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Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-3490-1777
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.
Series
Trita-REFR, ISSN 1102-0245 ; 13:01
Keyword [en]
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
Identifiers
URN: urn:nbn:se:kth:diva-117746ISBN: 978-91-7501-626-9 (print)OAI: oai:DiVA.org:kth-117746DiVA: diva2:602905
Public defence
2013-02-22, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
EFFSYS+EFFSYS2
Funder
The Swedish Energy AgencyStandUp
Note

QC 20130204

Available from: 2013-02-04 Created: 2013-02-04 Last updated: 2013-02-04Bibliographically approved
List of papers
1. Distributed Thermal Response Test on a U-Pipe Borehole Heat Exchanger
Open this publication in new window or tab >>Distributed Thermal Response Test on a U-Pipe Borehole Heat Exchanger
2009 (English)In: Proc. Effstock 2009, 11th International Conference on Thermal Energy Storage, Stockholm, Sweden: Academic Conferences Publishing, 2009Conference paper, Published paper (Refereed)
Abstract [en]

In a Distributed Thermal Response Test (DTRT) the ground thermal conductivity and boreholethermal resistance are determined at many instances along the borehole. Here, such a testis carried out at a 260 m deep water filled energy well, equipped with a U-pipe borehole heatexchanger, containing an aqueous solution of ethanol as working fluid. Distributed temperaturemeasurements are carried out using fiber optic cables placed inside the U-pipe, duringfour test phases: undisturbed ground conditions, fluid pre-circulation, constant heat injection,and borehole recovery. A line source model is used for simulating the borehole thermal response.Fluid temperature profiles during the test are presented. The results show local variationsof the ground thermal conductivity and borehole thermal resistance along the boreholedepth, as well as a deviation of the latter as compared to the one resulting from a standardthermal response test.

Place, publisher, year, edition, pages
Stockholm, Sweden: Academic Conferences Publishing, 2009
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-81315 (URN)
Conference
Effstock 2009, 11th International Conference on Thermal Energy Storage, Stockholm, June 14-17 2009
Note
QC 20120502Available from: 2012-02-10 Created: 2012-02-10 Last updated: 2013-02-04Bibliographically approved
2. Distributed Temperature Measurements on a U-pipe Thermosyphon Borehole Heat Exchanger With CO2
Open this publication in new window or tab >>Distributed Temperature Measurements on a U-pipe Thermosyphon Borehole Heat Exchanger With CO2
2010 (English)In: Refrigeration Science and Technology Proceedings, Sydney, Australia: International Institute of Refrigeration, 2010Conference paper, Published paper (Refereed)
Abstract [en]

In thermosyphon Borehole Heat Exchangers, a heat carrier fluid circulates while exchanging heat with the ground without the need of a circulation pump, representing an attractive alternative when compared to other more conventional systems. Normally, the fluid is at liquid-vapor saturation conditions and circulation is maintained by density differences between the two phases as the fluid absorbs energy from the ground. This paper presents some experimental experiences from a 65 meter deep thermosyphon borehole heat exchanger loop using Carbon Dioxide as heat carrier fluid, instrumented with a fiber optic cable for distributed temperature measurements along the borehole depth. The heat exchanger consists of an insulated copper tube through which the liquid CO2 flows downwards, and a copper tube acting as a riser. The results show temperatures every two meters along the riser, illustrating the heat transfer process in the loop during several heat pump cycles.

Place, publisher, year, edition, pages
Sydney, Australia: International Institute of Refrigeration, 2010
Keyword
Thermosyphon Borehole Heat Exchanger
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-73156 (URN)
Conference
9th IIF/IIR Gustav Lorentzen Conference on Natural Working Fluids. Sydney, Australia. April 12-14 2010
Note
QC 20120425Available from: 2012-02-01 Created: 2012-02-01 Last updated: 2013-02-04Bibliographically approved
3. Distributed thermal response tests on pipe-in-pipe borehole heat exchangers
Open this publication in new window or tab >>Distributed thermal response tests on pipe-in-pipe borehole heat exchangers
2013 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 109, no SI, 312-320 p.Article in journal (Refereed) [Artistic work] Published
Abstract [en]

Borehole Thermal Energy Storage systems typically use U-pipe Borehole Heat Exchangers (BHE) having borehole thermal resistances of at least 0.06 K m/W. Obviously, there is room for improvement in the U-pipe design to decrease these values. Additionally, there is a need for methods of getting more detailed knowledge about the performance of BHEs. Performing Distributed Thermal Response Tests (DTRT) on new proposed designs helps to fill this gap, as the ground thermal conductivity and thermal resistances in a BHE can be determined at many instances in the borehole thanks to distributed temperature measurements along the depth. In this paper, results from three heat injection DTRTs carried out on two coaxial pipe-in-pipe BHEs at different flow rates are presented for the first time. The tested pipe-in-pipe geometry consists of a central tube inserted into a larger external flexible pipe, forming an annular space between them. The external pipe is pressed to the borehole wall by applying a slight overpressure at the inside, resulting in good thermal contact and at the same time opening up for a novel method for measuring the borehole wall temperature in situ, by squeezing a fiber optic cable between the external pipe and the borehole wall. A reflection about how to calculate borehole thermal resistance in pipe-in-pipe BHEs is presented. Detailed fluid and borehole wall temperatures along the depth during the whole duration of the DTRTs allowed to calculate local and effective borehole thermal resistances and ground thermal conductivities. Local thermal resistances were found to be almost negligible as compared to U-pipe BHEs, and the effective borehole resistance equal to about 0.03 K m/W. The injected power was found to be almost evenly distributed along the depth.

Keyword
Borehole thermal resistance, Borehole thermal resistance, Coaxial, Pipe-in-pipe, Distributed thermal response test
National Category
Energy Systems
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-117745 (URN)10.1016/j.apenergy.2013.01.024 (DOI)000321724000035 ()2-s2.0-84879321257 (Scopus ID)
Funder
Swedish Energy AgencyStandUp
Note

QC 20130815

Available from: 2013-02-04 Created: 2013-02-04 Last updated: 2017-12-06Bibliographically approved
4. Evaluation of a coaxial borehole heat exchanger prototype
Open this publication in new window or tab >>Evaluation of a coaxial borehole heat exchanger prototype
2010 (English)In: Proceedings of the 14th ASME International Heat Transfer Conference, ASME Press, 2010Conference paper, Published 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
Coaxial Borehole Heat Exchanger
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-73125 (URN)10.1115/IHTC14-22374 (DOI)000307206500042 ()2-s2.0-84860503838 (Scopus ID)978-0-7918-2879-2 (ISBN)
Conference
14th ASME International Heat Transfer Conference. Washington, DC, USA. August 8–13, 2010
Note

QC 20120425

Available from: 2012-02-01 Created: 2012-02-01 Last updated: 2013-06-11Bibliographically approved
5. Distributed Thermal Response Tests on a Multi-pipe Coaxial Borehole Heat Exchanger
Open this publication in new window or tab >>Distributed Thermal Response Tests on a Multi-pipe Coaxial Borehole Heat Exchanger
2011 (English)In: HVAC & R RESEARCH, ISSN 1078-9669, E-ISSN 1938-5587, Vol. 17, no 6, 1012-1029 p.Article in journal (Refereed) Published
Abstract [en]

In a distributed thermal response test, distributed temperature measurements are taken along a borehole heat exchanger during thermal response tests, allowing the determination of local ground thermal conductivities and borehole thermal resistances. In this article, the first results from six heat injection distributed thermal response tests carried out on a new, thermally insulated leg type, multi-pipe coaxial borehole heat exchanger are presented. The borehole heat exchanger consists of 1 insulated central and 12 peripheral pipes. Temperature measurements are carried out using fiber-optic cables placed inside the borehole heat exchanger pipes. Unique temperature and thermal power profiles along the borehole depth as a function of the flow rate and the total thermal power injected into the borehole are presented. A line source model is used for simulating the borehole heat exchanger thermal response and determining local variations of the ground thermal conductivity and borehole thermal resistance. The flow regime in the peripheral pipes is laminar during all distributed thermal response tests and average thermal resistances remain relatively constant, independently of the volumetric flow rate, being lower than those corresponding to U-pipe borehole heat exchangers. The thermal insulation of the central pipe significantly reduces the thermal shunt to the peripheral pipes even at low volumetric flow rates.

Place, publisher, year, edition, pages
London: Taylor & Francis, 2011
Keyword
Ground Source Heat Pump, Borehole Heat Exchanger
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-73091 (URN)000299958700010 ()2-s2.0-84861615998 (Scopus ID)
Projects
EFFSYS2EFFSYS+
Note

QC 20120222

Available from: 2012-02-01 Created: 2012-02-01 Last updated: 2017-11-29Bibliographically approved

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