Size & simulate ground heat collectors

Cold district heating, part 6: size ground heat collectors as a heat source and simulate them in the network

Overview ▶ 0:10

Cold district heating network with ground heat collector as heat source — Starting point: sizing and simulating a ground heat collector field as the heat source for the cold district heating network

This tutorial demonstrates how to size a ground heat collector field as a heat source and simulate it over two years. Based on the results, it can be assessed whether the system works and whether the minimum temperatures are met.

Replacing the probe field with a collector ▶ 0:59

System editor: probe field is replaced with ground heat collector — Replacing the probe field with a ground heat collector in the system editor

Starting from the example project with a probe field:

Open the energy plant’s system in the system editor.
Remove the existing probe field.
Drag the ground heat collector into the system.
Reconnect the elements.

Parameterizing the collector ▶ 1:18

When you click the collector, the following options are available:

Ground model ▶ 1:25

Selection of the coupled ground model for the collector — Selection of the coupled ground model with a full geothermal simulation in the background

The coupled ground model is selected as the model. A complete geothermal simulation is then carried out in the background. Alternatively, a fixed temperature can be prescribed.

Soil type ▶ 1:43

Selection of the soil type with standard soil types and moisture settings — Selection of the soil type from the standard soil types with automatic moisture setting

Select the soil type matching the site from the standard soil types. If a soil report is available, the parameters can be entered more precisely. For each soil type, a typical moisture content is automatically set that can be adjusted if the actual moisture content is known.

Burial depth and geometry ▶ 2:21

Settings for burial depth, ground depth, and collector geometry — Geometry settings: burial depth (1.5 m), simulated ground depth, and optional two-layer collector

Burial depth: e.g. 1.5 m
Below that, 30 m of ground is simulated to ensure undisturbed conditions
Optional: Two-layer collector (single-layer in this example)
Optional: account for an edge zone (for the first simulation we recommend simulating without an edge zone; for a more detailed analysis, the collector width and edge-zone width can be specified to run a 2D simulation with additional heat gains from the edge zone)

Pipe loops ▶ 3:35

Pipe loop parameters with pipe dimension, pipe spacing, and loop length — Configuration of the pipe loops: 25 mm pipe, 20 cm spacing, 100 m length per loop

Pipe dimension: e.g. 25 mm pipe
Pipe spacing: e.g. 20 cm
Length per pipe loop: e.g. 100 m (typical length)

The number of parallel pipe loops is automatically calculated to cover the specified collector area.

Sizing the collector area ▶ 4:18

Pre-sizing dialog with heat demand, seasonal performance factor, and calculated collector area — Pre-sizing the collector area: heat demand, estimated heat gains, and calculated area (4,490 m2)

The collector area must be specified for the simulation. A pre-sizing helper dialog is available for this:

Click the sizing button.
The total heat demand of the consumers is determined automatically (e.g. 460 MWh/a).
A seasonal performance factor is assumed (for pre-sizing only).
Heat gains of the cold network are estimated (e.g. 120 kWh/m with a 500 m network length).
The coverage share of the collector is set (e.g. 100 %).
The estimated collector area is calculated (e.g. 4,490 m²).

Accept the value and start the simulation to verify the result.

Running the simulation ▶ 6:37

Simulation settings with climate data and 2 years of simulation duration — Starting the simulation: climate data, at least 2 years of simulation duration with geothermal coupling

Confirm the settings.
Select the climate data file for the site.
Set the simulation duration to at least 2 years (the first year serves as settling phase, the second year is evaluated).
Start the simulation.

Note on runtime: A thermo-hydraulic simulation coupled with a geothermal simulation can take half an hour to an hour or more, depending on project size. With particularly high heat extraction rates, 3 simulation years can also be reasonable.

Evaluating the results ▶ 7:57

Energy plant (collector field) ▶ 8:04

Results of the collector field with heat gains and ground temperature — Results of the collector field: heat gains, ground temperature with ice-formation effect, and inlet/outlet temperatures

The results of the collector field show:

Heat gains of the collector field
Ground temperature: In winter, a horizontal course is visible, caused by ice formation (accounted for in the simulation)
Inlet and outlet temperatures of the collector field

Checking the consumers ▶ 8:47

Temperatures at the consumers with minimum inlet temperature of about -2.6 °C — Temperatures at the consumers: minimum inlet temperature (approx. -2.6 °C) for evaluating system function

Critical for evaluating the system are the temperatures at the consumers:

Temperature difference: Should meet the configured 3 Kelvin (in summer the value fluctuates as the heat pumps switch off)
Inlet temperature into the heat pumps: e.g. minimum of approx. -2.6 °C

Based on the minimum inlet temperature, it can be judged whether the system will function under real-world conditions and whether the collector area has been adequately sized.