Sizing tool — GHE Analysis user manual

Sizing tool

Sizing tools have historically been widely used to determine the number and active length of the boreholes of a GSHP system. GHE Analysis allows you to easily size the GHE using ASHRAE's alternative method. The approach is particularly useful because it allows to easily determine the elements influencing the total length of the GHE and to modify them in order to reduce the size of the field. The active length H provided by the tool is therefore not used to simulate the GSHP system and calculate the values presented in the console.

GHE Analysis — Sizing tool panel — ASHRAE alternative method output showing required borehole active length broken down by pulse contribution
Sizing tool panel — ASHRAE alternative method output showing required borehole active length broken down by pulse contribution

Duration of yearly pulse (ty) - This is the period over which the average annual ground load is applied and should match the simulation duration. It captures the slow drift of the ground temperature caused by any imbalance between the heat injected and extracted over the years.

Duration of hourly pulse (th) - This is the duration of the peak-load block used to size for the worst case — typically a few hours (e.g. 4 to 6 h).

Summary & charts: ASHRAE three-pulse field length broken down by thermal contribution

The page contains a summary of the values useful for calculating the total length of the field by ASHRAE's alternative method. The charts represent the fraction of the total length as a function of the following four elements:

  1. The first column is mainly associated with the annual pulse qy. For balanced ground loads, this value is generally low. Otherwise, one should attempt to balance the ground loads or modify the geometry of the GHE.
  2. The second column is a function of the monthly response factor (Rgm) and the average monthly load (qm) of the month containing the thermal peak.
  3. The third column is dictated by the hourly peak and by the hourly response factor (Rgh).
  4. The last column is used to isolate the influence of Rb* on the total length of the GHE.

Read together, the four columns tell you why a field is the size it is — and therefore where to act. A tall first column points to an unbalanced load that hybridisation or load-side measures could correct; a tall last column points to a borehole resistance worth improving before adding length.

GHE in practice: using the sizing result as a starting point for full EFT simulation

It is important to keep in mind that the sizing tool assumes that EFT never exceeds TLim, which is not the case with the simulation model of GHE Analysis. Furthermore, two different lengths are calculated, one for the cooling mode and one for the heating mode. The longest value is usually a better guide of the length to use with GHE Analysis.

Treat the sizing tool as a fast, transparent starting point rather than the final word: it sizes for a temperature limit that is never exceeded, whereas the full simulation lets EFT move freely and reports what actually happens. A sound workflow is to size here, transfer the longer of the two lengths into the field layout, then confirm — and fine-tune — with the simulation, where load balancing, borehole resistance and field geometry can all be weighed against cost.