thermal flywheel home comfort for cloudy winter climates



Annualized Geo SolarAGS vs PAHSStrawblocksOdd BalesRice HullMica Peak AGSLiberty Lake AGSGreenleaf AGS

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.

Here’s a PDF of a paper I wrote recently, for those wanting a bit more detail:
Requested Paper for the Global Sustainable Building Conference 2005, Tokyo, Japan, Sept. 2005


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