Development and verification of simulation model of hybrid ground-coupled heat pump systems with inclined boreholes

Pao Yue-kong Library Electronic Theses Database

Development and verification of simulation model of hybrid ground-coupled heat pump systems with inclined boreholes

 

Author: Cui, Ping
Title: Development and verification of simulation model of hybrid ground-coupled heat pump systems with inclined boreholes
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Heat pumps.
Heating.
Department: Dept. of Building Services Engineering
Pages: xxvi, 218 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2239756
URI: http://theses.lib.polyu.edu.hk/handle/200/1842
Abstract: The technology of the ground-coupled heat pump (GCHP) offers a high energy efficient way to provide space heating and cooling as well as domestic hot water. Based on the restrictions of the GCHP application in warm-climate areas, this thesis proposed a new hybrid GCHP (HGCHP) system with a desuperheater and inclined ground heat exchanger (GHE) for the possibility of improving the application of the GCHP system in cooling-dominated buildings. This thesis focuses on developing a new simulation model of the novel HGCHP system and analyzing the energy and exergy performances of the system applied in warm-climate areas. Firstly, an analytical model of the inclined finite-line source has been developed to describe the transient and steady-state heat conduction processes in GHEs with inclined and vertical boreholes for long-term operation. An approximate method with satisfactory accuracy is proposed for the GHE design on the basis of the simulation model and recommended for engineering applications. The experimental validation shows that the analytical model is generally accurate to within +-12%, which is considered to be satisfactory for practical engineering. To compensate for the inaccuracy of the finite line source model for short-time scale, a two-dimensional numerical finite element model has been established within the ANSYS simulation environment. Comparisons between the numerical and analytical results show that the finite line-source model is capable of modeling the GHEs except for a few hours because of the line-source assumption. The numerical finite element model was validated by the measured U-tube wall temperatures during a short-time period. The results demonstrate a reasonable agreement between the numerical and the measured data. A steady-state, distributed parameter model of a water-to-water heat pump with a desuperheater (i.e. an HGCHP unit) has been developed. A comprehensive simulation program of the HGCHP system is thus developed, which is formed by the coupling of the heat pump unit and GHE models. A number of simulation cases were carried out, which demonstrate that the HGCHP system can offer considerable energy savings in the operating modes with DHW heating, especially in the case of using the desuperheater only to preheat hot water. The simulation model of the HGCHP system was extensively validated by experiments, covering four different operation modes which are commonly used in practice, i.e. cooling only, cooling with DHW, heating with DHW and DHW heating modes. The results show, on the whole, the simulation model is accurate to within +-15% of the experimental data, which demonstrates that it is suitable for research study and engineering applications with an acceptable accuracy. An exergy analysis of the HGCHP system is implemented on the basis of experimental data and the simulation model. Comparisons between the HGCHP and conventional systems (air-source heat pump, electric heater and gas-fired boiler) show that the HGCHP system possesses the highest exergy and energy efficiencies. An optimum operating condition of the HGCHP system can be found, which behaves high energy efficiency and simultaneously remains high exergy efficiency through co-analysis of the first and second laws of thermodynamics. Finally, an annual hourly simulation of the hybrid GCHP system is performed within the HVACSIM+ environment for a small residential apartment in Hong Kong. Compared with the conventional GCHP system, the hybrid system can effectively alleviate the unbalanced loads of the GHE on an annual basis. Furthermore, it can offer a significant energy saving of 67% for the DHW heating when compared with a conventional DHW heating system. In summary, the simulation model of the GHE can be used to analyze or design the GHEs with inclined boreholes. The simulation model of the HGCHP with DHW system can provide a useful and effective tool to investigate the system performance in a variety of operating modes. The simulation and experimental results obtained in this thesis demonstrate that the HGCHP system can improve energy performance and occupy less ground area, as well as reduce the hot water heating capacity of boilers or electric heaters. Based on the substantial simulation results, it is feasible and desirable to apply this kind of system to cooling-dominated buildings in Hong Kong and other southern regions of China.

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