Ground heat exchanger

GHE Analysis uses a high-performance simulation model that considers several factors, including the burial depth of the wellhead, the direction and magnitude of groundwater flow, and the position and thermal interactions between the boreholes of the ground heat exchangers (GHE). To simplify the design of GSHP systems on sites with complex geometries, the software allows users to import a base map and interactively position boreholes on it. Once the map is georeferenced, the coordinates of the wells can be exported directly in the project's Cartesian coordinate system. The software also includes a 3D borehole visualizer that makes it possible to view complex geothermal fields, including those with inclined boreholes.

One distinctive strength of GHE Analysis is its ability to incorporate groundwater flow directly into the thermal simulation. When a Darcy flux is present in the aquifer, flowing groundwater continuously replenishes the thermal energy drawn from or rejected to the formation, attenuating the long-term temperature build-up around the borehole field. Accounting for this effect produces more accurate EFT predictions and, on sites where groundwater is measurable, typically reduces the required total borehole field length — a direct capital cost saving. For large projects where significant groundwater flow may be present, the groundwater flux should be characterized as early as possible in the design process and incorporated into the model.

Base map and GHE layout - To streamline GHE design, the graphic scene includes a drop-down menu that provides quick configuration options for rectangular, hollow, L-shaped, elliptical, and inclined fields. Input boxes then allow you to interactively adjust the number of boreholes along the horizontal (H) and vertical (V) grid directions, as well as the spacing between wells. For elliptical or circular GHE layouts, you can easily adjust the number of boreholes along the periphery (P), as well as the horizontal and vertical diameters of the ellipse.

Move, rotate, or stretch a group of boreholes - Select one or more wells by creating a selection box with the mouse, click and hold the orange area, and then move the rectangle to the desired destination. To rotate the group, click and hold the black dot in the lower-right corner of the rectangle, and then move the point. To stretch the selection rectangle, click and hold the black dot to the right or bottom of the rectangle, and then move the point.

Adding or removing a borehole - To add a single borehole at a specific location, simply double-click the window with the mouse. To remove a well or a group of wells, first select the wells to be deleted, then press the Backspace key.

Undoing a change to the field - Every change to the borehole field can be taken back with the button at the bottom of the panel, or with Ctrl+Z while the drawing area has the focus; and Ctrl+Y put it back. The last twenty changes are kept.

Rotating the base map - To rotate the base map, press the Shift key and use the mouse wheel. Alternatively, you can press Shift+M or Shift+K simultaneously after selecting the base map.

Zooming - To zoom the base map, press the Ctrl key and use the mouse wheel. Alternatively, you can press Ctrl+K or Ctrl+L simultaneously after selecting the base map.

Changing the flow direction - To change the groundwater flow direction, press Shift+Ctrl and use the mouse wheel. Alternatively, you can press Shift+Ctrl+K or Shift+Ctrl+L simultaneously after selecting the base map. Note that rotation is done in 5° increments.

Add a base map - To add a base map, use the button and select an image.

Georeferencing the map - In order to georeference the base map, you must select 4 points on the map and provide the coordinates of each point using the button. Once this step is completed, the blue scale at the bottom of the map will be adjusted automatically.

Coordinates, azimuth and dip of inclined boreholes - To display the coordinates of each borehole in the local georeference system, click the button to show or hide the table of coordinates. You can also copy the coordinates using the dedicated button located below the table. To specify the azimuth (clockwise relative to geographical north) and dip (positive relative to horizontal), double-click the corresponding cell and enter a value. If a borehole is not perfectly vertical, GHE Analysis will automatically use a moving inclined finite line source model. Note that this model is slower, so long simulation times may occur.

3D borehole viewer - To activate the 3D borehole viewer, click the button. This visualization mode is particularly useful when designing GHEs with inclined boreholes, or when illustrating the size and layout of geothermal systems to non-specialists.

Thermal plume viewer - To activate the thermal plume viewer, click the button. Once enabled, a set of additional controls appears above the map: the quantity to contour, the minimum and maximum of the contour range, the number of contour levels, the time at which the plume is shown, and the opacity of the filled bands.

The first control chooses what the contour levels mean:

• Temp. perturbation — the temperature change caused by the operation of the GHE, around an undisturbed ground taken as the zero of the scale.
• Ground Temp. — the same field read as an absolute ground temperature, that is, the perturbation added to the undisturbed ground temperature Tg.

The two are the same computation; only the origin of the contour scale differs. Switching between them shifts the minimum and maximum of the range by Tg, so the levels you had chosen keep their meaning. That computation superposes the hourly load history in time with a very fast Duhamel superposition of the moving infinite line source. The ground model is a moving infinite line source in a uniform medium, with groundwater advection along the direction given by the G arrow and at the Darcy velocity vD. Boreholes are superposed spatially, each carrying its own share of the total load as computed by the g-function. The model does not represent the ground surface, layering, or any thermal interaction with a neighbouring GHE installation. By double-clicking a location on the map, the corresponding temperature at that coordinate will be displayed in the console under the field T(x,y).

The thermal plume viewer is a reading mode: while it is active, the simulation does not run, and everything that would feed it is frozen and greyed out. Deactivate the viewer with the button to edit again — the inputs reopen, and the simulation resumes. Leaving the panel for another page does the same thing on its own.

Saving and animating the view - The button beside the plume button saves the active view — same framing, same zoom, north arrow, scale bar and colour scale included, controls excluded — as a high-resolution image. When the thermal plume viewer is on, the same button turns into and records an animated GIF instead, because a still image only shows one instant of a field that evolves. The animation sweeps the whole simulated period set in the toolbar, in a fixed number of frames.

Active length (H) - The active length of each borehole is the distance from the top of the vertical pipes to the U-loop at the base of the well. This distance does not necessarily correspond to the length drilled on site. For Standing Column Wells, H corresponds to the saturated thickness along the SCW, that is, the groundwater static level minus the elevation of the base of the SCW.

Burial depth (D) - The burial depth is the depth to which the top of the vertical pipes will be installed below the final ground surface.

Ground thermal conductivity (λ_s) - This value is the average thermal conductivity along the well. For large projects, this value should be derived from a thermal response test conducted on site; the methodology for estimating thermal resistance and conductivity from TRT data.

Ground volumetric capacity (C_s) - This property is the product of the density (ρ in kg/m³) and the specific heat capacity (c_p in J/kg/K) expressed in millions (M) of J/m³/K. It is possible, but rather difficult, to deduce this value from a thermal response test. The use of a value taken from the literature is then recommended. The value of C_s is generally between 2.2 and 2.8 MJ/m³/K for most rocks (Waples and Waples, 2004).

Mean surface temperature (T₀) - The mean surface temperature is the average annual temperature observed at ground level. Together with the geothermal heat flux and ground thermal conductivity, this value is used to establish the undisturbed ground temperature between the surface and the base of the boreholes. To easily set the undisturbed ground temperature T_g, set the geothermal flux to 0.0; T₀ and T_g will then be equal.

Geothermal heat flux (q_g) - The geothermal heat flux, measured in milliwatts per square meter, is used to determine the undisturbed ground temperature T_g between the surface and the bottom of boreholes. The resulting undisturbed ground temperature T_g is shown below the input boxes.

Groundwater flow (v_D) - This value corresponds to the groundwater flux (Darcy flux), not the actual groundwater velocity. The input field accepts the base-10 logarithm of the flux in m/s, not the flux itself. Note that using a value of −12 allows the case of zero water flux (v_D = 0 m/s) to be numerically represented. The direction of water flow is illustrated on the map using the blue arrow (G).

Summary & charts: total field length, drilled depth and groundwater flow indicators

The page contains a summary showing the total length of the GHE, the total length drilled assuming that the wellhead starts at a depth D, as well as various values useful for the design of the GHE. Note that the direction of groundwater flow presented in the summary is calculated clockwise relative to the geographic north of the base map.

GHE in practice: ground property accuracy, thermal response tests and field layout strategy

Beyond the ground properties, the field geometry itself is a design lever the a designer controls directly. Wider spacing reduces the long-term thermal interference between boreholes — decisive for strongly unbalanced loads — but demands more land; tighter grids save area at the cost of a field that drifts warmer or cooler over the years if the ground loads are unbalanced. The layout should be coordinated early with the rest of the site (foundations, parking, services and any future construction), since a borehole field is effectively permanent once drilled.

TRT Measurements. Under certain conditions, a 10% error in the undisturbed temperature (T_g) or thermal conductivity (λ_s) can lead to a 10% oversizing of the GHE. For large-scale projects, these values should therefore be derived from a thermal response test carried out on a test well. It should be noted that the test well should be located so that it can be integrated into the final geothermal field.

Groundwater flow. Where groundwater flow has been measured or estimated, it should always be incorporated into the design. The thermal advection from flowing groundwater can substantially reduce the required total borehole field length: the moving water continuously replenishes the thermal energy drawn from or rejected to the ground, attenuating the long-term temperature drift around the field and allowing a more compact layout. If the groundwater flow is uncertain at the design stage, the conservative choice is to ignore it; any flow present in reality will only improve performance beyond the design prediction. Characterising groundwater flow in the field is costly — it generally requires installing piezometers and running pumping tests — so the investigation is seldom justified on small projects. On the other hand, for large geothermal projects where significant groundwater flow is suspected, early characterisation of groundwater flow can be highly cost-effective. Accurate quantification of groundwater flow may substantially reduce the required borehole field size, with drilling cost savings often far exceeding the cost of the investigation.

g-function. The g-function is the dimensionless thermal response function of the borehole field. It quantifies the temperature rise (or drop) at the borehole wall per unit of normalised heat flux applied uniformly to all boreholes, as a function of dimensionless time. Because the g-function is computed from the complete field geometry — borehole positions, active depth, burial depth, spacing — together with the geological and hydrogeological properties of the formation, it fully encodes the long-term thermal behaviour of a specific design. GHE Analysis recalculates the g-function automatically each time a borehole is added, moved, or removed, giving the designer immediate feedback on the thermal consequences of each layout decision. The maximum value reached by the g-function at the end of the design horizon is the single most informative performance indicator for the borehole field. A low maximum g-value means that the field can exchange heat with a smaller average temperature penalty at the borehole wall, which translates directly into a wider EFT margin from the temperature limits, a better seasonal heat pump COP, or a shorter total field length for the same performance target. Designers should therefore aim to minimise the maximum g-function value: wider borehole spacing, distributed or non-rectangular arrangements that reduce long-range thermal interference, and field configurations that exploit the local geology all contribute to flattening the g-function at long times. When groundwater flow is present, advective heat transfer further attenuates the long-term plateau, reinforcing the value of characterising the site hydrogeology early in the design process.