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  • 1.
    Acuna, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Fossa, Marco
    University of Genova.
    Monzó, Patricia
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Numerically generated g-functions for ground coupled heat pump applications2012In: Proceedings of the COMSOL Conference in Milan, 2012Conference paper (Refereed)
    Abstract [en]

    In most ground-coupled heat pump systems, Borehole Heat Exchangers (BHE) represent the typical engineering solution for utilizing renewable energy from the ground. The design of a complex BHE field is a challenging task, due the inherent transient nature of the thermal interaction between the heat exchangers and the surrounding soil. A computation effective method for solving the 3D transient conduction equation describing the ground response to a variable heat load profile is the temporal superposition of pre-calculated temperature response factors or g-functions. In this study Comsol heat conduction models have been developed to calculate g-function values for a borehole field with 64 boreholes. The aim of the investigation is to get an insight on the numerical generation of temperature transfer functions and to some extent provide new information on the Finite Line Source method for analytically generated g-functions as well as on those existing behind existing design software such as EED. The results generally showed a good agreement in lower time ranges. Further in time, the Comsol model revealed to be influenced either by the domain dimensions or the simulation end time.

  • 2.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Bergvärmepumpar Kan Göras Ännu Mer Effektiva2008In: Enegi&Miljö, ISSN 1101-0568, no 3Article in journal (Other (popular science, discussion, etc.))
  • 3.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes2013Doctoral thesis, comprehensive summary (Other academic)
    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.

  • 4.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Effektivare Utnyttjande av Energibrunnar för Värmepumpar Undersöks på KTH2010In: KYLA Värmepumpar, Vol. 6Article in journal (Other (popular science, discussion, etc.))
  • 5.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Forskningsprojekt Ska Ge Effektivare Bergvärme2009In: VVS Forum, ISSN 0346-4644, no 1Article in journal (Other (popular science, discussion, etc.))
  • 6.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Framtidens värmesystem med borrhålsvärmeväxlare2011In: Energi&Miljö, ISSN 1101-0568, no 2Article in journal (Other (popular science, discussion, etc.))
  • 7.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Improvements of U-pipe Borehole Heat Exchangers2010Licentiate thesis, monograph (Other academic)
    Abstract [en]

    The sales of Ground Source Heat Pumps in Sweden and many other countries are having a rapid growth in the last decade. Today, there are approximately 360 000 systems installed in Sweden, with a growing rate of about 30 000 installations per year. The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a Borehole Heat Exchanger (BHE), a closed loop in a vertical borehole. The fluid transports the heat from the ground to a certain heating and/or cooling application. A fluid with one degree higher or lower temperature coming out from the borehole may represent a 2-3% change in the COP of a heat pump system. It is therefore of great relevance to design cost effective and easy to install borehole heat exchangers. U-pipe BHEs consisting of two equal cylindrical pipes connected together at the borehole bottom have dominated the market for several years in spite of their relatively poor thermal performance and, still, there exist many uncertainties about how to optimize them. Although more efficient BHEs have been discussed for many years, the introduction of new designs has been practically lacking. However, the interest for innovation within this field is increasing nowadays and more effective methods for injecting or extracting heat into/from the ground (better BHEs) with smaller temperature differences between the heat secondary fluid and the surrounding bedrock must be suggested for introduction into the market.

    This report presents the analysis of several groundwater filled borehole heat exchangers, including standard and alternative U-pipe configurations (e.g. with spacers, grooves), as well as two coaxial designs. The study embraces measurements of borehole deviation, ground water flow, undisturbed ground temperature profile, secondary fluid and groundwater temperature variations in time, theoretical analyses with a FEM software, Distributed Thermal Response Test (DTRT), and pressure drop. Significant attention is devoted to distributed temperature measurements using optic fiber cables along the BHEs during heat extraction and heat injection from and to the ground.

  • 8.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Optimera med Rätt Kollektorval2010In: Borrsvängen, ISSN 1103-7938, no 2Article in journal (Other (popular science, discussion, etc.))
  • 9.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Slang intill bergväggen ger effektivare värmeväxling2009In: HUSBYGGAREN, ISSN 0018-7968, no 6Article in journal (Other (popular science, discussion, etc.))
  • 10.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Distributed Thermal Response Tests on a Multi-pipe Coaxial Borehole Heat Exchanger2011In: HVAC & R RESEARCH, ISSN 1078-9669, E-ISSN 1938-5587, Vol. 17, no 6, p. 1012-1029Article in journal (Refereed)
    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.

  • 11.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Evaluation of a coaxial borehole heat exchanger prototype2010In: 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.

  • 12.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Preben
    Palne Mogensen AB.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Distributed Thermal Response Test on a U-Pipe Borehole Heat Exchanger2009In: Proc. Effstock 2009, 11th International Conference on Thermal Energy Storage, Stockholm, Sweden: Academic Conferences Publishing, 2009Conference 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.

  • 13.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    A novel coaxial BHE: Description and first Distributed Thermal Response Test Measurements2010In: Proceedings World Geothermal Congress 2010, 2010, p. paper 2953-Conference paper (Refereed)
    Abstract [en]

    The thermal performance of a Borehole Heat Exchanger plays a significant role when defining the quality of heat exchange with the ground in Ground Source Heat Pumps. Different designs have been discussed and increased interest on innovation within this field has taken place during the last years. This paper presents the first measurement results from a 189 meters deep novel coaxial Borehole Heat Exchanger, consisting of an inner central pipe and an annular channel in direct contact with the surrounding bedrock. The measurements were taken during a distributed thermal response test using fiber optic cables installed in the energy well. Fluid temperature every ten meters along the borehole depth are presented and compared with similar measurements from a common U-pipe heat exchanger. A unique measurement of the borehole wall temperature in the coaxial collector illustrates how effective the heat transfer performance is through the annular channel.

  • 14.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Comprehensive Summary of Borehole Heat Exchanger Research at KTH2010In: IIR/Eurotherm Sustainable Refrigeration and Heat Pump Technology Conference, Stockholm: KTH Royal Institute of Technology, 2010, p. 69-Conference paper (Refereed)
    Abstract [en]

    A research project that aims at presenting recommendations for improving the COP of ground source heat pump systems by 10-20% through better design of Borehole Heat Exchangers (BHE) is described in this paper. Experiments are carried out with temperature measurements taken in different BHE types during heat pump operation conditions as well as during the thermal response tests. It is also expected to point out methods for having natural fluid circulation in the BHE, i.e. demonstrating that the heat carrier fluid can naturally circulate thanks to temperature induced density differences along the borehole depth, and thereby avoiding the use of electricity consuming pumps. A brief background presenting the most relevant work regarding BHE research around the world is first presented, followed by a comprehensive description of the current research at KTH. Some new measurements and obtained results are presented as an estimation of to what extent the project results have been achieved is discussed. An analysis on how the project results could allow reducing the borehole depth keeping today’s Coefficient of Performance is presented.

  • 15.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Distributed Temperature Measurements on a Multi-pipe Coaxial Borehole Heat Exchanger2011In: IEA Heat Pump Conference, International Energy Agency , 2011, p. 4.19-Conference paper (Refereed)
    Abstract [en]

    The first experiences with a multi-pipe borehole heat exchanger prototype consisting of an insulated central pipe and twelve parallel peripheral pipes are described. Secondary fluid distributed temperature measurements along the borehole depth, being the only ones of its kind in this type of heat exchanger, are presented and discussed. The measurements are carried out with fiber optic cables during heat injection into the ground, giving a detailed visualization of what happens both along the central and peripheral flow channels. The heat exchange with the ground mainly occurs along the peripheral channels and an indication of almost no thermal short circuiting, even while having large temperature differences between the down and upwards channels, is observed.

  • 16.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Distributed thermal response tests on pipe-in-pipe borehole heat exchangers2013In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 109, no SI, p. 312-320Article in journal (Refereed)
    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.

  • 17.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Experimental Comparison of Four Borehole Heat Exchangers2008In: Refrigeration Science and Technology Proceedings, Copenhagen: International Institute of Refrigeration, 2008, p. SEC09-W1-09Conference paper (Refereed)
    Abstract [en]

    The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a closed U-pipe loop in a vertical borehole. This fluid transports the heat from the rock to the ground source heat pump evaporator. The quality of the heat exchange with the ground and the necessary pumping power to generate the fluid circulation are dependent on the type of fluid and its flow conditions along the pipe. Four different borehole heat exchangers are tested using ethyl alcohol with 20% volume concentration. The fluid temperatures are logged at the borehole inlet, bottom, and outlet. The collectors are compared based on their borehole thermal resistance and pressure drop at different flow rates. The results indicate that the pipe dimensions play an important roll, spacers might not contribute to better heat transfer, and inner micro fins in the pipes improve the performance of the collectors.

  • 18.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    First Experiences with Coaxial Borehole Heat Exchangers2011In: Proceedings of the IIR Conference on Sources/Sinks alternative to the outside Air for HPs and AC techniques, International Institute of Refrigeration, 2011Conference paper (Refereed)
    Abstract [en]

    Some experiences with coaxial borehole heat exchanger prototypes are discussed here. Four different designs are described as they have been part of a research project at KTH: two pipe-inpipe annular designs, one multi-pipe and one multi-chamber design. A special focus is given to two of the prototypes, a pipe-in-pipe design with the external flow channel consisting of an annular cross section and partly insulated central pipe, and a multi-pipe design with twelve parallel peripheral pipes and an insulated central channel. The secondary fluid temperature profiles at low volumetric flow rates are presented for these two prototypes, measured with fiber optic cables during thermal response tests and allowing a detailed visualization of what happens along the heat exchanger depth. It is the first time this is carried out in these types of borehole heat exchangers. The measurements indicate good thermal performance and point at potential uses for these heat exchangers in different ground coupled applications.

  • 19.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Hill, Peter
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Characterization of Boreholes: Results from a U-pipe Borehole Heat Exchanger Installation2008In: Proceedings 9th IEA Heat Pump Conference 2008: Conference Proceedings, Zurich, Switzerland: International Energy Agency , 2008, p. 4-19Conference paper (Refereed)
    Abstract [en]

    Heat exchange with the bedrock for ground source heat pumps is commonly done with the help of U-pipe energy collectors in vertical boreholes. At the moment, there exist many uncertainties about how efficient the heat transfer between the rock and the collector is. For a complete performance analysis of these systems, a 260 m deep water filled borehole is characterized, by measuring the borehole deviation, the ground water flow and the undisturbed ground temperature. Significant attention is devoted to detailed temperature measurements along the borehole depth during operation providing a complete description of the temperature variations in time both for the secondary working fluid and for the ground water. The results show a deviated borehole from the vertical direction without any relevant ground water flow. The undisturbed ground temperature gradient varies from negative to positive at approximately half of the borehole depth. The transient response of the borehole during the heat pump start up is illustrated and it is observed that there does not exist any thermal short circuiting between the down and up-going pipes when the system is in operation.

  • 20.
    Acuña, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Khodabandeh, Rahmat
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Weber, Kenneth
    Distributed Temperature Measurements on a U-pipe Thermosyphon Borehole Heat Exchanger With CO22010In: Refrigeration Science and Technology Proceedings, Sydney, Australia: International Institute of Refrigeration, 2010Conference 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.

  • 21. Beier, R. A.
    et al.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, P.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Borehole resistance and vertical temperature profiles in coaxial borehole heat exchangers2013In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 102, p. 665-675Article in journal (Refereed)
    Abstract [en]

    Ground source heat pump systems are often coupled to the ground by circulating a fluid through vertical Borehole Heat Exchangers (BHEs). The design of a system requires estimates of the ground thermal conductivity and the borehole thermal resistance, which are usually determined by an in situ thermal response test on a completed borehole. The usual test interpretation methods average the inlet and outlet fluid temperatures and use this mean temperature as the average temperature along the borehole length. This assumption is convenient but does not strictly apply. For a coaxial heat exchanger this paper develops an analytical model for the vertical temperature profiles, which can be used instead of the mean temperature approximation to estimate borehole resistance. The model is verified with measured temperatures on a BHE, where an optical technique allows continuous measurements along a coaxial borehole during a distributed thermal response test. A sensitivity study shows that the proposed method corrects errors in the mean temperature approximation, which overestimates the borehole resistance in a coaxial borehole.

  • 22. Beier, R. A.
    et al.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, P.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Vertical temperature profiles and borehole resistance in a U-tube borehole heat exchanger2012In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 44, p. 23-32Article in journal (Refereed)
    Abstract [en]

    The design of ground source heat pump systems requires values for the ground thermal conductivity and the borehole thermal resistance. In situ thermal response tests (TRT) are often performed on vertical boreholes to determine these parameters. Most TRT analysis methods apply the mean of the inlet and outlet temperatures of the circulating fluid along the entire borehole length. This assumption is convenient but not rigorous. To provide a more general approach, this paper develops an analytical model of the vertical temperature profile in the borehole during the late-time period of the in situ test. The model also includes the vertical temperature profile of the undisturbed ground. The model is verified with distributed temperature measurements along a vertical borehole using fiber optic cables inside a U-tube for the circulating fluid. The borehole thermal resistance is calculated without the need for the mean temperature approximation. In the studied borehole, the mean temperature approximation overestimates the borehole resistance by more than 20%.

  • 23. Beier, Richard A.
    et al.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Paine
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Transient heat transfer in a coaxial borehole heat exchanger2014In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 51, p. 470-482Article in journal (Refereed)
    Abstract [en]

    Ground-source heat pumps often use vertical boreholes to exchange heat with the ground. A transient heat transfer model has been developed for a thermal response test on a pipe-in-pipe coaxial borehole heat exchanger. The analytical model calculates the vertical temperature profiles in the fluid flowing through the pipes, which are coupled to the surrounding grout and ground. The model is verified against measured vertical temperature profiles in the circulating fluid during a distributed thermal response test. The comparison with measured data indicates that the proposed model gives a more accurate estimate of the borehole thermal resistance than the conventional analytical model that uses a mean temperature approximation. The model demonstrates how strongly the shapes of the temperature profiles are dependent on the thermal resistance of the internal pipe wall and the flow direction.

  • 24.
    Björk, Erik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Granryd, Eric
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    Nowacki, Jan-Erik
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Weber, Kenneth
    Bergvärme på djupet: Boken för dig som vill veta mer om bergvärmepumpar2013 (ed. 100)Book (Other (popular science, discussion, etc.))
    Abstract [sv]

    I den här boken får du lära dig mer om bergvärmepumpar. Hur fungerar en värmepump? Hur gör man en lönsamhetskalkyl? Hur upphandlar man? Kan man trimma sitt system? Dessutom: lär dig mer om radiatorsystemet, berget och kollektorn.

  • 25.
    Derouet, Marc
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Monzó, Patricia
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Monitoring and Forecasting the Thermal Response of an Existing Borehole Field2015In: Proceedings World Geothermal Congress 2015, 2015Conference paper (Refereed)
    Abstract [en]

    Ground coupled heating and cooling systems have become very popular during the last decades in Sweden, with about 425000small Ground Source Heat Pumps (GSHP) and 400 large Borehole Thermal Energy Storage (BTES) systems. The largeinstallations have a total installed capacity of about 140 MW and deliver around 800 GWh of energy, out of which circa 80% areused for heating and about 20% for cooling. Normally, all installations are monitored to some extent. At most of them, temperaturesand energy flows on the building side are followed up and even logged. Electricity consumption is also known, as well as energyused by secondary back-up systems. On the ground side only temperatures going in and out from manifolds are followed up in thebest case. However, no information is recorded about how the thermal loads are distributed across the borehole field or along thedepth. This paper is the very beginning of monitoring activities where several new and existing GSHP installations are going to bestudied and forecasted during the next coming years in terms of their thermal response, the object being in this case an existingborehole field consisting of 26 boreholes located in Sweden that has been operating during 15 years. The boreholes are connectedto 3 heat pumps that provide space heating to 150 apartments. The field is divided in two sub-groups: one consisting of 14boreholes drilled in 1998 and connected to two of the heat pumps and a second group drilled in 2009 which is connected to the thirdheat pump. The layout of both fields is uneven (e.g. not following linear or rectangular pattern) and comprise vertical and inclinedboreholes, which is normal in Sweden. The predicted lack of thermal interaction between the borehole groups allowed theindependent study of each sub-borehole field. A method based on the finite line source theory was used to calculate the g-functionof both borehole fields and measured thermal loads were subsequently used as inputs to predict secondary fluid temperatures.

  • 26. Holmberg, Henrik
    et al.
    Acuna, Jose
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Naess, Erling
    Sonju, Otto K.
    Numerical model for non-grouted borehole heat exchangers: Part 2-Evaluation2016In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 59, p. 134-144Article in journal (Refereed)
    Abstract [en]

    In this paper a simplified and not fully discretized numerical model is used to simulate the performance of a non-grouted (water filled) borehole heat exchanger (BHE). The model enables simulation of the initial transient behavior of a BHE and gives transparent insight into the heat transfer mechanism acting during the startup and operation of the BHE installation. To account for the thermal effect of natural convection that arises in non-grouted BHEs, the model is complemented with a Nusselt-correlation. The model is presented in detail in Holmberg et al. (2014) and in the present paper it is evaluated based on distributed temperature measurements from Acuna (2010). The measurements were obtained during a distributed thermal response test (DTRT) and during heat pump operation, both on a 261 m deep borehole equipped with a U-tube collector and a distributed temperature sensing system. Despite the simplifications involved with the model, it agrees well with the measured data even on a time scale on the order of minutes. The Nusselt number related to natural convection in the borehole was found to be 6.4 during the DTRT and 3.68 during heat pump operation. This indicates the large differences in the borehole thermal resistance during heat injection and heat extraction.

  • 27. Holmberg, Henrik
    et al.
    Acuna, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Naess, Erling
    Sonju, Otto K.
    Thermal evaluation of coaxial deep borehole heat exchangers2016In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 97, p. 65-76Article in journal (Refereed)
    Abstract [en]

    This paper presents a performance study of deep borehole heat exchangers. The coaxial borehole heat exchanger (BHE) has been selected because for the present conditions it has a better performance than the conventional U-tube BHE. A numerical model has been developed to study the coaxial BHE. The model predictions are compared to detailed distributed temperature measurements obtained during a thermal response test. The model is found to accurately predict the behavior of a coaxial BHE. The influence of the flow direction of the mass flow is studied for BHE5 in the range 200 m-500 m. A parametric performance study is then carried out for the coaxial case with different borehole depths, flow rates and collector properties. The results clearly show a significant increase in the system performance with depth. In addition, it is shown that with increasing borehole depth, the heat load that can be sustained by the BHE is significantly increased. An overall performance chart for coaxial BHEs for the depths of 300-1000 m is presented. The chart can be used as a guide when sizing deep BHE installations.

  • 28.
    Ignatowicz, Monika
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mazzotti, Willem
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Melinder, Åke
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Methods of BHE flushing, charging and purging in Sweden2016In: Proceedings, 2016Conference paper (Refereed)
    Abstract [en]

    In Sweden, there are more than 500 000 small and about 500 relatively large ground source heat pumps (GSHP) having a total installed capacity of about 5.6 GW delivering approximately 15 TWh. yr-1 of heating and cooling energy in Sweden. The operational lifetime and reliability of any GSHP depends heavily on the way the system is designed, installed and operated. In order to provide a good system performance after installation, aspects such as borehole heat exchanger (BHE) system flushing, charging and purging, among others, should be taken into consideration. The aim of this work has been to review some existing methods of system flushing, charging and purging in order and make observations that may be applicable for the GSHP industry. Two Swedish case studies have been followed up and compared to existing strategies suggested by IGSHPA.The results show that there is a lack of specific recommendations regarding the flushing and purging procedures for BHEs in Sweden. A well-defined range or adaptation of similar IGSHPA standards could help in defining the minimum flush velocity. The two case studies showed different practices, with flushing velocities being significantly higher than the minimum flushing velocity recommended by IGSHPA. Flushing flow rates based on this standard are presented in this work for some typical BHE pipe sizes used in Europe.

  • 29.
    Ignatowicz, Monika
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Mazzotti, W.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Melinder, A.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Alternative alcohol blends as secondary fluids for ground source heat pumps2016In: Refrigeration Science and Technology, International Institute of Refrigeration , 2016, p. 610-617Conference paper (Refereed)
    Abstract [en]

    The most common secondary fluid used for the borehole heat exchangers in Sweden is an aqueous solution of ethyl alcohol (EA) due to its relatively good thermophysical properties and low toxicity. Commercially available ethyl alcohol based fluids in Sweden contain up to 10 wt-% denaturing agents in form of propyl alcohol (PA) and n-butyl alcohol (BA). The aim of this paper was to investigate the performance of the existing ethyl alcohol blend containing two denaturing agents and alternative alcohol blends in terms of the pressure drop and heat transfer in the BHE and comparison with ethyl alcohol based secondary fluid. Experimental results showed that the presence of these denaturing agents improves thermophysical properties such as specific heat capacity, thermal conductivity and dynamic viscosity when added in small concentration. EA18 + PA1.6 + BA0.4 and EA18.4 + PA1.6 present the best characteristics in terms of the heat transfer and pressure drop. Both blends are giving higher heat transfer coefficient by 9.4 % (EA18 + PA1.6 + BA0.4) and 8.11 % (EA18.4 + PA1.6) than pure EA20. Both blends are giving as well lower pressure drop than EA20 by up to 2.7 % (EA18 + PA1.6 + BA0.4) and 3 % (EA18.4 + PA1.6). EA18 + PA1.6 + BA0.4 gives 1.4 % higher heat transfer coefficient and EA18.4 + PA1.6 gives lower pressure drop by up to 0.4 % when these two blends are compared.

  • 30.
    Ignatowicz, Monika
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Mazzotti, Willem
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Melinder, A.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Different ethyl alcohol secondary fluids used for GSHP in Europe2017Conference paper (Refereed)
    Abstract [en]

    The most common secondary fluid used for the borehole heat exchangers in Sweden is aqueous solution of ethyl alcohol (EA). Commercially available ethyl alcohol based fluids in Sweden and other European countries contain various denaturing agents. Ethyl alcohol based secondary fluids in Sweden are distributed as ethyl alcohol concentrate, including up to 12 wt-% denaturing agents in form of propyl alcohol (PA) and n-butyl alcohol (BA). In other European countries, like Switzerland and Finland, the commercial products containing a mixture of methyl ethyl ketone and methyl isobutyl ketone (up to 4.5 vol-%) are used for GSHP application. The chemical character of these denaturing agents can in different ways affect the thermophysical properties. Therefore, the aim of this paper was to investigate the performance of commercially available alcohol blends in Europe in terms of pressure drop and heat transfer in the BHE. The results show that the most commonly used product in Sweden (EA18+PA1.6+BA0.4) presents the best characteristics in terms of higher heat transfer (up to 10 %) and lower pressure drop (up to 2.7 %) among different commercial products found in Europe. Another commercial product used in Switzerland showed second best performance in terms of higher heat transfer (up to 5 %) and lower pressure drop (up to 2 %). Moreover, other products containing higher concentrations of denaturing agents presented the worst performance in terms of lower heat transfer (up to 8 %) and higher pressure drop (up to 1 %) than EA20.

  • 31.
    Lazzarotto, Alberto
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration. KTH - Royal Institute of Technology.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration. KTH - Royal Institute of Technology.
    Monzó, Patricia
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration. KTH - Royal Institute of Technology.
    Analysis and modeling of a large borehole system in Sweden2016Conference paper (Refereed)
    Abstract [en]

    This paper presents a study on the thermal simulationof a large existing borehole thermal energy storage(BTES) system located in Stockholm, Sweden. Thebore field investigated presents an uneven pattern,which comprises vertical and inclined boreholes, for atotal of 130 units. Such complex bore field geometrycannot be modeled with the current availablecommercial design tools. The test case presented isutilized to explore the influence of boundaryconditions and level of detail utilized for representingthe model geometry on the output of the simulation.Two boundary conditions and three geometricalconfigurations were studied. The results show that, inthe considered case, the results obtained with thetested models give a marginal difference, hence alsothe greatest level of simplification can be utilizedwithout loosing accuracy in the analysis.

  • 32.
    Madani, Hatef
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Claesson, Joachim
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Lundqvist, Per
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    The Ground Source Heat Pump: A System Analysis With a Particular Focus on The U-Pipe Borehole Heat Exchanger2010In: 2010 14th International Heat Transfer Conference, Volume 4, 2010, p. 395-402Conference paper (Refereed)
    Abstract [en]

    The mass flow rate of the secondary refrigerant flowing in the borehole heat exchanger of a ground source heat pump is an influential system parameter whose variation can influence the pumping power, efficiency of the pump, heat distribution in the borehole, heat pump heat capacity, and above all, the system Overall Coefficient Of Performance (COP). The present paper uses both in-situ field measurements and modeling to evaluate these effects. From the field measurements, it can be concluded that the thermal contact between U-pipe channels increases as the brine mass flow rate decreases. Furthermore, the modeling results show that there is a certain optimum brine mass flow rate which gives a maximum overall system COP. Different optimum mass flow rates are obtained for different compressor speed and it is shown that their relation is almost linear. However, concerning system COP maximization, it can be concluded that a constant but carefully-selected brine mass flow rate can still be an appropriate option for the variable capacity heat pump unit studied in the present paper where the compressor frequency changes between 30Hz and 75Hz. Concerning the heat capacity maximization in the system, a variable speed brine pump can be used to help the insufficiently-sized compressor to cover the peak heat demand of the building.

  • 33.
    Monzó, Patrcia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuna, Jose
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Fossa, Marco
    University of Genova.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Numerical generation of the temperature response factors for  a borehole heat exchanger field2013In: Numerical generation of the temperature response factors for  a borehole heat exchanger field, 2013Conference paper (Refereed)
    Abstract [en]

    Ground Coupled Heat Pump (GCHP) systems connected to a set of vertical ground heat exchangers require short and long term dynamic analysis of the surrounding ground for an optimal operation. The thermal response of the ground for a multiple Borehole Heat Exchanger (BHE) field can be described by proper temperature response factors or “g-functions”. This concept was firstly introduced by Eskilson (1987). The g-functions are a family of solutions of the transient heat conduction equation and each of them refer to a given borehole field geometry. Furthermore the g-functions are the core of many algorithms for simulating the ground response to a GCHP system, including the well-known commercial software EED.

    Analytical approaches based on the Finite Line Source (FLS) model have been developed by Eskilson (1987), Zeng et al. (2002) and later by Lamarche (2007). Such solutions can be in principle applied together with space superposition to infer the thermal response for any BHE configuration.

    This study is a continuation of the previous work presented in Acuña et al. (2012), and a further investigation is devoted to optimize a numerical model of a squared configuration of 64 boreholes using the commercial software Comsol Multiphysics©. Symmetry conditions and different Fourier numbers have been applied and explored together with the effects related to the dimensions of the calculation domain with respect to the BHE depth and BHE field width. Furthermore, a parametric analysis is addressed to boundary conditions, which points out possible limits on the calculation domain extension. The results of the proposed numerical model are compared with the g-functions embedded within the EED software as well as those calculated by FLS method through the spatial superposition. In a closer approximation to reality, the numerical model is also studied accounting for an adiabatic part at the top of the BHE.

  • 34.
    Monzó, Patrcia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuna, Jose
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    A study of the thermal response of a borehole field in winter and summer2013In: : ICAE2013-524, 2013Conference paper (Refereed)
    Abstract [en]

    A Ground Source Heat Pump system is a well-known technology used to provide space heating and cooling in residential and commercial buildings. For large energy demands, a number of boreholes, which can vary between tens and hundreds, may be required. The boreholes can be arranged in linear, square, rectangular, or any other configuration not necessarily symmetric. The heat exchangers in the boreholes are typically connected in parallel. Recently, the idea of a more flexible configuration of multiple Borehole Heat Exchangers (BHEs) has been introduced in commercial applications, enabling the system to operate in a more versatile manner, dividing the ground into different thermal zones. In this new arrangement, the BHEs are connected into thermal sub-groups allowing them to operate separately as sub-systems, depending on the building energy needs and the seasonal periods.

     

    In this project, the temperature response of a multiple BHE configuration is obtained from simulations in a numerical model using FEM software, Comsol Multiphysics© under different operational conditions. First, the loads are imposed under the usual conditions so that all boreholes are operated to provide heating in winter and cooling in summer. The results of this study show that our numerical model presents a good agreement with the ones generated from EED when the system is balanced. Moreover, some hypothetical scenarios with respect to the BHEs arrangement and the operational mode are performed thanks to the flexibility of our numerical model. The hypothetical scenarios provide a first approach about the thermal behavior of the boreholes and their interactions within the field with respect to its wall temperature, previous operation and thermal storage. Further work will be devoted to study more realistic scenarios.

  • 35.
    Monzó, Patrcia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Bernier, Michel
    École Polytechnique de Montréal, Quebec, Canada.
    Acuña, Jose
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    A monthly based bore field sizing methodology with applications to optimum borehole spacing2016In: ASHRAE Transactions, ISSN 0001-2505, Vol. 122, no 1, p. 111-126, article id OR-16-009Article in journal (Refereed)
    Abstract [en]

    The required length of vertical ground heat exchangers(GHX) used in ground-coupled heat pump (GCHP) systems isdetermined so that the outlet temperature from the GHXremains within certain limits for the worst ground load condi-tions. These conditions may not necessarily occur after 10 or20 years of operation, as is usually assumed, but often occurduring the first year of operation.The primary objective of this paper is to develop a generalmethodology for the calculation of the total required bore fieldlength on a monthly basis during the first year of operationusing the framework of the ASHRAE bore field sizing method.Itisathreephaseprocess.Thefirstphaseconsistsofanalyzingandorderinggroundloadsaccordingtothefirstmonthofoper-ation.Next,afirstsetofrequiredlengthsisdeterminedbyusingthe analyzed ground load components and assuming atemperaturepenaltyTp=0.Then,aniterativeprocesstocalcu-latethetemperaturepenaltyattheendofeachmonthiscarriedout to obtain the final required length for the worst conditions.The methodology is exemplified in a particular case witha slight annual cooling thermal imbalance and with a highinfluence of the hourly peak in heating. For this particularcase, it is shown that the required bore field length occursduring the first year and that the starting month of operationhas a strong influence on the results.Finally,itisshownthatitispossibletoreducetheboreholespacing when the annual ground load is quasibalanced. In thecase studied here, the minimum length occurs for a borehole-to-borehole spacing of about 3.2 m (10.50 ft)

  • 36.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Analysis of the influence of the heat power rate variations in different phases of a Distributed Thermal Response Test2012Conference paper (Refereed)
  • 37.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Lazzarotto, Alberto
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    First Measurements of a Monitoring project on a BTES system2017In: IGSHPA Technical/Research Conference and Expo, Denver, March 14-16, 2017, International Ground Source Heat Pump Association , 2017Conference paper (Refereed)
    Abstract [en]

    Performance of Borehole Thermal Energy Storage (BTES) systems depends on the temperature of the secondary fluid, circulating through the ground-loop heat exchangers. Borehole systems are therefore designed in order to ensure that inlet and outlet temperatures of the secondary fluid are within given operational limits during the whole life-time of the system. Monitoring the operation of the bore fields is crucial for the validation of existing models utilized for their design. Measured data provides valuable information for researchers and practitioners working in the field. A first data-set from an ongoing monitoring project is presented in this article. The monitoring system comprises temperature sensors and power meters placed at strategic locations within the bore field. A distributed temperature sensing rig that employs fiber optic cables as linear sensors is utilized to measure temperature every meter along the depth of nine monitored boreholes, yielding data regarding both temporal and spatial variation of the temperature in the ground. The heat exchanged with the ground is also measured via power meters in all nine monitored boreholes as well as at the manifold level. The BTES system is located at the Stockholm University Campus, Sweden, and consists of 130 boreholes, 230 meters deep. After more than a year of planning and installation work, some selected measurements recorded in the BTES during the first months of operation are reported in this article.

  • 38.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Lazzarotto, Alberto
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Tjernström, Johan
    Nygren, Mikael
    Monitoring of a borehole thermal energy storage in Sweden2016In: CLIMA 2016-proceedings of the 12th REHVA World Congress: volume 3 / [ed] Per Kvols Heiseberg, Aalborg University, Department of Civil Enginnering, 2016, Vol. 3Conference paper (Refereed)
    Abstract [en]

    This paper presents the description of the first stage of a project consisting on the monitoring of a newly installed borehole thermal energy storage (BTES) system that started to operate during the autumn of 2015. The BTES system is designed for approximately 4 GWh per year of heat injection and 3 GWh per year of heat extraction and will provide heating and cooling to a set of institutional facilities at Stockholm University, Sweden. The energy storage system consists of a set of 130 borehole heat exchangers, 230 meters deep. Strategic locations within the bore field have been selected to carry out the measurements. The monitoring system comprises temperature and energy flow meters. The temperature measurements are performed using a distributed temperature sensing set-up which allows to measure temperature along the depth of the boreholes, providing a large amount of data for the characterization of the thermal processes in the ground. During the upcoming years, the measured data will be utilized to evaluate and optimize the actual operational condition of the system, and to test the validity of assumptions made during the design phase. Moreover, the measured data will be utilized for validation of current bore field design methods and to have a better understanding of the thermal interaction between neighboring boreholes.

  • 39.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Lazzarotto, Alberto
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mazzotti, Willem
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Borehole ThermalEnergy Storage: First stages of a monitoring  project2016In: Geo Outlook, Vol. 13, no 3, p. 14-20Article in journal (Other (popular science, discussion, etc.))
  • 40.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    A novel numerical model  for the thermal response of borehole heat exchangers fields2014Conference paper (Refereed)
    Abstract [en]

    The design of a borehole field should be based on a long-term simulation of its thermal response for specified energy loads. This response can be calculated from a pre-calculated dimensionless function, the g-function. This paper is focused on a new approach to the calculation of g-functions; in particular with a precise representation of the boundary conditions at the borehole wall. First, the almost constant temperature along the borehole heat exchanger requires a boundary condition of essentially isothermal boreholes along the depth. In a common case the borehole heat exchangers are connected in parallel, thus all boreholes should have the same temperature. Also, the total heat flow to the borehole field should be constant over time. A numerical model in which the boreholes are filled with a highly conductive material has been built, reproducing the isothermal condition. By thermally interconnecting the boreholes, the equal temperature condition is satisfied. The g-functions of some simple borehole field configurations are presented in this paper. The results show, in general, a good agreement with the existing solutions for a similar boundary condition.

  • 41.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Mogensen, Palne
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Ruiz-Calvo, Félix
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Montagud, Carla M.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    A novel numerical approach for imposing a temperature boundary condition at the borehole wall in borehole fields2015In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 56, p. 35-44Article in journal (Refereed)
    Abstract [en]

    The design of a borehole field should be based on a long-term simulation of its thermal response for the intended energy loads. A well-known method to evaluate the response is based on a pre-calculated dimensionless function, the g-function. When calculating g-functions, there are two commonly used approaches for treating the boundary condition at the borehole wall: a constant heat flux at every instant of time, or a uniform temperature at a constant total heat flow to the borehole field. This paper is focused on a new approach to model the thermal process of borehole fields; in particular with a precise representation of a uniform temperature boundary condition at the borehole wall. The main purpose of this model is to be used as a research tool to either generate g-functions for particular cases or handle situations that cannot be addressed by others methods. First, the almost constant temperature along the borehole heat exchanger in operation requires a boundary condition of essentially isothermal boreholes along the depth. In a common case, the borehole heat exchangers are connected in parallel, thus all boreholes should have the same temperature. Also, the total heat flow to the borehole field should be constant over time. For this purpose, a numerical model in which the boreholes are filled with a hypothetical highly conductive material has been built, reproducing the isothermal condition. By thermally interconnecting the boreholes, the equal temperature condition is satisfied. Finally, the specified total heat flow is fed into one spot at the highly conductive material. The model is validated by generating g-functions of some simple borehole field configurations. The g-functions present, in general, a good agreement with the existing solutions for a similar boundary condition. Moreover, the model is also tested against real experimental data from a 2. ×. 3 borehole field at an office building. The simulated daily fluid temperatures are compared with measured daily fluid temperatures for the sixth year of operation. The simulated values present, in general, a good agreement with the measured data. The results show that there are no significant differences with regard to the boundary conditions at the borehole wall, which for this specific case is due to the fact that the system is thermally balanced.

  • 42.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Puttige, Anjan Rao
    KTH.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    KTH.
    Cazorla, Antonio
    Instituto de Ingeniería Energética, Universidad Politécnica de Valencia, Camino de Vera s/n, Valencia 46022, Spain.
    Rodriguez, Juan
    EnergyLab, Fonte das Abelleiras s/n, Campus Universidad de Vigo, Vigo 36310, Spain.
    Montagud, Carla
    Instituto de Ingeniería Energética, Universidad Politécnica de Valencia, Camino de Vera s/n, Valencia 46022, Spain.
    Cerdeira, Fernando
    Universidad de Vigo, Maxwell 16, Vigo 36310, Spain.
    Numerical modeling of ground thermal response with borehole heat exchangers connected in parallel2018In: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 172, p. 371-384Article in journal (Refereed)
    Abstract [en]

    With bore fields for energy extraction and injection, it is often necessary to predict the temperature response to heat loads for many years ahead. Mathematical methods, both analytical and numerical, with different degrees of sophistication, are employed. Often the g-function concept is used, in which the borehole wall is assumed to have a uniform temperature and the heat injected is constant over time. Due to the unavoidable thermal resistance between the borehole wall and the circulating fluid and with varying heat flux along the boreholes, the concept of uniform borehole wall temperature is violated, which distorts heat flow distribution between boreholes. This aspect has often been disregarded. This paper describes improvements applied to a previous numerical model approach. Improvements aim at taking into account the effect of thermal resistance between the fluid and the borehole wall. The model employs a highly conductive material (HCM) embedded in the boreholes and connected to an HCM bar above the ground surface. The small temperature difference occurring within the HCM allows the ground to naturally control the conditions at the wall of all boreholes and the heat flow distribution to the boreholes. The thermal resistance between the fluid and the borehole wall is taken into account in the model by inserting a thermally resistive layer at the borehole wall. Also, the borehole ends are given a hemispherical shape to reduce the fluctuations in the temperature gradients there. The improvements to the HCM model are reflected in a changed distribution of the heat flow to the different boreholes. Changes increase with the number of boreholes. The improvements to the HCM model are further illustrated by predicting fluid temperatures for measured variable daily loads of two monitored GCHP installations. Predictions deviate from measured values with a mean absolute error within 1.1 and 1.6 K

  • 43.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Puttige, Anjan Rao
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mogensen, Palne
    Cazorla, Antonio
    Rodriguez, Juan
    Montagud, Carla
    Cerdeira, Fernando
    Universidad de Vigo.
    Numerical Modelling of Ground Thermal Response with Borehole Heat Exchangers Connected in ParallelManuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    With bore fields for energy extraction it is often necessary to predict the temperature response to heat loads for many years ahead. Mathematical methods, both analytical and numerical, with different degrees of sophistication, are employed. Often the g-function concept is used, in which the borehole wall is assumed to have a uniform temperature and the heat injected is constant over time. Due to the unavoidable thermal resistance between borehole wall and the circulating fluid and with varying heat flux along the boreholes, the concept of uniform borehole wall temperature is violated, which distorts heat flow distribution between boreholes. This aspect has often been disregarded. This paper describes improvements applied to a previous numerical model approach. Improvements aim at taking into account the effect of thermal resistance between the fluid and the borehole wall. The model employs a highly conductive material (HCM) embedded in the boreholes and connected to an HCM bar above the ground surface. The small temperature difference occurring within the HCM allows the ground to naturally control the conditions at the wall of all boreholes and the heat flow distribution to the boreholes. The thermal resistance between the fluid and the borehole wall is taken into account in the model by inserting a thermally resistive layer at the borehole wall. Also, the borehole ends are given a hemispherical shape to reduce the fluctuations in the temperature gradients there. The improvements to the HCM model are reflected in a changed distribution of the heat flow to the different boreholes. Changes increase with the number of boreholes. The improvements to the HCM model are further illustrated by predicting fluid temperatures for measured variable daily loads of two monitored GCHP installations. Predictions deviate from measured values with a mean absolute error within 1.1 and 1.6 K.  

  • 44.
    Monzó, Patricia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Ruiz-Calvo, Felix
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Acuna, Jose
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Montagud, Carla
    Instituto de Ingenieria Energetica, Universitat Politecnica de Valencia.
    Experimental Validation of a Numerical Model for the Thermal Response of a Borehole Field2014In: ASHRAE TRANSACTIONS 2014, VOL 120, PT 2, ASHRAE , 2014, Vol. 120, no 2Conference paper (Refereed)
    Abstract [en]

    The design of borehole fields for ground coupled-heat pump systems is often based on so-called g-functions, a pre-calculated dimensionless temperature response to a step heat load. The g-function is specific to the borehole field geometry. For a long-term analysis of a particular borehole field, the average borehole wall temperature is obtained from the g-function and a temporal superposition of its thermal load steps. There are two accepted provisions for the g-function regarding the boundary condition used at (and along) the borehole wall: a constant heat flux at every instant of time, or a uniform temperature at constant total heat flow to the borehole field, respectively. In this paper, a numerical model is built up according to the geometrical characteristics and ground thermal properties of a 2x3 borehole demo site at the Universitat Politecnica de Valencia, Spain. The model is separately studied with regard to the two boundary conditions. The models are first compared in terms of their g-function, which are verified against reference solutions. Then, the daily fluid temperatures are obtained from each of these models with measured daily loads during a six year period. The results are compared with measured daily fluid temperatures for the sixth year of operation. The simulated values present, in general, a good agreement with the measured data. The results show that there are no significant differences with regard to the boundary conditions at the borehole wall, which for this specific case is due to the fact that the system is thermally balanced. The simulated temperatures are more accurate during cooling periods.

  • 45.
    Penttilä, Jens
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Monzó, Patricia
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Temperature stratification of circular borehole thermal energy storages2014Conference paper (Refereed)
    Abstract [en]

    Circular borehole field geometries are sometimes preferred when designing borehole thermal energy storage systems, including controlled radial temperature gradients from the center and across the borehole field. A numerical model has been described in this paper in order to study the influence of connecting boreholes in radial zones with different thermal loads. The studied geometry consists of 3 concentric rings having 6, 12, and 18 boreholes, and the boundary condition at the wall of all boreholes can be flexibly changed. In this case, a realistic quasi-uniform temperature condition, observed using distributed temperature measurements, was applied at the borehole wall, giving a temperature gradient of 1 K from bottom to top in all boreholes. The boreholes are thermally connected with each other. Thermally connecting the boreholes is one of the alternatives of the Superposition Borehole Model (SBM). A few control strategies for using the circular borehole field are studied, both for balanced and unbalanced thermal load cases.

  • 46. Ruiz-Calvo, F.
    et al.
    De Rosa, M.
    Acuna, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Corberan, J. M.
    Montagud, C.
    Experimental validation of a short-term Borehole-to-Ground (B2G) dynamic model2015In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 140, p. 210-223Article in journal (Refereed)
    Abstract [en]

    The design and optimization of ground source heat pump systems require the ability to accurately reproduce the dynamic thermal behavior of the system on a short-term basis, specially in a system control perspective. In this context, modeling borehole heat exchangers (BHEs) is one of the most relevant and difficult tasks. Developing a model that is able to accurately reproduce the instantaneous response of a BHE while keeping a good agreement on a long-term basis is not straightforward. Thus, decoupling the short-term and long-term behavior will ease the design of a fast short-term focused model. This work presents a short-term BHE dynamic model, called Borehole-to-Ground (B2G), which is based on the thermal network approach, combined with a vertical discretization of the borehole. The proposed model has been validated against experimental data from a real borehole located in Stockholm, Sweden. Validation results prove the ability of the model to reproduce the short-term behavior of the borehole with an accurate prediction of the outlet fluid temperature, as well as the internal temperature profile along the U-tube.

  • 47. Zhang, L.
    et al.
    Zhang, Q.
    Acuna, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ma, X.
    Improved p(t)-linear Average Method for Ground Thermal Properties Estimation during in-situ Thermal Response Test2015In: Procedia Engineering, Elsevier, 2015, p. 726-734Conference paper (Refereed)
    Abstract [en]

    One potential problem for the ground coupled heat pump is that the ground thermal properties is hardly to be known due to the complicated ground construction. The p(t)-linear average method has been proved that it can improve the accuracy of borehole thermal resistance. However, the p(t)-linear fluid temperature distribution approximation is not agree well with the temperature profile measured by the fiber cable. Thus, in this paper, an improved p(t)-linear average method in which the fluid temperature distribution approximation based on the vertical temperature profile is proposed. With the new vertical temperature profile simulation model, the accuracy for the borehole thermal resistance estimation will be improved comparing to the true value. Besides that, the estimation results are sensitive with the distance between two pipes, and together with the borehole thermal resistance, the distance will be optimized by the outlet fluid temperature. The life cycle cost analysis results of a case study for an office building in Hunan University show that, although the operation cost will be increased, the total cost during the whole lifetime will be reduced with a lower initial investment.

1 - 47 of 47
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