Field test and numerical investigation on the heat transfer characteristics and optimal design of the heat exchangers of a deep borehole ground source heat pump system

Abstract Deep borehole ground source heat pump (DBGSHP) is a new type of heat pump heating system which extracts deep geothermal energy through heat exchange and can be applied for space heating in winter. To date, the development of deep borehole heat exchangers (BHEs) is limited to the cognized structure design and there is a lack of the experimental studies. This paper presents the investigation of the heat transfer characteristics of the heat exchanger of a DBGSHP heating system through both field test and numerical simulation. A field test was first carried out based on the DBGSHP implemented in a demonstration project. A numerical model was then developed to facilitate the evaluation of the heat extraction capacity and the outlet temperature of the coaxial deep BHEs. Based on the numerical model developed, a sensitivity study was further performed to examine the effect of the primary parameters including the inlet velocity, inlet temperature, flow pattern (one was that the circulating fluid flowed from the inner pipe to the annular space and the other was that the circulating fluid flowed from the annular space to the inner pipe) and pipe diameter on the performance of deep BHE. The results from the field test indicated that the average heat transfer capacity of each single borehole, the average COP of the heat pump unit and the DBGSHP heating system COP were 286.4 kW, 6.4 and 4.6, respectively. The simulation results matched well with the field test data, and showed that the inlet fluid velocity between 0.3 m/s and 0.7 m/s as well as the circulating fluid flowed from the annular space to the inner pipe can result in a better performance for the system of concern. The results from this study could be used as a reference basis for optimal design of coaxial deep BHE and to promote the utilization of deep geothermal energy.

[1]  Darius Mottaghy,et al.  Implementing an effective finite difference formulation for borehole heat exchangers into a heat and mass transport code , 2012 .

[2]  Michele De Carli,et al.  An analysis of solar assisted ground source heat pumps in cold climates , 2015 .

[3]  J. Lukes,et al.  Impact of global warming on performance of ground source heat pumps in US climate zones , 2015 .

[4]  John W. Lund,et al.  Direct utilization of geothermal energy 2010 worldwide review , 2011 .

[5]  Efstathios E. Michaelides,et al.  Geothermal power production from abandoned oil wells , 2009 .

[6]  Xuedan Zhang,et al.  A review of heat pump systems for heating and cooling of buildings in China in the last decade , 2015 .

[7]  Zeng Yu-chao,et al.  Progress of the enhanced geothermal systems(EGS) development technology , 2012 .

[8]  Ryozo Ooka,et al.  Optimization method for multiple heat source operation including ground source heat pump considering dynamic variation in ground temperature , 2017 .

[9]  Gongsheng Huang,et al.  A transient quasi-3D entire time scale line source model for the fluid and ground temperature prediction of vertical ground heat exchangers (GHEs) , 2016 .

[10]  Josef Weber,et al.  Deep Geothermal Energy Production in Germany , 2014 .

[11]  Pingfang Hu,et al.  Periodic heat flux composite model for borehole heat exchanger and its application , 2015 .

[12]  Ioan Sarbu,et al.  General review of ground-source heat pump systems for heating and cooling of buildings , 2014 .

[13]  Philippe Pasquier,et al.  Joint use of quasi-3D response model and spectral method to simulate borehole heat exchanger , 2014 .

[14]  Jinfeng Mao,et al.  Heat transfer analysis of a vertical ground heat exchanger using numerical simulation and multiple regression model , 2016 .

[15]  Jiliang Chen,et al.  Designing multi-well layout for enhanced geothermal system to better exploit hot dry rock geothermal energy , 2015 .

[16]  Y. Gu,et al.  Development of an equivalent diameter expression for vertical U-tubes used in ground-coupled heat pumps , 1998 .

[17]  Robert J. Moffat,et al.  Describing the Uncertainties in Experimental Results , 1988 .

[18]  T. Kujawa,et al.  Utilization of existing deep geological wells for acquisitions of geothermal energy , 2006 .

[19]  Yong‐Le Nian,et al.  Studies on geothermal power generation using abandoned oil wells , 2013 .

[20]  Fang Liu,et al.  Study on the thermal performance of several types of energy pile ground heat exchangers: U-shaped, W-shaped and spiral-shaped , 2016 .

[21]  Jianing Zhao,et al.  Emergy-based sustainability assessment of different energy options for green buildings , 2015 .

[22]  Seung-Rae Lee,et al.  Numerical evaluation of the effects of groundwater flow on borehole heat exchanger arrays , 2013 .

[23]  Dirk Müller,et al.  Combined simulation of a deep ground source heat exchanger and an office building , 2014 .

[24]  Andreas Zell,et al.  Geometric arrangement and operation mode adjustment in low-enthalpy geothermal borehole fields for heating , 2013 .

[25]  Marc A. Rosen,et al.  Deep Borehole Heat Exchangers — A Conceptual and Comparative Review , 2016 .

[26]  Philippe Pasquier,et al.  Fast fluid and ground temperature computation for geothermal ground-loop heat exchanger systems , 2008 .

[27]  Maoyu Zheng,et al.  Development of a numerical model for the simulation of vertical U-tube ground heat exchangers , 2009 .

[28]  Z. Fang,et al.  A finite line‐source model for boreholes in geothermal heat exchangers , 2002 .

[29]  Michele De Carli,et al.  Long-term performance of an irregular shaped borehole heat exchanger system: Analysis of real pattern and regular grid approximation , 2012 .

[30]  Thomas Kohl,et al.  System performance of a deep borehole heat exchanger , 2002 .

[31]  Xianbiao Bu,et al.  Geothermal energy production utilizing abandoned oil and gas wells , 2012 .

[32]  Marco Fossa,et al.  Improved Ashrae method for BHE field design at 10 year horizon , 2016 .

[33]  Xiaosong Zhang,et al.  Experimental thermal evaluation of a novel solar collector using magnetic nano-particles , 2016 .

[34]  Long Liu,et al.  Performance evaluation before and after solar seasonal storage coupled with ground source heat pump , 2015 .

[35]  Louis Lamarche,et al.  A new contribution to the finite line-source model for geothermal boreholes , 2007 .

[36]  Frédéric Kuznik,et al.  Numerical modelling of geothermal vertical heat exchangers for the short time analysis using the state model size reduction technique , 2010 .

[37]  Gioia Falcone,et al.  A systematic review of enhanced (or engineered) geothermal systems: past, present and future , 2013, Geothermal Energy.

[38]  Michele De Carli,et al.  Analysis of operating modes of a ground source heat pump with short helical heat exchangers , 2015 .

[39]  Liangliang Sun,et al.  Ground source heat pump system: A review of simulation in China , 2012 .

[40]  Ruggero Bertani,et al.  Geothermal power generation in the world 2005–2010 update report , 2012 .

[41]  Hywel Rhys Thomas,et al.  Optimization of operating parameters of ground source heat pump system for space heating and cooling by Taguchi method and utility concept , 2014 .

[42]  P. Olasolo,et al.  Enhanced geothermal systems (EGS): A review , 2016 .

[43]  Lifang Liu,et al.  Proposal and research on a combined heating and power system integrating biomass partial gasification with ground source heat pump , 2017 .

[44]  Xiaozhou Wu,et al.  Research of heat and moisture transfer influence on the characteristics of the ground heat pump exchangers in unsaturated soil , 2016 .

[45]  Per Eskilson Thermal analysis of heat extraction boreholes , 1987 .

[46]  Younes Noorollahi,et al.  Numerical simulation of power production from abandoned oil wells in Ahwaz oil field in southern Iran , 2015 .