How larger area should my friend allow for a heat field for a 12kW gshp.

I said pipes not slinkies all 2m deep and 2m apart

found some old spec that suggest 20-30m per 1kW for straight trench depending on gnd type

So far all I have is 3m apart for the trenches, probably have noisy sandy with hogging stones and clay mixed

Great, thanks

make much sense of this lot ? (see above link)

"4.4.2. Types of vertical GHEs
There are two basic types of vertical GHEs or borehole heat exchangers (BHE): U-tube and concentric- (coaxial-) tube system configurations (Figure 7). BHEs are widely used when there is a need to install sufficient heat exchanger capacity under a confined surface area, such as when the earth is rocky close to the surface, or where minimum disruption of the landscape is desired. The U-tube vertical GHE may include one, tens, or even hundreds of boreholes, each containing single or double U-tubes through which heat exchange fluid are circulated. Typical U-tubes have a nominal diameter in the range of 20–40 mm and each borehole is normally 20–200 m deep with a diameter ranging from 100 to 200 mm [17]. Concentric pipes, either in a very simple method with two straight pipes of different diameters or in complex configurations, are commonly used in Europe. The borehole annulus is generally backfilled with some special material (grout) that can prevent contamination of ground-water.
A typical borehole with a single U-tube is illustrated in Figure 8. The required borehole length L can be calculated by steady-state heat transfer equation as follows [13]:

L=q Rgtg−tf
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where q is the heat transfer rate, in kW; tg is the ground temperature, in K; tf is the heat carrier fluid (antifreeze, refrigerant) temperature, in K; Rg is the effective thermal resistance of ground per unit length, in (mK)/kW.
The GHE usually are designed for the worst conditions by considering that these needs to handle three consecutive thermal pulses of various magnitude and duration: yearly average ground load qa for 20 years, the highest monthly ground load qm for 1 month, and the peak hourly load qh for 6 h. The required borehole length to exchange heat at these conditions is given by [18]:

L=qhRb+qaR20a+qmR1m+qhR6htg−(tf+Δtg)
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where Rb is the effective borehole thermal resistance; R20a, R1m, R6h are the effective ground thermal resistances for 20 years, 1 month, and 6 h thermal pulses; Δtg is the increase of temperature because of the long-term interference effect between the borehole and the adjacent boreholes. Alternative methods of computing the thermal borehole resistance are presented by Bernier [18] and Hellström [19]."