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32 mm or 40 mm Loops?

Size is not everything

Never more true than in the dimensioning of geothermal probe diameters.
The liquid flowing through the geothermal probes must pick up the heat energy from the surrounding ground as efficiently as possible in order that the heat pump may more easily harvest as much heat as it can.

Several factors can influence the efficiency of the transfer of heat from the ground into the circulating fluid, some of these are,

  • Geology, i.e. physical thermogeological characteristics of the strata surrounding the borehole,
  • Presence or absence of groundwater and groundwater movement.
  • Thermal conductivity of the material (grout) in the borehole surrounding the probes
  • The overall dimensions of the borehole array and depth of the individual boreholes
  • The type of liquid flow within the geothermal pipe or probe.

It is the last of these factors that is the subject of this explanation. The first three factors can be estimated by a geological survey prior to drilling the boreholes.

Liquid flows through a pipe in one of two ways

Linear flow:
at lower velocities, think of the liquid as streams of spaghetti moving in parallel lines along the pipe. This is the most efficient flow in terms of frictional losses, but the most inefficient for heat transfer into the fluid as the streams of spaghetti do not intermingle and promote transfer of heat energy across the area of the pipe.

Turbulent flow: 
at higher velocities where the flow is chaotic, full of eddies across the pipe. This causes more frictional losses in the pipe and results in more energy loss in pumping. It does however increase the efficiency of heat transfer into the fluid across the full section of the pipe from outside the pipe if there is a temperature differential.

The trick in dimensioning geothermal probes is therefore to try to ensure that just enough turbulent flow is achieved, to allow efficient transfer of energy from the surrounding ground into all of the circulating fluid. At the same time not to have such high turbulence that the circulating pump has to expend disproportionate and inefficient amounts of energy just to circulate the fluid.

This is a balance that should be struck by the system designer.

There is some significant science to be applied, involving Reynolds Numbers, viscosities, specific weights, physical dimensions and flow velocities.

The narrower the pipe, the higher the velocity for a given volume and the higher the probability of turbulent flow.

However, too many metres of narrower 32mm pipe can lead to larger frictional losses requiring a more powerful circulation pump, thus wasting energy...

The designer must also bear in mind that the pipe diameter will directly determine the total volume of fluid circulating in the borehole array/system

This calculation process is not usually realistic for smaller installations, neither in terms of time nor cost. Therefore the following general rule can be followed.

Borehole depths below 110m may operate more efficiently at 32mm dia.

Borehole depths above 110m will operate more efficiently at 40mm dia.

At more extreme borehole depths, larger diameters i.e. 50mm beyond 200m may be more efficient.

Borehole depths of 100-120m can be served by both diameters 

IMPORTANT:

It is worth noting that the efficiency of the flow in the pipes and the resulting efficient harvesting of the heat from the ground must be matched by adequate dimensioning of the overall borehole array. Under-dimensioning will result in the pump and array trying to extract more and more heat from the ground which is itself being cooled by the abstraction process. A vicious circle may result with over-cooling, even freezing of the ground and the array resulting in ultimate failure of the system.

MATERIAL:

Geothermal loops should be made of PE100+ material, not PE80

PE100+ material is a superior grade material with a higher purity and durability than ordinary PE100

Geothermal loops should be SDR 11 (The ratio of external diameter to wall thickness is 11) Not SDR17

The loops should be sourced from a reliable supplier.

Synergy loops are pretested and individually certificated.


(Thanks to S. Costello of Geothermal Supplies Ltd., for his contributions to this page)