The required probe length $ l_{target} $ in accordance with VDI 4640 is calculated from aus
$$l_{soll} = \frac{P_{WP; Kälte}}{ \dot{q}_{l} }\qquad(1)$$

with
$ P_{WP,Kälte} $ = cooling capacity of the heat pump [W]

$\dot{q}_{l} $ = specific extraction power [W/m]

The specific extraction power is calculated from the density, heat
capacity and heat conductivity of the ground as a model.

#### Sizing the total drilling length

The total drilling length is correctly sized when the length of all
boreholes deviate from the required probe length by less than one
percent.

Effective for the target length is equation (1) and for the actual length
$$ l_{ist} = n_{Bohrungen} * t_{jeBohrung} \qquad(2)$$

wherein

$ n_{Bohrungen} $ = number of boreholes

$ t_{jeBohrung} $ = depth per borehole

For the relative deviation f, it applies that
$$f = \frac{|l_{soll} - l_{ist}|}{l_{soll}} \qquad(3)$$

Correct sizing exists if $f$ < 1% $(4)$

Undersizing exists if $l_{ist}$ < $l_{soll}$ ∧ $f$ >1% $(5)$

Oversizing exists if $l_{ist}$ < $l_{soll}$ ∧ $f$ >1% $(6)$

#### Sizing with maximum drilling depth

In addition to sizing the total drilling length in accordance with
equations 1 and 4, the maximum drilling depth
$ t_{max} $ set by you is also
taken into consideration.

The following applies for the maximum drilling depth
$ t_{max} $:

Drilling depth |
Significance |

400 m |
Up to 400 m is known as near-surface geothermal power, this is consequently also the maximum value which can be entered in GeoT*SOL®. |

99 m |
In Germany, **mining law** applies from 100 m drilling depth, requiring additional licensing procedures. \ Developers frequently avoid these procedures or want to use the boreholes for cooling in summer, with the result that they do not drill deeper. 99 m is therefore the standard setting in GeoT*SOL®. Deeper drilling is frequently encountered in Switzerland. |

< 99 m |
For certain geological formations, the authorities sometimes only approve boreholes considerably shorter than 99 m. It may also be the case that, for example, the conductivity of the ground is very poor from 80 m and it is therefore not worth drilling deeper. |