Theory

Extraction of heat from the soil can optionally be included in the model, as determined by the switch Heat pump in section “Soil Heat Flow”. Soil heat extraction rate from a specified layer, z_nhp, can be given as measured time series but may also be given as a function of air temperature according to governing rules for commercially available soil heat pump equipment:

                   (1.40)

where s_h1 is a constant heat extraction required for hot water purposes, s_h2 is a design parameter in the air temperature dependence and T_hpmax is the threshold temperature for the maximum heat extraction rate. See viewing function Heat pump extraction.

When the soil temperature drops below T_hpcut the extraction rate will be reduced according to

                                       (1.41)

where T_hp0 is the temperature at which the heat extraction reaches ceases. See viewing function Reduction of heat extraction.