies the house with fresh air, conditioned to the temperature
of the earth mass. So the air is cooled in the summer, warmed in the winter.
The fresh air flow guards against Radon, indoor pollutants, and excess moisture/dryness.
Extensions of PAHS:
It would be possible to construct more short-term heat storage areas, for
instance insulated gravel beds or water cisterns. It would also be possible
to use passive or active heat collectors, solar or otherwise, and channel
this heat through the earth mass or a short-term storage area. These options
could increase flexibility, and allow partial use of PAHS under less than
ideal conditions. ?
Further speculation and experiment with extending PAHS would seem to be a
very fruitful area.
This summary was written by Peter Payne based on posts from the Ferrocement discussion group.
______
Branched U-Tube Thermal Distributor (designing an earth-tube installation for a semi-buried home.)
See my nephew's research and discussion page on PAHS and PAHS precursors.
Joe Anderson, PE, through his Prime Design Engineering, has done some interesting thermal modeling for self heating houses. (I think Joe coined the term "self heating house"). Joe found that earth tubes can be minimal and even omitted for much of the USA. I interpret that one can almost remove floor insulation in a super insulated house to achieve self heating, self powered radiant flooring? Through some emails and perhaps unknowingly, Joe helped jog me to invent the square donut design.
Here is a Virginia man who achieves self heating without earth tubes, but does depend on an electrical heat exchanger.
Another North Westerner who has designed and built AGS homes reports on his work, but offers few picture-diagrams on the web.
The content on this website, http://harmoniouspalette.com, is placed in the public domain only as a free exchange of ideas and as a "hard studied wish to serve life". The author assumes no responsibility for the improper use of the concepts in these web pages, as all relevant laws of life and local codes should be verified and observed before any building or experimentation proceeds. discussion is welcome, please write. Bo Atkinson