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Capturing Heat While the Sun Shines,
to Warm Your Home Next Winter

(Annualized Geo-Solar Design)
By Don Stephens

In the Inland Northwest and other parts of the world where winter sun is
unpredictable, often cloud-obscured for days or even weeks at a time, the
conventional kinds of passive solar designs which have gotten so much
press in recent years can prove most disappointing. The typical direct-
gain solar house, for example, depends on DAILY recharge to carry it
comfortably through cold winter nights. And when sunshine fails to
materialize, avoiding chilly indoor temperatures means turning to a
back-up system, often by buying expensive and/or fossil-based power or
fuel.
More years back than I care to discuss, we were taught in grade school that
the oil and other geo-fuels, on which we had come to depend, were in
finite supply and faced depletion in our lifetimes. So while studying
architecture at the University of Idaho over forty years ago, I was
already conceiving solar and earth-sheltered designs to anticipate this
shortfall and the rising energy costs that would accompany it.
A number of solar strategies were developed in the 1960s and 70s for
Colorado and the southwest, which depended on their winter climate, with
its predictable daily deliveries of clear skies and full sun. But having
lived most of my life in northern Idaho and eastern Washington, where
below-zero winter nights and cloudy spells of a week or more at a time are
common, I soon realized that what worked there just wouldn't cut it in our
more demanding climate.

Over the last few years I've developed a very different solar heating
strategy based on capturing plentiful summer sunshine and storing it's
heat directly in the earth beneath the energy-efficient homes I design.
 >From there, it passively rises in winter through floor surfaces to
counteract the minimal remaining building-skin heat losses. Because most
of this usable heat remains in the earth's thermal flywheel for over six
months before providing benefit, I refer to this evolving technique as
Annualized Geo-Solar or AGS.

The first requirement for success with such an approach is a site which
receives ample sun in the summer. Fortunately this is far more likely
with the sun nearly overhead than in the winter when its low angle casts
long shadows from trees and surrounding topography.

The second need is soil of sufficient depth, ideally 6' or more, above
bedrock or the water table (although this storage mass can be built up in
various ways, if initially insufficient.)

Given such a site, the next challenge is to design a structure which will
not only meet the clients' needs and wishes within an affordable budget,
but also minimize the amount and rate of winter heat losses. This means
high wall and roof insulation levels as well as quality, energy-efficient
windows and doors. Because I have always been concerned with preserving
our natural environment and the health and well-being of my clients, I
limit my services to the design of non-toxic, environmentally-friendly
homes. This entails working within a palette of natural, local, durable
and/or recycled, often-"alternative" materials.

Thus I've become experienced in engineering and detailing with straw-
bales, "salvaged" and sustainably-harvested wood and standing-dead logs,
cob, adobe, rammed earth and soil/cement, earth-bags, used tires, recycled
glass, foam/cement bags and planks (like Rastra), as well as earth berming
and sodded roofs to attain the necessary high R-values, minimize life
cycle monetary and eco-system costs and visually blend structure with
site. I also like to use fire-resistant materials, such as earth-based
stuccos, cement/wood composites (like Hardyboard), insulating shutters and
metal roofing on exposed surfaces, particularly in potential wildfire
areas, so that the building itself is resistant and its natural setting
need not be compromised to accommodate typical-structure shortcomings.

At the same time, I'm visualizing how the structural system will work and
how the elements of annualized geo-solar will be incorporated. For heat
capture, I feel it's best to avoid depending on the "direct-gain" method
of bringing summer sun into the building itself through the windows as
this tends to present issues of both overheating and UV damage to
furnishings. Instead, "isolated-gain" devices like greenhouses and
sunspaces, as well as ground or roof-mounted flat-plate collectors capture
the required BTUs without such problems. In some cases, I also use
exposed metal roof surfaces themselves, with air flow beneath, as a heat
source. (And, soon, with a PV electricity-generating coating on top.)

Where possible, I like to design the flow from collector to earth mass to
occur passively by natural convection and solar chimneys, but where
circumstances don't support this, one or more fans, powered by small
photovoltaic units can give an assist whenever the sun shines and sensors
indicate heat is available. To maximize heat transfer, the air should not
travel too fast through the earth tubes running under the building, so
little fan power is needed.

Heat travels through dry earth at predictable rates per month, so the two-
season lag is designed in by the placement of the tubes relative to
uninsulated areas of floor near the home's above-grade perimeters. These
"exposure walls" are the high-loss interface between house and weather,
where windows alone can claim a majority of unprevented heat losses.

It's also essential to thwart "short-cut" losses between the under-floor
heated earth mass and the cold ground beyond and to prevent rain water
run-off and snow-melt from absorbing stored heat and carrying it down to
the water table. I accomplish this by calling for a sub-grade water-
diversion/insulation "cape" extending out around the building by 8 to 20
feet, depending on the configuration of the design.

This allows heat to build up under the building itself over a period of
years, protected by the surrounding buffer zone, so performance improves
over time. My instrumented thermal data and research experience show that
with proper collector size and an initial deep soil temperature in the 50
degree range (and IF the home were to remain otherwise unheated and
unoccupied) sun deposits the first summer would keep a properly insulated
interior above fifty-five degrees all the first winter of operation.
This can be anticipated to be over sixty the second year and nearing
seventy by the third.

What this means for the occupied home is that only enough fuel would be
required the first winter to heat it about 10 degrees, supplemented by
contributory warmth from people, pets, cooking, refer, hot water use and
lights. The second year would cut that heating energy demand by half and
in the third winter, the home should hover near 70 degrees with no
supplemental heat at all!

And the capital cost of such a system can be small - a heat source (a
greenhouse or sun space enjoyable in so many other ways or a simple flat
plate collector or the metal roof of the house itself), some 4" to 6" air
tubes in the ground, a solar chimney or small PV fan, and some sub-grade
perimeter insulation.

On the hillside project shown here, the owners made their own collector
out of re-used glass, some insulation, some used metal roofing and some
bargain concrete block. The pipe was bought at auction for 10 cents on
the dollar. Straw-bales, plastic sheeting and used carpet for protection
made the cape. The solar chimney required little in materials, they did
the labor themselves and the system needs no fan, as it flows by natural
convection. On a hot summer day air leaves the collector at over 160
degrees and after passing through the tubes, comes out near ambient!

It's all so simple and it works well in the winter sun-starved climates
where the other approaches don't.
DON STEPHENS, (509) 838-8222
Eco-home design consultant and construction innovator

www.greenershelter.org
P.O.Drawer 1441, Spokane WA 99210-1441

 









 

 

 

 

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