Geothermal energy & heat sources

Overview

These components integrate heat sources into the network – typically in the plant of the energy plant or as the source of a cold district heating network. Borehole heat exchangers and collectors are hydraulically flowed-through components with a ground boundary condition; the source heat exchanger couples any external source to the network via a UA value.

Borehole field

A field of vertical borehole heat exchangers. The model computes the pressure loss from the borehole geometry and the heat exchange against the borehole temperature (temperature of the grout), which is prescribed as a time series – usually from an external borehole-field simulation (e.g. EED) or from measurement data.

Parameter	Unit	Default	Meaning
Borehole depth	m	100	Depth of each borehole
Number of boreholes	–	10	Number of boreholes in the field
Borehole type	–	Two parallel pipes	Two parallel pipes (double-U) or Simple single pipe
Pipe	–	from pipe database	Borehole pipe (diameter, roughness)

The temporal development of the ground temperature over the operating years is not simulated internally – it is contained in the prescribed borehole temperature time series. For long-term considerations (regeneration, cooling-down) this time series must be generated accordingly.

Geothermal collector

A horizontal ground heat collector with one or two pipe layers. Two operating modes:

• Coupled ground model: a finite-volume model of the ground is generated and coupled with the network simulation. The model takes into account the climate boundary condition at the ground surface, heat conduction in the soil as well as ice formation; the soil moisture is assumed to be uniformly distributed. Optionally, edge effects are taken into account – the model then becomes two-dimensional and considerably more computationally intensive. For large collectors or first estimates, the one-dimensional model without edge effects is sufficient.
• Fixed temperature: the ground temperature is prescribed as a constant value or time series (fast, but without feedback of the heat extraction).

Parameter	Unit	Default	Meaning
Collector area	m²	1000	Occupied ground area
Length of one pipe loop	m	100	Length of one pipe circuit; the number of parallel circuits follows from area and pipe spacing
Collector width	m	50	Width of the field (for the edge-effect calculation)
Laying depth	m	1.5	Depth of the (upper) pipe layer
Pipe spacing	m	0.2	Spacing of parallel pipes within a layer
Number of horizontal layers	–	1	1 or 2 pipe layers
Layer spacing	m	1.0	Vertical spacing with two layers
Soil type	–	moderately loamy sand	Soil type according to DIN 4022; determines the thermal material properties
Soil moisture	m³/m³	0.246	Volumetric water content; influences thermal conductivity and ice formation
Edge width	m	10	Width of the co-modeled edge strip (only with edge effects)

The coupled ground model uses the same soil physics as the ground model of the routes.

Source heat exchanger

The source heat exchanger couples an external heat source to the network via a fixed UA value – suitable for waste heat, surface water or a lump-sum represented source:

$\dot Q = UA \cdot (T_{source} - T_{fluid})$

The source temperature $T_{source}$ is prescribed via the Heat exchange tab as a constant value or time series (required). If it lies above the fluid temperature, the network is heated; if it lies below, it is cooled – the component is thus also suitable for representing re-cooling.

Parameter	Unit	Default	Meaning
UA value	W/K	1000	Heat transfer capacity
Nominal volume flow	m³/h	2	Reference volume flow for the pressure loss
Nominal pressure loss	bar	0.5	Pressure loss at the nominal volume flow (quadratic scaling)
Fluid volume	L	3	Fluid volume

Notes

• The collector with a coupled ground model and edge effects is the most computationally intensive component – plan for longer simulation times or, for variant studies, do without the edge effects at first.
• The ground boundary conditions of the routes (pipe heat losses to the ground) are treated independently of this in the chapter Heat losses & ground.

Related topics

• Ground model
• Heat exchange types
• Component reference