If you've ever experienced the coolness of caves, cellars or even large above-ground stone structures such as European cathedrals during the hot summer months, you've exposed yourself to the dynamics underlying the concepts behind passive annual climate control systems. The cooling action you experience in these enclosed environments is a result of the heat being drawn away from your body to the surrounding air which transfers this thermal energy into the surrounding structures whose heat content is less than that of the adjacent air mass. The key concept to remember is that heat always flows from a warmer system (your body in the aforementioned example) to a cooler system (the surrounding air and walls). So if you are "warmer" than the surrounding air, the heat of your body will escape to the surrounding air until a temperature equilibrium is reached. Likewise, if the air inside the room is warmer than the surrounding walls, heat will be drawn out of the air into the walls, thus cooling the air (and warming the walls). Conversely, if the air inside the room is cooler than the surrounding walls, heat will be drawn out of the walls into the air thus warming the air (and cooling the walls). Passive Annual Heat Storage, PAHS for short, (term I believe was originally coined or at least rendered mainstream by John Hait) uses this thermodynamic principal in conjunction with bare earth to help control the climate within a man-made structure. For example, an earth sheltered dwelling will use the surrounding earth to regulate its temperature throughout the year. Such passive systems require an effective heat storage mechanism. Typically, materials with a high heat capacity such as earth or granite are effective at storing heat for a relatively long period of time. Heat capacity is a measure of a material's ability to store heat. This may be a misnomer since materials do not actually "store" heat like a 55 gallon drum stores water. Instead, heat is constantly flowing within the material from a hot region to a cold region until it reaches a seldom reachable equilibrium. What gives the material the apparent ability to store heat is the rate of heat flow. The longer it takes for heat to flow through a material, the longer this energy remains within that material. So in essence, stone walls of the cathedrals in Europe or the earth from bermed homes, slow the transfer of heat to a crawl thus foregoing the release of that energy at a later time. The key in effectively utilizing the storage mechanism of these dense materials is timing. If properly designed, a structure can be made to absorb the heat during the day and released at night for diurnal heat exchange, or, for long term use, heat can be stored during the summer months and released during the winter months. The same concept can be applied to warm regions of the planet whereby the stone walls or soil can be cooled during the night (or winter months) then used to extract heat from the room during the day (or summer months) A good example of the diurnal use of such system is the Iranian wind tower.
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