This section shows two different ways of modelling a Geothermal Heat Pump:

  • Modelling possibility 1: no regeneration of the ground

  • Modelling possibility 2: full regeneration of the ground

 

- Modelling possibility 1: no regeneration of the ground

In this case the modelling is similar to the Groundwater Heat Pump modelling. In this case, the regeneration potential must be calculated manually, by comparing the quantity of ‘geothermal heat’ imported and the ‘heat to geothermal’ exported.

 

 

Set-up summary

Energy Carriers

Energy Demands

Imports

Exports

Supply technology

Borehole
HP 1

Borehole
HP 2

Borehole
HP 3

Electricity

 

X

 

Primary Input

Primary Input

 

Geothermal heat

 

X

 

Input

 

(primary) Input

Geothermal for cooling

 

X

 

 

Input

 

Heat to geothermal

 

 

 

 

Output

Output

Heat 30-40°C

X

 

 

(primary) Output

 

 

Cooling 10-20°C

X

 

 

 

(primary) Output

(primary) output

 

 

- Modelling possibility 2: full regeneration of the ground

In this case, the geothermal source is modelled as a storage. This means that only what is stored can be used, and that a 100% of the heat used must be regenerated. One of the most common ways to regenerate boreholes (considered in this example), is to have a reversible heat pump and/or to do free cooling.

Let’s consider a case with a cooling as well as a heating demand. A reversible brine water heat pump is installed and has the three following modes (modes settings are detailed below):

 

  • Mode 1: Heating on geothermal (heat pump)

  • Mode 2: Heating on ambient air (heat pump)

  • Mode 3: Cooling (reversible heat pump)

 

Set-up summary

Energy Carriers

Energy Demands

Imports

Exports

Supply technology

Brine water HP 1

Brine water HP 2

Brine water HP 3

HEX waste heat to geothermal

Generic Borehole

Electricity

 

X

 

Primary Input

(primary) Input

Primary Input

 

 

Geothermal heat

 

 

 

Input

 

 

(primary) output

X

Ambient heat

 

X

 

 

Input

 

 

 

Heat 30-40°C

X

 

 

(primary) Output

(primary) Output

 

 

 

Active Cooling

X

 

 

 

 

(primary) output

 

 

Waste heat

 

 

X

 

 

Output

(primary) input

 

 

Set-up Implementation

 

 

 

 

It is not a given that the amount of heat rejected to and drawn from the borehole is the same over the year, in which case, there must be a possibility for the additional waste heat to exit the system / for the heat pump to work on another source of heat.

Therefore, the mode 2 is added (heat pump working on ambient air). This is an example, and any other technology could have been modelled. In the case of waste heat, this is the reason why an export of waste heat out of the system must be modelled.

 

A heat exchanger is modelled additionally to use the waste heat of the reversible heat pump to regenerate the ground. While the mode 1 of the heat pump draws heat from the borehole storage, the mode 3 combined with the HEX, allows to regenerate the borehole

 

 

Finally, the storage (storing the energy carrier ‘geothermal heat’) must be added. The maximum charging / discharging rate of the storage is a crucial parameter impacting the capacity of the system when considering geothermal systems. It can be approximated through the specific extraction potential in W/m, the borehole length, and the full load hours.

The charging and discharging efficiencies must be set to 100%. Indeed, as the storage technology has the possibility to charge and discharge at the same time, this may result in losses. If the borehole is fully charged, this principle may be used to ‘unload’ the system (by charging and discharging high quantities of energy).

 

e.g. based on data available on the Geoportal of Berlin:

For 1 x 100m Borehole with 1’800 full load hours and a specific capacity of the ground of 40 W/m:

→ 40 W/m * 100 m * 1’800 h = 7’200 kWh = 7.2 MWh

→ Discharge capacity = 4 kW / 7200 kWh = 0.00055 = 0.055 %/h

 

 

It should be noted that the storage cannot be forced

If there is the possibility, it will be bypassed. In this example, it means that if there is a cooling production (and hence a waste heat production) at the same time where there is a heating demand, the brine-water heat pump will directly use the waste heat.