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Geothermal heat exchangers out of composite pipe for heating and cooling purposes

Published at: Sep 11, 2011
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Cooling with heat pump and a ground probe field is an increasing market requirement that gain a wide acceptance and application. The ground field probe is dimensioned to cover the necessary services for cooling and heating.

What is often underestimated is the required energy, which is needed to cover the cooling demand. Depending on requirements, the needed account of energy for cooling demands can reach 50% or more of the entire energy! Since this is a complex realization, in which the geometry of the ground probe field, the drilling depths, the type of the ground probe and the calculation of the impact of heat storage play a decisive role, it is very wise to plan an experienced engineering company in such a realization. The details of such implementation decide about the efficiency and long-term investment protection of such a project.

 

Preface

Cooling with heat pump and a ground probe field is an increasing market requirement that gain a wide acceptance and application. The ground field probe is dimensioned to cover the necessary services for cooling and heating.

What is often underestimated is the required energy, which is needed to cover the cooling demand. Depending on requirements, the needed account of energy for cooling demands can reach 50% or more of the entire energy! Since this is a complex realization, in which the geometry of the ground probe field, the drilling depths, the type of the ground probe and the calculation of the impact of heat storage play a decisive role, it is very wise to plan an experienced engineering company in such a realization. The details of such implementation decide about the efficiency and long-term investment protection of such a project.

 

1. History

In the past 20 years, heat pumps experienced evolutions in great steps. Not only in terms of the technology contained therein, but also in the application and in longevity. Also, the drilling technique gets specialized in their range of applications and list a lot of improvements and optimizations.

The usage of ground probes is written down in many countries in guidelines and standards are a guide to protect the drinking water and the investment of customers, which have opted for a solution driven by shallow geothermal energy.

The material of the probe has experienced less development in recent decades, compared to the leaps in development of heat pumps and the drilling technique. There was a development to PE 100 and PE-X, which promises improved properties. There are also ground probe manufacturer, which manufactures ground probes out ​​of metal, but given to logistics and problems such as corrosion (stainless steel corrodes, albeit slower than normal steel) those ground probes didn’t find a wide application. With additional applications such as cooling, comes to the previously used ground probe material a requirement where the thermal characteristic of the material decides about efficiency and longevity of the ground probe field. Metals such as stainless steel would meet the thermal requirements, however, ground probes out of stainless steel for a drilling depth of 50 m would require an expensive logistical performance. Application area and the chemical composition of the surrounding medium determine how quickly or slowly stainless steel will rust, so there is no general composition of stainless steel, which can be used in any geology. Despite the problems mentioned here, stainless steel or metal alloy has certainly not yet reached the zenith of its potential applications in shallow geothermal solutions.

 

2. Heating and cooling with the same borehole

If customer demands such as heating and cooling are to be realized in a ground probe field, the customer can enjoy having a very efficient system. The heat stored in summer in the geology will result in an increasing efficiency of his heat pump during autumn and winter and a reduced amortization of his investment.

Quite apart from this encouraging news, the efficiency of such a ground probe field depends from the smart bridging of some technical conflicts.

In general, a ground probe made ​​of PE 100 is grouted (in Germany). Commercially available products are used for grouting, which mixed with water results in a relatively fluid suspension of cement, clay minerals, sand and water. The cured borehole is used to seal the borehole and seal pierced groundwater storeys against each other. Another task of the cured suspension is to store and release water - this task is handled by the clay constituents of the suspension. This property describe some suspension manufacturer with the function of a sponge, which also owns the property to store water, but otherwise the comparison with the sponge stops with this property. The hardened suspension has no elastic properties as the sponge! Some manufacturers mention a plastic property of their products. A Plastic property describes, under certain circumstances, a deformation of the material under pressure maintaining the deformation obtained by the pressure. But we don’t have a elasticity of the cured suspension.

Why is the consideration of these properties so important?

If we introduce het in the soil and extract the heat later, then we have differences in temperature of about 20 K, depending on country or application even more. Since we have connected permanently different materials in the borehole, it is relevant to now how well those materials cooperate thermally with each other.

Under thermal influence PE 100 expands about 18 times more than cement.

Under thermal influence PE-X expands about 13 times more than cement.

What does that mean?

- Therefore, the pipes that expands more than the hardened suspension, they exert a pressure on the hardened suspension. In this case, the suspension can get cracking. The hardened suspension can crumble, thereby downgrading the thermal transition and the sealing of groundwater storeys is not anymore guaranteed.

- Or the ground probe tube is damaged because the hardened suspension doesn’t drawback as the tube expand.

- Or the ground probe tube and the hardened suspension get damaged and will be limited in their functions.

- If the hardened suspension will experience only a few cracks because it have drawback, we have a gap formed as a ring around the ground probe tube, which decrease the thermal transition. Water penetrates through cracks in the ring gap and connects the groundwater storeys. When we face ice in this borehole, this can lead to further damage.

In countries/regions in which grouting is not required, the risks are not minimized, because in a borehole with granular soil around the ground probe tube damages to the tube, through the temperature-related expansion are very likely.

The potential losses are described here, are "possible" damage. The scale of these potential losses can certainly occur in very small steps (barely detectable) or develop in very small steps. I have no knowledge about long-term studies with the focus on those problems, but facing the material properties, these sorts of damages represent a scenario that is technically reproducible.

 

3. A possible solution

Given the problems outlined, it would make sense to use a ground probe tube, which acts similar like the hardened suspension. For this reason GERES has developed a ground probe of metal composite pipe.

Composite pipe expands only about 2.5 times more than cement.

Another relevant aspect of the cooling-application may be the drilling depth. In Germany, as in the whole Central European region, we have in depth between 25m - 40m depth, a constant temperature between 10 ° C - 11 ° C. At a depth of 100m a temperature of 12 ° C - 13 ° C. At 150m in depth, we have 14 ° C - 15 ° C. These are not absolute values, there might be some fluctuations in both directions. These temperatures are, however, the statistical average.

Surprisingly, most of ground probe fields with a cooling requirement, even where sufficient space is available, are implemented with drilling depth from 100m to 150m. However, a borehole up to 40m with temperatures of 10 ° C - 11 ° C rather speak for cooling than deeper boreholes. Not every property has enough surface to realize the demand for energy with holes up to 40m, but where you have enough size, it makes sense anyway.

 

4. Conclusion

The aspect of thermal compatibility in the borehole is explosive, because based on customer requirements (heating and cooling) is expected from the service provider to match the thermal properties of materials to each other, because the analysis, planning and implementation of the project is carried out by a specialist. Facing the above-mentioned damages, this would consider anyway difficult conversations with the customer. There are, unfortunately, no provider that offers an application-oriented materials selection (if you need just heating you should use this ground probe; if you intent to heat and cool a building use this ground probe and this grouting, etc.) so that the customer depends on the intelligent analysis and project implementation, the service provider offer.

The future of the industry depends, at least if all manufacturers and service providers (probe manufacturers, engineering offices, drilling companies), are able to optimize their products/services, taking into account new awareness and innovations. The common goal may be, to offer credible perspectives and related to ground probe fields, along the market requirements technically long-term solutions.

 

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