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geoKOAX Technology

geoKOAX, is the proprietary name of a different and innovative collector type developed by the German geoKOAX GmbH.

This is a coaxial vertical collector installed in boreholes of a larger diameter than those required for conventional duplex PE100+ SDR11 collector loops.

geoKOAX, presents a considerable advantage for sites which are restricted in the borehole depths being drilled. This is often the case in some european countries where governmental regulation of restricted borehole depths are imposed with the intention of protecting existing groundwater aquifers existing at known depths.

In the UK there are some areas, for example in the Oxford city areas, where strong artesian pressures are exhibited by some aquifers at relatively shallow depths, this also can inhibit the realistic drilling depths of geothermal boreholes.

In such cases the increased diameter of the geoKOAX probes as described can achieve higher heat harvesting from shallower boreholes.


The collector consists of a semi rigid cylindrical outer tube, within which, centrally located is a concentric smaller diameter tube which provides the return flow,

At intervals down the annulus between out and inner tubes spiral agitator vanes are mounted to give the fluid spiral turbulence. This enhances the transfer of heat from the outer collector wall to the fluid in the central inner tube.

The dimensions of the composite collector are such that a much larger volume (approximately 6.5x) of thermal fluid is present in every meter of collector compared to the current industry standard double 40mm loop collector.

The manufacturers claim that the heat collection efficiency of this probe is considerably greater than that of comparable conventional loops. This allows shallower boreholes to be used and less borehole meterage in total to be drilled (allbeit at a larger diameter). Typical borehole depths for the geoKOAX system are 30 to 100m.

geoKOAX collectors are supplied in stick form and are butt welded on site using special equipment (see below) to produce the required collector length.

Electrofusion Butt Welding Unit

geoKOAX collectors have been more extensively used in mainland Europe where restrictions are sometimes imposed upon maximum closed loop drilling depths to protect drinking water aquifers.

Synergy can offer this new technology which can be of particular advantage in the UK when borehole depths are limited by geological conditions, e.g. strong artesian pressures.

The key advantages of this system are (manufacturers description)


The core of the geoKOAX heat exchanger technology is passive turbulence. The continuous change from laminar flows to turbulent flows increases the heat transfer coefficient by 20% in the case of a 9ft 10in/ 3m probe and by up to 100% in the case of a 32ft 9in/10m probe  


The geoKOAX heat exchanger system features 6.5 times as much carrier fluid volume as conventional Duplex probe systems The coaxial tube is filled with a heat carrier liquid with particularly low viscosity and serves to store heat. This permits a precise release of the required energy at a constant temperature and also longer down-times of the heat pump. 

Contact Surface Area

The geometric concept of the geoKOAX system means that the con- tact surface area for geothermal heat is twice as large as that for conventional Duplex probe systems. As result of the homogenous, smooth coating, the space between the probe and the soil is even. Thus ensures a homogeneous filling of the annular space without any air pockets. 

Low Borehole resistance

The arrangement of multiple pipes, which is common with conventional probes, has proven to be suboptimal for transmission of energy, because the space between the pipes and the soil is uneven. The concentric orientation of the geoKOAX coaxial tube ensures that the space between the coaxial probe and the soil is even, thus leading to significantly lower borehole resistance. This makes for much better absorption of energy.