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Evaluation of a coaxial borehole heat exchanger prototype
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-3490-1777
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-9902-2087
2010 (English)In: Proceedings of the 14th ASME International Heat Transfer Conference, ASME Press, 2010Conference paper (Refereed)
Abstract [en]

Different borehole heat exchanger designs have been discussed for many years. However, the U-pipe design has dominated the market, and the introduction of new designs has been practically lacking. The interest for innovation within this field is rapidly increasing and other designs are being introduced on the market. This paper presents a general state of the art summary of the borehole heat exchanger research in the last years. A first study of a prototype coaxial borehole heat exchanger consisting of one central pipe and five external channels is also presented. The particular geometry of the heat exchanger is analyzed thermally in 2-D with a FEM software. An experimental evaluation consisting of two in situ thermal response tests and measurements of the pressure drop at different flow rates is also presented. The latter tests are carried out at two different flow directions with an extra temperature measurement point at the borehole bottom that shows the different heat flow distribution along the heat exchanger for the two flow cases. The borehole thermal resistance of the coaxial design is calculated both based on experimental data and theoretically.

Place, publisher, year, edition, pages
ASME Press, 2010.
Keyword [en]
Coaxial Borehole Heat Exchanger
National Category
Energy Engineering
URN: urn:nbn:se:kth:diva-73125DOI: 10.1115/IHTC14-22374ISI: 000307206500042ScopusID: 2-s2.0-84860503838ISBN: 978-0-7918-2879-2OAI: diva2:488564
14th ASME International Heat Transfer Conference. Washington, DC, USA. August 8–13, 2010

QC 20120425

Available from: 2012-02-01 Created: 2012-02-01 Last updated: 2013-06-11Bibliographically approved
In thesis
1. Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
Open this publication in new window or tab >>Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
2013 (English)Doctoral thesis, comprehensive summary (Other academic) [Artistic work]
Abstract [en]

U-pipe Borehole Heat Exchangers (BHE) are widely used today in ground source heating and cooling systems in spite of their less than optimal performance. This thesis provides a better understanding on the function of U-pipe BHEs and Investigates alternative methods to reduce the temperature difference between the circulating fluid and the borehole wall, including one thermosyphon and three different types of coaxial BHEs.

Field tests are performed using distributed temperature measurements along U-pipe and coaxial heat exchangers installed in groundwater filled boreholes. The measurements are carried out during heat injection thermal response tests and during short heat extraction periods using heat pumps. Temperatures are measured inside the secondary fluid path, in the groundwater, and at the borehole wall. These type of temperature measurements were until now missing.

A new method for testing borehole heat exchangers, Distributed Thermal Response Test (DTRT), has been proposed and demonstrated in U-pipe, pipe-in-pipe, and multi-pipe BHE designs. The method allows the quantification of the BHE performance at a local level.

The operation of a U-pipe thermosyphon BHE consisting of an insulated down-comer and a larger riser pipe using CO2 as a secondary fluid has been demonstrated in a groundwater filled borehole, 70 m deep. It was found that the CO2 may be sub-cooled at the bottom and that it flows upwards through the riser in liquid state until about 30 m depth, where it starts to evaporate.

Various power levels and different volumetric flow rates have been imposed to the tested BHEs and used to calculate local ground thermal conductivities and thermal resistances. The local ground thermal conductivities, preferably evaluated at thermal recovery conditions during DTRTs, were found to vary with depth. Local and effective borehole thermal resistances in most heat exchangers have been calculated, and their differences have been discussed in an effort to suggest better methods for interpretation of data from field tests.

Large thermal shunt flow between down- and up-going flow channels was identified in all heat exchanger types, particularly at low volumetric flow rates, except in a multi-pipe BHE having an insulated central pipe where the thermal contact between down- and up-coming fluid was almost eliminated.

At relatively high volumetric flow rates, U-pipe BHEs show a nearly even distribution of the heat transfer between the ground and the secondary fluid along the depth. The same applies to all coaxial BHEs as long as the flow travels downwards through the central pipe. In the opposite flow direction, an uneven power distribution was measured in multi-chamber and multi-pipe BHEs.

Pipe-in-pipe and multi-pipe coaxial heat exchangers show significantly lower local borehole resistances than U-pipes, ranging in average between 0.015 and 0.040 Km/W. These heat exchangers can significantly decrease the temperature difference between the secondary fluid and the ground and may allow the use of plain water as secondary fluid, an alternative to typical antifreeze aqueous solutions. The latter was demonstrated in a pipe-in-pipe BHE having an effective resistance of about 0.030 Km/W.

Forced convection in the groundwater achieved by injecting nitrogen bubbles was found to reduce the local thermal resistance in U-pipe BHEs by about 30% during heat injection conditions. The temperatures inside the groundwater are homogenized while injecting the N2, and no radial temperature gradients are then identified. The fluid to groundwater thermal resistance during forced convection was measured to be 0.036 Km/W. This resistance varied between this value and 0.072 Km/W during natural convection conditions in the groundwater, being highest during heat pump operation at temperatures close to the water density maximum.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 138 p.
Trita-REFR, ISSN 1102-0245 ; 13:01
Borehole heat exchangers, Distributed Thermal Response Test, Ground Source Heat Pumps, Coaxial, U-pipe, Multi-pipe, Pipe-in-pipe, Multi-chamber, Groundwater, Thermosyphon
National Category
Energy Systems Mineral and Mine Engineering Geotechnical Engineering Construction Management Energy Engineering Geophysical Engineering Other Civil Engineering
Research subject
SRA - Energy
urn:nbn:se:kth:diva-117746 (URN)978-91-7501-626-9 (ISBN)
Public defence
2013-02-22, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
The Swedish Energy AgencyStandUp

QC 20130204

Available from: 2013-02-04 Created: 2013-02-04 Last updated: 2013-02-04Bibliographically approved

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