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Passive annual heat storage
Passive annual heat storage (PAHS) is a building concept theorized to create a year-round constant temperature in an earth shelter by means of passive solar heating and a thermal battery effect lasting several months. It is claimed that an earth shelter designed according to PAHS principles would store the Sun's heat in the summer and release it slowly over the winter months without need for other forms of heating.
The main component of PAHS is an insulated and waterproof "umbrella" which extends out from the earth shelter for several meters in all directions. The earth under this umbrella is kept warm and dry relative to surrounding earth, which is subject to constant daily and seasonal temperature changes. This creates a large heat storage area of earth, effectively a huge thermal mass. Heat is gained via passive solar in the earth shelter and transferred to the surrounding earth by conduction. Thus, when the temperature in the earth shelter dips below the temperature in the surrounding earth, heat will return to the earth shelter. After a time, a stable temperature is reached which is an average of annual heat changes in the external environment.
Another feature of the PAHS system is the use of a coupled heat exchange, earth tube ventilation system which provides cooled fresh air in summer and warmed fresh air in winter.
This method was first described by inventor John Hait in his 1983 book "Passive annual heat storage: improving the design of earth shelters." The book contained a limited degree of scientific evidence regarding the efficacy of the building system such as dynamic heat flow computer simulations and a few example buildings where some of the PAHS principles had been applied. However, as of the 2013 revision, the textbook still does not document a real world example of a PAHS earth shelter designed exactly as prescribed, or data relating to the performance of such a structure in the field. As such, the capabilities of the PAHS system are largely unknown.