Ground Model

Overview

The ground model dynamically represents the heat exchange of buried supply and return lines with the surrounding soil. The heat losses or gains depend on laying depth, pipe spacing, soil type and soil moisture; the project’s climate data act as a boundary condition at the ground surface. Optionally, ice formation around the pipe is taken into account. The ground model is one of the model types of the pipe heat-exchange definition, see heat exchange of the routes.

Access

Database dialog of the pipe heat-exchange definitions: in the Heat exchange via pipes tab of the network properties window via Edit… or Assign from DB…, there select Type: Ground model.

Parameters

Parameter	Default	Range	Unit	Meaning
Soil type:	silty loam (Lu)	31 soil types	-	Determines the thermal and hygric properties of the soil (see below).
Laying depth of the pipes:	0.8	0.01 … 20	m	Vertical distance between the ground surface and the top of the pipe. A deeper laying dampens the influence of the outdoor air temperature on the fluid.
Spacing between the pipes:	0.5	0.05 … 20	m	Horizontal distance between the outer diameters of the supply and return lines. A smaller spacing increases the thermal interaction (short circuit) between the two pipes.
Moisture content:	0.25	0.01 … 0.9	m³/m³	Volumetric water content of the soil. A higher moisture content increases the thermal conductivity of the soil (faster heat exchange) and raises the potential for ice formation around the pipe at low fluid temperatures.
Setting the typical moisture content	-	-	-	Button: adopts the moisture content typical for the selected soil type at field capacity (pF = 1.8) from the soil data file.
Consider ice formation	disabled	-	-	Takes the freezing of the soil moisture around the pipe into account (release/absorption of latent heat) at fluid temperatures below 0 °C. Improves accuracy for low-temperature networks, but increases the computational effort and slows down the simulation.

Soil types

The Soil type: drop-down list comprises 31 soil types in the four main groups sand, loam, silt and clay, each with the pedological short codes:

Group	Soil types (short codes)
Sand	pure sand (Ss); weakly/moderately/strongly loamy sand (Sl2, Sl3, Sl4); silty-loamy sand (Slu); weakly/moderately clayey sand (St2, St3); weakly/moderately/strongly silty sand (Su2, Su3, Su4)
Loam	weakly/moderately/strongly sandy loam (Ls2, Ls3, Ls4); weakly/moderately clayey loam (Lt2, Lt3); sandy-clayey loam (Lts); silty loam (Lu)
Silt	pure silt (Uu); sandy-loamy silt (Uls); sandy silt (Us); weakly/moderately/strongly clayey silt (Ut2, Ut3, Ut4)
Clay	pure clay (Tt); loamy clay (Tl); weakly/moderately/strongly silty clay (Tu2, Tu3, Tu4); weakly/moderately/strongly sandy clay (Ts2, Ts3, Ts4)

Each soil type comes with a supplied data file containing the thermal and hygric characteristic functions: the moisture retention curve $\theta_l(\mathrm{pF})$ (created from van Genuchten parameters according to DIN 4220) as well as the moisture-dependent thermal conductivity $\lambda(\theta_l)$ according to Markert et al. 2017, plus the thermal conductivity of the frozen soil $\lambda(\theta_{ice})$. Via these characteristic functions, the set moisture content directly affects the thermal conductivity of the soil.

How the model computes

In the dynamic simulation, a separate numerical soil model is created for each ground-model definition used (and per discretization segment, see below). It computes the transient temperature field in the soil cross section around the pipes:

• Separately laid supply and return lines are jointly represented in one model with two pipes, so that the mutual thermal influence is captured. Twin pipes (supply and return in a shared insulation) are modeled as a single pipe.
• The network model and the soil models run as a co-simulation: the network model passes the heat flux of each pipe in W to the soil model, and the soil model returns the resulting ground temperature as a boundary condition to the network. The coupling time step: is defined in the simulation settings.
• The project’s climate data serve as the boundary condition at the ground surface; the mean annual outdoor air temperature from the climate data is applied as the initial temperature of the ground.
• With the option Discretization in multiple ground models along the network, routes are grouped into separate soil models based on their expected fluid-temperature interval; the fluid temperature interval: controls the fineness of the subdivision. Both options can be found in the simulation settings.

Notes

• The dynamic soil model is only computed in the simulation. In the steady-state calculation, the mean annual outdoor air temperature from the climate data is used instead as an approximation for the undisturbed ground temperature, see heat exchange of the routes.
• The Consider ice formation option is primarily relevant for cold district heating networks with fluid temperatures below 0 °C.

Related topics

• Heat exchange of the routes
• Climate data
• Simulation settings