Free-standing column of stones

for a heat store without fans

David M. Delaney, Ottawa, May 2005

ddelaney@sympatico.ca

keywords: thermal mass, thermal capacity, heat store, thermal closet, solar air heater, passive solar, solar greenhouse, sunspace, gabion, gabions, permeable sides, permeable retainer, bin of stones, rock bed, packed bed, bed of stones, column of stones, forced convection, unforced convection, natural convection, free convection, fan.

An enclosure filled with small stones may store heat from a solar air heater for later use. Warm air from the heater enters an opening in the enclosure, and passes through the stones. As the warm air moves through the cool stones, the stones heat and the air cools. Cool air passes out of the enclosure through a different opening and back to the air heater to be heated again.

If insulated or otherwise impermeable walls provide the only support for the sides of a mass of stones, air enters and leaves the mass only at the top and bottom. Unless the horizontal extent of the mass of stones (and the enclosure) is very large, fans may be needed to drive air in and out of the enclosure. When air can enter and leave the mass of stones at any point on its sides, the needed air movement may be driven entirely by the difference in weight between equal volumes of warm and cool air.

The remainder of this note describes the construction of a free-standing column of stones confined within a wire mesh..

Crushed stone packs too tightly, impeding air flow.
Diameter of stones is 38 mm to 68 mm. (1-1/2" to 2-1/2")

Volume of stones = 5 x 5 x (pi /4) x 8 = 157 ft3 = 4.45 m3
Void ratio = 0.4 (60% of the volume is solid stone).
Density of stone = 2400 kg/m3
Specific heat of stone = 880 J/kg.C

The thermal capacity of this column of stones is therefore
4.45 m3 x 0.6 x 2400 kg x 880 J/kg-C

= 5.64 MJ/C, or 2970 Btu/F

Wire mesh is supplied economically in rolls up to 48" (1219 mm) wide. The following instructions assume this format. The column shown above is 5 feet (1542 mm) in diameter and 8 feet (2438 mm) high. Two 20' (6.1 m) lengths of 48" (1219 mm) wide wire mesh form the compressive skin of the stone column.

This diagram displays a method of constructing a wire mesh cylinder to form the compressive skin of the column.

No overlap is needed between lower and upper horizontal panels. A second layer of wire mesh (see below) provides vertical tension strength.

The 2" OD pipes ("tension pipes") equalize the tension on the horizontal wires of the mesh and reduce stress concentration in the wires of the mesh near the vertical seam.

Draw the tension pipes together to close the cylinder. Bolt them together with U-bolts.

The flaps of mesh extending from the tension pipes to the interior of the cylinder fold back, D, E, against the interior of the cylinder. The flaps may be wired lightly to the outer layer of mesh to facilitate handling during construction, but they are held immobile against the tension of the outer layer of mesh entirely by the frictional forces caused by contact with the tension pipes and by the pressure of the stones filling the cylinder.

The vertical tension strength of the skin of the column can be greatly enhanced by inserting long panels of wire mesh ("vertical tension inserts") against the interior of the outer layer of mesh. The inserts will also greatly increase resistance to local blowouts. They may be wired lightly to the outer mesh for handling convenience during construction, but their resistance to vertical tension is due entirely to their tensile strength and friction between them and the mass of rock pressing them against the outer layer of mesh.

The vertical tension inserts will greatly compensate for any weakness of the outer layer of mesh at the horizontal seam between its two panels, and will oppose bending moments due to lack of straightness in the column, or due to horizontal acceleration of the column. The vertical tension inserts might be elaborated to resist overturning moments in earthquake country, by anchoring their lower ends to the ground.

End