# Geothermal energy in nPro

Note: Not all parts of the calculation logic shown correspond to the latest software version. However, most of the technical description is still applicable.

## Estimation of the length of geothermal probes

How much heat a borehole heat exchanger can extract from the ground depends on several factors. These include the **thermal conductivity** of the ground, the number of **operating hours** and, if applicable, the **influence of neighboring probes**. The higher the thermal conductivity of the soil, the more heat can flow to the probe from the surrounding area and the more heat can be extracted from the subsurface. In addition, **groundwater flow** can increase heat conduction in the ground. Ideally, the distance between two neighboring geothermal probes should **not be less than 7 m** (minimum: 5 m).

Ground type | 1800 Full load hours | 2400 Full load hours |
---|---|---|

Gravel, sand, dry | < 25 W/m | < 20 W/m |

Gravel, sand, water-bearing | 65-80 W/m | 55-65 W/m |

Clay, loam, moist | 35-50 W/m | 30-40 W/m |

Boulder clay | 45 W/m | 45 W/m |

## Estimation of the area of the ground collector

Geothermal collectors are plastic pipes that are laid directly under the earth's surface and extract heat from the ground in the area. The collectors are laid **below the frost line** at a **depth of approx. 1.2 to 1.5 m**. Ground collectors indirectly use the **solar radiation** on the ground as well as **heat from the ambient air**, which penetrates through heat conduction or through precipitation into the ground. For optimal utilization, areas with geothermal collectors **should not be built over**, so that as much solar radiation as possible as well as precipitation hits the area. For residential buildings, about 1.5 to 2 times the living area to be heated is required for heating with a heat pump. For each **kW of heating capacity, around 15 to 30 m² of collector area** is required (depending on the soil conditions).

Ground type | 1800 Full load hours | 2400 Full load hours |
---|---|---|

Dry, non-cohesive soil | 10 W/m² | 8 W/m² |

Cohesive soil, moist | 20-30 W/m² | 16-24 W/m² |

Water saturated sand/gravel | 40 W/m² | 32 W/m² |

## Geothermal model for heating and cooling

The geothermal model can be used as both a heat source and heat sink (cooling source). The energy flows of the model are shown in Figure 1.
In the upper figure, the energy flows are shown for the case when both a heat pump and a chiller are considered. This case is given,
for example, when a temperature of 40 °C is required to meet heat demands and a temperature of 2 °C is required to meet cooling demands,
but at the same time the temperature of the geothermal source is 8 °C. When using a heat pump and a chiller,
the **thermal extraction power** determined in the design calculation results
in
$$Q_\mathrm{ground} = \max \left(\max(Q_\mathrm{HP,evap,t}), \max(Q_\mathrm{CH,cond,t})\right)$$
Here, \(Q_\mathrm{HP,evap,t}\) is the annual heat profile at the evaporator of the heat pump and
\(Q_\mathrm{CH,cond,t}\) the annual heat profile at the condenser of the chiller.
The heat flow at the evaporator of the heat pump is
$$Q_\mathrm{HP,evap,t} = Q_\mathrm{HP,cond,t} \cdot (1-\frac{1}{COP_\mathrm{HP,t}})$$
and the heat flow at the condenser of the chiller is
$$Q_\mathrm{CH,cond,t} = Q_\mathrm{CH,evap,t} \cdot (1+\frac{1}{COP_\mathrm{CH,t}})$$

## Can a seasonal storage operation be modeled in the geothermal model?

In the geothermal model, a further model boundary condition can be defined, which allows to represent a seasonal storage operation.
This can be used, for example, for **aquifer storages** or to model **ice storages**. If heat and cold utilization is enabled in the model,
the ratio of heat extracted annually from the ground to heat injected annually into the ground can be defined by the **storage cycle
efficiency** \(\eta\):
$$\sum_t Q_\mathrm{HP,evap,t} = \eta \cdot \sum_t Q_\mathrm{CH,cond,t} $$
For aquifer storages, \(\eta\) = 70% is a commonly used assumption.

## This might also interest you

## Sources

- Planungshandbuch Wärmepumpen (Viessmann)
- Leitfaden zur geothermischen Nutzung des oberflächennahen Untergrundes
- Guideline VDI 4640: Thermische Nutzung des Untergrunds

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