Borehole designer

The effective borehole thermal resistance (R_b^*) of a borehole heat exchanger (BHE) significantly influences the heat exchange between the heat carrier fluid and the surrounding geological environment. R_b^* integrates the combined effect of borehole length, circulation flow rate and ground thermal conductivity into a single resistance value; it does not, however, account for the thermal capacity of the borehole components (pipes, grout, or fluid). By optimising the internal geometry of the BHE — pipe diameter, shank spacing, grout conductivity, and flow rate — the designer has direct control over R_b^*: a lower effective resistance reduces the required borehole field length and improves heat pump performance for the full system lifetime at a modest one-time investment.

Borehole heat exchanger type - The current version of GHE Analysis supports the simulation of the following five types of BHEs:

• Single U-Loop — one U-shaped pipe per borehole.
• Double U-Loop (a) — two U-loops with adjacent inlet/outlet pipes.
• Double U-Loop (d) — two U-loops with diagonal inlet/outlet pipes.
• Coaxial — concentric inner and outer pipes. (Intermediate tier and above.)
• Standing Column — a standing column well in center-in configuration. (Intermediate tier and above.)

Borehole diameter (d_b) - In this input box, provide a borehole diameter that can be drilled by local contractors. If the outer diameter of the pipes and their spacing do not correspond to a possible geometric configuration, the value provided in this field could be refused by GHE Analysis.

Pipe outer diameter (d_o) - Along with the pipe inner diameter (d_i), d_o is used to compute the pipe thickness and the pipe thermal resistance (R_p). The diameter and thickness of the pipe should match locally available pipes.

Pipe inner diameter (d_i) - This parameter is used to compute the Reynolds number (Re), which determines if a turbulent flow is fully developed in the pipes. This allows computation of the convective resistance (R_f) for any EFT. The pipe inner diameter is also used to compute the pipe resistance (R_p).

Pipe thermal conductivity (λ_p) - The pipe thermal conductivity is used to evaluate the pipe thermal resistance (R_p). For coaxial BHE, λ_p corresponds to the inner pipe. The most common material for piping is high-density polyethylene (HDPE) and the value provided for λ_p should come from the product data sheet.

Grout thermal conductivity (λ_g) - The grout thermal conductivity (or thermal conductivity of any filling material) is used to evaluate the grout thermal resistance (R_g).

Shank spacing (s) - In this field, the center-to-center distance between pipes must be provided.

Outer pipe outer diameter (d_oo) — coaxial BHE only - Along with d_oi, d_oo is used to compute the pipe thickness and the thermal resistance of the outer pipe (R_p).

Outer pipe inner diameter (d_oi) — coaxial BHE only - This parameter is used to compute the Reynolds number (Re) in the annulus, which determines if a turbulent flow is fully developed between the two pipes of the coaxial BHE. This allows computation of the convective resistance (R_f) for any EFT. Note that the Reynolds number indicated in the interface corresponds to the flow regime in the annulus, since the latter is generally more critical than that in the inner pipe.

Outer pipe thermal conductivity (λ_o) — coaxial BHE only - The outer pipe thermal conductivity is used to evaluate the pipe thermal resistance (R_p). For coaxial BHE, λ_o corresponds to the outer pipe.

Bleed flow rate (V_β) — standing column well only - The bleed flow rate is a key parameter of the efficiency of SCWs and is used to evaluate the borehole effective resistance (R_b^*). When Standing Column is selected, the valid range of certain parameters may be reduced, as the current simulation model supports a narrower parameter range.

Summary & charts: borehole thermal resistance breakdown and flow-rate sensitivity

GHE Analysis provides a summary to easily analyze the various resistances impacting heat transfer in a borehole heat exchanger. To facilitate the comparison of different configurations, the resistances R_f and R_p presented by GHE Analysis correspond to equivalent resistances obtained by dividing a given resistance by the number of pipes in the borehole.

The current version of GHE Analysis also shows the variation of R_b^* as a function of circulation flow rate for the limit temperature in heating mode. The red vertical line corresponds to the total circulation flow rate in heating mode divided by the number of wells (V_A/n_HP) and allows to easily illustrate the impact of the flow regime on R_f. The high values usually present on the left of the graph corresponds to laminar flow in the pipes, a situation that should be avoided during peak periods. If the flow rate per BHE is greater than 120 l/min, the R_b^* value at this flow rate is used for calculations.

Note that R_η is given by R_b^* − R_b. Also, for a coaxial BHE, the R_p value shown in the interface corresponds to the outer pipe resistance.

GHE in practice: minimising R*b through grout selection, pipe spacing and flow regime

A high circulation flow rate generally ensures turbulent flow and low convective resistance R_f. However, beyond a certain threshold, further increasing the circulation flow rate leads to a marginal reduction in R_f, but to a very significant increase in pumping costs and energy consumption. Designers should therefore pay particular attention to not using unnecessarily high pumping rates.

The borehole's internal geometry offers some of the largest controllable gains in R_b^*: thermally enhanced grout and pipes held apart by spacers rather than resting against one another can each shave meaningful resistance and, in turn, shorten the field. Standard bentonite-based grout has a thermal conductivity of 0.7–0.9 W/m·K; thermally enhanced grouts reach 1.5–2.5 W/m·K by incorporating conductive fillers such as high-conductivity sand. This conductivity increase typically reduces R_b^* by 20–40 %, with a roughly proportional reduction in the required borehole field length — a significant capital saving on larger installations where the cost of thermally enhanced grout is small compared to the cost of additional drilling. A lower R_b^* is almost always worth pursuing, since it improves heat transfer for the entire life of the system at a modest one-time cost.

GHE Analysis provides a live design environment for high-performance BHEs: every change to pipe geometry, grout conductivity, or flow rate is instantly reflected in the R_b^* summary and the flow-rate sensitivity chart. This immediate feedback lets the designer compare alternatives — double U-loop versus coaxial, standard versus thermally enhanced grout, or target flow rate versus installed pump capacity — and quantify the benefit of each choice in terms of borehole field length and operating cost before any hole is drilled. Note that R_b^* integrates the borehole length, circulation flow rate, and ground thermal conductivity, but does not account for the thermal capacity of the borehole components — a limitation shared by all quasi-steady-state borehole resistance models.

TRT Measurements. In the field, R_b^* is measured by conducting a thermal response test (TRT) on a test borehole. The R_b^* value obtained from a TRT is valid only for the specific conditions of that test — the circulation flow rate, the heat carrier fluid, and the geometry of the test borehole — and should not be applied directly to a design with different parameters. Because GHE Analysis recalculates R_b^* at every simulation hour as a function of the instantaneous fluid temperature and flow rate, it does not accept a single fixed experimental value from a TRT. The recommended workflow is to first enter the test conditions (borehole geometry, grout conductivity, pipe dimensions, fluid type and flow rate) so that the value computed by GHE Analysis matches the TRT measurement; once agreement is established, the designer can adjust individual parameters — for example increasing grout conductivity or exploring the effect of a higher design flow rate — to optimise the final borehole configuration. Comparing the TRT result with the value computed at the design flow rate can also help validate the grout thermal conductivity and pipe placement assumptions entered in the Borehole Designer.

Pipe placement and spacers. The R_b^* model assumes that pipes are symmetrically positioned inside the borehole with maximum shank spacing. In practice, pipes tend to cluster together once grouting begins unless spacers are systematically installed at regular intervals along the entire borehole depth. A set of pipes resting against one another can easily double the effective borehole resistance, erasing the benefit of a thermally enhanced grout. Specifying spacer installation in the contract documents and verifying it on site are therefore as important as the thermal design itself.

Grout quality and site execution. The thermal conductivity of the grout depends critically on the water-to-solids ratio and on thorough mixing. A poorly mixed or over-diluted grout can deliver a conductivity significantly below its product specification, permanently increasing R_b^* for the life of the installation. Site supervision during grouting is a worthwhile investment, particularly for projects where the borehole field was optimised using a high-performance grout specification.