2017 Proceedings of the IGSHPA Technical/Research Conference and Expo

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  • Publication
    Transient thermal resistance of borehole heat exchangers for hourly simulations of geothermal heat pumps systems
    (International Ground Source Heat Pump Association, 2017) Priarone, Antonella; Fossa, Marco
    The correct design of borehole fields requires the correct evaluation of the transient ground thermal response in time, but also the accurate estimation of the borehole (BHE) thermal resistance, expecially the grout contribution. Generally, the borehole thermal resistance is considered as steady-state; however, when considering the borefield hourly response to the building variable thermal loads, also the transient behavior of the grout thermal resistance plays an important role, which is quite often neglected. This study analyzes, with a dimensionless approach, the transient grout thermal resistance, with particular attention devoted to the effect of the boundary condition imposed to the internal tubes, namely imposed heat flux, imposed temperature and imposed convective coefficient, the last being the real operating conditions. In addition, the effects of grout to ground thermophysical properties and of shank spacing are analysed. The steady state numerical results are also compared with literature correlations. Finally, numerical evidences are given to demonstrate that the usual approach of calculating the overall BHE resistance just summing the grout resistance, numerical obtained by imposing a temperature on the tube surface, to the convective one can lead to meaningful errors at low Biot numbers.
  • Publication
    Thermal response of helix ground heat exchangers
    (International Ground Source Heat Pump Association, 2017) Fossa, Marco; Stutz, Benoit; Priarone, Antonella; Coperey, Antoine
    This paper is devoted to the thermal analysis of shallow ground heat exchangers with pipes arranged in a helix configuration. The pipes where the carrier fluid is circulated typically embrace a cylindrical volume that is filled by ground or concrete, the latter being the case of the so called geopiles. Other pipes dispositions include conic helices that can be easily inserted in proper excavations. The analysis of the transient thermal behavior of a helix/ground assembly is here carried out according to different approaches, including the exploitation of superposition techniques, the finite element modelling and experiments in a reduced scale mock up. Different geometrical configurations have been taken into account and also the variability of ground and concrete thermal properties have been considered. A detailed description of the experimental set up is provided and the model results have been processed in order to develop suitable temperature response factors (or g -functions) to be employed for predicting the ground heat exchanger behavior in different operating conditions.
  • Publication
    Investigation on the effects of different time resolutions in the design and simulation of BHE fields
    (International Ground Source Heat Pump Association, 2017) Fossa, Marco; Rolando, Davide; Priarone, Antonella
    The correct design of a field of Borehole Heat Exch angers (BHE) requires the knowledge of ground thermal properties, heat pump performance and building heating and cooling demand. The sequence o f heat pulses from (to) the ground by the heat pump can be described according to different time steps , from hours to months and even years. The monthly time step approach is often the preferred design choice which involves recursive calculations (temporal superposition techniques) and the availability of precalculated temperature response factors (or g-functions) for given BHE field geometries. Such a complex computing task is usually performed thanks to commercial codes in order to fulfil a carrier fluid temperature at the end of a given time horizon, typically 10 or 25 years. In this paper the monthly design approach (EED code and TecGeo proprietary code) is compared with the three thermal pulse approach (modified ASHRAE Method Tp8) and it is demonstrated that for a representative series of case studies the three pulse calculation, easy to be performed at engineering level, is able to provide the correct BHE field overall length with 8% accuracy with respect to the reference monthly calculations.
  • Publication
    Influence of ground heat exchanger zoning operation on the GSHP system long-term operation performance
    (International Ground Source Heat Pump Association, 2017) Yu, Mingzhi; Rang, Hongmei; Zhao, Jinbao; Zhang, Kai; Fang, Zhaohong
    To alleviate the ground heat accumulation after long term running of ground source heat pumps (GSHP), ground heat exchanger (GHE) zoning operation can be adopted. Two GHE operation modes - zoning operation and full running - are compared in a case where heat release to the ground in summer is larger than the heat extraction from the ground in winter. In this study the soil thermal conductivity, volumetric specific heat capacity, borehole depth and spacing are 2.0 W(mK)-1, 5.0x106 J(m3K)-1, 100m and 5m respectively with the boreholes arranged in a square 20x20 array. Under the given conditions the simulation results show that GHE zoning operation depresses the increase in amplitude of GHE outlet water temperature and so that the GSHP systems operate normally throughout the whole service life. By adopting GHE zoning operation, the energy consumption of the GSHP system is found to be reduced compared with that of a GHE operated without zoning. Operation without zoning shows that the GHE summer outlet water temperature increases faster than that with zoning operation and power demands are increased for the given GSHP load. Furthermore, in this case, the GSHP would not be able to run normally in the last several years due to the condensing temperature exceeding its upper limit.
  • Publication
    Fast and accurate calculation of the soil temperature distribution around ground heat exchanger based on a response factor model
    (International Ground Source Heat Pump Association, 2017) You, Tian; Li, Xianting; Shi, Wenxing; Wang, Baolong
    Ground heat exchanger (GHE) is an important component of ground coupled heat pump system (GCHP). To calculate the soil temperature around GHE accurately and fast, a refined response factor model (RF model) is proposed. It combines the heat transfer inside and outside the U pipe through the temperature of pipe wall and the heat flux of U pipe. For the RF model, after calculating the response factors by CFD simulation, the soil temperature can be calculated by the deduced analytical equations. The sandbox experiment is built up to validate the the RF model. Based on the experiment, this case is also studied by the numerical simulation and the RF model. Results show that the soil temperature differences between the RF model and the experiment are only -0.21°C ~0.69°C at the 96th time step. The relative errors of the soil temperatures between RF model and numerical simulation at the 1800th time step are only 1.86%~3.94%. RF model consumes 30% time of the numerical simulation for the soil temperature calculation with 1800 time steps and consumes only 1% time of the numerical simulation for that with 350400 time steps. Therefore, the RF model is accurate and fast to calculate the soil temperature around the GHE with fluid inside.
  • Publication
    Coupled heating optimization of hybrid GCHP system with heat compensation unit
    (International Ground Source Heat Pump Association, 2017) You, Tian; Li, Xianting; Wu, Wei; Shi, Wenxing; Wang, Baolong
    The coupled HCUT-GCHP system (integrating the GCHP with the efficient heat compensation unit (HCUT)) can operate in heat compensation mode and coupled heating mode to effectively eliminate the soil thermal imbalance and increase the heating supply at peak heating load. The coupled heating strategy directly influences the soil heat extraction not only at the peak load but also during the whole year. The system model is built in TRNSYS to investigate the optimal strategies of the coupled HCUT-GCHP systems with different boreholes. Results show that the systems with 80% and 100% boreholes can keep soil thermal balance well at different starting temperatures of coupled heating mode. Taking the system with 60% boreholes as an example, when the starting temperature of coupled heating mode increases from 0°C to 9°C, the power consumption for heating increases by 7.32MWh/°C, while the power consumption for heat compensation decreases by 9.40MWh/°C. For the systems with 100%, 80%, 60% and 40% boreholes, the optimal starting temperatures of coupled heating modes are respectively 9°C, 9°C, 6°C and 3°C and their annual system COPs are respectively 2.66, 2.64, 2.63 and 2.48 under the optimal strategies.
  • Publication
    Model for ground temperature estimations and its impact on horizontal ground heat exchanger design
    (International Ground Source Heat Pump Association, 2017) Xing, Lu; Spitler, Jeffrey D.; Li, Liheng; Hu, Pingfang
    The ground-source heat pump systems are highly efficient and energy saving. Its main disadvantage is a significantly higher installation cost compared to conventional systems. The length of the ground heat exchanger (GHX) piping, consequently, the first cost, depends on several factors; one key factor is the undisturbed ground temperature estimations. Xing and Spitler model was developed which provides a new set of ground temperature results for GHX design. There are two common methods in United States to be used for ground temperature estimations - ASHRAE Handbook method and ASHRAE district heating manual method. This paper presents the impact of Xing and Spitler model development on the horizontal ground heat exchanger (HGHX) design. An analytical HGHX simulation tool is developed. 12 geographically diverse sites in United States are chosen for the case study. Three different HGHX configurations are investigated. For each site, HGHX design length using the Xing and Spitler model estimated ground temperatures as inputs are compared to design results based on measured ground temperatures; the calculated HGHX design length percentage error are within ±18.9%. The calculated HGHX design length percentage error using the ASHRAE Handbooks results and ASHRAE district heating manual results are within ±38.3% and ±57.7% respectively.
  • Publication
    Three dimensional numerical simulation of double-U pile heat exchangers
    (International Ground Source Heat Pump Association, 2017) Wang, Zhonghao; Hu, Pingfang; Xing, Lu; Zhu, Na; Lei, Fei
    The Ground Source Heat Pump system with pile heat exchangers has been used in engineering in recent years. In this paper, a three-dimensional numerical model of double-U pile heat exchangers is established which was verified through the comparison with ground thermal response test results. The influencing factors for the heat exchange performance of double-U pile heat exchangers are analyzed including pile depth, inlet temperature, initial ground temperature and ground thermal conductivity. An improved method is proposed on the basis of the cylindrical source model, combining with the numerical model. The improved method which is simpler than general analytical model can be used to obtain the ground thermal properties and thermal resistance of energy pile.
  • Publication
    Numerical simulation of pile geothermal heat exchanger with spiral tube considering its thermo-mechanical behavior
    (International Ground Source Heat Pump Association, 2017) Wang, Deqi; Lu, Lin; Cui, Ping
    Pile geothermal heat exchanger (PGHE), which utilizes the building foundation piles as part of the geothermal heat exchangers (GHEs) for a ground-coupled heat pump (GCHP) system, has been attracting the interests of researchers and engineers. However, the continuous heat rejection/extraction of the PGHE to/from the piles will cause significant temperature variations (up to 25?) of piles and the surrounding soil, which can influence the mechanical behavior of the pile foundation severely. A modified direct shear apparatus has been developed to investigate the interface behavior between soil and pile. Then, based on the experiment results, the thermo-mechanical behavior of PGHE with spiral coils was investigated by a 3-D simulation model. The thermal loads induce additional compressive stress when the temperature rise, and the local compressive stress can reach to 9.35MPa near the heat exchanger pipe. Additionally, heat extraction led to a decrease of friction angle and normal contact pressure at the interface between soil and pile, and as a consequence, the shear force decreases with the temperature drop. Compared with no thermal disturbance, the ultimate friction resistance of pile is weakened by 15.37%.
  • Publication
    Heat extraction distributed thermal response test: A methodological approach and in-situ experiment
    (International Ground Source Heat Pump Association, 2017) Rolando, Davide; Acuna, Jose; Fossa, Marco
    The Thermal Response Test (TRT) is a worldwide adopted in-situ methodology able to estimate the ground thermal conductivity and borehole thermal resistance. During the test the carrier-fluid exchanges a constant heat flux with the ground while circulating in a pilot Borehole Heat Exchanger (BHE). During a Distributed Thermal Response Test (DTRT) the ground thermal conductivity and borehole thermal resistance are determined at different vertical sections along the borehole. The measured fluid temperature values are analysed with numerical or analytical approaches based on mathematical models which typically approximate the BHE. Those models are based on some strict assumptions, including pure conduction and constant heat transfer rate. During a heat extraction TRT the operating conditions to the ground are similar to the "winter mode" conditions of a working BHE system. In such case the estimated thermal behaviour of the borehole can differ from the result obtained by means of a heat injection TRT. This issue is of peculiar interest for water-filled boreholes, where the BHE thermal resistance is related to the water temperature and density gradient in the borehole filling-space. In this operating mode a heat pump is usually employed and the constant heat transfer rate condition required by the models can be difficult to be respected since the efficiency of the cooling-machine is dependent on the inlet carrier-fluid temperature to the evaporator. In this paper a methodology to perform a heat extraction DTRT with constant heat transfer rate to the ground is presented. The approach described has been applied in a real water-filled borehole installed in Stockholm, Sweden. Data analysis results are presented and the outcomes regarding the evaluation of the local borehole thermal resistance are discussed and compared with those from an erlier heat injection test performed in the same borehole.
  • Publication
    Analytical solution for optimal mass flow rate in primary circuit of ground-coupled heat pump systems
    (International Ground Source Heat Pump Association, 2017) Picard, Damien; Jorissen, Filip; Helsen, Lieve
    Ground source heat pump (GSHP) systems extract heat or cold from the ground by circulating a heat carrier uid (HCF) in a ground heat exchanger and inject this energy in buildings. This paper shows that there exists an optimal HCF ow rate which minimizes the energy use of such systems. The paper proposes an analytical solution for the optimal ow rate as a function of measurable variables, system parameters and data that can easily be derived from manufacturer data sheets. The analytical solution is validated using a detailed simulation model representing an existing GSHP system of 99 boreholes with a depth of 30m.
  • Publication
    New methods to spatially extend thermal response test assessments
    (International Ground Source Heat Pump Association, 2017) Raymond, Jasmin; Malo, Michel; Lamarche, Louis; Perozzi, Lorenzo; Gloaguen, Erwan; Bégin, Carl
    Thermal response tests (TRTs), used to evaluate the subsurface thermal conductivity when designing ground source heat pump systems, are spatially limited to the vicinity of the borehole where a test is carried out. The subsurface is heterogeneous and the thermal conductivity assessment provided by a TRT is likely to vary beyond the tested borehole. New methods have, therefore, been developed to extend subsurface assessments at the building site and the urban district scales. The first method relies on temperature profiles measured at equilibrium in ground heat exchangers that are reproduced with inverse numerical simulations to infer the terrestrial heat flow and the subsurface thermal conductivity beyond a first TRT. Inversion of temperature profiles was verified at a pilot site in the Appalachians where TRTs had been performed and showed a thermal conductivity estimate within less than 10 % for both approaches. The second method is based on geostatistical simulations to map the distribution of the subsurface thermal conductivity in areas where several ground source heat pump installations are anticipated. A first mapping exercise was achieved to the north of Montreal in the St. Lawrence Lowlands with fours TRTs and ten laboratory measurements interpolated with sequential Gaussian simulations.
  • Publication
    Minimum well separation for small groundwater heat pump (GWHP) systems in Korea: Preliminary analysis based on regional aquifer properties
    (International Ground Source Heat Pump Association, 2017) Park, Byeong-Hak; Ha, Seung-Wook; Lee, Kang-Kun
    Shallow geothermal energy has been estimated to have an excellent applicability in Korea, and its applications for space heating and cooling have steadily increased in recent years. Such application as ground source heat pump (GSHP) system can be classified into closed- and open-loop. In recent years, studies have been conducted to minimize the environmental impacts resulting from pumping/injection and to enhance the efficiency of groundwater heat pump (GWHP) system that is the open-loop system. These studies suggest that the characteristics of the aquifer have a significant role in designing efficient GWHP systems. This study considers various hydrogeological properties of Korea. An open-source numerical code called TRS was used for preliminary and sensitivity analyses of GWHP systems. In the analyses, arrival time when thermal plume arrives at pumping well and temperature change at pumping well were observed with different pumping/injection rates, hydraulic gradient, and well separation. Thus, we derived adequate well arrangement for efficient GWHP operation.
  • Publication
    Heat pump capacity effects on peak electricity consumption and total length of self- and solar-assisted shallow ground heat exchanger networks
    (International Ground Source Heat Pump Association, 2017) Eslami Nejad, Parham; Cimmino, Massimo; Hosatte-Ducassy, Sophie
    A new "self-assisted" Ground Source Heat Pump (GSHP) system configuration is proposed to address the relatively high peak electricity demand of undersized GSHP systems equipped with auxiliary electric heater. In this configuration, ground heat exchangers (GHE) have two independent circuits: the first circuit is used to inject the extra heat produced by the heat pump into the ground during off-peak operations, while the second circuit is used to extract heat in the winter and reject heat in the summer for space heating and cooling, respectively. This configuration is compared against a "solar-assisted" configuration and a conventional single U-tube configuration. An analytical model for shallow GHE networks is used to evaluate the effects of the heat pump nominal capacity and the borehole total length on the total electricity consumption and peak electricity demand of the three configurations. Results show that the self-assisted configuration reduces the peak electricity demand by 47%, in a case with a 29% undersized GHE network and a 16% undersized heat pump nominal capacity, while it increases the total energy consumption by 4.1%. Using a solar-assisted configuration for the same sizing parameters reduces the peak electricity demand by only 6.3% and the total energy consumption by 3.8%.
  • Publication
    Direct expansion ground source heat pump using carbon dioxide as refrigerant: Test facility and theoretical model presentation
    (International Ground Source Heat Pump Association, 2017) Eslami Nejad, Parham; Badache, Messaoud; Ouzzane, Mohamed; Aidoun, Zine
    In an attempt to address recent challenges on using natural refrigerants and to develop further knowledge and expertise in the field of direct expansion ground source heat pump (DX-GSHP), an experimental transcritical carbon dioxide (CO2) test bench was built at CanmetENERGY Research Laboratory. A previously developed theoretical model of the system was modified and validated against a set of experimental results and adopted to investigate the system performance in a wide operating range. A parametric analysis was also performed using the theoretical model for understanding the system and at exploring the performance improvement actions for future installations. Validation results showed that the model predicts the experimentation very well within the uncertainty of the measurement. Furthermore, parametric analysis showed that improper control of some parameters such as gas cooler CO2 outlet temperature and discharge compressor pressure can degrade the system performance by up to 25% and the heat pump heating capacity by 7.5%.
  • Publication
    First measurements of a monitoring project on a BTES system
    (International Ground Source Heat Pump Association, 2017) Monzo, Patricia; Lazzarotto, Alberto; Acuna, Jose
    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.
  • Publication
    Assessment of effective borehole thermal resistance from operational data
    (International Ground Source Heat Pump Association, 2017) Mikhaylova, Olga; Johnston, Ian W.; Narsilio, Guillermo A.
    Ground source heat pump (GSHP) systems use the ground as a source of sustainable thermal energy for heating and cooling of buildings. Efficient design of the ground heat exchangers (GHEs) for these systems is important so that long-term operation is adequate, efficient and cost-effective. Several design methods have been developed to size GHEs, and many of these methods, including the widely used ASHRAE method, use an effective borehole thermal resistance to model thermal processes in boreholes. A correct estimation of this parameter is crucial for an adequate sizing of borehole GHEs. This study estimates an experimental effective borehole thermal resistance of the borehole GHEs of an operating GSHP system based on monitoring data collected during the Elizabeth Blackburn School of Sciences full-scale shallow geothermal operational study in Melbourne, Australia. The experimental resistance is compared with the resistances predicted using several analytical and numerical methods. It was found that the experimental resistance can be significantly different from the resistances predicted by these other methods. The paper discusses possible reasons for such differences.
  • Publication
    Development of polyolefin compound and post-polymerization treatments for ground heat exchangers
    (International Ground Source Heat Pump Association, 2017) Kalantar Mehrjerdi, Adib; Naudin, Sebastien; Skrifvars, Mikael
    A ground source heat pump (GSHP) system can be used for both cooling and heating modes simultaneously for commercial, industrial and residential buildings virtually at any location with great flexibility to cover a wide range of demands all around the world. Polyethylene (PE) has been used as the main raw material in production of the Ground Heat Exchangers (GHE). This paper briefly reviews the history of polyethylene and development in polymerization process with emphasis on the third-generation bimodal structure. The characteristics of PE pipes used in GSHP systems are discussed. This paper is devoted to a critical review on the attempts in post-polymerization treatments of the PE, and GHEs to improve the performance of the systems. The experimental and simulated comparisons show that the enhancement of the thermal conductivity of the material can reduce significantly the overall borehole thermal resistance.
  • Publication
    Updated assessment of the technical potential of geothermal heat pump applications in the United States
    (International Ground Source Heat Pump Association, 2017) Liu, Xiaobing; Hughes, Patrick; Spitler, Jeffrey; Anderson, Arlene
    This paper presents an updated assessment of the technical potential of applying geothermal heat pump (GHP) systems in businesses and homes of the United States. The assessed technical potential includes energy savings, carbon emissions reductions, and consumer energy cost savings. This assessment is based on energy consumption data obtained from the latest survey of the energy consumption of residential and commercial buildings, conducted by the Department of Energy's Energy Information Administration. It uses energy savings data for GHP systems compared with existing conventional HVAC systems, which were obtained from the results of a series of computer simulations. The impacts of various climate and geological conditions, as well as the efficiency and market share of existing conventional HVAC systems, have been taken into account in the assessment.
  • Publication
    Storage of solar thermal energy in borehole thermal energy storage systems
    (International Ground Source Heat Pump Association, 2017) McCartney, John S.; Başer, Tuğçe; Zhan, Ni; Lu, Ning; Ge, Shemin; Smits, Kathleen
    This study focuses on the evaluation of solar thermal energy storage in a medium-scale soil-borehole thermal energy storage (SBTES) system installed in San Diego, CA. The SBTES system consists of an array of thirteen 15 m-deep, closely-spaced borehole heat exchangers installed in conglomerate bedrock. The entire site is above the water table, with relatively dry subsurface conditions. Instrumentation was included into the array to monitor temperature distributions with depth and radial spacing within the array, as well as water content fluctuations near the ground surface. A total of eight evacuated tube solar thermal panels with an absorber area of 4.16 m2 were connected in series to supply heat to a temporary heat storage tank. Results from a 4-month transient heat injection period into the SBTES system are presented in this paper. These include data on the characterization of the thermal properties of the SBTES system, the transient heat flux collected from the solar thermal panels, the corresponding transient heat flux into the subsurface, and the changes in ground temperature.