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Distributed thermal response tests on pipe-in-pipe borehole heat exchangers
KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.ORCID-id: 0000-0002-3490-1777
KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.ORCID-id: 0000-0002-9902-2087
2013 (Engelska)Ingår i: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 109, nr SI, s. 312-320Artikel i tidskrift (Refereegranskat) [Forskning på konstnärlig grund] 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.

Ort, förlag, år, upplaga, sidor
2013. Vol. 109, nr SI, s. 312-320
Nyckelord [en]
Borehole thermal resistance, Borehole thermal resistance, Coaxial, Pipe-in-pipe, Distributed thermal response test
Nationell ämneskategori
Energisystem
Forskningsämne
SRA - Energi
Identifikatorer
URN: urn:nbn:se:kth:diva-117745DOI: 10.1016/j.apenergy.2013.01.024ISI: 000321724000035Scopus ID: 2-s2.0-84879321257OAI: oai:DiVA.org:kth-117745DiVA, id: diva2:602902
Forskningsfinansiär
EnergimyndighetenStandUp
Anmärkning

QC 20130815

Tillgänglig från: 2013-02-04 Skapad: 2013-02-04 Senast uppdaterad: 2017-12-06Bibliografiskt granskad
Ingår i avhandling
1. Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
Öppna denna publikation i ny flik eller fönster >>Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
2013 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt) [Forskning på konstnärlig grund]
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2013. s. 138
Serie
Trita-REFR, ISSN 1102-0245 ; 13:01
Nyckelord
Borehole heat exchangers, Distributed Thermal Response Test, Ground Source Heat Pumps, Coaxial, U-pipe, Multi-pipe, Pipe-in-pipe, Multi-chamber, Groundwater, Thermosyphon
Nationell ämneskategori
Energisystem Mineral- och gruvteknik Geoteknik Byggproduktion Energiteknik Geofysisk teknik Annan samhällsbyggnadsteknik
Forskningsämne
SRA - Energi
Identifikatorer
urn:nbn:se:kth:diva-117746 (URN)978-91-7501-626-9 (ISBN)
Disputation
2013-02-22, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Projekt
EFFSYS+EFFSYS2
Forskningsfinansiär
EnergimyndighetenStandUp
Anmärkning

QC 20130204

Tillgänglig från: 2013-02-04 Skapad: 2013-02-04 Senast uppdaterad: 2013-02-04Bibliografiskt granskad

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