More on Passive Annual Heat Storage

"ANNUALIZED GEO-SOLAR HEATING" AS A SUSTAINABLE RESIDENTIAL-SCALE
SOLUTION FOR TEMPERATE CLIMATES WITH LESS THAN IDEAL DAILY HEATINGSEASON
SOLAR AVAILABILITY
Don STEPHENS, InA
Private Eco-shelter Design Consultant, Author, Teacher, Mentor and Alternative-construction Innovator,
P.O.B. 1441 Spokane WA 99210-1441 USA www.greenershelter.org , don@greenershelter.com
Vice President and Founding Member, Inland Chapter, Northwest EcoBuilding Guild
Architectural Advisory Board, Spokane Falls Community College, Spokane WA USA
Keywords: residential design, sustainable, annualized solar heating, geo-solar storage, time-lag, perimeter
insulation, AGS
Summary
Although the sun would, at first thought, seem a most sustainable source of cold-season building warmth for temperate climates, in many such regions its actual daily surface availability at those times of year is unreliable, at best. In these places, the traditionally advocated "short-cycle" solar techniques, dependent on DAILY cold-season radiant input through glass and using limited amounts of in-structure mass as storage,
have failed to provide effective solar fractions. This paper discusses a relatively simple and low-cost, but far more reliable alternative and supplemental strategy of solar capture, storage and return for those conditions, based upon ANNUAL, rather than daily, intervals.
In this case the far more predictable and plentiful SUMMER sun is the energy source which is tapped by any of a range of isolated collectors, low-velocity air is the typical transfer medium. Existing earth beneath the structure serves as the storage mass, while also facilitating a predictable and extended time-lag.
As a result,
peak delivery of that energy, up through the floor by conduction, only occurs six months later, when most needed to maintain WINTER warmth.
For sub-structure soil to reach and maintain optimal temperatures (a several year process), it must be buffered from winter's outdoor extremes by sub-grade perimeter insulation extensions and from precipitationtransferred losses by perimeter water-diversion devices.
Also, to maximize the system's efficacy in a given building, exposed portions of its shell must be constructed tightly and with high insulation values. (And to also best address other sustainability goals, such building-materials and insulation need also be chosen
which offer best compromises between the renewable, durable, non-toxic, locally available and/or those plentifully salvageable from the so-called "waste-stream".)
This "technique assemblage" of summer heat collection, sub-structure storage with designed-in time-lag,
winter return, and perimeter restriction of system losses, is what I have come to term "Annualized Geo-Solar" or AGS. It has been progressively developed and tested over more than thirty years in the inland northwestern portion of the United States, and continues to evolve, as knowledge of its methods is now being dispersed world-wide.

For my own part, in the US inland northwest, I was slowly evolving another, and I believe, preferred system, and one better suited to the waterproofed, wood-structured underground homes I'd found more economical to implement, easier for owner-builders to construct, and with far lower eco-impact than concrete.
I sought to provide greatest reliability with minimum initial cost, and with maximum material and design flexibility, without compromising performance.
The basic idea is conveyed by figure 1 below:
Figure 1 Figure 2
The result is a system utilizing sun-heated air as its in-soil distribution medium, passed through inexpensive polyethylene flex pipe.
It takes advantage of an extended and predictable time-lag which naturally occurs
when spring, summer and fall-deposited warmth disperses through defined distances in the dry, substructure earth. This heat, traveling either vertically from deep below the floor, or a similar distance horizontally directly beneath sub-slab insulation, reaches uninsulated conductive floor areas six months later.
(See figure 2 above). Here it then radiates from floor surfaces, up into living spaces, in response to the minimized indoor, cold-season heat-losses through windows and other surfaces. Although it takes several years (the number varying, due to differences in ratios between feasible collector size and output, indoor conditioned cubic footages and overall envelope insulation levels), when the soils do reach optimum temperatures, this system plus incidental indoor activity heat-inputs (from people, lights, cooking, other appliances, etc.) can provide, in winter-cloudy climates, an otherwise unattainable 100% solar heating fraction.
But to be that energy effective (as well as eco-appropriate), the structure must also have a very wellinsulated
envelope of renewable, healthy, recycled and/or salvaged-from-the-waste-stream materials, and
be detailed to minimize infiltration losses, in it's weather-exposed portions. For this, I choose planted-earth
or metal roofs (with strawbale or poured-in rice-hull insulation), and straw-bale, tire-bale, or ricehull-bag walls.
As a result, each design also sequesters, for the life of the structure, a number of tons of carbon. And
windows of highest performance standards, with PVC free-frames, also need effective manual or automatic
night insulation, to reduce heat losses in coldest periods.
Also, as figure 2 suggests, I’ve now adapted this approach to more conventionally-roofed and fully surfacebuilt
homes, as well. In either case, because I rely on the more-controllable, isolated-gain, solar capture
sources (sunspaces, greenhouses, thermosyphon collectors or "tuned" plenum spaces beneath any
exposed metal roof surfaces), windows and unbermed walls can remain under deep overhangs, precluding
the potential for summer over-heating and UV damage to furnishings, which are inherent in some other
approaches. In alternative-building circles and internet discussion groups, this unique solar technique has
come to be called AGS (short for Annualized Geo-Solar).
2. What ANNUALIZED GEO-SOLAR Actually Entails:
Starting with any site with soil of sufficient depth above bedrock or the water-table, and with sufficient midday
summer solar exposure for its chosen collection device, the essential elements of such an AGS design
specifically consist of the following:
2.1 An ISOLATED-GAIN SOLAR HEAT-SOURCE.
While such a source would typically be an air-based solar device (such as the sunspaces, greenhouses,
thermosyphon flat-plate collectors or sub-metal-roof-surface plenum spaces mentioned above), the storage
could, instead, also be charged by any of a range of other choices. These might include an outdoor,
summer-fired wood furnace or pottery kiln, water-filled extraction tubes running through a "hot" compost pile,
directly wind-powered electric resistance coils, etc. It's also possible to divert unwanted warm summer attic
or near-ceiling air into the poly dispersal tubes, thus storing this excess warmth for seasons when it will be
better appreciated, while also reducing or avoiding entirely, the need for costly air conditioning. This heat
source is connected to the...
2.2 INSULATED TRANSFER-DUCT SEGMENTS and UNSULATED HEAT-DEPOSIT TUBE
SEGMENTS
These carry that heated medium (air or whatever), with minimum losses, down into an adequate mass of dry
earth, for storage and time-lagged transmission, before reaching the underside of...
2.3 The CONDUCTIVE FLOOR MATERIAL
(In the Heat-return Zones), this facilitates upward heat transfer and radiates and convects warmth out into
the living spaces above, to replace losses occurring through windows and other perimeter surfaces.
2.4 A planned method of assuring the 6-MONTH CONTROLLED-LAG HEAT RETURN
This is accomplished by making the heat travel a predetermined number of feet (depending on soil type) in
the dry earth (either vertically, between deposit level and the slab above, or horizontally, between a deposit
site directly beneath the insulated center part of the floor and the nearest un-insulated perimeter slab areas,
where it can then conduct upward un-impaired ...(see again the flywheel options diagram above) ...This latter
approach is usually the easiest answer (unless, for other design reasons, deep compacted fill is being
placed beneath the floor, anyway) - no deep ditches / less "diggable" soil depth required.
2.5 Some OUTLET OPTION at the exhaust end of the deposit tube
A solar chimney (for a totally passive flow, where other factors make that possible) or an adjustable lowspeed
extraction fan (can be PV-powered), and a dampered exhaust outlet, or return of the medium to the
isolated heat source for re-warming.
2.6 A perimeter, sub-grade MOISTURE-DIVERSION MEMBRANE/INSULATION CAPE
This extends from the structure's walls to an outer edge a minimum of 20 feet [6.5 meters] away from the
nearest deposit tubes/ducts, to prevent "short-cutting" back to the outside ground surface, instead of coming
up, as wanted, through the floor, and to direct roof and surface rain/snow-melt run-off away, preventing it
from trickling down through the heat-storage and buffer-zone soil and robbing warmth stored there. (I
typically call for a layer of salvaged, used carpet atop that membrane, to protect it during top-soil placement
and planting - this is both a great positive re-use and a carbon sequestration tactic for a major waste-stream
item. The insulation itself can be conventional foamboard or, preferably, one of a range of salvaged
insulating materials.).
2.7 SIMPLE CONTROL SYSTEMS
These regulate when heat-flow to the deposit zone is active and when all exhaust convection is to be
blocked (to prevent the unwanted venting of precious, previously earth-stored heat.)
2.8 A Few Simple THERMAL SENSORS
These can be as simple as the inexpensive auto-supply-store digital thermometers with remote sensors on
thin wires, to be placed down 1" [2.5 cm] pipes...allowing annual monitoring of storage-zone temperatures, to
chart its year to year warming, and eventually, to help determine whether it may be necessary to restrict the
amount of summer heat input, just to prevent possible winter overheating. (Some clients also install a few
moisture-sensor stations in various places in the structure and the earth below, to check with a low-cost
wood moisture-meter from time to time, for informational purposes.)
All this sounds far more complex than it really is, once one develops a sense of how it all comes together.
And the cost of adding such a system can be so small, when compared with the savings it returns over
time...Just an enjoyable sunspace or simple collector (perhaps of salvaged materials), some corrugated
polyethylene pipe as air delivery tubes (avoid toxic PVC), a simple solar chimney (perhaps also an
opportunity for an interesting vertical design element?) or small PV fan to provide/assist flow. And in most
cases, cost of the perimeter insulation cape is more than off-set by savings it facilitates in using much
shallower footings, since frost then never reaches them.
3. Examples of how it can come together
Although the basic ideas of AGS are similar from project to project, they are impacted by floor plan(s) site
and materials to some degree. A couple of recently implemented designs below, show how it can come
together and materials and systems that have been included.
3.1 The Mica Peak Residence (Figure 3):
This 1,600 square foot (148.5 sq.meter) owner-built eco-home is also an opportunity for its occupants to
share with a continuing series of visitors, a wide range of sustainable, salvaged and recycled materials,
techniques and features. It is built into the hillside with an angle-of-slope "vertical crawlspace” behind, with
the roof extending poleward to meet the grade, which eliminates earth pressure against that house-wall,
gives easy access to utilities and provides inexpensive thermally-tempered storage space.
The AGS system is charged by a recessed flat-plate thermosyphon collector, built of salvaged tempered
glass, metal and other materials. The second-story "pilot house" is separable from its open stairs and the
house below, by swing-down glass doors, to prevent heat-losses up the stairs in winter while still providing
natural re-lighting to below. The solar chimney with thermal piston control louvers, behind the pilot-house,
permits totally passive, convective, AGS air-flow. The planted roof-top provides garden space and thermal
buffering above the roof-deck (and doubles the effective energy performance of) the R-35 roof insulation of
14 inch high by 18 inch wide( 35 by 46 cm) straw-bales, fitted between the webs and resting on the flanges
of 18 inch (46 cm) high composite-wood-I-beams at 19.2 inches on center (48.7 cm), with vent space
between the bale tops and underside of the waterproofed deck. The exposure walls are of in-fill straw-bales,
between salvaged-log posts, stuccoed with a mix of onsite sand/clay soil and white Portland cement at a
ratio of 15 to 1 and salvaged windows with track-enclosed baffle night-insulating shades . Other materials of
interest include rammed-earth interior sound-blocking partitions, rammed-earth "cinva" bricks, salvaged
cabinetry and glass block partitions, carpet-over-polysheet-over-earth floors, rice-hull insulation in the shop
roof, tire-bale and earth-rammed-tire retainment walls at shop/garage, artificial annualized-cooling "ice-cave"
walk-in freezer (behind garage into hill) with a high-density load-bearing strawblock (4,500 pounds per linear
foot) separation wall between ice cave and garage, stuccoed salvaged-carpet bank erosion-preventionbarrier,
straw-bale sub grade perimeter insulation (between salvaged-carpet-protected polysheet
membranes), all-fluorescent and natural lighting, salvaged windows and light fixtures, salvaged concrete
rubble patio and walk, stuccoed “earth-bag” wing-walls, separated grey-water, solar-pumped well-water
(gravity-fed from up-hill cistern) and grid-tied PVs. This house is in it's third winter of system operation and
dropped to a low of 65 degrees F. (18.33 C.) when unoccupied for several weeks, and its holdingtemperature
has improved about 5 degrees F.(~2.8 c.) each winter. (site deep-soil/well temp. 52 F..~11.1 C.)
Figure 3 Figure 4
3.2 The Liberty Lake Residence (Figure 4):
This 1,000 square foot (92.7 sq, meter) contractor-built home above a lake is heated by vertical time-lag
AGS and radiant infloor water tubes the heat of which is recovered from residual AGS heat in garage by an
air-to-water heat-pump water heater. It sits on a plinth of salvaged bead-foam and cement insulatedconcrete-
forms (ICFs) surrounding the earth-filled heat-storage vault, which is charged by outside air, preheated
in sunward solar collectors with salvaged aluminum-can absorption surfaces and by the heat at the
top of the sunspace, before being fed up into the under-metal-roof plenum for final heating, then drawn down
through insulated ducts into that earth-vault. The sunspace is glazed with salvaged sliding-door-inserts. A
solar sauna adjoining the main bath also provides clothes-drying (and space heat, if desired). "Bottle-wall"
elements in baths use savaged colored wine bottles to provide obscure natural light as do roof suntubes.
The collector metal is only on the east and south faces of the hip-roof, with planted-earth to go on the west
and north and over the underground garage. Interiors are divided by owner-made bamboo and paper
screens. Floors are finished with owner-made paper from onsite weeds, over red-painted floor slab, sealed
with no-VOC urethane. Permeable paving, green roof, rain-catchment planter, plantings and constructed
vernal pool address 100 % of precipitation on site (site will be fully restored to native plantings and pool will
provide ephemeral breeding habitat for amphibians and insects). Sub-surface insulation, berming and raincatchment
planter are of tire-bales and pet-yard retaining wall is of rammed tires. This summer was this
home's first charging season. (Well temperature is nearly the same as Mica Peak Residence, above).
4. Further AGS enhancements, currently in research:
4.1 Heat-pipes and Water Columns
I'm currently exploring minimally-expensive owner-made heat-pipes and polyethylene-barrel water columns,
installed adjoining perimeter walls, extending down about 5 feet (1.5 meters) to intercept heat moving
beyond the structure (below the level where it would otherwise move up through the floor, and draw it up into
the interior wall surface facing the living space, with insulation shutters to make this controllable, on an aswanted
basis.
4.2 Annualized Cooling and de-humidification
These have been two long-term goals I share with those in several alternative-building discussion groups,
which continue to receive thought and idea-exchange.
5. Conclusions
As my experience and continuing data collection confirm, with well-designed, energy-retaining structures,
and good collector-area to building volume / heat-loss ratios, AGS system performance over the years can
be expected to keep improving. With careful application, by the third to fifth annual cycle, one may
reasonably expect near 100% solar fractions in most temperate climates, with the balance made up by
incidental indoor heat sources (people, appliances, etc.).
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