Design / simulation

The professional design / dimensioning of a geothermal source system protects against the consequences of over- (e.g. high investment costs) as well as under-dimensioning (including inefficient operation or even failure of the geothermal system).

It requires detailed knowledge possible about a wide variety of project-specific conditions and requirements to ensure a sustainable and economical operation of the geothermal system. Relevant parameters for such a design are amongst others:

  • geological / hydrogeological and thermo-physical properties of the subsurface (e.g. thermal conductivity, ground temperature, hydraulic soil conductivity)
  • configuration / arrangement of the individual BHEs / energy piles (space or depth limitations)
  • thermal building equipment concept including e.g. heating and / or cooling operation, other heating systems, full load hours
  • quality of the thermal connection of the heat exchanger to the ground (including thermal borehole resistance, which is a possible result of the GRT)
  • specific requirements for the simulation (e.g. simulation time, temperature limits)

The variety of parameters and their different interactions require a computer-aided dimensioning of the geothermal source system. In order to simulate the dynamic storage and transport processes related to ground-based heat exchanger, special computer models and software solutions are used at HSW, for example Earth Energy Designer (EED), EWS, Pilesim, HEAT or Feflow.

Too often, geothermal source systems are designed solely based on soil thermal extraction rates taken from literature or “experience”. This may result is seriously erroneous dimensioning, since the extraction rate of geothermal source systems is influenced by many factors.

Thermo-hydro-dynamic numerical simulation (3D-FEM, FDM)

For complex design cases, such as inhomogeneous geological stratification (and therefore thermal conductivities), strong groundwater dynamics or non-stationary boundary conditions, numerical calculation methods are used. The use of numerical models allows a simulation and prediction of groundwater flow as well as the coupled transport of heat with water and the simulation of complex transport processes.

The thermo- hydro-dynamic 3D simulation with numerical models is state of the art for medium to large borehole heat exchanger, energy pile and groundwater heat pump systems (according to different recommendations and guidelines). It is demanded increasingly often by approving government authorities for the application to water rights permits.

Numerical models have significant advantages against analytical solution methods due to the possibly high temporal and spatial discretization in the assessment of geothermal source systems. Only a few specialized numerical models (e.g. FEFLOW, MODFLOW) take into account both the thermal conductivity in the soil matrix and the heat transport by groundwater.

In practice, numerical calculation methods have proven themselves, inter alia, for the solution of the following typical questions for geothermal plants:

  • forecast of influences on the temperature regime in the ground relative to real estate boundaries ("neighborhood problems") by operation of a geothermal system
  • optimization of complex geothermal source systems, taking into account a lateral heat transport with a local groundwater flow
  • determination of the storage utilization degree ratio of a geothermal plant with seasonal heat storage under groundwater influence
  • forecast of exploration and permission fields for geothermal energy in mining law proceedings

Further information can be found on our thematic website regarding geothermal simulations.

Design of ground heat collectors

The design of ground heat collectors up to about 10 kW is often done according to tabellaric literature values (for example from VDI guideline 4640).

In addition to thermophysical ground parameters (thermal conductivity, heat capacity and pore water content) the regionally different air temperature is one of the most important assessment criteria for ground heat collectors. The standard values for specific heat extraction rates given e.g. in the VDI guideline 4640 are therefore to be regarded critically depending on the geographic location of the planned collector.

For the dimensioning of medium to large soil collector systems and the derivation of design parameters for special applications (e.g. multi-layer trench collectors, collector spirals) we use numerical simulations (FDM-software HEAT) and specialized software called Ground Loop Design. The utilization of numerical models allows the consideration of site-specific ambient temperatures including seasonal changes as well as a discrete layer parameter discretization.

Opportunities for optimization exist due to new technical developments in the field of measuring technology. Thus, the response test for ground heat collectors via the thermal needle system allows an on-site measurement of thermal conductivities inside the level of installation and thereby contributes to an optimal design.

Design of thermo-active building components in contact with earth or water

For the simulation of thermally activated building components such as base plates, diaphragm walls and tunnels, HSW uses building physics software such as HEAT. It allows, inter alia, the consideration of material or construction specific heat transfer resistances and the mapping of heat flows.

In addition, HSW successfully designed thermal activations of water-wetted components such as pontoons, houseboats and sheet pile walls as well as roadways (“geothermal bridge” Berkenthin, “geothermal street”).

back to home page