as compared to PASSIVE ANNUAL HEAT STORAGE
by Don Stephens
Traditional "Passive Annual Heat Storage" (PAHS), as originally promoted by John Hait, in his 1983 book of that name, is a very simple-sounding, totally passive (meaning without fans, pumps or other mechanically-aided heat movement) DIRECT GAIN solar approach, designed to provide up to 100% of heating needs.
To achieve it, one builds a high-mass, UN-INSULATED (except where exposed to above-ground air-temperature fluctuations), largely earth- covered, usually poured-concrete house shell, with SEVERAL FEET of earth over it and earth against much of the walls. This is then covered with a moisture-barriered insulation "umbrella" over that roof earth and berms and extending out 20 feet (sub-grade) all around. Finally it requires a cap of sod and/or planted earth, or other durable material, protecting the umbrella from ultra-violet degradation and other trauma. With his designs, air exchange and supplemental soil heating are often achieved with thermo-driven passive air-flow through earth tubes.)
Solar heat comes in the windows, SUMMER and winter and is (hopefully) absorbed quickly enough, by the mass of the structure and surrounding earth, to prevent "excessive" overheating of living-space air. In cold "poleward" climates, they tend to work best if there's enough summer solar penetration to raise daytime indoor temps to 75° to 80°F. (24° to 27°C.), which is, unfortunately, warmer than many people enjoy for summer.)
When that solar-heated indoor air is warmer than the surrounding earth (summer and sunny winter days), heat moves through the earth-contact portions of the building shell (by conduction) out into the surrounding soil, to be stored there. In winter and cool spells, as heat is being lost through windows and exposed walls, causing the indoor air temperature to drop below that of the surrounding previously-heated earth, that stored warmth naturally moves (slowly) back through the shell and radiates into the living spaces, maintaining winter temperatures (optimally) up to 65° to 68°F. (18° to 20°C.), again, cooler than many people prefer in winter.
Because of its density, each cubic foot of dry earth holds over 1,000 times as many BTUs of heat, per degree, as a cubic foot of air, so the heated soil provides great reserves of warmth to resist the indoor air's slow cooling.
However, since "Classic PAHS" depends on a major overhead earth mass and rapid indoor-air-to-under-umbrella-earth conduction to prevent living spaces from overheating during solar gain, this approach doesn't really lend itself to building with wood or other more-insulating materials, to foam-sandwiched concrete thin-shells, or to above-grade designs. So, for those wanting to pursue these approaches, the potential for satisfactory performance from Hait's type of PAHS is seriously compromised, at best.
Another kind of passive annual heating technique has been advocated, for homes in milder climates, by members of the Baggs family of architects (and described in their their book, AUSTRALIAN EARTH-COVERED BUILDINGS, copyrighted 1991.) They use a "time-lag, surface-charged" approach derived, in part, from the experience of opal miners living in their excavated tunnels beneath the south-central Australian desert. To accomplish this, they advocate placing at least six feet of dry earth on top of the building. That way, summer sun warming the roof-top earth surface has to migrate downward for six months before passing through the uninsulated concrete ceiling, to warm the living spaces below in winter. (Likewise, winter cools the earth surface and that cooling draws warmth from the house below six months late for summer comfort.)
An advantage of this approach for hot-summer regions is that it allows designs to have overhangs sufficient to completely preclude warm-season direct sun penetration. On the other hand, major drawbacks include the massive structural demands of accommodating that much earth load and the costs of addressing them. And from an environmental perspective, there are major consequences due to the amount of concrete use this implies.
Still a different kind of annualized solar has been explored in England and elsewhere, involving ACTIVE summer heat collection by water-filled solar panels, and the transfer of that heated water into huge, insulated, usually- underground, storage tanks, from which that warmth is recovered six months later by various passive or mechanical means.
The Earth Centre by Zedworks is a fine example of this technique. This kind of "contained" and precisely-measurable system appeals to mechanical engineers, but the initial expense of constructing the huge vault and the often-complex collection and distribution systems can be major barriers.
Annualized Geo-Solar (AGS ), on the other hand, is both simpler and far less expensive. It takes advantage of the extended, predicable time-lag that naturally occurs when deposited heat disperses through defined distances in dry SUB-STRUCTURE earth and the ease with which it then radiates up from a conductive floor.
Since this warmth is typically acquired from ISOLATED GAIN solar- capture sources such as sunspaces, greenhouses, thermosyphon collectors or even plenum space beneath a metal roof surface, problems of unwanted indoor overheating during summer collection periods are avoided. And because the earth-charging transfer medium is usually air, it is more easily controlled and contained. As a result, far greater choices in design and materials are possible without compromising basic system performance.
(I say isolated solar is "typically" the heat source, but one can also use a range of others, such as an outdoor, summer-fired wood-stove or pottery kiln, extraction-tubes in a "hot" compost pile or what-have-you, for input to the system. One could also divert any unwanted attic or near-ceiling build-ups of summer heat into the under-slab dispersal tubes, thus storing this excess warmth for seasons when it will be more appreciated, while reducing or avoiding the need for costly air conditioning.)
The basic elements of an AGS system consist of:
- Any WELL-INSULATED (in the above-grade or shallow-earthed, planted portions) STRUCTURE, designed to minimize heat losses and gains, with a conductive floor that facilitates heat transfer, at least in heat-return zones .
- Some ISOLATED HEAT SOURCE (typically air-based summer solar, although one could use another energy supply and/or transfer media.)
- INSULATED TRANSFER DUCT/PIPE segments to carry the heated medium (air or whatever) from heat source to the dispersal zone earth beneath the house with minimum loss.
- UN-INSULATED DEPOSIT DUCT/TUBE SEGMENTS imbedded in the dispersal zone, where heat is transferred to...
- ...An adequate mass of DRY EARTH for storage and for time-lagged transmission, before moving up through....
- ...the CONDUCTIVE FLOOR MATERIAL and radiating into the living spaces.
- A CONTROL ON unwanted premature HEAT RETURN, either by the time required for it to travel through the VERTICAL DISTANCE between deposit site and the slab above, or by the HORIZONTAL DISTANCE between a deposit site directly beneath insulated areas of floor slab and the nearest un-insulated floor areas where one wants heat to conduct up through the floor. This latter approach is usually the easiest answer (no deep ditches/less diggable soil depth required); typically this means running the dispersal ducts/tubes under the insulated central portion of the floor, so the heat must travel out horizontally for 6 months (about 9'-10') before reaching perimeter uninsulated areas of the slab (the areas above which most of the heat loss through windows, doors and exposed walls also usually occurs.)
- An AIR OUTLET OPTION - either a solar chimney (for a totally passive flow, where other factors make that feasible), an extraction fan (sometimes PV-powered), and dampered exhaust outlet, or return of the medium to the isolated heat source, for rewarming.
- PERIMETER SUBGRADE MOISTURE-DIVERSION/INSULATION CAPE, extending from the structure's outside walls out to about 20'-24' from the deposit tubes/ducts, to prevent heat from short-cutting back outside, instead of coming up through the floor. (This often actually means just a 6' to 8' band of perimeter insulation, since most of that 20'- 24' distance is actually under the house - a major cost savings and landscape benefit not enjoyed with PAHS 20' edge extensions.)
- SIMPLE CONTROL SYSTEMS that regulate when the flow is activated and when all exhaust convection is blocked (to prevent the unwanted venting of precious earth-stored heat.)
- A few SENSOR POINTS to monitor performance and, eventually, determine whether it's necessary to restrict the amount of summer charging, to prevent possible winter over-heating.
All this may sound more complex than PAHS, but it's actually less expensive, more controllable and allows far more design and construction flexibility. And with its potential to meet 100% of your winter heating needs, while keeping you toasty warm in winter and cool in summer, it offers tremendous future freedom and long-term savings on energy bills!