Seasonal Thermal Store - Low-temperature Seasonal Heat Stores

Low-temperature Seasonal Heat Stores

One of the original motivations of early man's movement into caves was the ability of the earth to even out variations in temperature naturally. At depths of about 20 feet (6m) temperature is naturally “annualised” at a stable year-round temperature equivalent to the Mean Annual Air Temperature above ground.

With the development of modern passive solar building design, during the 1970s and 1980s a number of techniques were developed in the US that enabled thermally and moisture-protected soil to be used as an effective seasonal storage medium for space heating, with direct conduction as the heat return method.

Two basic techniques can be employed:

• In the Passive Annual Heat Storage (PAHS) and similar direct solar gain systems, solar heat is directly captured by the structure's spaces (through windows and other surfaces) in summer and then passively transferred (by conduction) through its floors, walls (and, sometimes, roof) into adjoining thermally buffered soil. It is then passively returned (by conduction and radiation) as those spaces cool in winter. These techniques were advocated in Daniel Geery's 1982 book Solar Greenhouses: Underground and John Hait's 1983 Passive Annual Heat Storage — Improving the Design of Earth Shelters.

• The Annualized Geothermal Solar (AGS) involves the capture of heat by isolated solar gain devices (rather than the building structure). From here it is deposited in the earth (or other storage masses or mediums) adjoining the building using active or passive technology. The depth at which the heat is deposited is calculated (according to soil type) to provide a controlled 6-month heat-return time-lag to the building through conduction as the building cools. This alternative was posed by Don Stephens.

These concepts are compared in greater detail at the Greener Shelter site.